Geology, geochemistry, and PbÁ/Pb zircon geochronology of

Transcrição

Precambrian Research 119 (2002) 189 /223
www.elsevier.com/locate/precamres
Geology, geochemistry, and Pb Pb zircon geochronology of
the Paleoproterozoic magmatism of Vila Riozinho, Tapajós
Gold Province, Amazonian craton, Brazil
/
Claudio N. Lamarão a, Roberto Dall’Agnol a,, Jean-Michel Lafon b,
Evandro F. Lima c
a
Group of Research on Granite Petrology, Centro de Geociências, Universidade Federal do Pará, Caixa Postal 1611, 66075-900 Belem,
PA, Brazil
b
Isotope Geology Laboratory, Centro de Geociências, Universidade Federal do Pará, Caixa Postal 1611, 66075-900 Belem, PA, Brazil
c
Centro de Estudos em Petrologia e Geoquı́mica, Instituto de Geociências, Universidade Federal do Rio Grande do Sul,
Caixa Postal 15001, 91501-900, Porto Alegre, RS, Brazil
Received 20 April 2001; received in revised form 15 October 2001; accepted 27 March 2002
Abstract
Pb /Pb zircon geochronologic data define two different periods of intense igneous activity in the Vila Riozinho
region, Tapajós Gold Province (TGP), south-central Amazonian craton. At /2.00 /1.97 Ga, the Vila Riozinho
volcanic sequence (20009/4 Ma, 19989/3 Ma) and the older São Jorge granite (19819/2 Ma, 19839/8 Ma) were formed.
At /1.89 /1.87 Ga, the younger São Jorge (18919/3 Ma), Jardim do Ouro (18809/3 Ma), and Maloquinha (18809/9
Ma) granites and the Moraes Almeida volcanic sequence (18909/6 Ma, 18819/4 Ma, 18759/4 Ma) were emplaced.
Similar age intervals are registered throughout the TGP that is thus a little younger than the Paleoproterozoic terranes
of the Maroni-Itacaiúnas Province in the Guiana Shield. Geochemical and geochronologic data demonstrate that the
São Jorge pluton is composed of two different granitoids, the older and younger São Jorge granites. These, as well as
the Jardim do Ouro granite, are I-type and magnetite series. The São Jorge granites are high-K calc-alkaline and show
significant geochemical variation, the Jardim do Ouro granite is more iron-rich and less oxidized. The Vila Riozinho
Formation is calc-alkaline to shoshonitic and geochemically similar to the São Jorge granites. The Maloquinha granite
and the Moraes Almeida Formation differ from the other studied rocks in petrographic and geochemical
characteristics, have aluminous A-type affinities, and were probably derived by low-temperature crustal melting. The
volcanic sequences of the studied area, formerly included into the Iriri Group of the Uatumã Supergroup, are divided
into the Vila Riozinho and Moraes Almeida sequences. The presence of these two sequences in the same tectonic
domain demonstrates the heterogeneity of the Uatumã Supergroup. The TGP registers accretionary processes related to
the formation of the Atlantica supercontinent at /2.00 Ga. This was followed (at /1.88 Ga) by an intracontinental
taphrogenic event that lasted throughout the Mesoproterozoic. The tectonic setting of the TGP was thus transitional
between a subduction-related magmatic arc and a stable continental block undergoing extension.
Corresponding author. Tel.: /55-91-211-1477; fax: /55-91-211-1609
E-mail address: [email protected] (R. Dall’Agnol).
0301-9268/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
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190
C.N. Lamarão et al. / Precambrian Research 119 (2002) 189 /223
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Paleoproterozoic; High-K calc-alkaline; Shoshonitic; A-type; Granitoids; Amazonian craton
1. Introduction
The Tapajós Gold Province (TGP; Faraco et al.,
1997) in the Central Brazil Shield, southern
Amazonian craton was included in the Central
Amazonian Province in early tectonic models
(Teixeira et al., 1989). Recently, Tassinari and
Macambira (1999) and Santos et al. (2000) identified a Paleoproterozoic magmatic arc in the southwestern part of the TGP and coined it VentuariTapajós (or Tapajós-Parima) Province, assuming
that it extends to the northwestern parts of the
Guiana Shield (cf. Fig. 1). According to these
models, the northeastern part of the TGP belongs
to the Central Amazonian Province. However, the
exact location of the border between the VentuariTapajós or (Tapajós-Parima) and Central Amazonian provinces is poorly defined.
In the last 45 years, /600 tonnes of gold have
been produced from the TGP. The deposits are
related to widespread plutonic and volcanic rocks
the relationships and geochemical signatures of
which are not clearly understood. Intermediate to
felsic volcanic sequences exposed in the TGP have
been included into the Uatumã Supergroup, a
major volcanic unit that covers large domains of
the Central Amazonian Province and also extends
to the neighboring provinces. Increasing evidence
suggests that this is an oversimplified picture,
however (Dall’Agnol et al., 1999a; Reis et al.,
2000; Santos et al., 2000).
The aim of this paper is to present new geologic,
geochemical and geochronologic data on the
volcanic sequences and granitoid plutons of the
Vila Riozinho region in the TGP. This area,
situated on the border of the Ventuari-Tapajós
and Central Amazonian tectonic provinces, is a
key area to test the proposed tectonic models. It
also displays diversified examples of the igneous
rocks exposed in the Tapajós province, including
large domains of the Uatumã volcanic sequences.
Finally, this is a suitable area to examine the
evolution of magmatic series in a Paleoproterozoic
orogenic to post-orogenic domain and their overall
significance for gold mineralization.
2. Geology of the TGP
Geologic knowledge of the TGP has improved
substantially in recent years. The Brazilian Geological Survey (CPRM) accomplished geologic
mapping on a 1:250 000 scale of a large part of
the province and presented a reinterpretation of its
stratigraphy, tectonic setting, and metallogenesis
(Klein and Vasquez, 2000; Coutinho et al., 2000;
Ferreira et al., 2000). Private companies have
developed gold prospecting projects in the province (Jacobi, 1999), and new more precise geochronologic data have also been obtained
(Lamarão et al., 1999; Klein and Vasquez, 2000;
Santos et al., 2000; Vasquez et al., 2000, Table 1).
The southwestern and western parts of the TGP
are included in the Paleoproterozoic VentuariTapajós Province that has been interpreted as a
magmatic arc related to subduction (Tassinari and
Macambira, 1999; Santos et al., 2000). The eastern
and northeastern parts of the TGP are situated in
the Archean Central Amazonian Province (Fig.
1a). However, tectonic structures that could delineate the contact of these two provinces have not
been identified and geologic structures are broadly
similar accross the border of the provinces.
The TGP is primarily composed of late Paleoproterozoic plutonic and volcanic rocks. Klein and
Vasquez (2000) distinguished an orogenic domain
Fig. 1. (a) Sketch map of the Amazonian craton (modified from Tassinari and Macambira, 1999) and (b) geological map of the TGP
(simplified from Klein et al., 2001). Inset in (a) shows the two exposed parts of the Amazonian craton */Guiana Shield in the north and
Central Brazil Shield in the south. Black dots in (b) refer to villages: J, Jacareacanga; JO, Jardim do Ouro; MA, Moraes Almeida; VR,
Vila Riozinho.
Fig. 1
191
192
Table 1
Geochronologic data of the Paleoproterozoic granitoids and volcanic sequences of the TGP and other regions of the Amazonian
craton
Geologic unit
Rock type
Age
Method
Reference
Tapajós region
Cuiú /Cuiú complex
Jacareacanga suite
Tonalite
Metaturbidite
20119/23 Ma
/2.1 Ga
U /Pb
U /Pb DtZr
Santos et al. (2000)
Creporizão
Creporizão
Creporizão
Creporizão
Granite
Granite
Granite
Granite
19579/6 Ma
19689/16 Ma
19979/3 Ma
19849/1 Ma
U /Pb
Pb /Pb Zr
Pb /Pb Zr
Pb /Pb Zr
Klein and Vasquez (2000)
Parauari suite
Cumaru
Parauari suite
Rosa de Maio
Tropas
Tropas
Penedo
Granite
Monzogranite
Granite
Monzogranite
Granodiorite
Granodiorite
Granite
18939/2 Ma
18839/8 Ma
18839/2 Ma
18799/11 Ma
18989/2 Ma
2010 Ma
18839/4 Ma
Pb /Pb Zr
Pb /Pb Zr
Pb /Pb Zr
U /Pb
U /Pb
U /Pb IZr
U /Pb
Santos et al. (2000)a
Maloquinha
Maloquinha
Caroçal
Caroçal
Santa Rita
Maloquinha
Maloquinha
Syenogranite
Granite
Granite
Granite
Monzogranite
Granite
Granite
18829/4 Ma
18729/4 Ma
/1.87 Ga
26569/6 Ma
24599/11 Ma
18709/4 Ma
26809/18 Ma
Pb /Pb Zr
U /Pb
Shrimp Zr
Pb /Pb IZr
U /Pb IZr
Shrimp Zr
Shrimp IZr
Ferreira et al. (2000)a
Ferreira et al. (2000)a
Iriri Volcanic Group
Rhyolite
Rhyodacite
Rhyolite
18889/2 Ma
18889/2 Ma
18889/6 Ma
Pb /Pb Zr
Pb /Pb Zr
Pb /Pb Zr
Dall’Agnol et al. (1999b)
Moura et al. (1999)
Cachoeira Seca suite
Crepori suite
Ingarana suite
Troctolite
Diabase
Gabbro
10469/50 /10729/18 Ma
16119/42 Ma
18879/3 Ma
K/Ar
K/Ar
Pb /Pb Zr
Klein and Vasquez (2000)a
Klein and Vasquez (2000)a
Roraima region
Pedra Pintada granite suite
Água Branca granite suite
Água Branca granite suite
Surumu Group
Surumu Group
(calc-alkaline granitoids and volcanic sequences)
Monzogranite
19589/11 Ma
Shrimp Zr
Monzogranite
19609/21 Ma
Pb /Pb Zr
Monzogranite
19389/37 Ma
Pb /Pb Zr
Shrimp Zr
Andesite
19779/8 Ma
Andesite
19849/7 Ma
Shrimp Zr
Reis et al. (2000)a
Almeida et al. (1997)
Almeida et al. (1997)
Reis et al. (2000)a
Reis et al. (2000)a
Pitinga region
Madeira granite
Madeira granite
Madeira granite
Iricoumé Group
Iricoumé Group
(A-type granites and volcanic sequences)
Granite
18349/6 Ma
hb-bt granite
18249/2 Ma
bt syenogranite
18229/2 Ma
Rhyodacite
19629/42 Ma
Rhyolite
18889/3 Ma
U /Pb Zr
Pb /Pb Zr
Pb /Pb Zr
U /Pb Zr
Pb /Pb Zr
Costi
Costi
Costi
Costi
Costi
Carajás region
Serra dos Carajás granite
Cigano granite
Pojuca granite
Musa granite
Velho Guilherme granite
Redenção granite
(A-type granites and volcanic sequences)
Granite
18809/2 Ma
Granite
18839/2 Ma
Granite
18749/2 Ma
Granite
18839/5 Ma
Granite
18749/34 Ma
Granite
18709/68 Ma
U /Pb Zr
U /Pb Zr
U /Pb Zr
U /Pb Zr
Pb /Pb wr
Pb /Pb wr
Machado et al. (1991)
Teixeira et al. (1998)a
Barbosa et al. (1995)
suite
suite
suite
suite
suite
suite
suite
suite
et
et
et
et
et
al.
al.
al.
al.
al.
