Sm–Nd isotopic investigation of Neoproterozoic and

Transcrição

Lithos 82 (2005) 345 – 377
www.elsevier.com/locate/lithos
Sm–Nd isotopic investigation of Neoproterozoic and Cretaceous
igneous rocks from southern Brazil: A study of magmatic processes
Maria do Carmo P. Gastala,T, Jean Michel Lafonb,
Léo Afraneo Hartmannc, Edinei Koesterd
a
Centro de Estudos em Petrologia e Geoquı́mica, Instituto de Geociências, UFRGS, P.O. Box 15 022, Porto Alegre, RS 91501-970, Brazil
b
Pará-Iso, Centro de Geociências, UFPA, P.O. Box 1611, Belém, PA 66 075-110, Brazil
c
Centro de Estudos em Petrologia e Geoquı́mica, Instituto de Geociências, UFRGS, P.O. Box 15 001, Porto Alegre, RS 91501-970, Brazil
d
Laboratório de Geologia Isotópica, Instituto de Geociências, UFRGS, P.O. Box 15 001, Porto Alegre, RS 91501-970, Brazil
Received 5 March 2004; accepted 4 January 2005
Available online 26 February 2005
Abstract
Nd-evolutionary paths for diversified igneous suites from southern Brazil are here re-evaluated using published results. We
interpret the e Nd paths considering the secondary fractionation of 147Sm/144Nd due to major petrogenetic processes. The
inclusion of Nd isotopes and geochemical data for Precambrian and Mesozoic basic rocks allow improving the discussion on
the subcontinental lithosphere beneath southern Brazil. Late Neoproterozoic rocks, mostly granitoids, are exposed in two
regions of the southern Brazilian shield, an eastern collisional belt and a western foreland. The latter included two geotectonic
domains amalgamated at this time, the São Gabriel Arc (900–700 Ma), and the Taquarembó cratonic block. Magma genesis
mainly involved mixture of crustal and incompatible-element-enriched mantle components, both with a long residence time.
Continental segments are the Neoarchaean–Paleoproterozoic lower crust (ca. 2.55 Ga) in the western foreland, and
Paleoproterozoic–Neoproterozoic recycled crust (2.1–0.8 Ga) in the collisional belt. Granitoids with a single crustal derivation
are limited in the southern Brazilian Shield. Mixing processes are well-registered in the western foreland, where the re-enriched
old mantle was probably mixed with a 900–700 Ma-old subducted lithosphere and a 2.55 Ga-old lower crust. The contribution
of the latter increased from the early 605–580 Ma to the later 575–550 Ma Neoproterozoic events, which may be due either to
crustal thickening or to delamination of the lithosphere. Magma sources were diversified in the 660–630 Ma collisional belt.
Initially, they involved the mixing between two components with similar Nd isotopic ratios, a 2.1–0.8 Ga-old recycled crust and
a subduction-processed old mantle. Regional heating and abundant production of granitic melts, with diversified contribution of
enriched mantle components, mark the end of the collisional period, at 630–580 Ma. We can also attribute this to the
delamination of the lithosphere, so that the same geodynamic process may explain the magmatism in the whole shield at the end
of the Dom Feliciano Orogeny. Mesozoic rocks include flood basalts from the Cretaceous Paraná Province and sub-coeval
alkalic suites. Multiple processes of metasomatism affected the lithospheric mantle, resulting in some complexity but they
mainly register two enriched-mantle components, both generated during Neoarchaean–Paleoproterozoic events. One end-
T Corresponding author. Tel.: +55 51 3316 6360; fax: +55 51 3316 7302.
E-mail address: [email protected] (M.C.P. Gastal).
0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2004.09.025
346
M.C.P. Gastal et al. / Lithos 82 (2005) 345–377
member has a more pronounced subduction signature. The other one probably resulted from the re-enrichment of the first
component at the end of the Camboriú collisional orogeny (~2.0 Ga).
D 2005 Elsevier B.V. All rights reserved.
Keywords: Nd isotopes; Southern Brazilian Shield; Magma production events; Subcontinental lithosphere; Mantle metasomatism
1. Introduction
Coherent Nd evolution patterns for igneous and
metamorphic rocks from the same region provide an
opportunity to investigate differences between the
magma production events, and to define crustal
provinces (Bennett and DePaolo, 1987). Neoproterozoic igneous associations from the southern Brazilian Shield, mostly granitoids, were formed during two
tectono-thermal events of the Brasiliano Cycle: the
São Gabriel accretionary orogeny (900–700 Ma) in
the west, and the Dom Feliciano collisional orogeny
(660–550 Ma) in the east. Nd-evolutionary patterns
for these associations show two distinct lithospheric
domains in the shield during the late Neoproterozoic
(660–550 Ma), as pointed out by Gastal et al. (in
press). They correspond to the eastern Dom Feliciano
collisional belt and the western Neoproterozoic foreland, composed at this time by the cratonic block
juxtaposed with the early Neoproterozoic São Gabriel
arc. The magma genesis for the late Neoproterozoic
igneous suites involved two old crust segments
(Neoarchaean–Paleoproterozoic and Proterozoic) and
distinct components of the lithospheric mantle. The
results of the previous review, despite some uncertainties, show that this type of approach is valuable for
the recognition of the regional characteristics of
mantle and crustal sources. This has stimulated further
investigations of Nd isotopes to place better constraints on the source materials, and on the timing of
the chemical modifications.
The interpretation of e Nd-evolutionary paths has
been usually based on the assumption that the studied
rocks remain closed with respect to the Sm–Nd
isotopic system during crustal processes (Bennett
and DePaolo, 1987; Cordani et al., 2000). However,
recent studies have shown that significant fractionation of the Sm/Nd ratio may occur during crustal
processes such as deformation and metamorphism
(Chavagnac et al., 1999), anatexis (Ayres and Harris,
1997; Davies and Tommasini, 2000), and even frac-
tional crystallization of granitic melts (Pimentel and
Charnley, 1991). Variations in both composition and
residence times of lithospheric mantle components
(enriched mantle—EM types) make it difficult to
interpret Nd model ages directly as crust-formation
ages. Further complexities may also be due to
different mixtures of distinct mantle components,
and between them and other crustal reservoirs (Arndt
and Goldstein, 1987).
To avoid these limitations, we address two main
issues: fractionation of 147Sm/144Nd during magma
genesis, and characterization of lithospheric mantle
components in the region. We examine the effects on
secondary fractionation of 147Sm/144Nd of major
processes affecting the sources during the generation
of magmas. Such processes include partial melting,
binary mixing of source materials (crust and mantle),
and metasomatism of the subcontinental lithosphere.
Mantle and crust end-members are considered on a
regional basis. We discuss the mantle components on
a comparative study of Proterozoic and Mesozoic,
basic to intermediate igneous rocks. Cretaceous (130–
100 Ma) associations include the extensive continental flood basalts from Paraná Magmatic Province and
sub-coeval alkalic rocks, which have mainly a lithospheric signature (Garland et al., 1996; Hawkesworth
et al., 1988, 1999; Marques et al., 1999). These basalts
and the late Neoproterozoic (660–550 Ma) igneous
rocks represent the two major magma-producing
events in the region, related respectively to the break
up and amalgamation of the western Gondwana
Supercontinent. Therefore, we consider that they
sampled main regional components of the lithosphere.
We also use isotope and geochemical data of selected
Proterozoic dykes from other regions next to southern
Brazil. Some Proterozoic tholeiites share many
compositional features with Cretaceous basalts in the
region, as discussed by Iacumin et al. (2003). This led
these authors to conclude that the regional heterogeneities in mantle sources were established since late
Archaean times, which substantiates the present study.
In summary, the main goal of the paper is to
improve the understanding of the regional lithospheric
sources of the late Neoproterozoic igneous associations. The inclusion of diversified rocks, with distinct
ages in e Nd-evolutionary paths, seems a challenging
but promising approach for studying the geodynamics
of the lithosphere beneath southern Brazil.
2. Geological and geochemical overview
2.1. Proterozoic
The southernmost portion of the Brazilian Shield is
continuously exposed from southern Brazil into
Uruguay (Fig. 1A; Hartmann et al., 2002). We
hereafter designate this region as southern Brazilian
and Uruguayan shields, which include the following
geotectonic domains: (a) the Rio de la Plata Craton
mostly in Uruguay, composed of (Archaean)–Paleoproterozoic terranes unaffected by Neoproterozoic
collisional orogeny (660–550 Ma); (b) the São Gabriel
magmatic arc limited to the northwestern portion of
the southern Brazilian shield and formed during the
Neoproterozoic accretionary orogeny (900–700 Ma);
and (c) the Dom Feliciano Belt in the east of shield
areas including several older crustal fragments (Paleoproterozoic and early Neoproterozoic), which was
reworked during the Neoproterozoic collisional orogeny (660–550 Ma). Two major magnetic discontinuities in the southern Brazilian Shield have N–S and
NNE–SSW trends, and they divide the shield into
three segments that fit roughly the main geotectonic
domains (Fig. 2A; Costa, 1997). The Porto Alegre
discontinuity separates the collisional belt into two
distinct segments (Fig. 2A–C; Fernandes et al., 1995).
The eastern portion includes mainly medium- to highK, calc-alkaline granitoids formed at 820–580 Ma,
and corresponds partly to the Pelotas batholith (Fig.
2C). The western portion of the collisional belt
consists of tectonically reworked Paleoproterozoic
basement rocks, including the Porongos schist belt.
This portion extends westwards making up the central
domain of the shield, which was largely covered with
sediments of the Camaquã Basin at 605–450 Ma (Fig
2A; Paim et al., 2000). Magmatic events and geological units, for which we compiled Nd isotope data,
are summarized in Table 1. Geochemical and isotope
347
data for selected samples of the basic-to-intermediate
Proterozoic rocks are listed in Table 2.
2.1.1. Neoarchaean–Paleoproterozoic domains
Geological units from the Rio de la Plata Craton
were mainly formed during the Trans-Amazonian
Cycle that includes the accretionary Encantadas
(2.25–2.10 Ga) and the collisional Camboriú (2.1–
2.0 Ga) orogenies (Hartmann, 2002). The Rio de la
Plata Craton comprises the Piedra Alta and Nico
Pérez terranes in Uruguay, and the Taquarembó
Block in southern Brazil (Fig. 1A). The southernmost extension of the craton crops out in Argentina–
Tandilia Belt (Fig. 1B), whereas its northernmost
exposure is the basement of the Asunción–Sapucai
graben in central Paraguay (ASU—Fig. 1B; Fulfaro,
1996). Some Archaean remnants were metamorphosed under granulite facies conditions during the
Trans-Amazonian orogenies. For example, the 2.55
Ga Santa Maria Chico Granulite Complex, in the
Taquarembó Block, is a tholeiitic island arc suite
metamorphosed at 2.03–2.02 Ga under high-grade
and high-P conditions (Table 1 and Fig. 2B;
Hartmann, 1998).
Paleoproterozoic associations include low- to highK calc-alkaline granite-gneisses, and metavolcanosedimentary rocks. These associations are well preserved in the Piedra Alta Terrane and Tandilia Belt
(Fig. 1A,B), and similar calc-alkaline associations are
exposed as basement to the Dom Feliciano Belt (Fig.
2C). The 2.23–2.06 Ga Buenos Aires Complex in the
Tandilia Belt includes three magmatic suites (Cingolani and Dalla Salda, 2000; Hartmann et al., 2002):
low- and high-K calc alkaline, and post-collisional
peraluminous (Table 1). All types were overprinted at
0.98–0.79 Ga by low-grade thermal events (Teixeira
et al., 2002). In the Dom Feliciano Belt, the 2.08 Ga
Arroio dos Ratos Complex, composed of medium- to
high-K calc-alkaline rocks, represents the Paleoproterozoic igneous associations (Table 1; Leite et al.,
2000). Older and more primitive calc-alkaline arc
associations also occur in this belt, as exemplified by
the 2.26–2.08 Ga Encantadas Complex (C.C. Porcher,
unpublished data; not shown in Table 1 or Fig. 2C).
The Arroio dos Ratos Complex, in the Pelotas
Batholith, was metamorphosed at ca. 0.63 Ga under
high-grade and low-P conditions (Leite et al., 2000).
Unmetamorphosed, intermediate to acid, 2.02 Ga
348
calc-alkaline dykes in the Tandilia Belt have an E–W
trend (TDD—Fig. 1A; Teixeira et al., 2002), and
represent a post-collisional event to the 2.1–2.0 Gaold Camboriú Orogeny (Table 1).
Intracratonic magmatic activity occurred in Uruguay and Argentina at the Paleoproterozoic–Mesoproterozoic transition (1.8–1.6 Ga). Dyke swarms
mostly composed of tholeiitic basalts and basaltic
andesites represent this event. The Uruguayan dykes
swarm, in the Piedra Alta Terrane, has an ENE–WSW
trend (UDS—Fig. 1A). For these dykes, the ages vary
from 1.79 to 1.73 Ga (Teixeira et al., 1999; Halls et
al., 2001), whereas in the Tandilia Belt younger 1.59
Ga-old dykes with an NE–SW trend occur (TDD—
Fig. 1A; Teixeira et al., 2002). The latter include lowand high-Ti tholeiites (Iacumin et al., 2001), while
UDS-basalts are mostly low-Ti tholeiites (Bossi et al.,
1993). UDS-tholeiites show geochemistry similar to
the 2.02 Ga calc-alkaline andesites from Tandil. Both
have evolved compositions, are CIPW quartz-normative, have extremely low TiO2 (b1 wt.%) and low high
field strength elements-HFSE (Zr, Nb) and Y contents. Incompatible-element ratios as Zr/Y, Y/Nb and
Zr/Nb are therefore in the same range of values for the
two sets of dykes (Table 2). Low Ti/Y and Ti/Zr ratios,
and high (La/Nb)N in these dykes suggest subductionmodified mantle sources (Fig. 3B–D). Mesoproterozoic Tandil tholeiites are CIPW hyperstene-olivine
normative and, contrarily, they were derived from an
enriched mantle less affected by previous subduction
events (Iacumin et al., 2001). Both the dominant lowTi, and the minor high-Ti Tandil tholeiites show low
(La/Nb)N ratios (V2, Fig. 3C). Low-Ti basalts,
however, have depleted compositions characterized
by high Ti/Zr and very low (La/Yb)N and (La/Nb)N
ratios, but high Ba/La ratios (Fig. 3B–D).
2.1.2. Neoproterozoic belts
The two major periods of tectono-magmatic
activity of the Brasiliano Cycle generated, respectively, the northwestern São Gabriel Arc (900–700
Ma) and the Dom Feliciano Belt (660–550 Ma) in
the east of the exposed southern Brazilian Shield
(Fig. 1A; Babinski et al., 1996; da Silva et al., 1999).
