Artigo Formato- - Degeo

Transcrição

Geo.br 1 (2002) 1-20
Irneu Mendes de Carvalho Júnior & Maurício Antônio
Carneiro.
ISSN1519-5708
http//:www.degeo.ufop.br/geobr
ULTRAMAFIC ROCKS PRESERVING CUMULATE TEXTURES: AN ARCHEAN
OCCURRENCE IN SOUTHERN SÃO FRANCISCO CRATON.
Irneu Mendes de Carvalho Júnior 1& Maurício Antônio Carneiro 1
1
Departamento de Geologia da Escola de Minas da Universidade Federal de Ouro Preto
(DEGEO/EM/UFOP)
ABSTRACT
The Ribeirão dos Motas Layered Sequence crops out in the southern portion of the São Francisco
Craton, Minas Gerais state, Brazil. This Archean sequence is formed by alternating peridotite
and pyroxenite layers exhibiting variable thickness and cumulate textures (e.g. orthocumulus,
adcumulus and heteradcumulus types). Subordinately, amphibolitized mafic rocks occur (e.g.
gabbronorite), but the genetic relationship between both amphibolitized mafic and ultramafic
rocks is yet unclear. Although the rocks of the layered sequence were metamorphosed and show
secondary mineral paragenesis ranging from high to low metamorphic grade, preserved igneous
textures and structures can be recognized. This paper presents some geological characteristics of
the Ribeirão dos Motas Sequence and also compares their peculiar characteristics with other
worldwide occurrences of mafic-ultramafic rocks (e.g. layered complex, alpine complex and
greenstone sequences). Additionally, this paper presents a petrologic-tectonic model to explain
the igneous origin of textures and structures of the sequence. Finally, as the previous statement,
the Ribeirão do Motas Layered Sequence probably represents a sui generis occurrence of maficultramafic rocks with peculiar characteristics in the transition between a layered complex and a
greenstone sequence.
RESUMO
A Seqüência Acamadada Ribeirão dos Motas aflora nos domínios da porção meridional do
1
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
Cráton São Francisco em Minas Gerais. Esta seqüência arqueana é constituída, principalmente,
por peridotitos e piroxenitos, que se alternam em camadas de espessura variada e exibem uma
ampla coleção de texturas cumuláticas (e.g. ortocúmulus, adcúmulus e heteradcúmulus).
Subordinadamente, são encontradas rochas máficas anfibolitizadas (e.g. gabronoritos), mas as
relações de contato com as ultramáficas são discutíveis. Apesar de suas texturas e estruturas
ígneas preservadas as rochas da Seqüência Acamadada Ribeirão dos Motas encontram-se
variavelmente metamorfisadas, de modo que paragêneses secundárias, variando de alto a baixo
grau, podem ser, localmente, identificadas. Neste trabalho, além da caracterização petrológica
das rochas desta seqüência, comparam-se as suas características peculiares com outras
ocorrências de ultramafitos mundiais (e.g. complexos estratiformes, complexos alpinos e
seqüências greenstone). Adicionalmente, apresenta-se um modelo petrotectônico para explicar a
origem de suas texturas e estruturas ígneas. Como conclusão preliminar acredita-se que a
Seqüência Acamadada Ribeirão dos Motas é uma ocorrência sui generis de ultramafitos, com
particularidades petrológicas e geoquímicas intermediárias entre um complexo estratiforme e
uma seqüência greenstone.
Keywords: Mafic-ultramafic rocks, layered complex, cumulates textures, São Francisco Craton, Archean.
2
Geo.br 1 (2002) 1-20
Carneiro.
occurrences. In particular, until recently
INTRODUCTION
The study of ultramafic rocks is important
for various aspects. It could reveals the
magmatic processes related to fractional
crystallization and crystal accumulation,
both in continental lithospheric or oceanic
environments. Additionally, these rocks
could contain significant contents of Au, Ni,
and PGE. Moreover, the ultramafic rocks
stand out due to their ultramelanocratic
nature, high density, less than 45% silica,
and high magnesium (MgO>12%) and low
aluminum (Al2O3<10%) contents. Common
examples of ultramafic rocks are peridotites
(l.s.) and pyroxenites (l.s.) from layered and
alpine
complexes
ultramafic
basalts
sequences.