(2000)a
(2000)
(2000)
(2000)a
(2000)
193
Table 1 (Continued )
Geologic unit
Rock type
Age
Method
Reference
Antonio Vicente granite
Jamon granite
Uatumã Supergroup
Granite
Granite
Andesite, rhyolite
18679/4 Ma
18859/32 Ma
18759/79 Ma
Pb /Pb Zr
Pb /Pb Zr
Pb /Pb wr
Teixeira et al. (1998)
Dall’Agnol et al. (1999c)
Teixeira et al. (1998)
Mato grosso region
Matupá
Moriru
(calc-alkaline granite and volcanic sequence)
Granite
18729/12 Ma
Ignimbrite
17789/5 Ma
Pb /Pb Zr
U /Pb Zr
Moura (1998)
Pinho et al. (2001)
Zr, zircon; IZr, inherited zircon; wr, whole-rock; DtZr, detrital zircon; hb, hornblende; bt, biotite.
a
Original age reference can be found in this paper.
in the southwest, containing the oldest sequences
and affected by compressive or transpressive
tectonics, and a post-tectonic (mainly extensional)
domain in the northeast where younger sequences
are dominant (Fig. 1b). The major tectonic lineaments follow a general NW-SE trend and most of
the granitic plutons are elongated in this direction
(Fig. 1b).
A geological map of the TGP and the stratigraphic sequence adopted by the CPRM (Klein et
al., 2001) are presented in Fig. 1b. The oldest
granitoids and gneisses in this region belong to the
Cuiú/Cuiú complex; the Conceição tonalite
yielded a U /Pb zircon age of 20119/23 Ma
(Santos et al., 2000, Table 1). The supracrustal
sequences of the Jacareacanga Group are considered broadly coeval with the Cuiú /Cuiú complex
and both units are related to early stages of
magmatic arc development (Ferreira et al., 2000;
Klein and Vasquez, 2000; Klein et al., 2001).
Detrital zircon ages of /2100 and /2875 Ma
have been obtained for the Jacareacanga Group
(Santos et al., 2000, Table 1). In stratigraphy, these
units are followed by the 1960/2000 Ma syn- to
late-orogenic calc-alkaline granitoids of the Creporizão suite (Table 1, Lamarão et al., 1999; Klein
and Vasquez, 2000; Vasquez et al., 2000). The
/1880 Ma Parauari suite corresponds to a
younger generation of post-orogenic, calc-alkaline
granitoids and is mainly exposed in the northeastern part of the region (Fig. 1b). Many /1880
Ma aluminous, A-type granite plutons (e.g. Caroçal and Maloquinha suite granites) are also found
in the province (Table 1).
Intermediate to felsic volcanic sequences are
widespread and have been included in the
/1880 Ma Iriri Group of the Uatumã Supergroup (Faraco et al., 1997; Klein and Vasquez,
2000, and references therein) or, more locally, in
the Bom Jardim Formation (andesitic flows and
related dikes; Klein et al., 2001, Fig. 1b). The Iriri
volcanic rocks show, however, geochemical and
petrologic differences and at least two different
ages (Dall’Agnol et al., 1999b; Lamarão et al.,
1999; Moura et al., 1999; Vasquez et al., 2000) that
are discussed in Section 6. 1880 Ma or younger
intermediate and mafic intrusions have been
recognized in the TGP (Table 1, Fig. 1b); lamprophyric rocks have also been identified (Klein and
Vasquez, 2000). Whole-rock K/Ar dating indicates a minimum age of 15369/31 Ma for these
rocks. Proterozoic and Phanerozoic sedimentary
rocks complete the stratigraphy of the province.
In the TGP, mineralization has been related to
two different periods: (1) at /1.96 Ga hosted by
the Cuiú/Cuiú complex, the Jacareacanga Group
and the Creporizão suite; and (2) at /1.88 Ga
hosted by the Parauari suite (sometimes also by
the Maloquinha suite, Iriri volcanic rocks, and
Ingarana mafic rocks) (Coutinho et al., 2000;
Klein et al., 2001). Coutinho et al. (2000) and
Klein et al. (2001) argued that most of the Tapajós
194
Fig. 2. Geological map of the Vila Riozinho region showing the distribution of the studied granitic rocks and volcanic sequences. A,
amphibole; B, biotite; MZG, monzogranite; LMZG, leucomonzogranite; SG, syenogranite; QM, quartz monzonite; MD,
monzodiorite; QMD, quartz monzodiorite.
195
Fig. 3. Q/A/P and Q/(A/P)/M diagram (Streckeisen, 1976) showing the modal compositions of the (a) older São Jorge granite,
younger São Jorge granite, and Jardim do Ouro granite and (b) Maloquinha granite. Lines 1, 2, and 3 denote, respectively, the
tonalitic-trondhjemitic calc-alkaline, granodioritic calc-alkaline, and shoshonitic series trends (cf. Lameyre and Bowden, 1982).
196
gold deposits can be classified as mesozonal to
epizonal orogenic (Groves et al., 1998) lode-gold
deposits hosted by granitoids. Other authors
emphasized presence of epithermal gold deposits
hosted by gabbros and volcanic rocks (Dreher et
al., 1998; Jacobi, 1999; Juliani et al., 2000; Nunes
et al., 2000).
except in the drill cores of the gold mineralized
area where it is locally brecciated or foliated
indicating a brittle /ductile deformation. The São
Jorge granites include frequent, 5/10 cm long,
oval-shaped mafic enclaves. They also display
local rapakivi texture characterized by sparse
crystals of K-feldspar mantled by plagioclase.
3.3. Moraes Almeida volcanic sequence
3. Geology of the Vila Riozinho region
The Vila Riozinho region in the easternmost
part of the TGP (Fig. 2) is covered by dense rain
forest and thus field relationships between different units are hard to decipher. There are gold
deposits associated with the São Jorge and Jardim
do Ouro granites. In the study area, the following
geologic units have been identified (Fig. 2):
3.1. Vila Riozinho volcanic sequence
This sequence is well exposed along the Jamanxim and Riozinho das Arraias rivers and is a low
relief region. It is composed of intermediate to
felsic flows, formerly included in the Iriri Group of
the Uatumã Supergroup. However, geochemical
and geochronologic data presented below demonstrate that this sequence is not coeval nor cogenetic
with the felsic volcanic sequence exposed in the
Moraes Almeida area.
Around Moraes Almeida, this sequence is
formed dominantly of reddish brown, strongly
welded and oxidized ignimbrites, rich in crystals
fragments and mm- to cm-sized lithic fragments.
To the northwest of Moraes Almeida, rhyolitic
flows are exposed and to the north there is a local
occurrence of trachytic rocks. Contact relationships between rhyolites and ignimbrites are not
visible in the field, but geomorphologic, petrographic, geochemical, and geochronologic data
(see below) suggest that they are related to the
same volcanic event. The granites exposed around
this volcanic sequence are leucogranites of the
Maloquinha suite. The circular shape of the
volcanic domain and the occurrence of a large
volume of pyroclastic rocks indicate that the
volcanic sequence is probably related to a caldera
(Cas and Wright, 1987; Lowell, 1991). The southern contact between the volcanic sequence and the
Maloquinha granite is marked by a /50-m thick
granite porphyry dike.
3.2. São Jorge granite pluton
3.4. Maloquinha granitic suite
South of Vila Riozinho, around the São Jorge
gold deposit, a granitoid pluton dominantly composed of amphibole /biotite monzogranite (Fig. 3)
is exposed. In the past, this pluton was interpreted
to comprise one granitoid series. However, petrographic, geochemical, and geochronologic data
discussed below indicate that the pluton is heterogeneous and composed of an older granitoid series
that makes up most of the pluton (older São Jorge
granite; Fig. 2) and a younger granite (younger
São Jorge granite; Fig. 2). Granite porphyries have
also been identified. The older São Jorge granite
and associated granite porphyries are massive,
displaying only local, non-penetrative foliation.
The same is true for the younger São Jorge granite,
A pluton of this suite around the volcanic
sequence of Moraes Almeida was studied (Fig.
2). This pluton is irregularly shaped and well
exposed along the roads and consists of isotropic
leucogranite that crops out in relatively high relief
areas.
3.5. Jardim do Ouro pluton
This pluton is situated around the Jardim do
Ouro village and is crossed by the Jamanxim
River. Only a small part of the pluton in the
northwestern section of the mapped area was
studied. It is composed of an isotropic or weakly
Table 2
Petrographic and mineralogic characteristics of the granitoids from the Vila Riozinho region
Accessory
minerals
Secondary
minerals
Mean magnetic
susceptibilitya
Oxygen fugacitya
Amphiboleb
/20%
Zircon
Epidote
20.9890 /10 3
Slv
Near
Mg-hastingsite
0.80 /
0.75
Tschermakite
0.79 /
0.71
Mg-hornblende 0.79 /
0.74
8.2262 /10 3
Slv
5.1634 /10 3
Slv
NNO and
0.64
Rare
Titanite
Amph-bt quartz monzonite/monzogranite
bt leucomonzogranite/
syenogranite
Granite porphyry
Mg/
Biotiteb Mg/
(Mg/Fe)
(Mg/
in amphb
Fe) in
btb
Mg-bio- 0.59 /
tite
0.53
Plagioclaseb
Andesine
7 /12%
Apatite
Carbonate
B/5%
Magnetite
Chlorite
/10%
Ilmenite
Sericite
Buffers
0.59
Fe-biotite
Zircon, titanite, apatite
Chlorite, car- 7.9652 /10 3
bonate, seri- Slv
cite
2.6596 /10 3
Slv
Near NNO
and HITMQ
buffers
actinolite
0.71
Mg-bio- 0.62 /
tite
0.57
(Andesine)
oligoclase
Lower than in Fe-hornblende
the São Jorge
granites
Fe-biotite
(Andesine)
oligoclase
Younger São Jorge
granite
Amph-bt monzogranite 8 /12%
bt leucomonzogranite
B/2%
Magnetite, ilmenite
Jardim do Ouro granite
7 /8%
Zircon, titanite, apatite,
magnetite
Chlorite, seri- 4.7988 /10 3
cite, epidote Slv
Maloquinha granite
1 /4%
Magnetite,
fluorite
Chlorite
Granite porphyry
/10%
Zircon, titanite, apatite,
magnetite
Chlorite, sericite, epidote
2.1534 /10 3
Slv
HITMQ
0.63 /
0.60
0.45
(Andesine)
oligoclase
Oligoclase albite
Oligoclase
Rare
Near NNO
buffer
0.40 /
0.33
Absent
Lower than in Fe-hornblende
the São Jorge
granites
0.32 /
0.29
Fe-biotite
0.43 /
0.40
Oligoclase,
albite
andesine oligoclase
197
bt, biotite; amph, amphibole.
a
Data from Figueiredo (1999).
b
Based on data obtained by electron microprobe at the Geosciences Institute of the Federal University of the Rio Grande do Sul */UFRGS.
Older São Jorge granite
bt-amph monzodiorite/
quartz monzodiorite
Mafic
minerals
198
Fig. 4. Classification diagrams (Leake, 1997) for amphiboles of the granitoids and volcanic sequences of Vila Riozinho, TGP.
Abbreviations as in Fig. 2.
foliated amphibole /biotite monzogranite that is
coeval with the younger São Jorge granite and
Maloquinha granite suite.
3.6. Granite porphyries and dikes
Several granite porphyries have been identified
in the mapped area. Those found in the São Jorge
pluton are similar in mineralogy to the older São
Jorge granite. Other granite porphyries cut the
rhyolites and Maloquinha granite. The gold deposits may be related to these porphyry systems
(Jacobi, 1999). Dacitic and andesitic dikes cut the
Jardim do Ouro pluton, the older São Jorge
granite and the rhyolites of the Vila Riozinho
sequence. This suggests recurrence of volcanic
episodes during the evolution of the province.