The late Neoproterozoic event was due to a northeastern migration of the belt along NE–SW mega-
349
shear zones, driven by one or several collisional
events located east or north of the region (Fernandes
et al., 1995; Leite et al., 2000; Bitencourt and Nardi,
2000). The western portion of the shield at this time
acted as a foreland, including two amalgamated
geotectonic domains: the São Gabriel Arc and the
Taquarembó Block (Chemale, 2000). Such a context
characterizes a post-collisional setting for most of the
granitic associations in the two domains of the shield
formed at 610–550 Ma, at the end of the Dom
Feliciano Orogeny. In the west, late Neoproterozoic
magmatic events were contemporaneous with the
beginning of deposition in the Camaquã foreland
basin (Fig. 1A).
The São Gabriel magmatic arc includes metavolcano-sedimentary sequences—the Vacacaı́ Group,
intruded by low-K calc-alkaline metagranitoids, and
the Cambaı́ Complex (Table 1). The studied units are
from two regions, south and north (Fig. 2B). In the
first one, the Cambaı́ Complex and the Cerro
Mantiqueiras Ophiolite were formed mostly at 750–
730 Ma, and all units were metamorphosed in the
middle to upper amphibolite facies conditions at 730–
700 Ma (Leite et al., 1998). The latter is a segment of
the subarc oceanic crust including harzburgites
interpreted as a refractory mantle component, and
amphibolites that are island arc basalts (IAB) and
basaltic andesites (Leite, 1997). In northern region,
the Vacacaı́ sequences have similar crystallization
ages of ca. 760 Ma, but the Cambaı́ metagranitoids
have a younger U–Pb age of 704F13 Ma (Table 1;
Babinski et al., 1996; Remus et al., 1999, 2000a).
Metavolcano-sedimentary sequences occur juxtaposed
with mafic–ultramafic bodies, and these units were
metamorphosed at 700 Ma under low-grade conditions. In Uruguay, Treinta Y Tres basaltic dykes
probably represent the post-tectonic magmatic events
of the São Gabriel Orogeny. These dykes have an
Fig. 1. Geological maps of Precambrian and Cretaceous rocks in the southern Brazil. A—Geotectonic units from southern Brazilian and
Uruguayan shields, modified from Hartmann et al. (2000) and Santos et al. (2003). B—Paraná Magmatic Province and sub-coeval alkalic rocks,
showing the distribution of regional units and chemical types of tholeiites (Peate et al., 1992; Kirstein et al., 2000; Morbidelli et al., 1995;
Comin-Chiaramonti et al., 1997). Acid volcanics and sedimentary units above basalts are not shown; heavy dot-dashed line for low-Ti
Esmeralda tholeiites; numbers in (B) for T DM(Nd) model ages (Ga) of some Cretaceous units and Proterozoic dykes. Cretaceous alkalic rocks:
ASU—Asunción-Sapucaı́ Graben (potassic and sodic types), Eastern Paraguay Alkalic Province (Comin-Chiaramonti et al., 1997), and PCA—
Passo da Capela Province (Viero, unpublished data). Proterozoic dykes: TTD—Neoproterozoic (ca. 750 Ma) transitional to alkalic, Treinta Y
Tres basalts in Uruguay (Mazzucchelli et al., 1995; Girardi et al., 1996); TDD—Mesoproterozoic (1.59 Ga) tholeiites and Paleoproterozoic
(2.02 Ga) calc-alkaline andesites from Tandil, Argentina (Iacumin et al., 2001); and UDS—Paleoproterozoic (1.7–1.8 Ga) Uruguayan dyke
swarm (Bossi et al., 1993). In (A) and (B), the southern Brazilian shield detailed in Fig. 2 is highlighted; dashed lines for international limits.
350
Table 1
Sketch of magmatic events and geological units for the main geotectonic domains in southern Brazilian, Uruguayan and Argentinian shields
Events
Geological and chemical
features
A. Camaquã Basin–Neoproterozoic to Eopaleozoic
Ordovician intraplate
Restricted tholeiitic volcanic
volcanism (490– 450 Ma)
activity.
Neoproterozoic post-collisional
associations—Dom
Feliciano Orogeny
(605–550 Ma)
(1) Late magmatic episode
(575–550 Ma)
Young and large granitic
Post-tectonic, alkaline
bodies (570–550 Ma)
metaluminous granites.
Syn-tectonic, high-K
calc-alkaline granites.
Post-tectonic volcano-plutonic
association (575–570 Ma)
Alkaline signature, ranging
from metaluminous to weakly
peralkaline compositions.
(2) Early magmatic episode
(605–580 Ma)
Post-tectonic volcano-plutonic
association with shoshonitic
signature, and subordinate
alkaline granites.
B. Dom Feliciano Belt—Pelotas Batholith—Neoproterozoic
Neoproterozoic post-collisional
associations—Dom
Feliciano Orogeny
(660–580 Ma)
(1) Post-tectonic granitoids
Highly evolved calc-alkaline
(615–580 Ma)
granites.
Alkaline and metaluminous
granitoids.
(2) Late- to post-tectonic
granites (620–600 Ma)
High-K calc-alkaline granites.
(3) Syn-transcurrent granitoids Peraluminous, two-mica
(660–610 Ma)
granite.
High-K calc-alkaline granites.
e Nd(t)
Geological units/lithologies
Age (Ma)
Rodeio Velho Formation—basalts and
basaltic andesites.
470F19
(2r)1
14.0 to
9.619
São Sepé granitic complex—
monzogranites and syenogranites.
Jaguari granite—monzogranites
and syenogranites.
Caçapava do Sul granitic
complex—granodiorites, monzogranites
and leucogranites.
Leões Ring Complex—diorites, qz.
monzodiorites, and qz. syenites.
Acampamento Velho Formation—
trachytes, rhyolites and volcanoclastics
rocks.
Santo Antônio granitic massif—
monzonites to monzogranites.
Lavras do Sul Intrusive Complex—
monzodiorites, monzonites, monzogranites
and alkaline syenogranites; minettes and
olivine minettes as dykes.
Lavras do Sul Intrusive Complex—
perthite granites.
Hilário Formation—trachyandesites,
spessartites and volcanoclastics rocks.
558F8 (2r)2
567F4 (2r)3
15.3 to
11.719
12.619
561F6 (2r)4
–562F8 (2r)5
20.2 to
10.313
572F3 (2r)6
13.9 to
11.46
12.6 to
12.519
583F2 (2r)6
9.019
599F7 (2r)3
–599.5F1.4
(2r)3–7
4.3 to
0.26,13,20
585F4 (2r)3–5–7
3.213
Capão do Leão granites—monzogranites
and syenogranites.
Encruzilhada do Sul Intrusive Suite:
Monzonites, qz. monzonites, and qz.
syenites.
Monzogranites and syenogranites.
Arroio Moinho granite—monzogranites
and syenogranites.
Pinheiro Machado Suite (young-PMS)—
syeno and monzogranites produced by
remelting of old units during a late
tectono-thermal event.
Cordilheira-type metagranites—
monzogranites and leucogranites.
Quitéria metagranite—granodiorites.
9.0 to
2.319
583F3 (2r)8
3.79
611F3 (2r)8
10.89
594F5 (2r)9
595F1 (2r)9
15.69
4.69
613F6 (2r)9
6.6 to
5.79
638F6 (2r) to
617F8 (2r)10
658F4 (2r)10
7.4 to
5.421
7.5 to
7.321
351
Table 1 (continued)
e Nd(t)
Events
Geological and chemical
features
Geological units/lithologies
Age (Ma)
Neoproterozoic flat-lying
association—São Gabriel
Orogeny (900–700 Ma)
Medium- to high-K
calc-alkaline granitoids.
Pinheiro Machado Suite (old-PMS)—
tonalites and granodiorites, with abundant
qz. diorites enclaves, hybrid rocks and
xenoliths of the basement.
Piratini Complex—tonalitic gneisses as
xenoliths.
806F17 (2r)11
8.0 to
5.39
781F5 (2r)11
8.611
C. São Gabriel Arc—Neoproterozoic
Neoproterozoic island arc
Low-grade
associations—São Gabriel
volcano-sedimentary
Orogeny (900–700 Ma)
sequences and
mafic–ultramafic intrusions.
Vacacaı́ Group—Campestre Formation:
753F2 (2r)12
low-K calc-alkaline, intermediate-to-acid
metavolcanics.
Mata Grande Gabbro—stratified
mafic–ultramafic intrusive body including
peridotites, troctolites, leucogabbros
and anorthosites.
Low-K calc-alkaline granitoids Cambaı́ Complex—metadiorites,
North: 704F13
and mafic–ultramafic
metatonalites and metatrondhjemites.
(2r)13
sequences, metamorphosed in
South: 879F14
amphibolite facies conditions.
(2r)
to 735F10 (2r)5
Cerro Mantiqueiras Ophiolite—
733F10 (2r)5
serpentinized harzburgites, amphibolites
and Mg-rich schists.
D. Reworked cratonic segments—Pelotas Batholith—Paleoproterozoic
Paleoproterozoic flat-lying
Medium- to high-K
Arroio dos Ratos Complex—
association—Camboriú
calc-alkaline granite-gneisses. metagranodiorites and metatonalites.
Orogeny (2.1–2.0 Ga)
E. Rio de la Plata Craton—Archaean to Paleoproterozoic
Mesoproterozoic intraplate
activity.
event, Tandilia Belt
(1.59 Ga)
Paleoproterozoic intraplate
event, Piedra Alta Terrane activity.
(1.79–1.73 Ga)
Paleoproterozoic post-tectonic Restricted calc-alkaline
event, Tandilia Belt—
volcanic activity.
Camboriú Orogeny
(2.1–2.0 Ga)
Paleoproterozoic flat-lying
Low- to high-K calc-alkaline,
association, Tandilia Belt— and peraluminous
Camboriú (2.1–2.0 Ga) and granite-gneisses.
Encantadas (2.25–2.12 Ga)
orogenies
Neoarchaean island arc
Granitoids, mafic–ultramafic
association, Taquarembó
and sedimentary rocks
Block (ca. 2.70–2.55 Ga)
metamorphosed in granulitic
facies conditions at the
Camboriú Orogeny
(ca. 2.03 Ga).
+7.813
+5.213
+5.2 to
+2.813
+6.3 to
+5.822
+6.2 to
+1.222
2078F13 (2r)11
+6.611
1588F11 (2r)14
7.2 to
+4.725T
Uruguayan dyke swarm—basaltic andesites 1727F10 (1r)15
and andesites.
and
1790F5 (2r)16
Tandil dykes (E–W)—basaltic andesites,
2020F24 (2r)14
andesites and rhyolites.
5.6 to
1.215T
Buenos Aires Complex—metagranites,
metatonalites, metatrondhjemites,
migmatites and amphibolites, and minor
schists, marbles and metavolcanics.
4.3 to
0.117
Tandil and Azul dykes (NW–SE)—
basalts and basaltic andesites.
2234F15 (2r)
to
2065F9 (2r)17
Santa Maria Chico Granulitic Complex— ca. 255018
depleted-LILE bimodal basic–acid
association, with metatonalites,
metatrondhjemites, metabasalts, piroxenites
and metasedimentary rocks.
3.6 to
2.725T
+3.3 to
+1.123–24
352
estimated age of 750–700 Ma and crosscut the old
granitoids from the Dom Feliciano Belt with a N–S
trend (TTD—Fig. 2B; Mazzucchelli et al., 1995;
Girardi et al., 1996). They are tholeiites and transitional basalts with low (La/Nb)N and high Ti/Y ratios,
plotting close to the Mesoproterozoic high-Ti basalts
from Tandil in Fig. 3A–D.
The Pelotas Batholith comprises the 806–613 Ma
Pinheiro Machado Suite (da Silva et al., 1999), and
several younger 660–580 Ma granite suites (Fig. 2C).
The Pinheiro Machado Suite is a high-K calc-alkaline
association with a flat-lying structure, and abundant
mafic enclaves and xenoliths (Table 1). Two age
groups of granitoids compose the Pinheiro Machado
Suite (PMS), interpreted as injection migmatites by da
Silva et al. (1999). According to these authors, the 613
Ma-old syenogranites and monzogranites correspond
to anatectic melts (young-PMS), while the less
evolved 806 Ma facies comprises the mesosomes
(old-PMS). The younger and more abundant granitoids can be grouped into three petrotectonic associations (Philipp et al., 2002; Koester et al., 2001): (1)
syn- to late-transcurrent, high-K calc-alkaline and
peraluminous, (2) late- to post-tectonic, high-K calcalkaline, and (3) post-tectonic, highly evolved calcalkaline and alkaline (Table 1). Syn-transcurrent suites
related to the Dorsal de Canguçu Shear Zone comprise
high-K calc-alkaline granodiorites (658F4 Ma), and
two-mica leucogranites (638 to 617 Ma; Frantz et al.,
2003). The emplacement of the late- and post-tectonic
granite suites occurred in a shorter time (615–580
Ma), with a marked peak at ca. 600 Ma (Babinski et
al., 1997; Koester et al., 2001; Philipp et al., 2002).
Magmatic events in the Camaquã foreland basin
have diversified geochemical affinity (Table 1; Fig.
2B). Voluminous intermediate-to-acid, shoshonitic,
alkaline and high-K calc-alkaline magmatism characterizes its initial stages at 605–550 Ma. At the end, the
deposition of sediments was intercalated with the
localized 470 Ma–old tholeiitic volcanism (Table 1).
Two major magmatic events have been identified, at
605–580 Ma and at 575–550 Ma, in this basin (Gastal
and Lafon, 1998, 2001). The first one includes the
shoshonitic, post-tectonic volcano-plutonic association and subordinate alkaline granites. Volcanic rocks,
mostly trachyandesites, are best exposed nearby the
Lavras do Sul Intrusive Complex, which includes a
diversified group of ca. 599 Ma granitoids (Gastal et
al., submitted for publication). Minettes and spessartites occur as dykes or domes associated with these
intrusive and volcanic rocks. The late magmatic event
includes two groups of rocks (Gastal and Lafon,
2001): (a) the post-tectonic, alkaline silica oversaturated volcanic suite, and minor subvolcanic
bodies formed at 573–572 Ma, and (b) large granitic
bodies formed at 570–550 Ma, including post-tectonic
alkaline metaluminous and syn-tectonic high-K calcalkaline types.