In
all
and
komatiites
and
from
greenstone
these
lithotectonic
associations the composition of the mantle
from were they derive is recorded. They are
associated
with
ISSN1519-5708
extensional
tectonic
processes that have occurred in the earth’s
crust since the Paleoarchean. For this reason
the distribution of this type of magmatism is
worldwide and not restricted to a specific
geotectonic environment. However, they are
more conspicuous in the cratonic terranes, as
is the case of the São Francisco Craton
only the ultramafic volcanics of the Rio das
Velhas Supergroup greenstone sequence
(RVS), outcropping in the Quadrilátero
Ferrífero (Padilha 1984; Schorscher 1992;
Sichel 1983), were known in the southern
portion of the Craton (Figure 1). These
ultramafic rocks underwent such an intense
greenschist facies metamorphism that their
primary textures and mineral assemblages
are invariably obliterated. Carneiro et al.
(1996a; b) reported for the first time the
presence of Archean ultramafic rocks with
cumulate textures and preserved primary
mineralogy in the domains of southern São
Francisco Craton (SSFC), later named
Ribeirão dos Motas Layered Sequence
(RMLS - Carneiro et al. 1997a; b; c;
Carvalho Júnior et al. 1997, Carvalho Júnior
et al. 1998, 1999, Carvalho Júnior 2001).
The petrography of RMLS is presented in
this work and also general geologic aspects
are discussed on a preliminary basis such as
the mineralogy, petrology and chemistry
together
with
the
possible
tectonic
environment for this sequence. Additionally
a crystallization model for its rocks is
presented based in their mineralogical and
3
Geo.br 1 (2002) 1-20
Carneiro.
Quadrilátero
textural study.
ISSN1519-5708
Ferrífero
Archean
and
Proterozoic supracrustal sequences (RVS
GEOLOGICAL ASPECTS
and Minas Supergroup), fact that would
RMLS crops out in the SSFC domains, in
therefore characterize a more widespread
the central-southern region of the Minas
original geographic distribution of these
Gerais state (Figure 1). This region, which
sequences. In a general sense, RMLS rocks
has been studied since the mid-90’s
are widespread for a large portion of the
(Carneiro et al. 1996a; b; 1997a; b; c;
SSFC. However, it is in the Ribeirão dos
1998a; b; Carvalho Júnior et al. 1997; 1998,
Motas region that crops out the largest
and Fernandes et al. 1997), has as
volume of RMLS rocks (Figure 1C). In this
predominating lithotectonic unit the Campo
region the ultramafic body presents a
Belo Metamorphic Complex (Fernandes &
segmented irregular shape so that its
Carneiro 2000, Oliveira & Carneiro 2001,
fragments are preferentially aligned along
Teixeira et al. 1996). The Campo Belo
NE/SW. A strong internal framework can be
Metamorphic Complex is composed by
observed in this body, which is interpreted
igneous or metamorphic nature rocks, with
as being a primary layering (Figure 2A),
felsic, mafic and ultramafic composition,
constituted by peridotitic and pyroxenitic
corresponding respectively to high-grade
rocks, where idiomorphic pyroxene crystals
gneisses
granitoids
can be observed (Figure 2B). These
(gneissified or not), gabbros (sometimes
structures are sub horizontal, as seen in
amphibolitized), peridotites and pyroxenites.
Figure 2A, or even sub vertical (Figure 2C).
Locally, sillimanite quartzite and banded
However, the bedding strike is invariably
iron formation (with orthopyroxene) are
NE/SW.
(granulite
facies),
found that may correspond to remains of the
4
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
Figure 1. A) SSFC morphotectonic domain. Keys: 1 – Archean metamorphic complexes, 2 – Rio das Velhas Supergroup, 3 –
Minas Supergroup, 4 – Espinhaço Supergroup, 5 –Araçuaí Belt, 6 – Brasília Belt, 7 – Bambuí Supergroup; B) Regional
geotectonic context identifying the studied region; C) Simplified geological map of the studied region emphasizing RMLS
occurrences.
5
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
illustrated in Figure 3. A third rock suite,
constituted
by
amphibolitized
gabbroic
rocks, here named Amphibolitic Suite (AS),
which
systematically
occurs
associated
together PES and PIS ultramafic rocks.