4. Petrography and mineral chemistry
4.1. Granitoid plutons
Petrographic and mineralogical characteristics
of the studied granitoids are summarized in Table
2. The older São Jorge is composed of biotite /
amphibole monzodiorite and quartz monzodiorite
to amphibole/biotite monzogranite and leucomonzo- or syenogranite (Fig. 3a). The latter two
are light gray or pink gray and dominant in the
pluton. Monzodiorite, quartz monzodiorite, and
amphibole/biotite monzogranite display mediumor coarse-, even-grained granular hypidiomorphic
textures. Leucomonzogranite and syenogranite are
medium- to fine-grained.
The younger São Jorge and Jardim do Ouro
granites are more homogeneous and composed
dominantly of amphibole/biotite monzogranite
(Fig. 3a). In the mineralized area, the primary
minerals of the younger São Jorge granite are,
generally, intensively altered. Quartz-rich leucomonzogranite and microgranite are subordinate
phases. Granite porphyries are monzogranitic and
have quartz, plagioclase, alkali feldspar, amphibole, and biotite phenocrysts in a felsitic, finegrained groundmass.
The Maloquinha suite is composed of reddish,
medium- and even-grained leucosyenogranite with
subordinate leucomonzogranite (Fig. 3b). Except
for occasional amphibole pseudomorphs in quartz
199
boles and biotite of the São Jorge granites and
associated granite porphyry 59 (Figs. 4 and 5,
Table 2) are similar to the amphiboles and biotite
of calc-alkaline or ‘subalkaline’ series granitoids */
they were formed in relatively oxidizing conditions. The Jardim do Ouro granite and granite
porphyry 18a were formed in less oxidizing conditions and do not have a typical ‘calc-alkaline’
character.
Plagioclases of the amphibole /biotite monzogranite facies of the São Jorge and Jardim do Ouro
granites are quite similar (Table 2), displaying a
conspicuous normal, oscillatory zoning. In the
hydrothermally altered parts of the younger São
Jorge granite, plagioclase is altered. Plagioclase of
the Maloquinha granite is sodic oligoclase to albite
approaching in composition that of the leucomonzogranites of the São Jorge pluton.
4.2. Volcanic sequences
Fig. 5. (a) Total Al versus Mg (Nachit et al., 1985) and (b)
MgO versus Al2O3 (Abdel-Rahman, 1994) diagram of biotite
from the studied granitoids. BLMZG, biotite leucomonzogranite of the older São Jorge granite, other symbols as in Fig. 4.
monzonite and quartz syenite, the leucogranites
have chloritized biotite as the main mafic phase.
Magnetite and fluorite are common accessory
minerals.
The São Jorge, Jardim do Ouro, and Maloquinha granites show relatively high magnetic
susceptibility (Table 2), and are classified as
magnetite series granites (Figueiredo, 1999; Ishihara, 1981). The Jardim do Ouro granite crystallized at a somewhat lower oxygen fugacity than
the other granites (Figueiredo, 1999). The amphi-
In the TAS diagram (Fig. 6), the Vila Riozinho
sequence samples plot mostly in the fields of
basaltic andesite, basaltic trachyandesite, trachyte,
and rhyolite along the boundary between alkaline
and subalkaline fields. The rhyolites and ignimbrites of Moraes Almeida are enriched in silica
compared to the rhyolites of Vila Riozinho
sequence. The fields of the São Jorge and Maloquinha granites correspond to those of the Vila
Riozinho and Moraes Almeida volcanic rocks,
respectively.
Basaltic andesite and basaltic trachyandesite
have phenocrysts of zoned plagioclase and clinopyroxene, late amphibole and biotite, and grains
of opaque minerals and zircon in a microgranular
or pilotaxitic groundmass. The mafic phases are
often completely replaced by chlorite, epidote,
actinolite, secondary titanite, and opaque minerals. Trachyte is flow-foliated and has phenocrysts
of saussuritized plagioclase and subordinate alkali
feldspar, biotite, and pseudomorphosed amphibole, in a quartz-poor, felsitic groundmass. Rhyolites have phenocrysts of sericitized plagioclase,
alkali feldspar, and subordinate quartz in a felsitic,
sometimes granophyric groundmass. Biotite and
pseudomorphosed amphibole are the main mafic
phases.
200
Fig. 6. TAS diagram (Le Maitre et al., 1989) for the Vila Riozinho and Moraes Almeida volcanic sequences showing the fields of the
studied granitoids for comparison. Dashed line separates alkaline and subalkaline fields (Irvine and Baragar, 1971).
The ignimbrites surrounding the Moraes Almeida village (Fig. 2) are reddish brown, densely
welded crystal-rich lapilli tuffs. Quartz, alkali
feldspar, and subordinate plagioclase crystal fragments, as well as cm-sized lithic fragments, lie in a
fine-grained felsic matrix with abundant welded,
stretched, and recrystallized shards. The rock is
strongly oxidized and scarce biotite and amphibole
crystals are entirely pseudomorphosed by iron
oxides. To the west of Moraes Almeida (Fig. 2),
the dominant rock is a porphyritic, locally ignimbritic light brown rhyolite that has phenocrysts of
quartz, alkali feldspar, and rare plagioclase in a
felsitic groundmass. A gray porphyritic trachyte is
found north of Moraes Almeida and has phenocrysts of plagioclase, quartz, augite, Fe /hornblende, and rare alkali feldspar in a fine-grained
felsitic, sometimes granophyric, matrix.
5. Geochemistry
5.1. Analytical procedures
Chemical analyzes were done at the LakefieldGeosol laboratories in Belo Horizonte, Brazil.
SiO2, TiO2, Al2O3, total Fe2O3, MnO, MgO,
CaO, K2O, Na2O, P2O5, Rb, Sr, Ba, Zr, Y, Nb,
Ga, and V were analyzed by X-ray fluorescence
combined with atomic absorption spectrometry.
Rare earth elements (REE) were analyzed by
inductively coupled plasma atomic emission spectrometry. FeO was analyzed by wet chemistry at
the Geosciences Center of the Federal University
of Pará. Additional information about analytic
methods can be obtained from the authors by
request.
Table 3
Chemical composition of the granitoids and volcanic rocks of the Vila Riozinho region
Older São Jorge granite
Facies
BAQMD to
GP
ABQM to ABQS
ABMZG
103
27a
BLMZ/SG-south
BLMZ/SG-center
68a
85
Younger São Jorge granite
Maloquinha granite
ABMZG
BLMZG
ABQS
BLMZ/SG
70
54a
22a
GP
JOG
ABMZG
BAMD
Sample
SiO2 (wt.%)
TiO2
58
102
54.5
0.81
32
28
73a
74
35a
59
9/10
7/3
21
06a
18a
03a
58.40
64.50
66.80
67.80
68.90
71.60
72.50
73.20
74.20
75.10
68.60
67.50
68.20
73.90
70.30
74.80
76.10
76.60
67.40
0.79
0.43
0.50
0.39
0.37
0.26
0.27
0.24
0.19
0.19
0.41
0.40
0.39
0.08
0.26
0.21
0.07
0.11
0.48
71.10
0.28
18.4
15.90
17.80
16.00
16.20
15.30
14.70
14.70
14.30
14.10
13.50
15.20
15.80
16.10
14.00
14.80
13.50
13.00
12.50
15.80
14.60
Fe2O3
4.7
3.00
1.87
1.80
1.86
1.71
1.37
1.23
1.03
0.94
1.00
1.59
2.70
2.50
0.49
0.84
1.14
0.77
1.18
1.54
1.29
FeO
2.34
3.42
1.20
0.90
0.94
0.98
0.39
0.42
0.33
0.14
0.12
1.18
0.99
1.20
0.20
1.40
0.32
0.21
0.20
1.40
1.00
MnO
0.10
0.11
0.07
0.07
0.06
0.07
0.05
0.06
0.05
0.05
0.04
0.05
0.04
0.04
0.03
0.08
0.04
0.05
0.02
0.06
MgO
3.10
4.30
0.88
0.62
0.81
0.83
0.46
0.33
0.28
0.09
0.11
1.10
1.20
1.30
0.12
0.21
0.14
0.09
0.09
0.33
0.57
CaO
6.90
5.20
2.80
1.80
2.30
2.00
1.40
0.99
0.91
0.54
0.65
2.30
2.40
2.40
0.61
1.10
0.40
0.50
0.33
1.60
1.70
0.03
Na2O
4.00
3.40
4.70
4.50
4.40
4.00
4.00
4.30
4.40
4.10
4.10
4.20
4.40
4.20
4.60
4.40
4.20
4.40
3.60
4.40
3.90
K 2O
2.50
3.30
4.40
5.20
4.20
4.30
4.90
4.80
5.10
5.00
5.00
4.50
4.00
4.50
4.60
5.60
5.30
4.60
4.90
5.10
4.90
P2O5
0.37
0.27
0.18
0.12
0.14
0.11
0.08
0.06
0.05
0.02
0.02
0.14
0.14
0.13
0.01
0.04
0.03
B/ 0.01
B/ 0.01
0.12
0.05
LOI
1.51
1.34
0.64
0.50
0.61
0.80
0.39
0.51
0.21
0.45
0.38
0.71
0.57
0.62
0.32
0.47
0.53
0.74
0.61
0.72
0.55
Total
99.23
99.43
99.47
98.81
99.71
99.37
99.60
100.10
99.82
100.21
99.98
100.14
101.58
98.96
99.50
100.61
100.53
100.14
98.95
99.97
100.7
Trace elements (ppm)
Ba
1319
1881
2230
1278
1312
1236
1053
959
675
409
423
1364
1730
1958
477
834
446
82
73
1667
Rb
54
112
135
234
155
163
150
212
158
217
190
145
100
101
182
246
251
492
387
140