The two age groups of Neoproterozoic basic-tointermediate rocks from the foreland basin have
evolved compositions, plotting in the field of silicasaturated alkalic suites (Fig. 3A). All have low Ti/Zr
and high Zr/Nb ratios (Fig. 3B; Table 2). The high
Ba/La and Ba/Nb ratios reflecting elevated concentrations of Ba (900 to 1900 ppm) show that these
rocks are distinct from the other Proterozoic basic
rocks (Fig. 3C; Table 2). They have high (La/Yb)N
ratio implying garnet retention in the source, and
show higher and varied (La/Nb)N, up to 7 (mostly
z3; Fig. 3C,D). The least evolved minette (olminette, mg 69%), however, show the lowest (La/
Nb)N ratio close to those of most of the Neoproterozoic Trienta Y Tres transitional dykes (Table
2). The minette (mg 58%) shows higher (La/Nb)N
and more extreme contents of incompatible elements
(LREE, P2O5, Ba, Sr, Ta, Nb). This suggests the
presence of distinct source materials since the
fractional crystallization does not have any signifi-
Notes to Table 1:
References: 1—U–Pb, Hartmann et al. (1998); 2—U–Pb, Remus et al. (1999); 3—207Pb/206Pb, Gastal et al. (submitted for publication); 4—U–
Pb, Remus et al. (2000a); 5—U–Pb, Leite et al. (1998); 6—207Pb/206Pb, Sm–Nd, Gastal and Lafon (2001); 7—U–Pb, Remus et al. (2000b);
8—207Pb/206Pb, Philipp et al. (2002); 9—U–Pb, Sm–Nd, Babinski et al. (1997); 10—U–Pb, Frantz et al. (2003); 11—U–Pb, Sm–Nd, da Silva et
al. (1999); 12—U–Pb, Machado et al. (1990); 13—U–Pb, Sm–Nd, Babinski et al. (1996); 14—Ar–Ar and U–Pb, Teixeira et al. (2002); 15—
Ar–Ar, Sm–Nd, Teixeira et al. (1999); 16—U–Pb, Halls et al. (2001); 17—U–Pb, Hartmann et al. (2002); 18—U–Pb, Hartmann et al. (1999);
19—Sm–Nd, Chemale et al. (unpublished data); 20—Sm–Nd, Gastal et al. (2003); 21—Sm–Nd, Frantz et al. (1999); 22—Sm–Nd, Leite (1997);
23—Sm–Nd, Hartmann (1987); 24—Sm–Nd, Mantovani et al. (1987); 25—Sm–Nd, Iacumin et al. (2001). T—igneous suites for which Sm–Nd
isotopes are detailed in Table 2. e Nd(t) For CHUR (Chondritic Uniform Reservoir) values from Goldstein et al. (1984 in Rollinson, 1993).
353
Fig. 2. Geological maps of the southernmost Brazilian shield (A) that is separated into two major domains: the western Neoproterozoic foreland
(B) and the Dom Feliciano collisional belt (C). A—Geotectonic units for the shield as in Fig. 1A. Main fault zones and major magnetic
discontinuities are also shown (I—Porto Alegre and II—Caçapava magnetic discontinuities, from Fernandes et al., 1995; Porcher and Lopes,
2000). Minor occurrences of Cretaceous rocks: Passo da Capela Province (PCA) and dykes and sills (Rondina–Palmas subvolcanics—Viero,
1998). DCSZ—Dorsal de Canguçu shear zone. B—Western Neoproterozoic foreland, modified from Gastal and Lafon (1998): 1—São Sepé
Granite Complex; 2—Cambaı́ Complex—northern region and Vacacaı́ Group (Mata Grande Gabbro); 3—Caçapava do Sul Granite Complex;
4—Santo Antônio Granite Massif; 5—Acampamento Velho Formation (rhyolites), Taquarembó Plateau; 6—Leões Ring Complex; 7—Jaguari
Granite; 8—Cambaı́ Complex—southern region and Cerro Mantiqueiras Ophiolite; 9—Lavras do Sul Intrusive Complex; 10—Hilário
Formation (trachyandesites); 11—Rodeio Velho Formation (tholeiites); and 12—Santa Maria Chico Granulite Complex. C—Dom Feliciano
collisional belt, modified from Phillipp et al. (2000) and Koester et al. (2001), including the Porongos Belt and the Pelotas Batholith: 1—
Encruzilhada do Sul Intrusive Suite; 2—Arroio dos Ratos Complex; 3—Quitéria and Cordilheira metagranites; 4—Arroio Moinho Granite; 5—
Pinheiro Machado Suite (PMS) and Piratini gneiss (T DM Nd ages for old- and young-PMS, see text for details); 6—Capão do Leão Granite. In
(B) and (C), numbers are for T DM(Nd) model ages (Ga) of the studied units. Post-Devonian sediments as in Fig. 1A, and white for sedimentary
units of the Camaquã Basin. HKCA—high-K calc-alkaline; CA—calc-alkaline. For references see text and Table 1.
cant effect on the (La/Nb)N ratio (Weaver, 1991).
Nevertheless, intrusive basic-to-intermediate rocks in
the two age groups show larger variations in trace
elements, probably due to open-system differentiation in shallow magma reservoirs.
2.2. Mesozoic
The Cretaceous tholeiitic magmatism of the
Paraná Province covered the Phanerozoic Paraná
Basin sedimentary sequences, deposited after the end
of the Brasiliano Cycle. It was erupted during a
period of ca. 10 my, with a marked peak at 133–129
Ma (Stewart et al., 1996; Hawkesworth et al., 1999).
Alkalic magmatism including sodic and potassic
rocks is present in several provinces, and carbonatites are confined to discrete centres surrounding the
basin (Fig. 1B). These rocks were coeval with and
postdated the tholeiites. Table 3 summarizes the
Mesozoic igneous events from southernmost Brazil,
Sample
Lithology
SiO2
TiO2 FeOt
P2O5 Sr
mg TDM
(Ga)
e Nd(t) f Sm/Nd Rb/Sr I Sr
La/NbNa La/YbNb Ba/Rb Ti/Y Zr/Y Ba/Nb Zr/Nb Reference
A. Neoproterozoic post-collisional associations—Dom Feliciano Orogeny (605–550 Ma)—Camaquã Basin
Early magmatic episode, shoshonitic volcano-plutonic association—Lavras do Sul Intrusive Complex and Hilário Formation (599
Kl 647E Minette
44.51 2.51 11.13 1.61 1948 58 1.48
4.36 0.50 0.057 0.705432 2.8
64.5
Kl 647A Ol minette
47.33 2.54 9.35 0.88 1210 69 1.39
2.68 0.47 0.099 0.7047
1.6
41.1
And 5T Trachyandesite
56.25 0.99 6.63 0.25 1285 53 1.97
8.96 0.43 0.267 0.70472 –
24.9
Lp 401 Sperssatite
57.9 0.91 7.13 0.44 687 59 –
–
–
–
–
3.1
20.5
Ba 7T
Basaltic
53.53 1.03 8.69 0.21 710 63 1.42
2.32 0.45 0.019 0.704963 3.7
39.4
trachyandesite
Lp 21T Spessartite
56.95 0.94 6.45 0.48 803 63 1.93
8.82 0.44 0.06 0.705254 3.4
24.7
Ma).
37.8
19.4
31.0
26.8
18.1
488
750
–
287
247
23.4
14.6
21.6
–
9.7
11.2
51.1
37.4
–
97.7
77.5
5.5
7.9
–
13.2
20.0
177
5.9 70.3
12.7
1–2
3–4
Ma)—Dom Feliciano Belt
1.32
+2.17
+3.73
+2.87
0.28
0.19
0.12
0.12
0.062
0.021
0.021
0.025
0.70364
0.70433
0.70441
0.70453
1.5
1.0
1.6
1.2
9.2
8.2
8.2
3.9
33.9
24.8
11.9
15.7
359
549
464
395
8.1
7.0
7.0
7.0
31.8
15.0
12.8
27.5
14.3
10.4
15.0
23.7
5–6
0.70489
0.70879
0.71077
0.70312
–
0.7044
0.9
2.2
1.8
0.6
3.7
0.7
–
–
–
1.2
2.6
1.3
11.5
12.9
10.7
5.8
1.3
4.4
361
469
474
297
212
269
6.3
7.3
7.0
3.0
3.4
3.4
19.8
37.8
29.0
16.5
68.0
6.2
8.3
13.1
9.8
12.8
44.5
14.8
7–8
0.70549
0.70346
–
0.70444
2.3
1.8
2.1
2.2
5.9
5.0
5.8
6.0
6.4
7.0
7.1
7.9
219
232
234
237
5.9
5.6
5.6
5.7
38.6
28.7
28.6
30.7
17.0
13.5
13.8
15.0
9–10
10.1
–
–
3.4
8.1
7.2
161
228
200
4.7 57.4
6.0 75.4
5.2 39.2
15.0
12.6
13.2
7–8
C. Mesoproterozoic and Paleoproterozoic, intraplate tholeiitic events (1.8–1.6 Ga)—Rio de la Plata
Azul and Tandil dykes (1.59 Ga), Tandilia belt.
A 38
High-Ti basalt
47.87 3.73 14.72 1.47 309 40 2.32
1.10 0.33 0.263
MT 70 High-Ti basalt
48.58 1.96 14.93 0.98 422 43 2.65
7.19 0.43 0.084
MT 67 High-Ti basalt
48.2 1.9 14.59 0.96 448 46 –
–
–
0.097
A2
Low-Ti basalt
50.1 1.29 12.09 0.12 154 51 –
–
–
0.11
A 16
Low-Ti basalt
50.39 0.92 10.19 0.1
171 53 2.6
0.25 0.18 –
A5
Low-Ti basalt
50.63 0.99 10.7 0.11 138 57 2.6
+4.68 +0.01 0.043
Uruguayan dyke swarm (1.73 Ga), Piedra Alta Terrane.
UR 4
Andesite
57.53 0.95 9.3 0.14 207 40 3.01
5.58 0.28 0.242
UR 28
Basaltic andesite 55.57 0.93 9.91 0.14 198 44 –
–
–
0.207
UR 46 Basaltic andesite 55.12 0.86 9.17 0.13 225 50 2.48
2.03 0.35 –
UR 33 Basaltic andesite 54.66 0.83 9.29 0.12 189 52 2.42
1.22 0.34 0.164
Craton
D. Paleoproterozoic post-tectonic, calc-alkaline event—Rio
A 54
Andesite
59.69 0.78 8.51 0.32
A 41
Andesite
58.12 0.8
8.44 0.29
A 40
Basaltic andesite 56.46 0.77 9.38 0.39
dykes (2.02 Ga)
0.70386 2.7
0.70504 2.6
0.70376 3.3
de la Plata Craton,
375 43 2.76
666 53 2.71
564 57 2.66
Tandilia Belt–Tandil
3.61 0.41 0.39
3.50 0.43 0.089
2.73 0.43 0.073
Major elements on volatile-free basis (wt.%); mg=100*MgO/(MgO+FeO), mol% and FeO=0.85*FeOt; e Nd(t) and I Sr—calculated for the mean age referred in each case, T DM(Nd)
model age for DMM values from Goldstein et al. (1984 in Rollinson, 1993), and both e Nd(t) and f Sm/Nd for CHUR values from Goldstein et al. (1984, in Rollinson, 1993). Ratios
normalized for: a—primitive mantle (Hofmann, 1988), b—chondrite (Boynton, 1984 in Rollinson, 1993). Bold for samples taken as the reference component of the UDS-like
enriched mantle type.
References: 1—M.C. Gastal (unpublished data), 2—Gastal et al. (submitted for publication), 3—de Lima (1995), 4—F. Chemale Jr. (unpublished data), 5—Girardi et al. (1996), 6—
Mazzucchelli et al. (1995), 7—Iacumin et al. (2001), 8—Teixeira et al. (2002), 9—Bossi et al. (1993), 10—Teixeira et al. (1999). *—Samples of the Hilário Formation (de Lima,
1995), whose Nd–Sr isotopes are of equivalent samples from F. Chemale Jr. (unpublished data).
B. Neoproterozoic post-tectonic association—São Gabriel Orogeny (820–700
Transitional to alkaline basalts—Treinta Y Tres dykes (750 Ma).
U 122
Basalt
47.08 3.47 16.06 1.47 502 35 1.82
U 136
Basalt
46.73 3.02 12.06 0.68 605 54 1.66
U 140
Basalt
46.77 2.17 11.15 0.39 670 59 1.67
U 127
Basalt
47.05 1.78 10.23 0.25 550 65 1.83
354
Table 2
Nd–Sr isotopes and chemical data for representative samples of Proterozoic post-tectonic and intraplate, basic to intermediate rocks from southern Brazil, Uruguay and Argentina
12
150
A
TP
B
TA
PT
BTA
100
TB
6
High-Ti
PM
BAS
8
OIB
50
Alkaline
2
Subalkaline
0
Low-Ti
Ti/Zr
Na 2O+K 2O
10
4
355
LC
B
BA
A
0
41
46
51
56
61
0
200
400
SiO2
50
600
800
1000
Ti/Y
C
D
80
Slab fluid
Slab fluid
ER
Ba/La
(La/Yb)N
LC
10
ER
OIB
LC
OIB
10
PM
7
0.4
1
(La/Nb)N
8
Western Neoproterozoic foreland
Early magmatic event (605 - 580 Ma)
Minette dykes
Basaltic trachyandesites
Trachyandesites
Opx-bearing diorites and monzodiorites
Spessartites
Late magmatic event (575 - 550 Ma)
Opx-bearing diorites and monzodiorites
1
0.4
PM
1
(La/Nb)N
8
Proterozoic basic-intermediate dykes
Calc-alkaline Tandil dykes
Uruguayan dyke swarm (UDS)
Low-Ti basaltic andesites
High-Ti andesites
Tandil tholeiitic dykes
Low-Ti basalts
High-Ti basalts
Treinta Y Tres dykes
Transitional to alkalic basalts
Fig. 3. Major and trace-element ratios for representative samples of basic to intermediate rocks from Proterozoic associations (MgON4 wt.%,
anhydrous basis and FeO=0.85*FeOt). A—Total alkalis vs. silica diagram with the fields from Le Maitre (1989): B—basalt, BA—basaltic
andesite, A—andesite, TB—trachybasalt, BTA—basaltic trachyandesite, TA—trachyandesite, BAS—basanite, PT—phonolitic tephrite, and
TP—tephritic phonolite; dashed line is the dividing of alkaline and subalkaline compositions (Irvine and Baragar, 1971). B—Ti/Zr vs. Ti/Y with
the fields for high-Ti and low-Ti, Cretaceous Paraná basalts as detailed in Fig. 4B. C and D—Ba/La and (La/Yb)N (chondrite normalized—
Boynton, 1984) vs. (La/Nb)N (primitive mantle normalized—Hofmann, 1988) diagrams, respectively. PM—primitive mantle (Hofmann, 1988),
LC—lower crust (Wedepohl, 1995), slab fluid and ER—melt derived from an eclogite restite (Tatsumi, 2000), OIB—ocean island basalts (Sun,
1980 in Rollinson, 1993). For OIB, (La/Yb)N is the average for selected samples of alkalic basalts (MgON4 wt.%) from Hawaii (GEOROC
database, http://georoc.mpch-mainz.gwdg.de/).
356
Table 3
Sketch of cretaceous magmatic events and geological units in southern Brazil, Uruguay and Paraguay
Province/units/locality
Age (Ma)
Geochemical/geological types
A. Late Cretaceous to Eocene/Oligocene (99–32 Ma)
Eastern Paraguay Province—Asunción
Sapucai graben.
Asunción
70–32
Ultra-alkaline sodic rocks
Passo da Capela Province.
Piratini, Brazil
99–76
Sodic alkalic rocks
B. Early Cretaceous (138–120 Ma)
Eastern Paraguay Province—Asunción
Sapucai graben.