However it has not been possible until now
to establish the nature of the genetic
relationship between the ultramafic and
amphibolitic suites because AS rocks are
always found as boulders spreading out in
the areas where ultramafic rocks occur.
Geographically
the
PIS
rocks
are
preferentially found at the ultramafic body
rims whereas the PES rocks predominate
within the body.
Petrographically
the
RMLS
ultramafic
rocks can be grouped in two suites:
Peridotitic (PES) and Pyroxenitic (PIS),
according to the classification diagram
illustrated in Figure 3. A third rock suite,
Figure 2. A) RMLS igneous bedding, represented by
rocks of the alternating Peridotitic and Pyroxenitic Suites
(PES and PIS); B) Whitish pyroxene phenocrysts, product
of a magmatic accumulation in the PES rocks; C) Layer of
the PIS in a vertical (sub vertical) bedding.
Petrographically the RMLS ultramafic
rocks can be grouped in two suites:
Peridotitic (PES) and Pyroxenitic (PIS),
according to the classification diagram
constituted
by
amphibolitized
gabbroic
rocks, here named Amphibolitic Suite (AS),
which
systematically
occurs
associated
together PES and PIS ultramafic rocks.
However it has not been possible until now
to establish the nature of the genetic
relationship between the ultramafic and
amphibolitic suites because AS rocks are
6
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
always found as boulders spreading out in
preferentially found at the ultramafic body
the areas where ultramafic rocks occur.
rims whereas the PES rocks predominate
Geographically
within
the
PIS
rocks
are
the
body.
Figure 3. Classification diagram for ultramafitites, according Streckeisen (1974), illustrating the modal composition of the
RMLS rocks from the Peridotitic (PES) and Pyroxenitic (PIS) Suites.
inequigranular, anhedral to subhedral. The
PERIDOTITIC SUITE
The
PES
holocrystalline
medium-grained.
rocks
varying
The
are
from
essentially
fine
crystals
to
are
essential igneous mineral composition is
olivine (Fo:87-91%), amphibole (magnesian
hornblende) and pyroxenes (enstatite and
7
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
clinoenstatite – En: 87-90%). Brown spinel
(oikocrysts) or in the matrix in adcumulus
is an accessory mineral. Secondary minerals
form. The mesh texture can also be
are represented by green spinel, clinochlore,
observed, resulting from the alteration of
serpentine and opaque minerals. The PES
olivine adcumulate horizons. The olivine
rocks present cumulate textures of the
intercrystal contact geometry is maintained
adcumulate and heteradcumulate types. The
and it usually shows peculiar features of
adcumulate
is
post-cumulate overgrowth such as zones of
of
opaque mineral concentration in the olivine
orthocumulus olivine crystals without any
ghost crystals. In this rock suite pyroxene
filling of intercumulus phases between the
and amphibole appear as interstitial mineral
crystals.
texture
phases of cumulus phases, being therefore
(Figure 4B) is characterized by the presence
characterized as more hydrated intercumulus
of
phases that developed post-cumulus, as
texture
characterized
The
by
the
4A)
presence
heteradcumulate
orthocumulus
(chadacrysts)
(Figure
either
olivine
inside
crystals
pyroxenes
described by Tribuzio et al. (1999).
8
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
minerals of the intercumulus phase orthopyroxene,
clinopyroxene
and
amphibole (post-cumulus) – showing only
spinel, olivine and amphibole inclusions in
the form of chadacrysts in orthopyroxene
and clinopyroxene crystals (Figure 4B).
Thus amphibole occurs in two ways in these
rocks: as inclusions in the intercumulus
phases, and as sub horizontal monomineralic
clusters alternating with pyroxenes, olivine
and serpentine layers in the matrix. The
pyroxenes
present
identical
optical
characteristics except for the extinction
angle
around
15°
in
the
case
of
clinopyroxenes. These minerals (ortho- and
clinopyroxenes) sometimes show straight
faces and internal fracturing pattern at 90°.
The spinels appear as granular crystals
sometimes
having
rectangular
and
hexagonal cross-sections. These minerals
present high relief, and are olive green to
Figure 4.