236
Sr
1101
711
859
360
622
563
390
333
212
123
129
636
1215
1350
255
148
49
21
19
247
187
Zr
191
274
368
358
246
240
197
264
208
188
198
265
199
196
97
358
241
142
206
470
194
4
9
15
12
7
11
5
17
13
12
21
11
6
4
4
16
14
25
26
13
14
19
25
25
55
35
40
25
39
27
46
29
30
11
11
10
60
54
94
139
55
38
24
24
25
25
24
25
20
24
22
20
20
21
27
27
24
24
22
24
24
26
22
165
159
48
44
40
44
12
12
10
B/10
B/10
57
49
54
B/10
B/ 10
B/ 10
B/ 10
B/ 10
B/10
Nb
Y
Ga
V
765
20
La
18.45
22.32
24.69
58.46
28.13
42.54
25.54
25.84
20.38
31.86
12.36
34.91
26.78
29.50
13.64
66.74
77.98
20.56
114.60
39.98
30.39
Ce
37.06
46.68
51.36
122.20
49.89
71.37
49.70
49.75
43.26
63.45
24.87
57.55
53.70
53.25
20.88
116.40
120.50
54.90
186.60
71.90
61.60
Nd
16.18
19.07
19.19
42.40
20.18
25.94
16.75
17.70
15.71
22.01
9.02
21.51
18.34
15.93
5.63
33.46
50.58
26.68
52.36
24.97
23.61
Sm
3.33
3.55
3.23
7.27
3.46
4.58
2.96
3.07
2.87
3.97
1.71
3.65
3.22
2.77
0.92
6.45
9.95
8.39
11.23
4.28
5.08
Eu
0.96
0.79
0.79
1.20
0.67
0.81
0.52
0.49
0.42
0.49
0.24
0.65
0.77
0.70
0.17
0.62
0.66
0.30
0.35
0.72
0.72
Gd
2.38
2.51
2.18
4.97
2.57
3.19
1.98
2.16
2.06
2.62
1.22
2.43
1.64
1.50
0.49
3.63
5.17
6.80
7.29
2.87
3.83
Dy
1.47
1.57
1.30
3.21
1.62
1.92
1.46
1.28
1.38
1.65
0.94
1.31
0.82
0.62
0.21
2.18
2.87
7.14
4.44
1.69
2.61
Ho
0.22
0.29
0.26
0.64
0.29
0.39
0.26
0.20
0.25
0.26
0.20
0.21
0.10
0.10
0.04
0.41
0.55
1.52
0.82
0.30
0.48
Er
0.72
0.74
0.73
1.66
0.75
0.95
0.80
0.55
0.70
0.73
0.58
0.55
0.20
0.16
0.11
0.89
1.23
4.44
1.83
0.75
1.32
Yb
0.43
0.50
0.57
1.35
0.53
0.69
0.66
0.46
0.59
0.54
0.49
0.35
0.20
0.16
0.10
0.70
0.91
4.89
1.26
0.45
1.06
Lu
0.06
0.07
0.13
0.20
0.08
0.11
0.10
0.09
0.11
0.08
0.08
0.05
0.03
0.03
0.02
0.10
0.13
0.69
0.17
0.06
0.16
Rb/Sr
0.05
0.16
0.16
0.65
0.25
0.29
0.38
0.64
0.75
1.76
1.47
0.23
0.08
0.07
0.71
1.66
5.12
23.43
20.37
0.57
1.26
Ba/Rb
24.43
16.79
16.52
5.46
8.46
7.58
7.02
4.52
4.27
1.88
2.23
9.41
17.30
19.39
2.62
3.39
1.78
0.17
0.19
11.91
3.24
Rb/Zr
(La/Yb)n
0.28
0.41
0.37
0.65
0.63
0.68
0.76
0.80
0.76
1.15
0.96
0.55
0.50
0.52
1.88
0.69
1.04
3.46
1.88
0.30
1.22
28.89
30.01
29.44
29.27
35.82
41.37
26.00
37.75
23.32
39.97
17.06
67.71
92.22
122.16
96.88
64.26
57.90
2.84
61.39
60.24
19.32
Eu/Eu
0.99
0.77
0.86
0.58
0.65
0.62
0.63
0.55
0.50
0.43
0.48
0.53
0.92
0.95
0.68
0.36
0.25
0.12
0.11
0.60
0.48
A/CNKa
0.84
0.85
1.01
0.98
1.01
1.03
1.02
1.04
0.99
1.07
1.01
0.95
0.99
1.00
1.03
0.97
1.01
0.99
1.06
1.01
0.99
FeOtot/MgOb
0.68
0.59
0.77
0.80
0.76
0.75
0.78
0.82
0.82
0.92
0.89
0.70
0.74
0.73
0.84
0.91
0.91
0.90
0.93
0.89
0.79
Al2O3
201
Facies
BA
BTR
Sample
90b
MV-154
SiO2 (wt.%)
Moraes Almeida volcanic sequence
Trachyte
36c
202
Vila Riozinho volcanic sequence
106a
Rhyolite
107
92a
62a
98
93a
101
115
TRACH
Rhyolite
48
52
Ignimbrite
12c
39a
51
15a
111
46
14
38
17
54.40
54.70
61.20
64.70
68.20
68.40
69.40
69.70
71.10
71.80
67.60
72.00
74.80
75.20
75.80
72.70
73.60
75.20
75.40
75.90
0.89
1.00
1.60
0.41
0.54
0.36
0.44
0.37
0.33
0.37
0.35
0.54
0.37
0.24
0.18
0.24
0.17
0.14
0.11
0.15
0.15
76.10
0.15
Al2O3
14.60
16.10
15.90
18.80
16.50
15.60
15.70
16.30
16.00
15.60
14.70
16.00
13.80
13.00
12.60
12.80
13.70
13.80
13.00
12.60
12.70
12.80
Fe2O3
2.54
3.43
4.50
1.93
2.63
1.56
1.80
1.66
1.66
1.38
1.78
1.11
1.10
0.87
1.06
0.89
0.91
1.88
1.28
1.58
1.38
1.48
FeO
5.72
4.83
4.86
1.05
1.41
0.49
0.72
0.58
0.31
0.11
0.11
1.70
0.99
0.39
0.22
0.28
0.80
0.11
0.11
0.11
0.11
0.11
MnO
0.16
0.13
0.14
0.10
0.14
0.05
0.05
0.06
0.08
0.02
0.05
0.08
0.10
0.06
0.05
0.07
0.03
0.04
0.03
0.03
0.03
0.04
MgO
5.80
5.50
3.60
0.95
1.20
0.59
0.57
0.37
0.44
0.52
0.25
0.61
0.40
0.24
0.25
0.25
0.28
0.24
0.21
0.18
0.22
0.14
CaO
7.80
7.30
6.40
1.60
2.90
0.86
2.30
1.20
0.90
0.68
1.20
2.00
0.70
0.54
0.40
0.45
0.89
0.91
0.31
0.79
0.80
0.70
Na2O
2.90
3.20
3.10
6.20
4.10
5.30
4.40
4.30
4.80
5.60
4.20
5.10
2.70
3.80
3.70
3.80
3.70
3.60
4.00
3.40
3.70
3.60
K 2O
2.10
2.50
3.10
5.80
4.40
5.50
4.30
4.80
5.00
2.90
4.80
5.10
6.50
5.40
5.00
5.30
5.40
5.60
4.80
4.90
4.90
4.80
P2O5
0.29
0.32
1.20
0.16
0.19
0.08
0.09
0.09
0.08
0.07
0.05
0.13
0.09
0.02
0.02
0.02
0.04
0.04
0.02
0.03
0.01
0.02
LOI
1.81
1.02
0.80
1.55
0.97
0.49
0.68
0.79
0.76
1.28
0.64
0.07
1.05
0.73
0.47
0.62
0.92
0.65
0.53
0.96
0.51
0.52
Total
99.41
99.73
99.90
99.75
99.68
99.08
99.45
99.92
100.06
99.63
99.93
100.04
99.80
100.09
99.15
100.52
99.54
100.61
99.60
100.13
100.41
100.46
Trace elements (ppm)
Ba
912
1260
1154
2864
1338
1481
1352
2275
1592
1349
1410
2641
471
267
56
241
500
667
214
439
327
354
Rb
41
67
151
140
120
176
176
161
163
120
195
143
358
262
298
282
220
252
309
248
285
240
Sr
723
688
496
431
633
388
433
462
333
508
275
293
42
36
20
39
82
122
56
88
82
69
Zr
173
192
281
610
286
420
297
408
421
370
292
636
322
317
236
301
231
298
187
231
234
198
Nb
Y
Ga
6
5
12
10
10
20
6
12
12
15
15
16
18
17
21
18
11
14
20
14
16
11
23
19
32
20
27
28
38
29
35
21
41
39
52
56
61
49
42
204
167
53
103
58
23
23
28
20
26
18
22
21
19
17
21
22
23
19
20
22
20
22
21
19
24
21
V
206
195
198
61
67
14
32
24
22
23
17
B/ 10
10
B/ 10
B/ 10
B/ 10
B/ 10
B/ 10
B/ 10
B/ 10
B/ 10
B/ 10
Th
B/5
B/5
B/5
B/5
B/5
12
B/5
B/5
9
11
B/5
B/ 5
16
25
24
18
22
21
35
25
20
21
La
17.95
17.27
43.16
22.26
48.48
23.36
34.94
32.46
27.05
42.71
34.69
61.85
142.10
Ce
39.31
35.85
87.26
40.59
93.68
51.98
71.55
64.53
53.22
83.85
70.37
114.70
215.20
Nd
16.50
16.53
36.09
13.42
33.36
19.83
24.48
22.19
16.91
35.59
25.16
38.54
86.19
Sm
3.44
3.52
6.36
2.41
6.05
3.62
4.20
4.28
3.27
7.51
5.88
8.06
18.76
Eu
0.81
0.92
1.52
0.45
1.10
0.72
0.88
0.79
0.49
2.05
0.23
0.71
1.06
Gd
2.58
2.77
4.02
1.43
3.50
2.64
2.74
2.72
2.20
6.01
4.35
5.89
15.40
Dy
1.59
2.00
2.07
0.76
1.83
1.92
1.54
1.72
1.40
4.75
2.93
3.72
9.75
Ho
0.31
0.42
0.40
0.12
0.33
0.37
0.30
0.31
0.24
0.93
0.54
0.75
1.84
Er
0.78
1.01
0.92
0.27
0.82
0.99
0.74
0.85
0.65
2.54
1.36
1.98
4.29
Yb
0.50
0.71
0.51
0.18
0.66
0.75
0.54
0.77
0.47
2.36
1.20
1.55
3.12
Lu
0.07
0.11
0.07
0.03
0.09
0.10
0.09
0.11
0.08
0.33
0.17
0.23
0.41
Rb/Sr
0.06
0.10
0.30
0.32
0.19
0.45
0.41
0.35
0.49
0.24
0.71
0.49
8.52
7.28
14.90
7.23
2.68
2.07
5.52
2.82
3.48
3.48
Ba/Rb
22.24
18.81
7.64
20.46
11.15
8.41
7.68
14.13
9.77
11.24
7.23
18.47
1.32
1.02
0.19
0.85
2.27
2.65
0.69
1.77
1.15
1.48
0.23
0.39
1.11
0.83
1.26
0.94
0.95
0.85
1.65
1.07
1.22
1.21
Rb/Zr
(La/Yb)n
Eu/Eu
A/CNKa
FeOtot/MgO
b
0.24
0.35
0.54
24.28
16.42
56.79
0.42
0.42
0.59
0.39
83.47
49.96
20.97
43.84
0.32
0.67
0.22
28.60
39.01
12.23
19.51
26.86
0.14
30.73
0.80
0.87
0.86
0.69
0.67
0.69
0.75
0.66
0.53
0.91
0.69
0.76
0.79
0.97
0.98
0.96
0.98
1.13
1.07
1.15
1.03
0.91
1.08
0.99
1.03
1.00
1.01
0.30
1.01
1.05
0.19
1.02
0.99
1.03
0.58
0.59
0.71
0.75
0.76
0.76
0.80
0.85
0.80
0.72
0.87
0.82
0.83
0.83
0.82
0.81
0.85
0.88
0.86
0.89
0.86
0.91
A, amphibole; B, biotite; QMD, quartz monzodiorite; MD, monzodiorite; MZG, monzogranite; SG, syenogranite; QM, quartz monzonite; LMG, leucomonzogranite;
QS, quartz syenite; GP, Granite Porphyry; JOG, Jardim do Ouro Granite; BA, basaltic andesite; BTR, basaltic trachyandesite; TRACH, trachyte.
a
Molecular Al2O3/(CaO/Na2O/K2O).
b
FeOtot denotes total Fe as FeO.
54.80
TiO2
5.2. Results
Representative analyses of the studied granitoids and volcanic rocks are presented in Table 3.
The older São Jorge granite and the Vila Riozinho
volcanic sequence consist of a compositionally
wide series of rocks with SiO2 between 54.5 and
75.1 wt.%. The other studied granitoids and the
Moraes Almeida volcanic sequence have a more
limited SiO2 range (67.5 /76.6 wt.%). The São
Jorge and Jardim do Ouro granites and the Vila
Riozinho volcanic sequence are transitional between metaluminous and slightly peraluminous
(Fig. 7a) and show FeOtot/(FeOtot/MgO) between 0.6 and 0.9 (Fig. 7b). The younger São
Jorge granite is a little more enriched in Mg that
the older São Jorge granite. The Maloquinha
granite and the Moraes Almeida volcanic sequence
are both dominantly peraluminous (Fig. 7a) and
have FeOtot/(FeOtot/MgO) higher than 0.8 (Fig.