San Juan Bautista region
Alkali basalt–trachyte suite
120
130–118
Basanite–phonolite suite
138–120
Paraná Magmatic Province.
Eastern Uruguay.
Lavalleja and Aiguá series
132–124
Treinta Y Tres serie
Southern Brazil and Uruguay.
Rondina-Palmas, Brazil
134–130
137–123
Esmeralda unit
Palmas unit
Gramado unit
Urubici unit
129
133–127
132
Paranapanema unit
138–133
Lithology
Nephelinites, phonolites and tephrites.
Phonolites and tephritic phonolites.
Sodic alkalic rocks.
Potassic to high potassic,
alkalic rocks.
Potassic to high potassic,
alkalic rocks.
Nephelinites.
Alkali gabbros, syenoggabros, syenodiorites,
trachybasalts and trachyandesites.
Nepheline gabbros, ijolites and Ne
monzoggabros.
Acid volcanics, and subvolcanic
bodies.
Low-Ti tholeiites.
Trachydacites, trachytes, rhyolites and syenites.
Alkalic basalts, as dykes or sills in
shield areas.
Low-Ti tholeiites.
Low-Ti acid volcanics.
Low-Ti tholeiites.
High-Ti tholeiites, interbedded
with Gramado unit.
High-Ti tholeiites.
Basaltic andesites and basalts.
Olivine diabase.
Rhyolite and rhyodacites.
Basalts.
Basalts.
Acid volcanics are not shown in Fig. 1B. Data from Peate et al. (1992), Stewart et al. (1996), Comin-Chiaramonti and Gomes (1996), CominChiaramonti et al. (1997), Viero (1998), and Turner et al. (1999a).
Uruguay and Paraguay. Geochemical and isotope
data for selected samples of, respectively, Paraná
tholeiites and the potassic alkalic rocks from Eastern
Paraguay Province, are in listed Table 4. For
tholeiites, we selected major magma types in southern Brazil and use representative samples from
Garland et al. (1996), and from Kirstein et al.
(2000) in eastern Uruguay.
The Paraná Magmatic Province includes voluminous flood basalts with minor silicic volcanics
capping the sequence, and dyke swarms. Dominant
tholeiitic, basalts and basaltic andesites were subdivided into two chemical groups (low- and high-Ti),
which are geographically distributed into regional
units as shown in Fig. 1B (Peate et al., 1992). High-Ti
magma types have older Ar–Ar ages and predominate
in the north and west, while younger low-Ti types
occur in the south and east of the province (Fig. 1B
and Table 3; Stewart et al., 1996). However, as
discussed by Garland et al. (1996), the field relationships show that the chemical units are not chronostratigraphic. They are diachronous, so that the
chemical variations in time and space reflect the
distribution of source materials mostly within the
lithospheric mantle as indicate the slightly high (La/
Nb)N ratios (Fig. 4C,D). In southern Brazil, high-Ti
units include Paranapanema and Urubici basalts, and
Gramado and Esmeralda tholeiites are the low-Ti
types. The dominant Cretaceous Treinta Y Tres unit in
eastern Uruguay and the low-Ti Gramado tholeiites
are broadly similar (Kirstein et al., 2000; Turner et al.,
1999b). Transitional to alkali basalts with low (La/
Nb)N and Zr/Nb ratios are very localized in the south
of the province. They are reported in eastern Uruguay
Table 4
Nd–Sr isotopes and chemical data for representative samples of Cretaceous, basic to intermediate rocks from southern Brazil, Uruguay and Paraguay
Sample
Lithology
SiO2
TiO2 FeOt
P2O5 Sr
mg T DM
(Ga)
e Nd(t)
f Sm/Nd Rb/Sr I Sr
Tholeiites—Paraná Province (130 Ma)
Eastern Uruguay, Treinta Y Tres serie, low-Ti types.
93L96
Basalt
51.92 1.24 9.89 0.14 248 50 1.98
93L95
Basalt
51.32 1.24 9.96 0.15 577 51 2.13
502-1008
Basaltic and. 54.02 1.18 9.91 0.16 190 52 1.2
93L93
Basaltic and. 53.69 1.23 10.22 0.14 286 53 1.87
Southern Brazil—Western Uruguay, Gramado unit, low-Ti types.
MG-2
Basaltic and. 55.95 1.26 9.56 0.2
208 46 1.46
DUP-08
Basaltic and. 53.23 1.38 11.3 0.23 287 47 1.41
GB-40b
Basalt
50.74 0.92 9.75 0.13 208 59 1.7
GB-20a
Basalt
51.03 0.83 9.33 0.07 193 63 1.73
Southern Brazil, Urubici unit, high-Ti types.
DSM-17b
Basaltic and. 52.95 4.2 12.16 0.63 879 38 1.16
DSM-10
Basalt
51.84 3.7 12.01 0.56 826 41 1.18
DSM-30
Basaltic and. 52.32 3.65 11.71 0.5
657 44 1.32
Southern Brazil—Western Uruguay, Paranapanema unit, high-Ti types.
CB276
Basalt
51.54 3.22 15.55 0.39 206 32 1.41
CB234
Basalt
51.73 2.68 13.44 0.37 327 40 1.2
CB747
Basalt
50.48 1.96 13.01 0.24 173 47 1.35
1
0.094
0.07
0.073
0.059
0.70712
0.707
0.70745
0.70727
3.1
2.6
1.9
2.1
48.9
53.5
35.1
28.3
11.1
17.2
13.7
13.4
0.026 0.70716 3.6
0.102 0.70725 2.2
0.1
0.70735 2.3
27.7
41.4
72.8
832
655
637
1109
10.2
14.8
19.1
29.1
22.6
47.8
25.9
36.2
6.6
6.8
6.4
6.7
18.9
11.2
12.3
564 10.2 62.7
525 13.9 36.1
572 14.8 41.6
14.0
7.2
5.9
1
2–3
7.06
7.51
0.19
8.81
0.28
0.26
0.29
0.34
0.089
0.017
0.083
0.035
0.71064
0.71023
0.7117
0.71161
2.2
2.2
2.3
2.8
3.2
3.3
5.0
4.4
11.7
27.1
17.5
33.0
232
240
227
231
4.3
4.3
5.2
5.0
36.7
38.7
32.9
47.1
19.7
18.9
19.3
22.7
4.66
4.43
3.96
3.62
0.35
0.36
0.26
0.25
0.308
0.098
0.063
0.088
0.71215
0.70856
0.70746
0.70877
2.1
1.6
1.4
1.7
6.3
4.6
3.4
4.3
6.6
12.6
13.9
6.6
236
277
275
310
5.7
5.0
4.4
4.6
32.7
28.2
25.9
21.3
14.0
11.9
12.7
13.8
1.68
2.23
3.27
0.36 0.035 0.70495 1.6
0.37 0.07 0.7052 1.5
0.35 0.055 0.70517 1.5
10.7
10.1
8.9
23.8
10.2
15.3
614
616
591
8.6 23.5
8.5 20.8
7.5 22.0
11.2
10.7
11.2
2.17
1.27
3.17
0.29 0.175 0.70552 1.4
0.33 0.049 0.7059 1.3
0.34 0.046 0.70607 1.4
3.6
3.5
4.2
10.2
15.3
17.0
402
357
436
4.8 20.0
4.6 15.0
5.1 11.1
12.6
12.8
11.2
4
Alkalic potassic rocks—Eastern Paraguay Province—Asunción Sapucai graben (127 Ma)
Basanite–phonolite suite.
56-PS268
Ne gabbro 43.4 3.04 12.44 0.89 735 64 1.68
14.27 0.53
77-PS245
Ijolite
45.11 2.06 9.36 0.77 1624 65 1.55
12.32 0.53
D211-3088 Basanite
48.86 1.69 8.23 0.54 1216 69 1.74
12.17 0.45
D207-PS111 Tephrite
52.94 1.7
4.57 0.4 1836 76 1.95
11.81 0.39
Alkali basalt–trachyte suite.
208-3341
Alkaligabbro 46.93 2.06 11.04 0.95 2057 49 1.83
13.34 0.45
D159-PS9
Trachybasalt 50.64 1.69 8.57 0.35 1163 62 2.29
16.09 0.39
47-PS263
Syenogabbro 53.26 1.81 7.3 0.61 1618 62 2.33
16.86 0.39
La/NbNa La/YbNb Ba/Rb Ti/Y Zr/Y Ba/Nb Zr/Nb Reference
4
4
References: 1—Comin-Chiaramonti et al. (1997), 2—Kirstein et al. (2000), 3—Turner et al. (1999a), and 4—Garland et al. (1996).
Conventions as in Table 2. Bold for samples taken as the reference component of the BP-like enriched mantle type.
357
358
12
150
TP
A
B
PT
8
Paranapanema
PM
TA
BAS
Pitanga
100
OIB
BTA
Ti/Zr
Na2O+K 2O
10
TB
6
Esmeralda
Urubici
Alkaline
4
50
Gramado
2
LC
Subalkaline
0
41
B
46
BA
51
A
56
0
61
0
200
400
SiO2
600
800
1000
Ti/Y
80
50
C
D
Slab fluid
Slab fluid
(La/Yb)N
ER
Ba/La
LC
10
ER
LC
OIB
OIB
10
PM
7
0.4
1
8
(La/Nb)N
1
0.4
PM
1
(La/Nb)N
8
40
Slab fluid
E
Cretaceous
Paraná Magmatic Province
ER
LC
High-Ti Paranapanema unit
Ba/La
High-Ti Urubici unit
Low-Ti Esmeralda unit
Low-Ti Gramado Unit
Low-Ti Trienta Y Tres Unit
Transitional to alkali basalt dykes (Rondina-Palmas)
OIB
10
PM
Eastern Paraguay Province
ASU-K basanite-phonolite suite
ASU-K alkali basalt-trachyte suite
7
10
80
Ba/Nb
Fig. 4. Major and trace-element ratios for selected samples of basic to intermediate rocks from Cretaceous provinces (MgON4 wt.%, anhydrous
basis and FeO=0.85*FeOt). A—Total alkalis vs. silica diagram with the fields from Le Maitre (1989), as in Fig. 3A. B—Ti/Zr vs. Ti/Y diagram
with the subdivisions and fields for Cretaceous Paraná basalts according to Peate et al. (1992), and Peate and Hawkesworth (1996). High-Ti
types: Pitanga, Paranapanema and Urubici, and low-Ti types: Gramado and Esmeralda. C and D—Ba/La and (La/Yb)N vs. (La/Nb)N diagrams,
as in Fig. 3C–D. E—Ba/La vs. Ba/Nb diagram. Mantle and crust components as in Fig. 3.
(Kirstein et al., 2000), and occur as dykes or sills in
southern Brazil (Table 3, Figs. 2A and 4A,B; Viero,
1998).
High- and low-Ti tholeiites from the lava field
have evolved CIPW quartz-normative compositions
(mgb65%), and they experienced extensive differentiation in low-pressure magma reservoirs (Garland
et al., 1996; Peate and Hawkesworth, 1996). High-Ti
basalts are more evolved, being characterized by
elevated values of FeOt, P2O5, Ti/Y, Ba/Rb, Ti/Zr and
Zr/Y, and low Ba/La and Ba/Nb ratios (Table 4; Fig.
4B–E). Differences among diverse high-Ti magma
types can result from distinct degrees of previous
depletion caused by partial melting (Peate et al.,
1992, 1999). High-Ti Paranapanema tholeiites have
chemistry comparable to that of the Neoproterozoic
Trienta Y Tres transitional dykes, and the two show
similar values of Ti/Y, (La/Nb)N, Ba/Nb and Ba/La
(Tables 2 and 4; Figs. 3 and 4). Southern Paraná,
low-Ti tholeiites are less fractionated (mg of 40–
65%) and characterized by low Ti/Y ratios (Fig. 4B).
They show a marked enrichment of LILE over HFSE
and LREE (Peate et al., 1992). Compared with the
high-Ti types, they have lower Sr and P2O5 contents
and Ba/Rb ratio, and higher (La/Nb)N and Zr/Nb
ratios (Table 4; Fig. 4C–E). However, Esmeralda
tholeiites exhibit more depleted trace-element compositions showing high Ti/Zr and low (La/Yb)N ratios
(Fig. 4B,D). The enriched low-Ti Gramado basalts in
turn have trace-element contents broadly similar to
those from the Paleoproterozoic Uruguayan tholeiites
(UDS), as mentioned by Iacumin et al. (2003).
Compared with these Paleoproterozoic dykes, they
are slightly enriched in Ti, Ba, Nb, Zr, Y and heavy
rare earth elements—HREE. They show similar Ti/Y,
Ba/Nb and Ba/La ratios, but higher Ti/Zr and Ba/Rb
and lower Zr/Y, (La/Nb)N and Zr/Nb ratios (Tables 2
and 4; Figs. 3 and 4).
Alkalic provinces include mainly mafic potassic
rocks, formed from early Cretaceous up to late
Cretaceous/Paleocene (Gibson et al., 1996; CominChiaramonti et al., 1997). Two compositional groups
are recognized based on TiO2 and trace-element
contents, and Sr–Nd isotope ratios. They also show a
north–south provinciality, with the high-Ti group in
north–northeast and the low-Ti types in the south of
the Paraná lava field (Gibson et al., 1996). In the
Eastern Paraguay Province, the potassic rocks from the
359
Asunción–Sapucai graben best represent the low-Ti
potassic types (ASU—Fig. 1B; Comin-Chiaramonti et
al., 1997). They consist of moderately to strongly
potassic rocks, and form two suites, basanite-tophonolite (BP-ASU) and alkali basalt-to-trachyte
(ABT-ASU) and their intrusive analogues (Fig. 4A).
The two suites have mostly low TiO2 content and Ti/Zr
ratio that suggest a previous Ti-depletion (Fig. 4B;
Table 4). They also have Zr/Y, Ba/La, Ba/Nb and Zr/
Nb ratios higher than those of ocean island basalts—
OIB (Fig. 4C). The BP-ASU rocks often exhibit a
more fractionated rare earth elements (REE) pattern,
higher Ti/Y ratio and higher contents of Rb, K, Zr, Ti
and Y (Fig. 4B,D). This more incompatible-elementenriched chemistry is comparable to that of minette
dykes from the western Neoproterozoic foreland in the
southern Brazilian Shield, which show similar values
of TiO2, FeOt, P2O5, REE, Rb, Sr, Y, Rb/Sr, Ti/Y, Zr/
Nb and Ba/Nb, but more pronounced enrichment of
Ba, Ta, Nb and Zr (Tables 2 and 4). Late Cretaceous/
Oligocene sodic rocks in eastern Paraguay postdated
potassic suites in the province (Table 3). However,
they are high-Ti types showing lower (La/Nb)N ratios
(Comin-Chiaramonti et al., 1997). Sodic alkalic rocks
with high (La/Nb)N form the late Cretaceous Passo da
Capela Province in southernmost Brazil (Figs. 1B and
2A; Barbieri et al., 1987; Viero, 1998).