A) Olivine orthocumulus in adcumulate
organization and spinel orthocumulus in PES rocks. Keys:
Ol - Olivine, Es –Spinel (photomicrography under plane
light); B) Olivine orthocumulus chadacrysts characterizing
its heteradcumulate quality in PES rocks. Keys: Ol –
Olivine, Px - Orthopyroxene, Es – Spinel, Amp –
Amphibole (photomicrography under polarized light); C)
Intercumulus texture, marked by the coexistence of two
intercumulus phases, where the intercumulus amphibole
chadacrysts are enclosed in intercumulus pyroxene
oikocrysts in PIS rocks. Keys: Px - Clinopyroxene, Amp –
Amphibole (photomicrography under polarized light).
A curious association takes place between
greenish brown, having opaque minerals at
the rims. Possibly this color pattern marks a
compositional variation that starts with a
brownish green, Cr- and Mg-rich, Fe-poor
spinel, changing to a green, considerably Alrich spinel, and ending as a Fe-rich spinel
and
opaque
mineral
(magnetite).
9
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
habit.
present modal olivine and spinel, the
Systematically it encloses opaque and
secondary mineral assemblage is composed
colorless to slightly yellow phases. The
of clinochlore, amphibole and opaque
relief is low and its interference color is
minerals. Otherwise, only amphibole makes
anomalous. It is always associated with
up the secondary paragenesis. In textural
spinel. Serpentine (chrysotile) appears in the
terms the rock predominantly presents
form of mineral aggregates resultant from
intercumulus texture. This type of texture is
the alteration of olivine and pyroxenes,
characterized by pyroxene oikocrysts with
being pale yellow to practically colorless.
amphibole, olivine, spinel and/or garnet
Between
chadacrysts,
Clinochlore
shows
olivine
tabular
ghost
crystals
(relict
being
the
amphibole
hexagonal crystal forms of serpentinised
chadacrysts (Figure 4C) representative of a
olivine), it is common the presence of zones
minor intercumulus phase (Carvalho Júnior
of
(magnetite)
& Carneiro 1999). The orthocumulate
concentration, which is a serpentinisation
texture occurs when the pyroxenite presents
sub product.
olivine orthocumulus in the pyroxene-
opaque
mineral
amphibole-rich matrix, being comparatively
PYROXENITIC SUITE
quite rare. The (ortho- and clino-) pyroxenes
essentially
are the most abundant minerals in the
holocrystalline, fine- to coarse-grained, with
pyroxenites. Their main diagnostic features
subhedral to anhedral crystals. The essential
are high relief, practically colorless crystals
mineral composition is almost exclusively
with shades of light pink in some thin
(ortho-
pyroxenes,
sections, distinctive 90° cleavage pattern
predominating orthopyroxenes (En:79-85%)
and first-order gray to second-order orange
and amphibole (magnesian hornblende to
polarization color. Amphibole can either be
tremolite). Olivine and spinel, when present,
enclosed in pyroxenes, or disseminated as
are less abundant than 5% and constitute the
clusters showing 120° polygonal contacts,
accessory mineral assemblage. Garnet is
moderate relief, second-order orange to
also an accessory mineral in PIS, but it is
violet polarization colors and pale yellow
The
PIS
rocks
and
are
clino-)
extremely rare. When the pyroxenites
10
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
shades. Despite rare, green spinel can also
intergranular texture is also present, in
be observed. Its isotropic characteristic
which amphibole and pyroxene crystals
under polarized light is distinctive. In
cluster as crystalline agglomerates between
general the rims are altered to an opaque
lath-shaped feldspars. The rocks of this suite
mineral. Olivine, when present in the rocks
are composed of plagioclase, pyroxene and
of this suite, occurs as granular crystals
amphibole. Garnet, apatite, zircon and
enclosed in pyroxenes. Most of the time
opaque minerals are present as accessory
olivine appears as ghost crystals in the
minerals. Pyroxene uralitization is very
matrix. Garnet is light pink and isotropic,
common. In this reaction silica is originated
generally enclosed in pyroxene crystals.
from pyroxene by means of pyroxene
Clinochlore is a secondary mineral. Its
hydration and transformation to amphibole
diagnostic features are tabular habit, pale-
that
yellow colored to colorless, undulatory
xenomorphic quartz droplets. Plagioclase
extinction, and anomalous birefringence.
appears
The opaque minerals are more abundant in
saussurite. It is colorless, shows ghost
the pyroxenitic rocks with modal spinel and
polysynthetic twinning and subhedral to
olivine. These opaque minerals are regularly
anhedral shapes and sometimes intercepts
distributed in the matrix, generally at the
pyroxene crystals. Pyroxene (diopside) is
spinel rims and in the serpentine interstices.