7b).
In the Rb versus (Y/Nb) plot (Fig. 7c), the São
Riozinho sequence samples plot dominantly in the
volcanic arc field (VAG). The Maloquinha granite
and the Moraes Almeida volcanic sequence samples plot mostly in the within plate granite (WPG)
field. Most of the analyzed samples fall in the postcollisional field of Pearce (1996) which could
reflect transition from calc-alkaline to alkaline
series in orogenic to post-orogenic tectonic settings
(Bonin, 1990; Barbarin, 1999) or increase in arc
maturity (Brown et al., 1984).
In the (K2O/Na2O)/CaO versus Zr/Nb/
Ce/Y diagram (Fig. 7d), the São Jorge and
Jardim do Ouro granites are like normal and
fractionated S- and I-type granites. Their oxidized
character, metaluminous to marginally peraluminous nature, and VAG character (Fig. 7c) point to
strong I-type granite affinity. Samples from the
Vila Riozinho sequence straddle the I- and A-type
granite boundary. This reflects the higher zirconium contents (Table 3) and more alkaline character of these volcanic rocks compared to the older
São Jorge granite. The Maloquinha granite and
the Moraes Almeida volcanic sequence samples
are transitional between fractionated I-type granite and A-type granite (Fig. 7d). Nevertheless, their
203
high FeOtot/(FeOtot/MgO), WPG affinity (Fig.
7c) and mildly metaluminous to peraluminous
character (Fig. 7a) suggest that they are geochemically similar to evolved aluminous, A-type granites
(King et al., 1997; Dall’Agnol et al., 1999a; Rajesh,
2000).
The Maloquinha granite and the Moraes Almeida volcanic sequence have higher Rb/Zr and
Rb/Sr ratios than the other studied rocks (Fig. 7e
and f). The Rb/Zr ratio increases from the Vila
Riozinho sequence to the younger São Jorge
granite, the older São Jorge granite is intermediate
between the two. This indicates that these rocks
are not comagmatic. A similar conclusion is
suggested by the geochronologic data discussed
below. The granite porphyry associated with the
older São Jorge granite is similar to the São Jorge
granites with similar SiO2 contents (Fig. 7). The
granite porphyry associated with the Maloquinha
granite and the Moraes Almeida ignimbrites has a
common geochemical affinity with the trachyte
(sample 48) of the Moraes Almeida volcanic
sequence.
In the log[CaO/(Na2O/K2O)] versus SiO2 plot
(Fig. 8a), the São Jorge and Jardim do Ouro
granites and the Vila Riozinho sequence samples
indicate calc-alkaline affinity and transitional
character between normal and mature arc series.
In the K2O versus SiO2 diagram (Fig. 8b) most of
the granitoid samples plot in the high-K field. The
Vila Riozinho volcanic sequence has a transitional
high-K to shoshonitic character. In the TAS
diagram (Fig. 6) the Vila Riozinho sequence is,
on average, slightly more alkaline than the São
Jorge granites (Table 3, Fig. 7e and f). The calcalkaline character of the São Jorge granites is also
indicated by their relatively oxidized nature (Figs.
4 and 5; cf. Figueiredo, 1999). The Jardim do Ouro
granite constitutes a more iron-rich amphibole and
biotite (Figs. 4 and 5) and was probably crystallized from a less oxidized magma (Figueiredo,
1999).
The REE contents of the studied rocks are
relatively low (Fig. 9), except in a few samples of
the Maloquinha granite and Moraes Almeida
ignimbrites. The REE patterns of the São Jorge
granites (Fig. 9a) and associated granite porphyries are enriched in the LREE and the younger São
204
Fig. 7
Jorge granite shows lower contents of the HREE
than the older São Jorge granite. Negative Eu
anomalies (Fig. 9a) are very small in the younger
São Jorge granite, in the older São Jorge granite
moderate. The Vila Riozinho volcanic sequence
REE pattern (Fig. 9c) is broadly similar to that of
the older São Jorge granite. The Maloquinha
granite and Moraes Almeida volcanic rocks (Fig.
9b and d) are enriched in total REE compared to
the São Jorge granites and the Vila Riozinho
volcanic sequence. Overall, the REE patterns are
similar but the Maloquinha granite shows higher
HREE contents than the Moraes Almeida volcanic rocks (Table 3). Negative Eu anomalies are
remarkable in both the Maloquinha granite and
the Moraes Almeida rhyolites and ignimbrites
(Fig. 9b and d). The REE signature of the Moraes
Almeida trachyte (sample 48) differs from the Vila
Riozinho trachytes by higher content of the HREE
(Fig. 9d). The Jardim do Ouro granite and granite
porphyry 18a have REE patterns similar to those
of the low-silica samples of the Maloquinha
granite (Fig. 9b; Table 3).
Primordial mantle-normalized trace element
patterns of the studied rocks with more than 60
wt.% of SiO2 (Fig. 10) corroborate the geochemical similarities of the São Jorge granites, Vila
Riozinho volcanic sequence, and the calc-alkaline
granitoids of normal arcs (cf. Brown et al., 1984).
All are depleted in Nb, P, and Ti and do not show
significant Sr and Ba depletion as observed for
alkali-calcic granitoids of more mature arcs
(Brown et al., 1984). The Vila Riozinho samples
display a discrete positive Ba anomaly and the
younger São Jorge granite (Fig. 10c) differs from
the older São Jorge granite (Fig. 10a) by small
positive Ba and Sr anomalies and depletion in Yb.
The Maloquinha and Jardim do Ouro granites and
Moraes Almeida volcanic sequence (Fig. 10d and
e) are similar and show strong depletion in Ba, Nb,
205
Sr, P, and Ti. The patterns of the granite porphyry
(18a) and trachyte of the Moraes Almeida sequence display less pronounced negative anomalies of Sr, P, and Ti, small positive Ba anomaly,
and enrichment in Zr. These patterns are broadly
similar to those of mature continental arc granitoids (Fig. 10f).
6. Geochronology
6.1. Analytical procedures
Zircon crystals from volcanic and granitic rocks
of the São Jorge region were dated by the single
grain Pb evaporation method (cf. Kober, 1986,
1987) at the geochronological laboratory of Universidade Federal do Pará, Belém, Pará, Brazil.
Isotope analyses were performed on a Finnigan
MAT262 mass spectrometer in dynamic mode
using the ion counting detector. When the Pb
signal exceeded the ion counting saturation threshold, the isotopic measurements were done in static
mode on Faraday cups. 207Pb/206Pb ratios were
corrected by a mass discrimination factor of 0.129/
0.03% determined by repeated analyses of the
NBS-982 Pb standard. In the Amazonian craton
and specifically in the Tapajós Province,
207
Pb/204Pb ratios may be slightly higher than the
Stacey and Kramers (1975) model. However, the
calculation of common Pb corrections using the
mean Pb values of sulphides associated with
primary gold deposits from the Tapajós Province
(Coutinho et al., 2000) provided ages similar to
those obtained when common Pb corrections were
done using the Stacey and Kramers (1975) model.
Analyses with 206Pb/204Pb ratios lower than 2500
were rejected, thus minimizing the effects of
common Pb correction. The age of each sample
was obtained using the mean of 207Pb/206Pb ratios
Fig. 7. Geochemical diagrams for the granitic rocks and volcanic sequences of Vila Riozinho region. (a) Molecular Al2O3/(CaO/
Na2O/K2O) versus SiO2; (b) FeOtot/(FeOtot/MgO) versus SiO2 (FeOtot denotes total iron as FeO); (c) Rb versus Y/Nb, fields from
Pearce et al. (1984): syn-COLG */syn-collision granites, WPG */within plate granites, VAG */volcanic arc granites, ORG */ocean
ridge granites; (d) (K2O/Na2O)/CaO versus Zr/Nb/Ce/Y; average composition of A-type (A), M-type (M), S-type (S), and I-type
(I) granites and fields for fractionated felsic granites (FG) and unfractionated M, I, and S type granites (OGT) according Whalen et al.
(1987); (e) Ba/Rb versus Rb/Zr; (f) Rb/Sr versus Rb/Zr.
206
Fig. 8
207
Fig. 9. Chondrite-normalized (Evensen et al., 1978) patterns of selected REE for (a) the São Jorge granites and associated granite
porphyry; (b) Maloquinha and Jardim do Ouro granites and granite porphyry; (c) Vila Riozinho volcanic sequence; and (d) Moraes
Almeida volcanic sequence.
of at least four crystals at the highest step of
temperature. When different heating steps of the
same grain gave similar ages, all of them were
included in the age calculation. Grains that yielded
lower ages (probably because of Pb loss after
crystallization) were discarded. Weighted mean
and errors on the ages were calculated following
Gaudette et al. (1998). The ages are presented in
Table 4 at the 2s level.
The Pb evaporation method provides a
Pb/206Pb age, which is a minimum age as Pb/
U ratios are not determined. However, the assumption that they represent a ‘concordant’ crystallization age of zircons from magmatic rocks is
probably valid in the case when repeated measurements of 207Pb/206Pb do not vary between different
crystals or at different heating step in one grain
(Kröner et al., 1999; Costi et al., 2000, and
207
Fig. 8. (a) log [CaO/(Na2O/K2O)] versus SiO2 plot (Brown et al., 1984) and (b) K2O versus SiO2 plot for the São Jorge and Jardim do
Ouro granites, granite porphyry, and the Vila Riozinho volcanic sequence. In (b), the field of Vila Riozinho volcanic sequence is shown
for comparison. Low-K, medium-K, and high-K calc-alkaline fields and shoshonite fields in (b) from Peccerilo and Taylor (1976),
modified by Rickwood (1989). Symbols as in Figs. 6 and 7.
208
Fig. 10. Primordial mantle-normalized spidergrams (Wood, 1979) for the (a) older São Jorge granite and associated granite porphyry;
(b) volcanic sequence of Vila Riozinho; (c) younger São Jorge granite; (d) Maloquinha granite, Jardim do Ouro granite, and granite
porphyry; (e) volcanic sequence of Moraes Almeida; and (f) primitive, normal, and mature arc granitoids (Brown et al., 1984).