3. Theoretical Sm–Nd approach
3.1. e Nd-evolutionary patterns
Fig. 5 depicts the Nd isotope systematics for the
definition of crust-formation events and the evolution
of granitic magmas, adopted by DePaolo and coworkers (DePaolo, 1981; Farmer and DePaolo, 1983;
Nelson and DePaolo, 1985). The slope of the Ndevolution line will change if further fractionation of
Sm from Nd occurs, as exemplified in this figure. The
fractionation factor a Sm/Nd between Sm/Nd values
from melt (m) and protolith (p) respectively describes
the magma evolutionary histories. If a Sm/Ndb1, the
T DM(Nd) age of the magma will be lower than those at
t 1, and the opposite occurs when a Sm/NdN1. The first
case commonly corresponds to the evolution of the
melt, and the latter to that of the refractory residue
a Sm/Nd factor will be 1 when the fractionating phases
360
New crust at t2
DMM
10
Crust formation
5
Mixture
old crust +
juvenile materials
0
dεNd/dt
8
-5
εNd
αSm/Nd > 1
TDM > tCF
-10
-15
αSm/Nd< 1
TDM < tCF
Partial melting
at t2
Total melting
Nd
Sm
or D /D ≈ 1
TDM = tCF
-20
-25
fSm/Nd
αSm/Nd = Sm/Nd
m
/Sm/Nd
p
t1 = tCF
t2
-30
0
0.5
1
1.5
2
T (Ga)
Fig. 5. Schematic diagram of Nd systematic according to Farmer and DePaolo (1983). The mantle evolution curve is for the depleted mantle—
DMM from Goldstein et al. (1984). Old crust produced at t 1, and the effects of partial melting and mixing between the old crust and juvenile
magmas are shown at time t 2. Growth lines for crustal segments are for a nearly constant value of f Sm/Nd~ 0.40 (Bennett and DePaolo, 1987).
Solid lines represent the isotopic evolution of the material produced at time t 1 and t 2, and dashed lines are the extrapolation of growth lines
backward in time until they intercept the DMM curve (t CF—age of the crust formation; a Sm/Nd—fractionation factor, superscript m and p for
melt and protolith, respectively; and D Nd and D Sm—weighted mean of partition coefficients for all fractionated phases).
have little effect on the Sm/Nd ratios, or in particular
cases of total melting.
A powerful petrogenetic approach is available
when the crystallization age of the rock is known
from other methods. The bulk of the continental crust
has values of the fractionation factor ( f Sm/Nd, DePaolo
and Wasserburg, 1979) that are 40% lower than those
at depleted MORB mantle—DMM, resulting in
contrasting e Nd-growth paths (Farmer and DePaolo,
1983; Nelson and DePaolo, 1985; Bennett and
DePaolo, 1987). The alignments of samples with
different ages along continuous paths, clustering
around the average crustal rocks, thus define crustal
or tectonic provinces (Bennett and DePaolo, 1987).
Discontinuities in this pattern, marked by increasing
e Nd values at successively younger ages, can be due to
new inputs of mantle-derived magmas (Fig. 5). For
these purposes, however, the simplified btwo-stageQ
model for crust formation is adopted, in which the
evolution first occurs in a depleted mantle and next in
the continental crust (DePaolo, 1981).
We have enlarged the compiled Nd-database of
Gastal et al. (in press), to improve the character-
ization of Paleoproterozoic source materials. Nd
isotopic values were selected from geological units
whose crystallization ages are well constrained by
U–Pb, Ar–Ar or Pb-evaporation methods. In e Ndevolutionary diagrams, the main crust and mantle
components correspond to e Nd-growth lines of
samples taken as reference in each case (Fig.
6A,B). For other samples, the slope of e Nd-growth
lines can be evaluated as the enrichment factor on
f Sm/Nd vs. e Nd(t) diagrams (Fig. 6C). The inclusion of
basic rocks with diverse ages in these diagrams gives
better information on the mantle sources over geological time.
3.2. Evaluation of the Sm/Nd fractionation
Since we can estimate the fractionation factor
(a Sm/Nd) for processes such as partial melting and
fractional crystallization, we can investigate the modifications caused by igneous differentiation to understand the significance of Nd isotopes. The analysis is
made on the deviations introduced by the Sm–Nd
fractionation on e Nd(0) values and T DM(Nd) ages,
A
Early Brasiliano
Orogeny
B
Transamazonian
Orogenies
10
Brasiliano subduction-related
lithosphere
DMM
DMM
IAB
5
361
W
0
CV
εNd
-5
-10
Paleoproterozoic to
Neoproterozoic
crust in collisional belt (CC)
-15
UDS
Trans-Amazonian
subcontinental lithosphere
BP
-20
Neoarchean-Paleoproterozoic
lower crust (LC)
-25
-30
0
0.5
1.5
1
3
2.5
2
1
0.5
T (Ga)
-0.1
1.5
2.5
2
T (Ga)
C
D
W
W
IAB
IAB
-0.2
fSm/Nd
-0.3
UDS
ABT
UDS
CV
-0.4
-0.5
BP
-0.6
-20
-15
-10
-5
0
εNd(t)
5
-20
-15
-10
Santa Maria Chico Granulite Complex (LC)
0
Buenos Aires Complex
Uruguayan dyke swarm (UDS)
Low-Ti basaltic andesites
High-Ti andesites
Tandil tholeiitic dykes
Low-Ti basalts
High-Ti basalts
Neoproterozoic - Eopaleozoic
5
Dom Feliciano Belt
Pinheiro Machado Suite (CC)
Trienta Y Tres dykes
São Gabriel Arc
Cambaí-Vacacaías sociation (CV)
Cerro Mantiqueiras Ophiolite
IAB
E
TC
W
-5
εNd(0)
Basic-intermediate dykes
Calc-alkaline Tandil dykes
0
5
Neoarchaean-Mesoproterozoic
Arroio dos Ratos Complex (CC)
-5
εNd(t)
-10
-15
BP
-20
UDS
ABT
Harzburgites (W)
Amphibolites (IAB)
Camaquã Basin
Minettes
Rodeio Velho Formation
-25
-30
0.08
0.10
0.12
147
0.14
0.16
0.18
144
Sm/
Nd
Fig. 6. Sm–Nd isotopic ratios for Neoarchaean to early Neoproterozoic, crustal rocks and for Paleoproterozoic to Cretaceous, basic-to-intermediate
igneous associations highlighting the main regional crustal and mantle segments. A and B—Nd isotopic evolutionary diagrams for respectively
crust and mantle-derived rocks; Brasiliano subduction-related lithosphere includes IAB—island arc basalts, CV—Cambaı́–Vacacaı́ associations,
and W—mantle wedge with old components. C and D—f Sm/Nd vs. e Nd(t) diagrams for the same associations as in (A) and (B). E—e Nd(0) vs.
147
Sm/144Nd diagram for basic to intermediate rocks with diverse ages, and some of the old crustal associations. TC—basalt from the Tristan da
Cunha plume (Inaccessible Island, Cliff et al., 1991). In (B), (D) and (E), heavy and dotted lines delimit the fields respectively for high- and low-Ti
Paraná tholeiites, black star for low-Ti Trienta Y Tres tholeiites, and symbols for alkalic suites from the Eastern Paraguay Province as in Fig. 4.
362
respectively, De Nd(0)=(e Nd(0)p e Nd(0)m) and DT DM=
p
(T DM T m
DM) (superscript p and m for protolith and
melt, respectively). These deviations are related to
the time of the subsequent event (t 2), as shown in
Fig. 5, but the controls act in an opposite way. The
difference of enrichment factors between protolith
p
and melt ( f Sm/Nd f m
Sm/Nd) describes the change in
the slope of the e Nd-growth line, with t 2 determining
the amount of the deviation on e Nd(0) values. The
older t 2 is, the larger is De Nd(0) as defined by the
p
relation De Nd(0)cQ*Ndt*2 ( f Sm/Nd f m
Sm/Nd ) ( Q Nd =
k*Sm104*[( 147Sm/ 144Nd)/( 143Nd/ 144Nd)] CHUR). For
T DM(Nd) ages, the deviation introduced is proporp
tional to the time between t 1 and t 2 when t 1=T DM,
p
as expressed by the relation: (T m
DM t 2)c(T DM
p
m
DM
t 2)*( f Sm/Nd f DM
Sm/Nd)/( f Sm/Nd f Sm/Nd). So the longer
p
the residence time (T DM t 2) is, the greater are the
deviations on T DM(Nd) ages.
Two-component mixing can involve either bulk
assimilation (melt-rock) or two melts derived from
distinct sources (mantle or crust). Selective partial
melting of the veined-subcontinental lithosphere and
metasomatism of the mantle are both variants of these
processes. Binary mixing processes always cause
modifications on initial 143Nd/144Nd ratios of the
source materials. Therefore, we directed our discussion to the effect on Sm/Nd ratio depending upon the
timing about the melting–crystallization event. In the
equations for two-component mixing, F is the weight
fraction of component A and (1 F) that of the
contaminant B (Langmuir et al., 1978; DePaolo and
Wasserburg, 1979). Variations on e Nd(0) values and
T DM(Nd) ages are complementary, being described by
a single relation (superscript M for the mixture):
A
B
A
(T ADM T M
e Nd (0) M )/
DM )/(T DM T DM )c[(e Nd (0)
A
B
B
DM
M
(e Nd(0) e Nd(0) )]*[( f Sm/Nd f Sm/Nd)/(f Sm/Nd f DM
Sm/
A
e Nd(0)M)/(e Nd(0)A e Nd(0)B)c
Nd), where (e Nd(0)
NdB*(1 F)/NdM. The effects on the Nd isotope
composition of the magma caused by adding a small
weight fraction of the contaminant can be better
assessed made (1 F)~0. This reveals that the deviations are independent of the time, except e Nd(t). They
depend upon the isotope composition, and are strongly
controlled by chemical parameters such as NdB/NdA
and f BSm/Nd/f A
Sm/Nd. For compositions with very different incompatible elements patterns (depleted and
enriched), the T DM(Nd) age of the mixture approximates that from the enriched end-member. The closest
approximation occurs for strong chemical and weak
isotope contrasts, when the T DM(Nd) age stays nearly
constant and the e Nd values change regularly by
varying F. In this case, as the younger the mixing
event is, the larger the deviations on e Nd(t) are (see Fig.
9A). When the two end-members are chemically more
similar, the e Nd values and T DM(Nd) ages vary
regularly depending upon the initial isotope differences. This is the usual situation of mixing between
juvenile and old crustal segments (Fig. 5).
4. Nd isotopes in Cretaceous igneous rocks and
evidence for different mantle sources
Trace-element ratios and Sr–Nd–Pb isotopes of
Paraná basalts have been attributed to incompatibleelement-enriched components in the subcontinental
lithosphere, though crustal contamination is also
reported (Gibson et al., 1996; Garland et al., 1996;
Peate and Hawkesworth, 1996; Comin-Chiaramonti
et al., 1997; Hawkesworth et al., 1999). Such
features can be due to contamination of the asthenosphere-derived magmas as they pass through the
lithosphere, or may result exclusively from partial
melting of the subcontinental lithosphere. OIB-like
asthenospheric sources were reported for some
basalts in eastern Uruguay (Kirstein et al., 2000),
and for transitional and alkali basalts from dykes
and sills in southernmost Brazil (Viero, 1998). Sodic
alkalic rocks from both the Passo da Capela and the
Eastern Paraguay provinces have young Nd model
ages (1.1–0.6 Ga, Fig. 1B). An analogous origin
may be inferred for the two provinces, but in the
former the high (La/Nb)N ratios imply the involvement of lithospheric mantle materials (Barbieri et al.,
1987).
High- and low-Ti tholeiites from the lava field and
the potassic alkalic rocks from Eastern Paraguay
Province (ASU) all have T DM(Nd) ages older than
0.9 Ga, and (La/Nb)N ratios higher than 1 (Figs. 1 and
4C–D; Table 4). ASU-potassic rocks show the lowest
e Nd(t), implying mantle sources with a longer residence time (Fig. 6B,D). They also have high average
I Sr value of 0.706–0.707, so that they are a useful endmember for magmas derived from a previously
modified subcontinental lithosphere. According to
Comin-Chiaramonti et al. (1997), the modifications
of mantle sources of ASU-potassic rocks occurred in
an old subduction-related environment, but the enrichment events were promoted by very small degrees of
peridotite melting (~0.2%). The alkali basalt–trachyte
suite from this province shows slightly lower e Nd(t)
( 13.3 to 16.0) and older T DM(Nd) ages of 2.3–1.7
Ga (ABT—Fig. 6B,D; Table 4). The least fractionated
samples from this suite plot close to the Paleoproterozoic UDS-tholeiites on the e Nd(0) vs.147Sm/144Nd
diagram (Fig. 6D,E). Their patterns of incompatible
elements mimic those from the UDS-tholeiites that
have a subduction signature (Gastal et al., in press).
Most of the ASU-potassic rocks are roughly aligned
between the two end-members on e Nd(0) vs.147Sm/
144
Nd (BP and UDS—Fig. 6E), and the same occurs
in the e Nd(0) vs. 1/Nd diagram (not shown). Although
the geochemical contrasts between the two ASUpotassic suites are small (Fig. 4; Table 4), Nd isotopes
suggest distinct degrees of re-enrichment of a
previously subduction-modified mantle. Two of the
less evolved potassic rocks of the basanite–phonolite
suite from this province have T DM ages of 1.7–1.5 Ga
and low 147Sm/144Nd (Fig. 6E; Table 4). They are
taken as representative of melts derived from the previously subduction-modified old mantle later enriched in incompatible elements (BP—Fig. 6B,D–E;
Table 4).
Contrasts of Nd–Sr isotopes and trace-element
ratios between and within the two chemical groups
of Paraná basalts suggest distinct parental magmas
and mantle sources (Peate et al., 1992; Garland et
al., 1996; Hawkesworth et al., 1999). High-Ti
tholeiites (Paranapanema and Urubici units) are
isotopically homogeneous with T DM(Nd) ages ranging from 1.5 to 0.9 Ga and have a Nd–Sr primitive
isotope signature (Fig. 1B; Table 4). Contamination
with crustal components was subordinate in these
tholeiites, and its origin has been related to smalldegree silicate melts, either during the magmatism
or in the development of REE-enriched source
materials (Garland et al., 1996; Hawkesworth et
al., 1999). Nonetheless, Urubici basalts exhibit Sr–
Nd–Pb isotopes close to those of anomalous basalts
from the Walvis Ridge, which define the EM-I
component in the nomenclature of Zindler and Hart
(1986), as discussed by Peate et al. (1999). These
authors attributed this isotope signature to the
involvement of the Brasiliano lithosphere. Low-Ti
363
basalts from southern Paraná (Esmeralda, Gramado
and Trienta Y Tres units) exhibit varied Sr–Nd–Pb
isotope compositions and older T DM(Nd) ages of 2.4
to 1.3 Ga (Figs. 1B and 5B,D). Gramado and
Treinta Y Tres basalts are isotopically enriched with
low e Nd(t) (0 to 8) and high I Sr (N0.707). They
are regionally heterogeneous with striking evidence
for open-system differentiation at shallow levels. In
spite of this, it has been proposed that they derived
by extensive melting of a previously depleted
lithospheric mantle that was subsequently REEenriched (Peate and Hawkesworth, 1996; Kirstein
et al., 2000). However, the mantle source of these
tholeiites is still a controversial question. CominChiaramonti et al. (1997, 1999) suggested their
derivation from the same mantle end-member of
mafic potassic suites from the Eastern Paraguay
Province (ASU). Nevertheless, Kirstein et al. (2000)
rejected this hypothesis based on modelling of traceelement ratios such as Th/Nd and Ti/Y. Esmeralda
tholeiites have more depleted trace-element and
isotopic compositions, probably resulting from the
mixture between an asthenospheric MORB-like
source and the evolved and contaminated Gramado
basalt (Peate and Hawkesworth, 1996).