strongly fractured and most of the time
encloses
silica
almost
in
the
completely
form
of
altered
to
shows exsolution lamellae and amphibole
AMPHIBOLITIC SUITE
fringes around its rims. It has pale green
are
shades and second-order blue polarization
holocrystalline varying from fine to coarse
color. Hornblende appears as subhedral to
grained, showing anhedral to subhedral
anhedral crystals and also as alteration
crystals. They present glomeroporphyric
product from pyroxene, characterizing the
texture. The corona texture can be observed
uralitization process. Garnet is common as
in the pyroxene crystals, being this mineral
anhedral
transformed to amphibole at the rims by an
fractured,
The
amphibolitic
incipient
uralitization
suite
rocks
process.
Relict
crystals;
it
presenting
is
isotropic
quartz
and
and
11
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
plagioclase inclusions. Apatite appears as
haloes in host amphibole. Opaque minerals
colorless crystals, with perfect hexagonal
(iron oxides) are almost always associated
basal sections and is enclosed by plagioclase
with pyroxene and amphibole, constituting
and amphibole. Zircon occurs as elongated
subhedral, sometimes dendritic crystals.
granular
crystals,
producing
pleochroic
tholeiites or basaltic komatiites (Arndt &
DISCUSSION
A series of fundamental questions still
remains concerning the genesis and nature
of the RMLS rocks. One of them refers to
the crystallization environment of the rocks
of the sequence (plutonic? volcanic?).
Another is related, for instance, to the
tectonic environment. Systematically the
PES and PIS rocks present several cumulate
textures, as illustrated in Figure. 4A, B and
C. The development of such texture type
usually
characterizes
layered
igneous
complexes, such as Bushveld, Stillwater,
Fisknaenesset and Skaegaard. However, the
literature of the greenstone sequences also
reports, besides spinifex, the presence of
cumulus textures in the basal portions of the
sub
volcanic
to
volcanic
occurrences
(komatiites). Chemically, they present MgO
contents higher than 18% (Arndt & Nisbet
1982). The komatiites are in general
associated with more aluminous and more
Fe-enriched rocks, corresponding to Mg-rich
Nisbet 1982) that, similarly to the more
magnesian terms, present spinifex texture.
Cumulate textures are also observed in
harzburgitic, lherzolitic, dunitic, pyroxenitic
and
peridotitic
rocks
of
the
alpine
complexes (Windley 1995). The genesis of
such complexes involves distinct tectonic
environments. Firstly, they are the product
of crustal rifting that leads to the generation
of an oceanic crust. Then, this type of
complex undergoes a convergent margin
regime, where fragments of an oceanic crust
(ophiolites), molasses sequences and flysch
pile up on the adjacent continental crust.
Due
to
this
genetic
complexity,
the
ultramafic rocks of the alpine complexes
present peculiar characteristics related, for
instance,
to
geochemical
signature
(incompatible and REE elements) and initial
87
Sr/86Sr
ratios.
crystallization
is
On
the
other
dependent
on
hand,
P-T
conditions and volatiles fugacity, at the
12
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
same time that nucleation and settlings are
lower MgO contents (Carneiro et al. 1999,
taking place in the whole lava pile, evolving
Carvalho Júnior 2001) are not associated
with the spreading center (Sparks &
with sedimentary rocks (Table 1) like
Parmentier 1994). Consequently, in that case
greenstone sequences worldwide. Texturally
the fractional crystallization and crystal
RMLS rocks do not quite resemble the
settling is a short time magmatic process. It
alpine complexes. Besides, the ultramafic
is different from what occurs, for example,
rocks of the alpine complexes are in general
in stratiform complexes, where long time
more depleted in light REE and, as the
crystallization
promoting
greenstone sequences, associations typical
abundant settling crystals and adcumulate
of alpine complexes have never been
textures. However, in this type of complex P
observed in RMLS (Table 1). When
and T oscillation regulates the mineral
compared to the stratiform complexes,
composition of the solid solutions, in terms
RMLS ultramafic rocks present, in general
of Mg, Fe, Cr and Ni phases (Naslund &
terms, the same textural types of the basal
MacBirney 1996). These P and T variations
terms of such complexes, except for the
make the existence of a sharp contact
cryptic variation in the ultramafic layers
between
(study not yet performed) and the presence
rock
processes
layers
of
different
in
of more differentiated terms (Table 1). In
successive
fact, AS (amphibolitized) gabbronorites are
magmatic injections in the crystallization
found with chemical composition (Carneiro
chamber favor the formation of repetitive
et al. 1997a; b) suggesting, however, the
and thick lithologically distinct layers.
same magmatic evolution, where the PES
compositions
stratiform
possible.
complexes,
Moreover,
the
GENERAL COMPARISONS
and PIS terms would be the most primitive.