Table 4
Zircon single-crystal evaporation Pb isotopic data from the granites and volcanic rocks of the TGP
Zircon
Vila Riozinho volcanic sequence (sample 62a */trachyte)
62a/3
Temperaturea Number of
(8C)
ratios
206
Pb/204Pb
208
Pb/206Pb 2s
207
Pbb/206Pb 2s
18 868
25 000
0.22716
0.28489
364
71
1766
1977
18
3
62a/5
1450 F
1480 F
1520
38
134
72
23 810
43 478
28 571
0.31348
0.40531
0.43786
91 0.12294
125 0.12305
110 0.12232
62a/6
1480
1480 F
18
70
15 625
31 250
0.15653
0.15637
80
71
62a/7
1450
16
6623
0.25633
125
62a/8
1450 F
1480 F
75
108
23 810
111 111
0.24879
0.26016
62a/9
1450 F
1480 F
167
137
12 195
33 333
0.21612
0.19347
136
87
62a/10
1450 F
1500 F
49
152
20 833
34 483
0.26782
0.261490
109 0.12200
33 0.12276
33 1986
26 1997
5
4 1997
62a/14
1500
1500 F
88
55
18 519
45 455
0.30966
0.31038
75
109
27
30
3
3
34 2000
19 2001
53 1991
5
3
8 2000
0.20303
0.20333
56
71
2851
2853
5
6
0.12148
59
1978
44 0.12276
40 0.12252
0.12395
0.12494
0.17322
0.17353
73 1997
36 1993
34
15
2014
2028
2589
2592
4
9
11
5 1994
5
5
2
:
4
1998
3
1450
1500
1550
54
88
50
52 632
250 000
200 000
0.22479
0.23548
0.25652
150 0.12256
97 0.12263
85 0.12287
23 1994
40 1995
43 1999
3
6
6 1995
3
107/2
1500
84
166 667
0.22771
262 0.12253
26 1994
4 1994
4
107/3
1450
1500
1550
80
88
86
21 739
21 277
200 000
0.26976
0.30348
0.29800
69
70
77
0.12198
0.12208
0.12209
22
19
28
1986
1987
1987
3
3
4
107/4
1450
1500
84
84
4854
40 000
0.27940
0.31178
76
109
0.12226
0.12198
18
16
1990
1986
3
2
107/5
1450
1500
1550
34
86
42
100 000
66 667
76 923
0.11436
0.18156
0.26939
103 0.12338
310 0.12333
88 0.12297
48 2006
17 2005
28 2000
7
2
4 2004
4
107/6
1450
1500
82
88
6211
5102
0.08648
0.15006
78 0.12194
308 0.12288
17 1985
22 1999
2
3 1999
3
107/8
1450
1550
88
90
12 500
142 857
0.27741
0.29889
22
19
3
3
Mean (526 ratios; USD / 2.7) :
91
69
0.12224
0.12213
1989
1988
2000
18
90
Mean (579 ratios; USD / 1.6)
106
19
Age (Ma) 2s
Crystal
1450
1500
Vila Riozinho volcanic sequence (sample 107 */trachyte)
107/1
0.10796
0.12141
Age (Ma) 2s
step
4
209
210
Zircon
Older São Jorge granite (sample 27 */hornblende /biotite monzogranite)
27/1
(8C)
ratios
206
Pb/204Pb
208
Pb/206Pb 2s
207
Pbb/206Pb 2s
Age (Ma) 2s
step
Age (Ma) 2s
Crystal
76
5556
0.38319
67 0.12158
36 1980
5 1980
5
1450
1500
99
96
6098
12 048
0.30364
0.34729
53 0.12167
158 0.12204
15 1981
46 1987
2
7 1982
3
27/5
1450
1550
12
67
11 905
8197
0.32340
0.40542
304 0.12179
106 0.12158
186 1983
74 1980
27
11 1980
10
27/6
1450
1550
34
90
20 000
24 390
0.34918
0.36009
139 0.12162
84 0.12148
110 1980
24 1978
16
3 1978
3
1981
2
Older São Jorge granite (sample 35 */biotite leucomonzogranite)
35/1
:
1500
1550
84
18
4219
2695
0.32102
0.43022
73 0.12114
222 0.12137
26 1973
57 1977
4
8 1974
3
35/2
1450
1500
1550
34
18
16
3817
5525
4348
0.24192
0.29916
0.30033
754 0.12054
153 0.12115
175 0.12108
32 1965
95 1974
59 1973
5
14
9 1973
7
35/3
1450
1550
86
84
8130
8197
0.32950
0.37180
124
306
30
82
4
12
35/4
1450
1500
84
68
8475
13 514
0.52811
0.57281
120 0.12102
532 0.12150
24 1972
32 1979
4
5 1974
7
35/5
1450
1500
47
88
7042
22 727
0.22892
0.26499
132 0.12094
45 0.12247
64 1970
16 1993
9
2 1993
2
1983
8
0.11975
0.11912
1953
1943
Younger São Jorge granite (sample F09 */hornblende /biotite monzogranite)
F09/1
1475
1500
1550
17
78
27
47 619
25 641
18 519
0.20933
0.21075
0.21480
125 0.11536
37 0.11544
73 0.11576
30 1886
13 1887
63 1892
5
2
10 1887
2
F09/2
1500
1550
8
82
3704
17 544
0.22023
0.20681
137 0.11527
138 0.11530
47 1885
28 1885
7
4 1885
4
F09/3
1425
1475
1550
36
88
80
13 514
9346
17 544
0.13981
0.26128
0.22682
156
126
56
41
25
23
6
4
3
F09/4
1500
85
12 987
0.18099
32 0.11610
29 1897
5 1897
5
F09/5
1425
1475
86
17
35 714
33 333
0.20538
0.21770
35 0.11536
641 0.11706
24 1886
38 1912
4
6 1893
24
F09/6
1500
95
32 258
0.19728
50 0.11589
54 1894
8 1894
8
F09/7
1450
77
5102
0.21933
39 0.11547
29 1888
5 1888
5
F09/8
1450
30
43 478
0.22055
89 0.11587
21 1894
3 1894
3
0.11588
0.12031
0.12032
1894
1961
1961
1500
27/4
Zircon
F09/10
(8C)
ratios
1550
81
206
Pb/204Pb
33 333
208
Pb/206Pb 2s
0.23068
207
Pbb/206Pb 2s
44 0.11592
Age (Ma) 2s
step
16 1894
3 1894
3
1891
3
Moraes Almeida volcanic sequence (sample 39a */rhyolite)
39a/1
Age (Ma) 2s
Crystal
25
2755
0.37874
145 0.11452
61 1873
10 1873
10
1450
1500
36
52
2519
3077
0.27689
0.41960
187 0.11664
138 0.11558
61 1906
39 1889
9
6 1894
15
39a/6
1500
88
3636
0.34135
311 0.11563
24 1890
4 1890
4
39a/7
1450
1500
18
52
14 706
83 333
0.40385
0.39994
752 0.11435
210 0.11567
206 1870
36 1891
33
6 1891
6
1890
6
1865
1874
1877
1879
7
4
3
5 1876
3
Moraes Almeida volcanic sequence (sample 48 */trachyte)
48/1
1450
1500
1533
1550
34
90
82
82
4831
7874
15 873
8547
0.2762
0.29191
0.30457
0.32278
98
72
73
92
0.11404
0.11458
0.11480
0.11491
45
28
22
34
48/3
1500
86
11 905
0.29172
67 0.11493
19 1879
3 1879
3
48/5
1450
1500
84
70
71 429
62 500
0.16951
0.16781
47 0.11522
52 0.11543
34 1884
23 1887
5
4 1886
3
48/6
1450
1500
52
84
4167
17 241
0.27616
0.26711
88 0.11554
73 0.11553
49 1889
24 1888
8
4 1888
3
1881
4
Moraes Almeida volcanic sequence (sample 15 */ignimbrite)
15/1
1450
1550
84
68
7092
7407
0.17298
0.17343
46 0.11467
111 0.11418
17 1875
19 1867
3
3 1872
8
15/2
1450
1500
32
54
8547
8197
0.18970
0.19640
113 0.11531
102 0.11498
36 1885
29 1880
6
5 1882
5
15/3
1500
52
58 824
0.20383
108 0.11479
47 1877
7 1877
7
15/4
1450 F
1500 F
76
89
12 987
37 037
0.16161
0.20417
59 0.11447
34 0.11498
27 1872
23 1880
4
4 1876
8
15/5
1500 F
94
38 462
0.23107
76 0.11468
24 1875
4 1875
4
1875
4
Jardim de Ouro granite (sample 03 */hornblende /biotite monzogranite)
03a/1
84
4695
0.28539
158
1450
1500
18
88
55 556
111 111
0.11505
0.11842
133 0.11476
59 0.11503
0.12141
113 1876
20 1881
23
1977
18
3 1880
3
3
03a/3
1500
88
31 250
0.16258
120 0.11482
44 1877
7 1877
7
211
1500
03a/2
1550
39a/5
212
Zircon
(8C)
ratios
206
Pb/204Pb
208
Pb/206Pb 2s
207
Pbb/206Pb 2s
03a/4
1450 F
1500
35
52
40 000
15 625
0.12184
0.14995
63 0.11497
74 0.11476
55 1880
25 1876
9
4 1877
4
03a/5
1450
1500
84
88
11 765
21 277
0.11258
0.16585
39 0.11265
38 0.11524
16 1843
23 1884
3
4 1884
4
1880
3
Age (Ma) 2s
step
Age (Ma) 2s
Crystal
Maloquinha granite (sample 06a */biotite leucomonzogranite)
06a/1
1500
88
7463
0.22017
55 0.11555
21 1889
3 1889
3
06a/5
1550
82
2985
0.22842
60 0.11426
32 1869
5 1869
5
06a/6
1500
86
5102
0.17092
72 0.11471
21 1876
3 1876
3
06a/7
1450
82
4292
0.34699
98 0.11486
50 1878
8 1878
8
1880
9
Mean (338 ratios; USD /0.9)
Note: Analyses excluded from age calculation in italic font.
a
F denotes analysis in static mode using Faraday cups.
b
Corrected for common Pb.
references therein). This assumption is reinforced
by the fact that TIMS and SHRIMP U/Pb zircon
data published from the same region (Santos et al.,
2000) are consistent with our Pb evaporation
results (cf. Table 1).
6.2. Results
6.2.1. Vila Riozinho volcanic sequence
Two samples of trachytes were dated from Vila
Riozinho (Fig. 2). Sample 62a shows two different
zircon populations, clear and brown. In total,
eight grains were dated. Two brown zircon grains
gave ages of 28529/4 Ma and 25919/3 Ma. Six
grains of the clear population gave Paleoproterozoic ages; one of these was excluded from age
calculation. The remaining crystals yielded an age
of 20119/12 Ma with a large scatter as indicated
by the unified standard deviation (USD) of 8.8.
Grain 9 (Table 4) is responsible for the scatter as it
gave a 30 Ma older age. This may reflect a small
inherited Pb component. The four remaining
grains have 207Pb/206Pb indicating an age of
19989/3 (USD /1.6) that we assume as the
crystallization age of the 62a trachyte. Archean
ages of 2.59 and 2.85 Ga obtained for the brown
zircon grains (6 and 14) indicate an inherited
component with a minimum age of 2.85 Ga.
Zircons recovered from sample 107 are clear. Pb
isotopic determination of seven grains gave an age
of 19939/4 (USD /4.3) and elimination of the
three youngest grains yielded an age of 20009/4
Ma (USD /2.7). This is interpreted as the crystallization age of the trachyte. The ages of 19989/3
Ma and 20009/4 Ma of the two trachytes are quite
similar and are interpreted as the crystallization
age of the Vila Riozinho volcanic sequence.
6.2.2. Older São Jorge granite
Two samples of the Older São Jorge granite
were dated, sample 27 from the dominant
hornblende /biotite monzogranite facies and sample 35 from the more evolved biotite leucomonzogranite facies (Fig. 2). Zircons from sample 27
were clear, light pink, and well-formed. Four
grains were dated and these yielded similar ages
at each step of heating. Their mean age is 19819/2
213
Ma (USD /0.9), which is interpreted as the
crystallization age of the granite.
Zircons from sample 35 were also clear and wellformed. Five grains were analyzed and they all had
low 206Pb/204Pb ratios. Grain 35/3 (Table 4)
provided a /20/30 Ma lower age than the four
other ones and was discarded. The remaining
grains yielded an age of 19839/8 Ma (USD /5).
Grain 35/5 gave a slightly older age (19939/2 Ma)
at the highest temperature (1500 8C) */this is
responsible for the high value of USD. If 35/5 is
excluded, an age of 19749/2 Ma (USD /1.1) is
obtained. The older age of 19939/2 Ma may be
related to the presence of a small amount of
inherited Pb. The slightly lower age of 19749/2
Ma is in good agreement with the fact that it
corresponds to a more evolved facies than the
19819/2 Ma old hornblende /biotite monzogranite. However, as no definitive criteria allow to
exclude the highest heating step of grain 35/5, the
age of 19839/8 Ma is used for the biotite leucomonzogranite. The older São Jorge granite was
thus emplaced slightly after the Vila Riozinho
volcanic sequence.