In summary, the Paraná magma types and subcoeval alkalic rocks tapped a regionally heterogeneous and stratified subcontinental lithosphere and
had a subordinate contribution from MORB- or OIBlike asthenospheric sources. In southern Brazil and
Uruguay, these rocks demonstrate that the subcontinental lithosphere is heterogeneous and was
intensely modified during old events, as initially
cited by Hawkesworth et al. (1988). In addition, the
similarity for geochemical and isotopic data between
some Cretaceous and Proterozoic basic rocks, as
extensively discussed by Iacumin et al. (2003),
seems to constrain the time of the main modifications to the lithospheric mantle in this region to the
Neoarchaean–Paleoproterozoic.
5. Nd isotopes in Proterozoic basic-to-intermediate
igneous rocks and the lithospheric mantle
components
Like the Cretaceous basic-to-intermediate igneous
rocks in the region, most of the Proterozoic rocks
364
exhibit high (La/Nb)N ratios (Fig. 3C,D; Table 2),
implying subduction-related mantle materials or contamination with the old, recycled crust. Igneous suites
with a younger isotope signature are produced during
the early Neoproterozoic subduction event (Fig. 6A;
Table 1), whereas those with an older signature are
derived from the subcontinental lithosphere modified
during the Trans-Amazonian or older orogenies.
e Nd(0) lower than 9 and T DM(Nd) ages older than
1.4 Ga (mostly N1.6 Ga) characterize the latter (Fig.
6B). They also have distinct f Sm/Nd resulting from the
fractionation of Sm–Nd during the magma genesis, or
from diverse REE-enriched mantle materials (Fig.
6D). Two extreme compositions, based on Nd
isotopes and the trace-element behaviour, suggest at
least two end-members for the mantle modified during
Neoarchaean–Paleoproterozoic subduction events as
proposed by Gastal et al. (in press). One end-member
has a strong signature of these events, while the other
one has a more enriched, non-subduction-related
isotope and geochemical signature.
Besides the two main end-members, we may
speculate about the presence of other REE-enriched
mantle components less affected by Proterozoic
subduction events. They would be represented by
rocks with low (La/Nb)N ratios (V2, Fig. 3C,D), but
distinct Sr–Nd isotope ratios. These rocks include the
more radiogenic, Mesoproterozoic high-Ti tholeiites
from Tandil and the Neoproterozoic Treinta Y Tres
transitional dykes that have more juvenile Sr–Nd
signature (Fig. 6B,D). The two sets of dykes, however,
have a much-localized occurrence. In addition, most of
the Mesoproterozoic Tandil dykes are low-Ti tholeiites
characterized by a primitive Sr–Nd isotope signature
(Fig. 6B; Table 2), low (La/Yb)N ratios (Fig. 3D), and
REE patterns like E-MORB as argued by Iacumin et al.
(2001). They were derived from a mantle extremely
depleted by partial melting, which might be equivalent
to the asthenosphere or else to the DMM-like matrix of
the Paleoproterozoic subcontinental lithosphere. This
mantle could also include incompatible-elementenriched domains with varied origins, such as that
responsible for the generation of the coeval high-Ti
tholeiites. A similar situation in the early Neoproterozoic subcontinental lithosphere could be responsible
for the characteristics of the Neoproterozoic Treinta Y
Tres transitional dykes, and some of the Cretaceous
high-Ti Paranapanema basalts.
5.1. Old subduction-related lithospheric mantle type
(UDS-like component)
Paleoproterozoic calc-alkaline Tandil-andesites and
the Mesoproterozoic low-Ti UDS-tholeiites, both have
T DM(Nd) ages older than 2.4 Ga, low e Nd(t) values and
typically a subduction geochemical filiation (Figs.
3C,D and 6B–D; Table 2). Their origin has been
related to the mantle modified during the Neoarchaen–
Paleoproterozoic orogenies. The 2.02 Ga-old Tandil
dykes plot on the same e Nd-growth line of the other
Paleoproterozoic associations thought to derive from
the lithospheric mantle produced during the Neoarchaean accretionary orogenies (Fig. 6A). However,
the two set of dykes have high values of both (La/Nb)N
and (La/Yb)N ratios. To explain their geochemistry
Iacumin et al. (2001, 2003) argued that the enrichment
events had been promoted by melts derived from the
subducted material present in mantle sources. In the
UDS-tholeiites, decreasing e Nd(t) values and the
increase of Sm/Nd occur with the differentiation
(Fig. 6D,E). Then the less evolved basaltic andesites
show the highest 143Nd/144Nd initial ratios, and only
the most evolved and enriched UDS-andesites plot in
the same e Nd-growth line of calc-alkaline Tandil dykes
(Fig. 6B). This suggests the contamination of UDS
melts with old REE-enriched materials with Nd
isotopes akin those of the Paleoproterozoic Tandil
dykes, corroborating the proposition of Iacumin et al.
(2001, 2003). Despite these differences, trace-element
ratios for the two sets of dykes show that they were
produced from similarly enriched mantle materials.
The wide occurrence of UDS makes the low-Ti
tholeiites the best representatives of magmas derived
from the subcontinental lithosphere strongly modified
during the Neoarchaean–Paleoproterozoic subduction
events. The two least evolved UDS-basaltic andesites
are thus considered as equivalent to melts derived from
this mantle end-member (UDS—Fig. 6B–E; Table 2).
Based on Nd isotopes, analogous mantle materials
seem to be also involved in the generation of the
Cretaceous enriched, low-Ti basalts and the alkali
basalt–trachyte ASU suite. The higher e Nd(t) values in
the former also indicate the contribution of more
juvenile source materials (Fig. 6B). Even so, the
geochemical and isotope features of these Proterozoic
and Cretaceous rocks allow the conclusion that the
subcontinental lithosphere beneath the southern Bra-
fractionated compositions, with Sm–Nd isotopes and
patterns of incompatible elements similar to those of
the more enriched Cretaceous basanite–phonolite ASU
suite (Tables 2 and 4; Figs. 3B–D, 4B–B and 6B). The
two least evolved potassic rocks from the latter, as
previously cited, are thought to represent the endmember of an old mantle in which the subduction
signature was reduced in further events (BP—Fig. 3).
The enrichment now should involve some less-hydrous
fluids or melts to justify the increase of trace elements
such as Nb and Ta. These two potassic rocks are
zilian and Uruguayan shields is dominated by an
enriched mantle with a strong signature of Neoarchaean–Paleoproterozoic subductions. In the region,
these events correspond to the major crust-formation
period (Cordani et al., 2000; Hartmann et al., 2000).
5.2. Re-enriched old lithospheric mantle type (BP-like
component)
Minette dykes from the western foreland of the Dom
Feliciano collisional orogeny (660–550 Ma) show
São Gabriel
Orogeny
A
365
B
Dom Feliciano
Orogeny
10
DMM
CV
5
DMM
IAB
W
εNd
0
CC
-5
BP-ASU
-10
LC
-15
UDS
-20
Foreland
Pelotas Batholith
-25
-30
0
0.5
1.5
1
2
0.5
-0.1
1
1.5
2
T (Ga)
T (Ga)
C
Late Neoproterozoic igneous associations
Pelotas Batholith (660 - 580 Ma)
IAB
-0.2
Syn-transcurrent granitic suites
W
Post-tectonic, calc-alkaline and alkaline granitic suites
Alkaline granites
fSm/Nd
-0.3
-0.4
UDS
Western foreland
Early magmatic event (605 - 580 Ma)
LC
CC
Shoshonitic rocks, and akaline granites
CV
Late magmatic event (575 - 550 Ma)
Alkaline, metaluminous granitoids and volcanics
-0.5
High-K calc-alkaline granites
BP- ASU
-0.6
-20
-15
-10
-5
εNd(575 Ma)
0
5
10
Fig. 7. Sm–Nd isotopic ratios for late Neoproterozoic magmatic associations from respectively the Pelotas Batholith and the western foreland. A
and B—Nd isotopic evolutionary diagrams showing the main crustal and mantle segments as in Fig. 6; arrows indicate the evolution in time in
each region. C—f Sm/Nd vs. e Nd (575 Ma) diagram for the two groups of rocks, compared with some older suites. Symbols for crust and mantle
regional components as in Fig. 6. Crustal segments: LC—Neoarchaean–Paleoproterozoic lower crust, CC—Paleoproterozoic to Neoproterozoic
crust in the collisional belt, and CV—Cambaı́–Vacacaı́ association (São Gabriel Arc). Trans-Amazonian subcontinental lithosphere: UDS-like
and BP-like enriched-mantle components, and the Brasiliano subduction-related lithosphere (IAB, W and CV).
366
thought to have formed from incompatible-elementenriched veins in a Neoarchaean–Paleoproterozoic
subduction-processed matrix of the subcontinental
lithosphere. The Neoproterozoic minettes, however,
show younger T DM(Nd) ages (~1.4 Ga) and higher
e Nd(t) values ( 2 to 4), implying also the participation of some more juvenile materials. Differences in
the behaviour of trace elements and Sr–Nd–Pb isotopes
between the two minettes suggest distinct mantle
sources. Gastal et al. (2003), based on Pb isotopes,
pointed out that the mantle wedge modified by fluids
during the 900–700 Ma early Neoproterozoic orogeny
is a viable source for the olivine minette, but source
materials with a longer residence time are required for
the minette. The last has high (La/Nb)N similar to those
of the Cretaceous BP-potassic rocks, while in the olminette this ratio is lower and akin those of the
Neoproterozoic Trienta Y Tres transitional dykes characterized by more juvenile Sr–Nd ratios (Figs. 3C,D,
4C,D and 6B; Tables 2 and 4). This raises the possibility that the non-subduction enrichment of mantle
sources also occurred at the end of the early Neoproterozoic orogeny. Differences in Nd isotopes between
the Neoproterozoic minettes and the Cretaceous BPASU rocks may be thus due to distinct re-enrichment
events of the subcontinental lithosphere already modified during old orogenies. In this way, the two
Neoproterozoic minettes could be registering multiple
processes of mantle metasomatism in that lithosphere at
respectively the Paleoproterozoic and Neoproterozoic.
This would justify their higher contents of Nb, Zr, Ta
and Ba compared with Cretaceous BP rocks.
In the western Neoproterozoic foreland of southern
Brazilian Shield, Ordovician basalts and other Neoproterozoic basic–intermediate rocks show Nd isotope
ratios that are similar either to Neoproterozoic
minettes or to Cretaceous BP-potassic rocks (Figs.
6B and 7B,C; Tables 2 and 4). Minettes and some
basaltic trachyandesites have Nd isotope ratios
(e Nd(t)N 4) that plot between two mantle endmembers, the BP-like enriched and the IAB-like early
Neoproterozoic subcontinental lithosphere, respectively. This corroborates their genesis through a binary
mixing of these two mantle materials, but the
involvement of crustal sources is registered in
trachyandesites, spessartites and other intrusive basic
rocks, which have e Nd(t) higher than 9 and older
T DM(Nd) ages (N1.7 Ga) (Table 2). Furthermore, the
high (La/Nb)N and (La/Yb)N in these rocks may be
suggesting that the mixing processes occurred into
deeper mantle levels (Fig. 3D). In spite of the genetic
particularities, Nd isotopes of these basic–intermediate rocks are consistent with the major presence of a
BP-like enriched mantle end-member in the region at
the end of the Neoproterozoic collisional orogeny.
6. Crustal source materials for late Neoproterozoic
igneous associations
Three juvenile crustal segments formed in an arc
setting are recognized in the southern Brazilian shield,
and they have distinct Rb/Sr and geochemical signatures. They show T DM(Nd) ages close to the respective
crystallization age, positive e Nd(t) values (+1 to +8),
and typically crustal values of f Sm/Nd (Fig. 6A,C). The
Neoarchaean–Paleoproterozoic lower crust in the Rio
de la Plata Craton (LC—Fig. 6A) is equivalent to the
2.55 Ga Santa Maria Chico Granulite Complex that has
a low I Sr (~0.702 at 2.03 Ga). Paleoproterozoic
associations from the Tandilia Belt show Nd isotope
ratios consistent with their derivation from the reworking of this old lithosphere (Hartmann et al., 2002),
plotting in the same e Nd-growth of the LC-segment
(Fig. 6A). Continental crust in the Dom Feliciano
collisional belt is still poorly constrained in terms of Nd
isotopes (CC—Fig. 6A), which are delimited by one
sample from the Paleoproterozoic 2.08 Ga Arroio dos
Ratos Complex, and by the sample with the highest
e Nd(t) (~ 5) from the 800 Ma Pinheiro Machado Suite
(old-PMS, Table 1). The Arroio dos Ratos Complex is a
continental calc-alkaline arc association with high I Sr
(0.711 at 723 Ma). Therefore, its e Nd-growth line was
taken as the upper limit for the Nd isotopic evolution of
a Paleoproterozoic crust, since primitive arc suites with
similar ages also occur in the collisional belt. The oldPMS samples mostly plot on the e Nd-growth path of the
Arroio dos Ratos Complex (Fig. 6A), but they exhibit
lower Rb/Sr, and Sm and Nd values. The old-PMS can
thus represent the products of reworking of the
Paleoproterozoic crust induced by new input of
mantle-derived magmas (da Silva et al., 1999). The
juvenile crust produced during the São Gabriel accretionary orogeny is represented by a sample of metadiorite from the Cambaı́–Vacacaı́ association (CV—
Fig. 6A), which has the value of f Sm/Nd close to the bulk
continental crust. Most of CV association displays a
low I Sr (0.7032–0.7040). e Nd-growth line for the CV
sample and those representatives of the 733 Ma Cerro
Mantiqueira Ophiolite delimit the Brasiliano subduction-related lithosphere (Fig. 6A). References for an
IAB, and a subarc mantle wedge (W) are taken from the
more primitive samples of respectively amphibolites
and harzburgites from the ophiolite (Fig. 6A).