From this comparison it is preliminarily
In general terms, if compared with the
concluded that RMLS does not keep great
komatiitic peridotites from the greenstone
similarities,
sequences, the RMLS rocks stand out
complexes. Likewise, its petrographically
because they: a) do not present textures
characteristics do not absolutely match
indicative of fast cooling; b) have relatively
for
instance,
with
alpine
13
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
with those of the greenstone sequences. The
sequence. Bearing this in mind, its rocks
similarities are more compatible with the
could have originated from partial melting
layered complexes (Table 1). Regionally,
events in the mantle that succeeded the
the
if
melting that originated the mafic-ultramafic
compared to the geochemical pattern of the
rocks of a greenstone sequence. This would
RVSG greenstone sequence (Carneiro et al.
explain, for instance, their lower MgO and
1999, Carvalho Júnior 2001), indicates that,
higher FeO contents in relation to RVS
once its rocks are more depleted in MgO
komatiites (Carneiro et al. 1999, Carvalho
and REE, the two rock sequences could
Júnior 2001). RMLS melting events would
have
primitive
then come from a depleted mantle and
magmatic system where RMLS would
would be trapped in deep crustal portions,
correspond to a depleted member, originated
never reaching the surface, corresponding to
from the same mantle from where the RVS
a
komatiitic
extracted.
Considering that the existence of continuous
However, Ti would not behave in an
magma injections is necessary for the
incompatible
magma
development of successive strata of settled
evolution, once TiO2 contents are higher for
minerals (Carvalho Júnior & Carneiro
RMLS
1999),
RMLS
geochemical
belonged
to
magmas
rocks
manner
a
pattern,
single
were
for
(Carneiro
this
et
al.
1999,
possible
it
is
RVS
greenstone
possible
that
root.
continuous
Carvalho Junior 2001). Therefore, being a
incompatible element enrichment in the
sui generis sequence, it is believed that
magma coming from a depleted mantle
RMLS
magmatic
would explain the fact that the RMLS rocks
crystallization interface located between a
are richer in incompatible element s and
stratiform
alkalis then the RVS rocks.
corresponds
complex
to
and
a
a
greenstone
14
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
agglomerate in the form of polyhedral
CRYSTALLIZATION MODEL
adcumulate crystals (Figure 5B) with spinel
orthocumulus
(Figure
2C).
With
the
In this context, the development of the
impoverishment
RMLS igneous strata, based on research on
composition of the magma for olivine and
the evolution of magmatic chambers by
spinel crystallization, the nucleation of the
Wager & Brown (1968), Campbell (1978),
intercumulus
Maalφe (1978), Irvine (1980), Paktunç
pyroxenes, becomes active. But, due to the
(1984), Sparks et al. (1993) and Thériault &
convection
Fowler (1996) among others, can be
chamber (Sparks 1993), not all olivine and
explained according the following evolution
spinel settle down, so that some crystals are
model (Figure 5). Initially, olivine and
retained by the crystallization of the
spinel crystallize and settle down in layers
intercumulus
phase
from the lower portions of the evolving
retention
cumulate
magmatic chamber (Figure 5A). The very
imprints the heteradcumulate character on
dense olivine crystals would settle fast and
the
of
the
material,
inherent
of
olivines.