6.2.3. Younger São Jorge granite
The dated sample F09 is from a drill core in the
mineralized part of the younger São Jorge granite
(Fig. 2) and represents a hornblende /biotite
monzogranite without evidence of significant hydrothermal alteration. Zircon crystals were euhedral, zoned, clear, and light pink. Nine grains
yielded good reproducibility except for one grain
(F09/5) that was slightly older at the highest
temperature step (Table 4). All nine crystals
yielded an age of 18919/3 (USD /3.2). The older
19129/6 Ma age from grain F09/5 may indicate
small amount of inherited Pb. Exclusion of this
from the age calculation would not modify the
result much (18909/3 and USD /2.3). Therefore,
we consider 18919/3 Ma as the crystallization age
of the younger São Jorge granite. This is /90 Ma
younger than the two samples from the older São
Jorge granite.
6.2.4. Moraes Almeida volcanic sequence
Three volcanic samples were dated from the
Moraes Almeida sequence. A rhyolite (39a) with
214
euhedral to sub-euhedral prismatic, elongated, and
clear zircons was collected along the Transgarimpeira road (Fig. 2). Four grains were analyzed and
provided low 206Pb/204Pb ratios. The mean
207
Pb/206Pb age for the four grains is 18909/6
Ma (USD /2.5) and is interpreted as the crystallization age of the rhyolite. Four grains were also
analyzed from a trachyte (48) north of Moraes
Almeida (Fig. 2). These yielded an age of 18819/4
Ma (USD /2.7) that is considered as the age of
crystallization. The third analyzed sample (15) is
an ignimbrite near Moraes Almeida (Fig. 2).
Zircons were elongate, clear, and euhedral. Five
grains gave similar ages at all steps of heating with
a mean age of 18759/4 Ma (USD /2.8). This is
interpreted as the age of crystallization of the
ignimbrite.
6.2.5. Maloquinha granite
The Maloquinha granite sample 06a was collected along the road between Jardim de Ouro and
Moraes Almeida villages (Fig. 2). Zircons were
subhedral clear and dominantly prismatic with a
lot of inclusions and fractures. Four grains were
analyzed, all of them had low 206Pb/204Pb ( B/7500;
Table 4). The mean of Pb isotopic ratios corresponds to an age of 18809/9 Ma (USD /0.9),
which is considered as the emplacement age of the
granite.
6.2.6. Jardim do Ouro granite
For the Jardim de Ouro granite, sample 03a
from the Jardim do Ouro village was dated (Fig.
2). Zircons were dominantly clear, prismatic and
zoned. Five grains were selected for dating and
four gave reproducible results with an age of
18809/3 Ma (USD /1.4). This is considered as
the crystallization age of the granite. Grain 03a/1
gave a significantly older age of 19779/3 Ma and
was excluded of the age calculation. This age is
within the range of ages obtained for the Vila
Riozinho volcanic rocks and Older São Jorge
granite (2.0 /1.98 Ga) and could represent inheritance.
7. Discussion
7.1. Magmatic series and granitoid typology
The São Jorge and Jardim do Ouro granites
display the geochemical characteristics of I-type
granitoids. The classic I-type granitoids of the
Lachlan Fold Belt, as envisaged by Chappell and
White (1992) and Chappell et al. (2000), were
produced by partial melting of an igneous crustal
protolith of intermediate composition, and were
not associated with subduction processes. The ICordilleran type granites, as defined by Pitcher
(1983, 1987), correspond more strictly to the I-type
granites generated in a subduction-related tectonic
setting. Pitcher (1983, 1987) introduced the ICaledonian type as a variant of the I-Cordilleran
type. Blevin and Chappell (1995) considered that
the petrogenesis of the I-type granitoids of the
Lachlan Fold Belt is not related to active subduction processes and that they are similar to the ICaledonian type granitoids in this respect.
The I-type character of the Tapajós granitoids
does not necessarily imply a subduction-related
environment and the I-type classification adopted
here is preliminary. The Maloquinha granite displays within plate or A-type affinities in some
diagrams (Fig. 7c) but in others a more ambiguous
character is evident (Fig. 7d). It probably corresponds to evolved leucogranites of the aluminousA type series (King et al., 1997; Dall’Agnol et al.,
1999a; Rajesh, 2000). These magmas can result
from low-degree melting of crustal quartz-feldspatic rocks and this is our preferred interpretation
also.
All of the studied granitoids contain magnetite,
display relatively high magnetic susceptibility
values, and were formed at relatively oxidizing
conditions, near the NNO buffer (Figueiredo,
1999). Accordingly, they can be defined as magnetite series granitoids (Ishihara, 1981). The compositions of amphiboles and biotite also suggest that
oxidizing conditions prevailed during the crystallization of the São Jorge and Jardim do Ouro
granites. The Vila Riozinho and Moraes Almeida
volcanic sequences are petrographically and geochemically similar to the São Jorge granites and
the Maloquinha granite, respectively.
Our mineralogical and geochemical data indicate that the São Jorge and Jardim do Ouro
granites and the Vila Riozinho sequence have
calc-alkaline affinities. They follow the classic
calc-alkaline trend in the AFM and log[CaO/
(K2O/Na2O)] versus SiO2 diagrams (Fig. 8a)
and their incompatible trace-element signatures
are consistent with this. All these rocks are K2Orich and most of them plot in the high-K2O field in
the K2O versus SiO2 diagram (cf. Peccerilo and
Taylor, 1976; Rickwood, 1989). Some of them
could be classified as shoshonites (Fig. 8b).
In northeastern and southern Brazil, Brasiliano
(Pan-African) granitoids are widespread and many
calc-alkaline or shoshonitic batholiths have been
identified (Nardi and Lima, 1985; Nardi, 1986;
Sial, 1986; Sial and Ferreira, 1990; Lima and
Nardi, 1998; Sial et al., 1999). Nevertheless, the
distinction between high-K2O calc-alkaline and
shoshonitic granitoids is also controversial. Mariano and Sial (1993) showed that, in northeast
Brazil, shoshonitic rocks and the high-K2O calcalkaline granitoids partially overlap in several
geochemical plots. The best chemical separation
is found on a Ba versus Rb plot (Mariano and Sial,
1993, Fig. 6). The Ba and Rb contents of the São
Riozinho sequence (Table 3) overlap those of the
high-K2O granites of northeast Brazil. The older
São Jorge granite approaches the Itaporanga
batholith, while the younger São Jorge granite is
enriched in Ba and Sr compared to the former and
is more similar to the Bodocó batholith. The REE
patterns of the older São Jorge and younger São
Jorge granites are similar to those of porphyritic
quartz monzonite and granite, respectively, of the
Serra da Lagoinha and Acari batholiths (Neves
and Mariano, 1997). Despite the lower REE
contents, the REE pattern of the Vila Riozinho
sequence rocks does not differ significantly from
that of the biotite diorites associated with the Serra
da Lagoinha and Acari high-K calc-alkaline
granitoids (Neves and Mariano, 1997). It is also
similar to the REE patterns of the basalts and
shoshonites of the Lavras do Sul shoshonitic
association (Lima and Nardi, 1998).
It is clear that the criteria for separation of highK2O and shoshonitic series are not unequivocal.
215
The absence of mafic compositions ( B/54 wt.%
SiO2) in the studied rocks precludes evaluation of
a possible rapid K2O increase with magmatic
differentiation in low silica rocks (cf. Meen,
1987). However, the calc-alkaline granodioritic
trend followed in the QAP diagram (Fig. 3) and
geochemical analogy with the high-K calc-alkaline
granitoids of northeast Brazil point to a dominant
calc-alkaline rather than shoshonitic character for
the São Jorge granites. The same is probably true
for most of the Vila Riozinho volcanic sequence,
but samples with exceptionally high K2O, Na2O
and Zr (Table 3) may be shoshonitic. This suggests
that the Vila Riozinho sequence is transitional
between high-K2O and shoshonitic or that it does
not comprise a consanquineous volcanic sequence.
The post-tectonic setting of the TGP would
certainly be typical of shoshonitic series (cf. Lima
and Nardi, 1998).
7.2. Volcanic sequences of the Vila Riozinho region
and the Uatumã volcanism
The volcanic sequences of the Tapajós province
have been included mostly in the Iriri Group or,
more locally, in the Bom Jardim Formation
(Faraco et al., 1997; Klein et al., 2001). Available
geochronologic data (Santos et al., 2000; Vasquez
et al., 2000, Table 1 and references therein)
indicate an age of /1.88 Ga for these volcanic
sequences. However, Lamarão et al. (1999) and
Dall’Agnol et al. (1999b) emphasized the presence
of volcanic sequences with different ages and
geochemical affinities and stressed that the Uatumã Supergroup should be redefined.
The data obtained by Lamarão et al. (1999)
together with our new data reveal at least two
volcanic sequences in the Vila Riozinho region.
The /2.00 Ga Vila Riozinho volcanic sequence is
composed of a high-K calc-alkaline to shoshonitic
transitional series. The /1.88 Ga Moraes Almeida sequence is geochemically similar to the
evolved leucogranites of the Maloquinha granite
suite and has aluminous A-type affinity. The Vila
Riozinho sequence is composed of basaltic andesite, trachyte, and rhyolite with similar geochemical signatures. The rhyolites are clearly distinct
from the felsic volcanic rocks of the Moraes
216
Almeida sequence and the chemical and chronologic differences justify the separation of these
sequences. The two studied volcanic sequences
should be considered as independent units and
the names Vila Riozinho Formation and Moraes
Almeida Formation are proposed for them. The
latter can probably be included in the Iriri Group,
but the former is older and related to another
magmatic event.
The identification of the two volcanic series has
important implications for the understanding of
the magmatic evolution of the Amazonian craton
in late Paleoproterozoic and early Mesoproterozoic time. In fact, Dall’Agnol et al. (1987, 1994,
1999a) argued that the Uatumã volcanism that
covers large regions of the Amazonian craton was
unlikely to be composed of a single magmatic
sequence that could be coeval with the associated
widespread anorogenic granite magmatism. They
stressed the distinct geochemical signatures of
many of the calc-alkaline volcanic sequences that
were included in the Uatumã Supergroup, and the
typical A-type signature of the anorogenic granites. Conversely, Sidder and Mendoza (1991)
considered the Uatumã-related volcanic sequences
of the northern Guiana Shield late- to postorogenic rather than anorogenic. In this same
region, Reis et al. (2000) demonstrated that the
/1.96 Ga Surumu volcanic rocks, formerly
correlated with the Uatumã Supergroup, have a
calc-alkaline character and are geochemically
similar to the /1.96 Ga Pedra Pintada granitoids
(Table 1). Costi et al. (2000) reported ages of
/1.82 Ga for the tin-mineralized A-type, anorogenic granites of Pitinga, whereas the felsic volcanic country rocks of the Iricoumé Group, also
included in the Uatumã Supergroup, yielded ages
of /1.96 and /1.88 Ga (Table 1). These data
indicate that the anorogenic granites of the Central
Amazonian Province are not necessarily coeval (or
comagmatic) with the spatially associated intermediate to felsic volcanic rocks. Santos et al.
(2000) also noted that the Uatumã Supergroup
should be redefined and proposed that the Uatumã Supergroup should be reserved for the
/1.88 /1.87 Ga volcanism of the Central Amazonian and Tapajós provinces.