7. Nd isotope signatures of late Neoproterozoic
igneous associations
7.1. Pelotas batholith
The Pelotas Batholith includes a homogeneous
group of granitoids with high I Sr (mostly 0.710 to
0.740). All but the alkaline granitoids (Encruzilhada do
Sul Intrusive Suite—ESIS, Table 1) have e Nd(t) values
in a restricted range ( 3.7 to 7.5), but large variations
of f Sm/Nd ( 0.06 to 0.57) (Fig. 7A,C). Except ESIS,
the magma genesis probably involved a mixture of two
components with similar Nd isotope composition: the
Paleoproterozoic–Neoproterozoic crust—CC, and the
enriched subduction-processed mantle—UDS-like
component. This is valid for the two age groups of
granitoids formed respectively at ca. 800 and at ca. 600
Ma. Peraluminous metagranites show the highest I Sr
(~0.740) and anomalous Nd isotopic ratios, both
consistent with its formation by partial melting of
CC-segments (Fernandes and Koester, 1999). For all
high-K calc-alkaline granites, the increase of e Nd(t)
values at younger ages suggests the contribution of a
primitive mantle component at the end of the postcollisional period (~600 Ma—Fig. 7A). The primitive
mantle could have been a DM-component or the BPlike end-member, but the last is more suitable to explain
the geochemical affinity of the post-tectonic granitoids.
The alkaline ESIS is comparable to analogous western
granites produced from distinct mixed sources (Fig.
7A–C). Nd isotope data also substantiate some type of
homogenization due to a major period of regional
heating that occurred at ca. 640–630 Ma in the batholith
(Gastal et al., in press). This event affected all but the
ESIS alkaline suite on the western side of the Dorsal de
Canguçu shear zone (Fig. 2C). It could be due to
underplating of the voluminous amounts of mantlederived magmas, caused by post-collisional tectonic
367
relaxation or lithosphere delamination at the end of the
Dom Feliciano Orogeny (~600 Ma).
7.2. Western Neoproterozoic foreland
Most of the igneous associations with diversified
Nd isotope compositions have low I Sr (~0.704–
0.706). Nonetheless, Nd isotope ratios are similar
for both granitoids and basic–intermediate rocks in
each of the two age groups of magmatic suites (605–
580 Ma and 575–550 Ma), defining two major mixing
lines in e Nd(0) vs.147Sm/144Nd (not shown) and f Sm/Nd
vs. e Nd(575) diagrams (Fig. 7C). The two mixing lines
apparently share the same more fractionated endmember, equivalent to the BP-like mantle component.
This led Gastal et al. (in press) to propose that the two
age groups are related to the mixing of this REEenriched mantle with diversified components: the
early Neoproterozoic IAB-like lithosphere and the
Neoarchaean–Paleoproterozoic LC-segment. The
605–580 Ma-old rocks show the highest values of
e Nd(t), varying from 9 up to ~0, and T DM(Nd) ages
from 2.0 to 1.3 Ga (Table 2; Fig. 7B). The magma
genesis for these rocks, including also granitoids,
involved the early Neoproterozoic IAB-like lithosphere as the other end-member of the mixture. This is
particularly valid for those intrusive and volcanic
rocks occurring near the Lavras do Sul Intrusive
Complex (Fig. 2B; Tables 1 and 2). In this case, the
IAB-like contaminant may be the subduction-related
subcontinental lithosphere, or else the arc granitoids.
The 575–550 Ma-old granitoids and acid volcanics
show lower e Nd(t) values ( 10 to 20), older
T DM(Nd) ages (3.0–1.9 Ga) and higher I Sr (0.705–
0.713). Alkaline associations have Nd isotope ratios
consistent with the old lower crust (LC) being the
other end-member of the mixing processes. The wellcorrelated linear array on f Sm/Nd vs. e Nd(575) for these
rocks support this idea (Fig. 7C). Such LC-contaminant seems to be also involved in the generation of
some rocks of the early magmatic event (605–580
Ma), but its major participation in the magma genesis
certainly occurred in the late 575–550 Ma-old event.
The younger, high silica alkaline and high-K calcalkaline granites have unradiogenic initial Pb ratios
and Pb inheritance up to 2.4 Ga, reinforcing this
conclusion (Gastal et al., in press). These authors
ascribed the increasing LC-contribution at the end of
368
the post-collisional period to some crustal thickening.
However, as previously mentioned, trace-element
ratios for basic–intermediate rocks suggest mixing
processes at deeper mantle levels, which could
indicate that delamination of the lithosphere occurred
at this time. This is an interesting hypothesis, because
we can explain the magma genesis at the end of the
600–550 Ma Dom Feliciano collisional orogeny
through the same geodynamic process in both the
western foreland and the collisional belt.
8. Sm–Nd fractionation during igneous processes
8.1. Partial melting
Deviations due to partial melting are assessed taken
non-modal batch melting models and assuming melting–segregation–extraction–emplacement processes as
synchronous. Using the equations from Shaw (1970),
the fractionation factor (a Sm/Nd) makes the variations
on Sm/Nd ratios of melts proportional to D Nd/D Sm
(D Nd and D Sm are the weighted mean of partition coefficients—Kds for all fractionating phases). The Sm/
Nd variations will be maximum and equal to a Sm/Nd at
very small degree of melting ( F—melt fractionc0). In
crustal assemblages with an intermediate composition
and without residual garnet, a Sm/Nd is equal to or higher
than 0.80 (~1.0) for the wide range of degree of melting
referred in experiments (10–50%). In mantle assemblages (Kds from McKenzie and O’Nions, 1991),
residual garnet and amphibole result in larger fractionation (D Nd/D Smc0.4 and 0.6), and the effect of
pyroxenes and olivine is smaller (D Nd/D Smc0.7 and
0.8). Taken the composition of the garnet lherzolite
from McKenzies and O’Nions (1995), the maximum
a Sm/Nd is of 0.7–0.6 for degrees of melting smaller
than 2%. However, a Sm/Nd is of 0.9 for 15% of
melting, more suitable for the production of the
commoner basalts. For the garnet–phlogopite peridotite (GPP—Erlank et al., 1987; HP3—Tatsumi and
Kogiso, 1997), the main source of alkalic potassic
Table 5
Deviations due to partial melting on e Nd(0) values and T DM(Nd) ages for crustal components in the southern Brazilian shield
Geological
units/samples
Age Crustal protolith
(Ga) e (t) 147Sm/144Nd f
Nd
p
Sm/Nd
e Nd(0)p T pDM
(Ga)
a Sm/Nd t 2* e Nd(t 2)
(Ga)
Neoarchaean-to-Paleoproterozoic lower crust in cratonic segments—LC
Santa Maria Chico Granulitic Complex
H34,
2.55 +3.33 0.1196
0.39
21.96 2.65
0.80
metabasalt1
Paleoproterozoic-to-Neoproterozoic crust in the collisional belt—CC
Arroio dos Ratos Complex
G3, biotite 2.08 +6.58 0.1101
0.44
16.54 2.01
tonalite2–3
Pinheiro Machado Suite (old-PMS)
Rs-7b,
0.80
5.26 0.1132
0.42
13.85 1.86
granodiorite
gneiss3–4
Neoproterozoic crust in the São Gabriel Arc—CV
Cambaı́ Complex—Northern region
Rs-20h,
0.70 +3.38 0.1162
0.41
metadiorite5
2.73 1.03
Produced melts
f
m
Sm/Nd
e Nd(0)m T m
DM
(Ga)
Deviations
De Nd(0) DT DM
(Ga)
2.0
0.8
0.6
2.19
14.11
16.08
0.51
0.51
0.51
28.06 2.52
24.40 2.28
23.79 2.24
6.10
2.44
1.83
0.13
0.38
0.42
0.80
0.8
0.6
7.71
9.92
0.55
0.55
18.79 1.80
18.23 1.76
2.25
1.69
0.21
0.25
0.80
0.6
7.46
0.54
15.58 1.63
1.73
0.23
0.80
0.6
+3.44
0.53
4.51 0.95
1.78
0.08
The value of 0.80 for a Sm/Nd is taken as the maximum fractionation for crustal protoliths without residual garnet; f Sm/Nd=
p
m
m
[((147Sm/144Ndp*a Sm/Nd)/147Sm/144NdCHUR) 1]; De Nd(0)=(e Nd(0)p e Nd(0)m)ct 2*Q Nd*( f pSm/Nd f m
Sm/Nd); DT DM=(T DM T DM); [(T DM
p
m
DM
1
DM
t 2)/(T DM t 2)]c[( f pSm/Nd f DM
)/(
f
f
)],
for
Q
=25.08
Ga
and
f
=0.087.
Sm/Nd
Sm/Nd
Sm/Nd
Sm/Nd
Nd
References: 1—Hartmann et al. (1999); Hartmann (1987); 2—da Silva et al. (1999); 3—Leite et al. (2000); 4—Babinski et al. (1997); 5—
Babinski et al. (1996).
contribution is required to explain the high e Nd(t)
values (Fig. 8A). Some late Neoproterozoic granitoids, with anomalous T DM(Nd) ages, also show
abrupt variation of Sm/Nd ratios (Fig. 7C). Such
features occur in granites that have unquestionable
crustal contribution, and hence they could be
ascribed to major or accessory phase-driven fractionation processes (Ayres and Harris, 1997; Davies
and Tommasini, 2000). This would take place in the
syn-transcurrent peraluminous and highly evolved
calc-alkaline granites from the Pelotas Batholith,
and it could have been important in some 575–550
Ma-old granites from the western Neoproterozoic
foreland. Based on Nd isotopes, therefore, we
conclude that granitoids with only a crustal contribution are limited in the southern Brazilian
Shield, since the mantle contribution is almost
always required.
For basic rocks taken as representative of mantle
end-members with a Trans-Amazonian signature
(UDS- and BP-like types), we want to know how
closely their Nd isotope compositions reflect those of
the sources. We took the crystallization ages and the
values of a Sm/Nd suitable for the melting-degree in
each case (Table 6). The error on e Nd(0) values would
rocks and minettes, we obtained similar values of
a Sm/Nd (0.87–0.81: Table 6).
In the three crustal segments, we took a a Sm/Nd of
0.80 as the maximum fractionation factor and three
periods of magma genesis: 2.0, 0.8 and 0.6 Ga (Table
5; Fig. 8A). The maximum deviations would be for
melts derived from the LC-segment, 6 e Nd-units at 2.0
Ga and 0.4 Ga on T DM ages at 0.6 Ga. For melting
events during the ca. 0.6 Ga-old Dom Feliciano
Orogeny, errors of two e Nd-units could occur in the
present-day isotope ratios of melts derived from all
crust segments—LC, CC and CV. Deviations on
T DM(Nd) ages would be lower than the resolution
limit of ca. 0.2 Ga for those derived from the young
CC- and CV-segments. Small errors in e Nd(0) and
T DM(Nd) ages due to melting events during this
orogeny could be an argument for the generation of
granitoids in the Pelotas Batholith only by reworking
of the CC-crust, as postulated by da Silva et al.
(1999). Nevertheless, the behaviour of 147Sm/144Nd
and e Nd(t) of most granitoids makes this difficult to
accept (Figs. 7C and 8A). Only some highly
fractionated, calc-alkaline granites (young-PMS and
Arroio Moinho) may have been derived by partial
melting of this CC-segment. Even so, the mantle
10
5
Rs20h
CV
G3
Paleoproterozoic
basic rocks
DTDM ≤ 0.15 Ga
B
DTDM ≤
0.42 Ga
DMM
DMM
BP-like
mantle
H34
CC
0
εNd
Transamazonian
Orogenies
Early Brasiliano
Orogeny
A
369
LC
-5
UDS-like
mantle
-10
-15
BP-ASU
t2 = 2.0 Ga
t2 = 0.6 Ga
-20
ABT-ASU
t2 = 0.8 Ga
-25
-30
0
DεNd(0) ≤ 6.10
0.5
1
1.5
T (Ga)
2
2.5
3
0.5
Cretaceous basic rocks
DεNd(0) ≤ 0.35
1
1.5
2
2.5
3
T (Ga)
Fig. 8. Deviations on e Nd(0) values and T DM(Nd) ages due to Sm–Nd fractionation caused by partial melting respectively of the crust (A) and
mantle (B), regional components. A—Partial melting of the three crustal segments–LC, CC and CV–is modeled for events at t 2 of 2.0, 0.8 and
0.6 Ga. The Nd isotopic diagram is according to the results shown in Table 5; symbols for crustal segments as in Figs. 6 and 7 for granitoids
from the Pelotas Batholith. B—Compositions of the mantle sources for the two main groups of basic rocks with a Trans-Amazonian signature
(UDS, ABT- and BP-ASU), estimation taken from the crystallization age in each case. Nd isotopic diagram according to the results shown in
Table 6; symbols for Cretaceous and Paleoproterozoic, basic rocks as in Figs. 4 and 6, respectively. Dotted lines for the extrapolation of growth
lines as schematized in Fig. 5.
370
Table 6
Deviations on e Nd(0) values and T DM(Nd) ages for the calculated mantle sources of basic rocks taken as representative of the lithospheric
components with a Trans-Amazonian signature, southern Brazilian shield
Geological
units/samples
Age
(Ga)
Magma composition
e Nd(t)
147
Sm/144Nd
Paleoproterozoic Uruguayan dyke swarm—UDS
UR46, basaltic 1.73
2.03 0.1275
andesite
UR33, basaltic
1.22 0.1292
andesite
F
f Sm/Ndm
e Nd(0)m
T DMm
(Ga)
0.35
17.32
2.48
0.34
16.15
2.42
Cretaceous potassic alkalic province, Asunción Sapucaı́ graben, Paraguay
Alkali basalt–trachyte suite—ABT-ASU
47-ps263,
0.127
16.86 0.1195
0.39
18.10 2.33
syenogabbro
d159-ps9,
16.09 0.1203
0.39
17.32 2.29
trachybasalt
Basanite–phonolite suite—BP-ASU
77-ps245,
0.127
12.32 0.0921
0.53
14.01 1.55
ijolite
56-ps268,
14.27 0.0925
0.53
15.96 1.68
Ne gabbro
0.15
0.11
0.06
a Sm/Nd
0.90
0.87
0.81
Mantle sources
Deviations
f Sm/Ndp
e Nd(0)p
T DMp
(Ga)
De Nd(0)
DT DM
(Ga)
0.28
14.19
2.63
+3.13
+0.15
0.27
12.98
2.57
+3.17
+0.15
0.30
17.81
2.85
+0.29
+0.52
0.30
17.03
2.81
+0.29
+0.52
0.42
13.66
1.86
+0.35
+0.31
0.42
15.61
2.02
+0.35
+0.34
F for melting degree; a Sm/Nd=[(D Nd*(1 F))+F]/[(D Sm*(1 F))+F], for non-modal batch melting; the values of F for Cretaceous potassic rocks
are from Comin-Chiaramonti et al. (1997); other references as in Tables 4 and 5.
be negligible (b0.5 e Nd-units) for the Cretaceous
potassic rocks (BP- and ABT-ASU), but larger for
T DM(Nd) ages (0.3–0.5 Ga). Therefore, these Cretaceous rocks provide good approximations of the Nd
isotope composition of mantle sources, so that the
variations in their e Nd(t) values seem to reflect
processes that took place in mantle environments,
reinforcing previous discussions. In the Paleoproterozoic UDS-basaltic andesites, a reverse relation is
found with larger deviations of e Nd(0) values (~3) and
smaller for T DM(Nd) ages (b0.15 Ga). If no other
major fractionation process occurred, the Nd DMmodel ages of UDS-tholeiites best reflect the time of
the extraction of the mantle source: 2.4–2.6 Ga. This
result corroborates that the subduction signature in
these tholeiites was acquired during the Neoarchaean–
Paleoproterozoic orogenies.
8.2. Binary mixing of source materials
We discard the possible effects of partial melting
by considering the studied rocks as representative of
the melts. In all situations, we assumed melting–
mixing–crystallization as synchronous, and that no
subsequent fractionation occurred. More prominent
cases of binary mixing reported in the studied
igneous suites, all include the BP-like mantle as one
end-member and the other end-member has diversified origins: the Neoarchaean–Paleoproterozoic
lower crust—LC, the early Brasiliano mantle
wedge—IAB, and the UDS-like subduction-processed mantle, respectively. Since the BP-like endmember is more REE-enriched than the other
components—UDS, LC and IAB, it strongly influences the isotopic composition of the mixed melts.
Thus, small variations on e Nd and 147Sm/144Nd
correspond to significant increments of the contaminant (Fig. 9A,B). This is more accentuated when
the other component is the depleted and young early
Neoproterozoic IAB-component (NdB/NdA~0.09 and
f Sm/NdB/f Sm/NdA~0.3). For the two other contaminants,
UDS and LC, isotopic and chemical contrasts are
smaller (NdB/NdA~0.2 and f Sm/NdB/f Sm/NdA~0.6–0.8),
resulting in regular variations of hybrid melts. The first
case would explain the origin of some 605–580 Maold granitoids and volcanic rocks in the western
foreland of the Dom Feliciano collisional orogeny,
through the binary mixing of the BP-like mantle with
A
10
B
IAB-magma
Contaminant
5
50
70
0
30
90
95
90
70
LC-melt
Contaminant
UDS-magma
Contaminant
70
-10
DMM
50
BP-magma
80
εNd
DMM
90 95
95
-5
371
-15
BP-magma
-20
-25
-30
1
0.5
T (Ga)
1.5
2
2.5
0.5
1.5
1
Dom Feliciano
Orogeny
-0.1
C
IAB
Camboriú
Orogeny
D
10
W
DMM
5
-0.2
MM
MM
0
UDS-like
mantle
95
-0.3
εNd
-5
fSm/Nd
-0.4
3
2.5
2
T (Ga)
90
LC
95
-10
80
90
80
-0.5
BP-like
mantle
-15
CV
70
70
MM - addition of
30% melt
-20
50
-25
BP- ASU
-0.6
-20
-15
-10
-5
εNd(575 Ma)
0
5
10
-30
0
0.5
1
1.5
2
2.5
3
T (Ga)
Fig. 9. Nd isotopic diagrams shown the deviations on Sm–Nd system due to binary mixture of distinct mantle- and crust-derived magmas (A–
C), and the effects of metasomatism on an UDS-like mantle (D). A to C—Binary mixing models in which the main end-member is the BP-like
enriched mantle, and the contaminant includes respectively: (A) the early Brasiliano IAB and the Paleoproterozoic–Neoarchaean subduction
processed UDS-like mantle. The first model explains the early magmatic association from the Dom Feliciano western foreland (605–580 Ma),
mostly the Lavras do Sul Intrusive Complex, and the second model some Cretaceous potassic ASU rocks; and (B) the Neoarchaean–
Paleoproterozoic lower crust—LC, whose results explain the late magmatic associations from the western foreland. In (A) and (B), dotted lines
for increments of the contaminant. C—f Sm/Nd vs. e Nd(575 Ma) diagram illustrating the solutions that explain the contrasting sources for the twoage groups of igneous rocks from the Dom Feliciano western foreland, shown in (A) and (B); ticks for the proportion (%) of the contaminant.
D—Metasomatism of the Paleoproterozoic–Neoarchaean subduction processed UDS-like mantle (as in Fig. 8B). Such process is modeled for
events at 2.0 and 0.6 Ga through the addition of 30% of a melt, which was calculated through 0.2% melting from a garnet lherzolite with a DMisotopic initial composition (McKenzies and O’Nions, 1995). Mantle components as in Fig. 8; symbols for Cretaceous tholeiites as Fig. 6,
Neoproterozoic rocks Fig. 7, and Fig. 3 for Proterozoic basic-to-intermediate dykes.
70–80% of the IAB component (Fig. 9A). The
scattering of some samples including minettes seems
also to be registering the more enriched nature of the
subcontinental lithosphere in this region, as early
discussed (Fig. 9C). The same type of mixing with
90–95% or higher proportion of the IAB component
would resolve the origin of some Cretaceous low-Ti
Paraná tholeiites (Fig. 9A). Otherwise, the 575–550
Ma-old igneous rocks in the western foreland can have
derived through the mixing of 70 up to 95% of the
Neoarchaean lower crust, LC, with the BP-like mantle
(Fig. 9B,C).
372
8.3. Metasomatism of the mantle
We evaluate the re-enrichment of a subcontinental
lithosphere processed during the Neoarchaean subduction (UDS-like component), in an attempt to
study the origin of basic rocks with a TransAmazonian signature. The effects are analysed for
events occurred during the Trans-Amazonian and
Brasiliano orogenies. From the assumed average
composition of the UDS-mantle, we calculated the
weighted fraction of melts or fluids added to obtain
the BP-like mantle. Slab-derived fluids are from
Tatsumi (2000), and the melts are derived through
0.2% melting from the garnet lherzolite (McKenzies
and O’Nions, 1995), and for both we assumed a
DM–I Nd. The slab-derived fluid is more fractionated
(Sm/Nd of 0.187) and has higher Nd and Sm than
the melt (Sm/Nd of 0.216), so 10 times more melt
are required (30:3). Different Sm/Nd ratios of the
added materials do not result in diversified effects on
both T DM(Nd) ages and e Nd(0) values, so Fig. 9D
displays alone the addition of melts. In addition, the
metasomatizing agent whether OIB melt or slab fluid
must be discriminated on the base of incompatibleelement ratios (Weaver, 1991).
The results illustrate conclusions already advanced by others authors (de Hollanda et al.,
2003). If the metasomatic event is relatively recent
(just before crystallization) the present-day Nd
isotope ratios are high, but when it occurs in old
events a low 143Nd/144Nd may develop with time.
So, metasomatic events occurred during the TransAmazonian orogenies would cause small deviations
on e Nd(0) values (b0.5) and larger on T DM(Nd) ages
(~0.5 Ga). During the ca. 0.6 Ga Dom Feliciano
Orogeny, these events would promoted very large
increases of e Nd(0) values (De Nd~13.5) and decreases of T DM(Nd) age (DT DM~1.7 Ga). In both
cases, Nd DM-model ages of the mantle sources
reflect the timing of the metasomatic event,
particularly in cases of low Sm/Nd ratio of the
added material and shorter residence time of the
original mantle. However, T DM(Nd) ages of magmas derived from the enriched mantle will not
always return the timing of these events due to the
effect of partial melting. This will occur particularly
when a long time elapsed between the melting and
enrichment events, which could be the case of the
Cretaceous potassic ASU rocks. The best resolution
to produce the BP-like component would be that
involving the re-enrichment through the addition of
melts from an original UDS-like mantle during the
2.0 Ga Camboriú collisional orogeny (Fig. 9D).
During the intraplate event, at 1.7–1.6 Ga, similar
process approximates but does not totally resolve
the mantle source for the Neoproterozoic minette
dykes. These dykes have high e Nd(t) values requiring most added melts, or a younger event of
metasomatism, or else the contribution of more
juvenile materials (Fig. 9D).
9. Final considerations
The combined assessment of the nature of crustal
and mantle sources, and the timing of chemical
modifications provide a reliable approach to study
the geodynamic of the lithosphere beneath southern
Brazil. Geochemical and isotope data for basic to
intermediate rocks improve previous discussions
about the composition of the subcontinental lithosphere. Likewise, the review of the effects on Sm–Nd
isotopes due to major processes affecting the sources
during the generation of the studied rocks revealed
that they are important parameters to consider into
petrogenesis. Igneous differentiation processes may
cause significant deviations on Sm/Nd isotopic ratios,
making more difficult a single and direct petrogenetic
interpretation of Nd model ages and e Nd values. In
spite of the simplicity and vulnerability of the adopted
Nd isotope modeling, the results commonly substantiate the previous interpretations on e Nd-evolutionary
patterns built for diversified lithologies, that is, basic
rocks and granitoids with different ages. In some
cases, the behaviour of trace elements also supports
these results. The present review justifies future
isotope, geochemical and geological integrated studies
in this region, including a more varied spectrum of
igneous rocks. Certainly, this will substantially
improve the ideas here proposed.
Multiple processes of metasomatism affected the
lithospheric mantle in southern Brazilian and Uruguayan shields, resulting in zones with extreme
isotope and chemical composition that are typical of
cratons and adjacent orogens (O’Brien et al., 1995).
Proterozoic and Cretaceous igneous rocks largely
register at least two incompatible-element-enriched
mantle components, despite some regional complexity. The dominant end-member, UDS-like mantle
type, has a strong isotope signature of Neoarchaean–
Paleoproterozoic subduction events that correspond to
the major period of the crust formation in these
shields. In the other end-member, BP-like mantle
type, this previous signature was reduced during
tectono-thermal events at the end of the 2.0 Ga
Camboriú collisional orogeny through a new process
of OIB-like enrichment. The Paleoproterozoic Uruguayan dyke swarm (UDS) best represents the first
component, also sampled by the Paleoproterozoic
calc-alkaline Tandil dykes, and probably by the
Cretaceous low-Ti tholeiites from southern Paraná
Province. This mantle type apparently was important
in the generation of some 800 and 660 Ma-old
granitoids from the Dom Feliciano Belt in southern
Brazil. The Cretaceous BP-suite from the Eastern
Paraguay Province best represents the re-enriched
end-member, also registered in the 600–550 Ma
Neoproterozoic igneous rocks formed at the end of
the Dom Feliciano Orogeny. The studied Neoproterozoic rocks show little evidences of mantle enrichment events during the Brasiliano orogenies, but these
events can have been important in source materials of
Cretaceous rocks. Therefore, the bulk of data allow
proposing that the major events of metasomatism in
the subcontinental lithosphere beneath the region took
place mainly during the Neoarchaean–Paleoproterozoic. This conclusion is distinct from the proposal of
Comin-Chiaramonti et al. (1997, 1999) based on
T DM(Nd) ages. These authors considered that the
source materials of Cretaceous tholeiites and alkalic
rocks resulted from two chemically distinct mantle
metasomatic events occurred during the Neoproterozoic (1.1–0.5 Ga) and Mesoproterozoic (1.6–1.3 Ga).
We interpret the differences between the two propositions based on the Nd isotope modelling. The
T DM(Nd) ages of magmas not always return the
timing of the metasomatic event of the source,
particularly when a long time elapsed between the
melting and enrichment events. This can be the case
of some Cretaceous rocks, such as the potassic ASU
rocks. Nonetheless, we discuss the metasomatism of
the mantle in terms of addition of melts or fluids,
commonly referred as OIB- and subduction-related
events. Other processes can also promote similar
373
chemical and isotope signatures, such as the delamination of the subcontinental lithosphere into the
deeper asthenospheric mantle (Zindler and Hart,
1986). Now, the enriched nature of the mantle can
be due to eclogitic–granulitic subducted material or
eclogite–granulite-derived melts (Hirshmann and
Stolper, 1996; Cordery et al., 1997; Tatsumi, 2000).
According to Iacumin et al. (2003), this type of
process can explain the chemical signature of some
enriched tholeiites in southern Brazil and Uruguay.
During the 660–550 Ma Dom Feliciano collisional event, the magma genesis involved distinct
mantle and crustal source materials, defining two
main domains that experienced different geodynamic
evolution: the eastern collisional belt and the western
foreland. Both regions also had different evolution
during the early Neoproterozoic, as reported by
Babinski et al. (1996), with the reworking of old
crustal segments in the east and the development of
the juvenile magmatic arc in the west. The 660–550
Ma-old magmatic events in the collisional belt
involved the mixture of two components with similar
Nd isotopes, the 2.1–0.8 Ga recycled crust (CCsegment) and the Neoarchaean–Paleoproterozoic
subduction-processed mantle (UDS-like mantle
type). The diversified contribution of the REEenriched old-mantle (BP-like mantle type) marked
the end of the collisional period in this belt, at 630–
580 Ma. This enriched BP-mantle type in turn
appears to have been dominant in magma genesis
of late Neoproterozoic igneous suites in the western
foreland. Now, the mixing processes also involved
the 900–700 Ma subduction lithosphere (IAB-segment) and the 2.55 Ga lower crust (LC-segment).
The contribution of the CC-segment increased from
the early (605–580 Ma) to the young (575–550 Ma)
events, which can be due either to crustal thickening
or to delamination of the lithosphere. The delamination of the lithosphere at the end of the 660–550 Ma
collisional orogeny is a suitable model because we
can explain the magma genesis through the same
geodynamic processes in the whole southern Brazilian Shield. In the collisional belt, this process would
have promoted the thinning of the lithosphere,
resulting in regional heating and abundant production of granitic melts. Based on Nd isotopes, we
conclude that most of Mesoproterozoic T DM(Nd)
ages in the studied Neoproterozoic igneous rocks
374
result from binary mixing processes of diversified
and old source materials (mantle and crust). Such
processes are well registered in the western Neoproterozoic foreland, where they exemplify the
prominent effect of a REE-enriched mantle with a
long residence time. Here, the BP-like mantle endmember strongly controls the Nd isotopes of the
binary mixtures either including the juvenile lithosphere (IAB) or the lower crust (LC).
Another major conclusion is concerning the origin
of the abundant, late Neoproterozoic granitoids with
diversified geochemical affinities from the southern
Brazilian Shield. Those with a single crustal derivation are limited to syn- to late-transcurrent peraluminous types. Post-tectonic, high silica, highly evolved
calc-alkaline and alkaline metaluminous types also
have a major crustal contribution. The same is valid
for syn-tectonic high-K calc-alkaline granitoids in
western foreland. However, in most cases, Nd
isotopes require some type of mantle contribution in
their genesis.
Acknowledgments
This study was financially supported by the
Fundação de Amparo à Pesquisa no Estado do Rio
Grande do Sul (FAPERGS nos. 00/2366.3 and 03/
0321-3) and PRONEX/CNPq. We are grateful to F.
Chemale Jr.(UFRGS) and A. P. Viero (UFRGS), who
provided some the unpublished isotopic data used
here. Bernard Bonin and Arto Luttinen are thanked for
the constructive comments and suggestions, which
substantially improved the manuscript. We are also
grateful to V.P. Ferreira, A.N. Sial and I. McReath for
the opportunity to contribute to the special issue of
this journal.
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