7.3. Magmatic evolution of the Vila Riozinho
region and its implication for the tectonic evolution
of the TGP
Geochronologic data obtained for the Vila
Riozinho region are summarized in Fig. 11. The
studied rocks were formed during two distinct time
intervals, at 2.01 /1.97 Ga and 1.90 /1.87 Ga.
These two periods of intense magmatic activity
dominated the entire Tapajós Province (cf. Table
1). The oldest age, 20119/23 Ma, was obtained for
the Conceição tonalite of the Cuiú /Cuiú complex
(Table 1). Older inherited zircons have been
identified in the Jacareacanga supracrustal sequence (Santos et al., 2000), the Vila Riozinho
sequence (Lamarão et al., 1999), and granites of
the Maloquinha suite (Santos et al., 2000). There is
a gap of 70 m.y. and also a difference in the
composition between the two magmatic events
(Fig. 11). If an overall subduction-related tectonic
setting is assumed for both events, the older event
could be syn- to late-orogenic and the younger
event post-orogenic.
The geochemical characteristics of the studied
rocks can be used to evaluate whether the Tapajós
Province records a transition from an orogenic to
a post-orogenic setting. Normally, an increase in
alkalinity is expected in the rocks from primitive to
more mature arcs (Brown et al., 1984) or from
orogenic to post-orogenic settings (Bonin, 1990;
Barbarin, 1999). A transition from low-K calcalkaline to high-K calc-alkaline and alkaline is
normally encountered (Barbarin, 1999). Moreover,
Brown et al. (1984) considered that the increase of
arc maturity could be indicated by Rb/Zr and Nb
behavior, as both display a positive correlation
and increase parallel to the arc evolution. Low-K
calc-alkaline series have not been identified in Vila
Riozinho, but they could be represented by the
Cuiú /Cuiú tonalites. The K-rich Vila Riozinho
sequence and São Jorge granites, whose ages are
between 2.00 and 1.88 Ga (Table 4; Fig. 11), do
not show a trend of increasing arc maturity in a
(Rb/Zr) versus Nb plot (Fig. 12). Conversely, these
rocks show decreasing Nb and increasing Rb/Zr
with decreasing age. However, the Moraes Almeida sequence and associated Maloquinha granites are more alkaline than the other studied rocks
217
Fig. 11. Summary of geochronologic data on the magmatic rocks of Vila Riozinho. Horizontal bars show the 2s error in the ages of
the dated samples.
and could represent mature arc rocks. Nevertheless, this cannot account for the strong geochemical contrasts between the approximately
coeval younger São Jorge, Jardim do Ouro, and
Maloquinha granites.
High-K calc-alkaline magmas are generally
associated with subduction (Brown et al., 1984;
Barbarin, 1999). Roberts and Clemens (1993) and
Clemens (1999) discussed experimental evidence
for the origin of high-K, I-type granitoids and
stated that they can be only derived from partial
melting of hydrous mafic to intermediate crustal
rocks of calc-alkaline or high-K calc-alkaline
character. In their model, the thermal effects of
underplated or intraplated basaltic magma into
continental crust (Huppert and Sparks, 1988)
would be responsible for fluid-absent partial melting of fertile metaluminous protoliths and generation of high-K magmas. Neves and Mariano
(1997) adopted a similar model to explain the
origin of the abundant high-K calc-alkaline granitoids of the Borborema Province in northeast
Brazil. They proposed that these granitoids were
emplaced in an intracontinental setting. The heat
218
source was a mantle plume impinging the base of
the continental lithosphere and the K-rich calcalkaline granitoids were not subduction-related.
This alternative model may be applicable for the
TGP, at least to its northeastern part.
The Vila Riozinho region is located near the
boundary between the Central Amazonian and
Tapajós-Parima provinces. Nd model ages (Sato
and Tassinari, 1997; Santos et al., 2000) indicate
that the Central Amazonian Province was formed
during the Archean, the Tapajós tectonic province
in the Paleoproterozoic. If the regional tectonic
models are correct, the evolution of the southwestern and northeastern sectors of the TGP (Fig.
1) are distinct. Moreover, a subduction-related
setting for at least part of the province is suggested
by the presence of abundant gold deposits (Coutinho et al., 2000; Klein et al., 2001). These are
generally classified as orogenic-type gold deposits
(Groves et al., 1998) and associated with ‘calcalkaline’ sequences.
Brito Neves (1999) discussed the tectonic evolution of South America from the supercontinent
point of view and stressed the importance of two
events at 2.159/0.05 Ga and 2.009/0.05 Ga. They
can be distinguished on a continental scale and are
attributed to the Transamazonian orogeny. The
Paleoproterozoic evolution of the Northern Guiana (or Maroni-Itacaúnas) Province (Fig. 1) has
been well documented in French Guiana and in
the Amapá state of Brazil. According to Vanderhaeghe et al. (1998), the Paleoproterozoic terranes
of French Guiana were assembled between 2.17
and 2.08 Ga. Similar ages have been obtained in
Amapá (Lafon et al., 2000) where slightly younger
granitoids (/2.05 Ga) are also found. For the
TGP, available U/Pb and Pb /Pb zircon data
suggest that its evolution could be related to the
2.00 Ga continental event, not to the 2.15 Ga
event. However, the few available Nd model ages
are only a little older than 2.0 Ga (Santos et al.,
2000). Therefore, this hypothesis should be reevaluated when more geochronologic and isotopic
information is available.
In South America, the Paleoproterozoic supercontinent was rifted mostly during the Mesoproterozoic (Brito Neves, 1999). Extensional events
led to the formation of dike swarms and sedimen-
Fig. 12. Rb/Zr versus Nb diagram showing the composition of
the magmatic rocks of Vila Riozinho. Increasing arc maturity
trend from Brown et al. (1984). Symbols as in Figs. 6 and 7.
tary and volcanic platform covers. The /1.88 Ga
Uatumã Supergroup and widespread cratonic (or
anorogenic) granite magmatism in different provinces of the Amazonian craton could represent
the oldest magmatism related to these events. The
/1.88 Ga event is well documented in several
provinces, also by the anorogenic granites of the
Carajás region (Table 1). It is concluded that this
event was extensional and affected not only the
new accreted terranes but also the older, stabilized
Archean domains, such as Carajás (Dall’Agnol et
al., 2000).
A model for the evolution of the Vila Riozinho
region (Fig. 13) involves a first stage of subduction-related magmatism at 2.01 /1.97 Ga (Fig. 13a)
and two hypotheses for /1.90 /1.87 Ga: a second
event of subduction (b1) would give origin to the
magmas of the Jardim do Ouro, Maloquinha, and
younger São Jorge granites and Moraes Almeida
volcanic rocks, or a period of intracontinental
magmatism related to a distensional event (b2)
could have followed the /2.0 Ga subduction
event. This distensional event would mark the
beginning of the taphrogenesis that would continue throughout the Mesoproterozoic. The Vila
Riozinho region would represent a transitional
area between the orogenic and the intracontinental
219
Fig. 13. Schematic tectonic model for the Vila Riozinho and TGP. (a) At 2010 /1970 Ma oceanic crust subducts at the southwestern
margin of the Central Amazonian Province. Melting of the subducted oceanic crust generates magmas and fluids that ascent to the
overlying mantle wedge and lower crust. Different degrees of interaction with lower crust will result in the Vila Riozinho and older São
Jorge magmas. Two hypotheses are envisaged to explain evolution at 1900 /1870 Ma: (b1) At a more advanced stage of subduction, a
new series of magmas is generated. These magmas are related to a more mature arc and are emplaced at the border between the
accreted Paleoproterozoic terranes and the Central Amazonian Province. (b2) After the Transamazonian event (a), the Amazonian
craton experiences a major distensional event. Mantle plumes cause melting of the lithospheric mantle and the resulting mafic magmas
are emplaced at the mantle-crust boundary. The lower crustal rocks melted yielding felsic magmas. These magmas ascend and form the
Maloquinha granite and the Moraes Almeida volcanic sequence. The younger São Jorge and Jardim do Ouro granites could derive
from mafic crustal sources or result from interaction between mantle and crustal sources.
220
domain. The younger high-K calc-alkaline granitoids could be associated with the late subductionrelated processes or, alternatively, represent intracontinental tectonic evolution (cf. Neves and
Mariano, 1997). In this model, the felsic Maloquinha granites and the Moraes Almeida volcanic sequence correspond to melts derived by
anatexis of the older crustal rocks. At this stage,
we are more inclined for the second hypothesis
(Fig. 13b2), but this question should be reevaluated when Nd and Pb isotopic data are available.
8. Concluding remarks
The Pb /Pb zircon data for the volcanic and
plutonic rocks of the Vila Riozinho region demonstrates two Paleoproterozoic periods of intense
magmatic activity. At 2.00 /1.97 Ma, the Vila
Riozinho volcanic formation and the older São
Jorge granite were formed. During the second
period, between /1.90 and 1.87 Ga, the Jardim
do Ouro, younger São Jorge, and Maloquinha
granites and the Moraes Almeida Formation were
formed. The age intervals defined for the rocks of
Vila Riozinho are largely coincident with ages
throughout the TGP, demonstrating that they are
relevant also on a regional scale. The studied
terranes of the Tapajós region are a little younger
than the Paleoproterozoic terranes of the Northern Guiana Province indicating that the tectonic
processes in these provinces were not coeval.
The older São Jorge, younger São Jorge, and
Jardim do Ouro granites display the broad characteristics of I-type and magnetite series granites.
The São Jorge granites are high-K and calc-alkaline. The Jardim do Ouro granite is more iron-rich
and less oxidized than the São Jorge granites. The
Vila Riozinho Formation displays a calc-alkaline
to shoshonitic character and is geochemically
similar to the São Jorge granites. The Maloquinha
granite and the Moraes Almeida Formation are
aluminous, A-type rocks, probably derived from
low degrees of melting of crustal sources.
The volcanic sequences of the studied area,
formerly included in the Iriri Group of the
Uatumã Supergroup, are divided in two sequences
with different ages and geochemical signatures.
The Vila Riozinho and Moraes Almeida sequences
are considered as two independent formations and
demonstrate that the huge continental Paleoproterozoic volcanic event of the Amazonian craton is
more complex than previously thought.
The evolution of the TGP records accretionary
processes related, on a global scale, to the formation of the Atlantica supercontinent in the midPaleoproterozoic ( /2.00 Ga). This initial step was
followed a (/1.88 Ga) by an intracontinental
taphrogenetic event that extended to the Mesoproterozoic. The tectonic setting of the TGP is
thus transitional, in time and space, between a
subduction-related setting, involving the generation of a magmatic arc, and a stable continental
block affected dominantly by extensional tectonics.
Acknowledgements
M.A.B.M. Figueiredo and R.M.K. Borges participated actively in field work and, together with
L.H. Ronchi, H.T. Costi, A.A.S. Leite, E.S. Farias
and C.E.M. Barros, contributed to the Tapajós
research project. Rio Tinto Zinc Company gave
support in the field, authorized sampling in the
mineralized area and transmitted unpublished
information about the São Jorge gold deposit
area. M.L. Vasquez, E.L. Klein and M.T.L.
Faraco, from CPRM-Belém, contributed to regional mapping, discussions about the Tapajós geology or furnished unpublished information.
ADIMB stimulated the developing of this research. It received support from PADCT/FINEP/
FADESP (8/8/98/0400/01), CNPq (000400038/99
and 463196/00-7) and UFPA. This paper is a
contribution to PRONEX/CNPq (Proj. 103/98 */
Proc. 66.2103/1998-0) and IGCP-426.
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Geology, geochemistry, and PbÁ/Pb zircon geochronology of

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