With
to
chemical
represented
the
(Figure
magmatic
5C).
mineral
the
by
This
phases
exclusive
15
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
crystallization of ortho- and clinopyroxenes
to magmatic injections in the previous phase
(more pyroxenitic strata), the intercumulus
ends the crystallization cycle (Figure 5D)
phase becomes dominant (Figure 5C). For
and, therefore, establishes the layered
Thériault & Fowler (1996) the thickness of
pattern of the studied rocks. According to
the peridotitic and pyroxenitic layers is
this model, the cyclicity of the magmatic
related to the cyclicity of the magmatic
injections would explain the alternating
injections in the chamber that allowed
peridotitic and pyroxenitic strata.
feeding of the phases depleted in the first
fractionation
injections
events.
allow
New
olivine
magmatic
and
spinel
CONCLUDING REMARKS.
crystallization and at the same time enrich
the fluid phase (Schneidder & Eggler 1986)
Invariably the RMLS rocks show features
contributing
nucleation
generated under low-, medium- and high-
(Figure 5C). The amphiboles nucleated
grade metamorphism. These features are
during the development of the intercumulus
characterized by the development of mineral
phase occur as chadacrysts immersed in
assemblages composing mesh-type textures,
pyroxene oikocrysts (Figure 4A). These
formed
amphibole crystals correspond, however, to
serpentinisation, and granoblastic textures,
minor intercumulus phases, once they
containing
cannot be
orthocumulus,
contacts. The Cr-spinel transformation to
because they are not accumulation products,
aluminous spinel, generating clinochlore and
but an in situ growth, together with
opaque minerals, also marks a metamorphic
pyroxene crystallization (e.g. Tribuzio et al.
event. This transformation takes place in the
1999 & Andersen et al. 1984). Olivine and
presence of magnesian olivine, enstatite and
spinel accumulation and agglutination, due
a fluid phase that contains enough water,
to
amphibole
classified
as
during
magnesian
tremolite,
with
olivine
polygonized
16
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
Figure 5. Schematic model illustrating the main generation stages of the Ribeirão dos Motas Layered Sequence, inside a
magmatic chamber; A) Magmatic injection followed by olivine and spinel nucleation; B) Settling and accumulation of olivine
and spinel during the crystallization of pyroxene intercumulus; C) Crystallization of pyroxene intercumulus during new
magmatic injection, yielding amphibole nucleation in the form of chadacrysts within the pyroxene oikocrysts; D) Olivine and
spinel nucleation due to magmatic injections in the earlier phase, followed by their accumulation and agglutination, closing the
crystallization cycle and this way establishing the layered pattern of the studied rocks.
occurring in the medium- to high-grade
paragenesis under high grade, during its
interface. However, despite undergoing
emplacement in the crust in a lower
more than one type of metamorphic
continental crust environment, compatible
reworking during its crustal evolution,
with the granulite facies. Finally, in terms of
RMLS keeps relict features unequivocal of
Brazilian occurrences, RMLS is similar to a
the original igneous paragenesis. Likewise,
sequence of mafic-ultramafic rocks that
the layered structures and cumulate igneous
crops out in the Amazonian Craton domains,
textures may remain practically unaltered,
more precisely in the Xingu Metamorphic
which means a re-equilibrium of the main
17
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
Complex, in Pará State. This occurrence is
and Luanga rocks were previously emplaced
known as Luanga Basic-Ultrabasic Complex
in distinct crustal portions. In the case of
and encompasses a sin-volcanic body,
RMLS, it is an ultramafic occurrence of
intrusive in the Rio Novo greenstone
komatiitic affinity (Carneiro et al. 1999) and
sequence (Suíta 1988). Similarly to RMLS,
deep
Luanga ultramafic rocks present cumulate
texture and granulite-facies metamorphism).
textures, but their minerals are a little richer
The Luanga Complex, in its turn, is a
in Fe than RMLS (olivine: Fo:84-87%,
shallow plutonic occurrence of tholeiitic to
pyroxene: En:76-78%; Suíta 1988). Despite
calc-alkaline
similar in the way they occur, in texture and
Nevertheless,
part of the mineral composition, the RMLS
development of greenstone sequences.
plutonic
crystallization
affinity
both
are
(cumulus
(Suíta
linked
1988).
to
the
REFERENCES
Andersen, T.; O’reilly S.Y.; Griffin, W.L. 1984. The
trapped fluid phase in mantle xenoliths from
Victoria, Australia: implications for mantle
metasomatism. Contrib. Mineral Petrol. 88:7285.
Arndt ,T. & Nisbet, E.G. 1982. Komatiites. 1rt. Ed.
London, Allen Unwin. 526p.
Campbell, I.H. 1978. Some problems with the
cumulus theory. Lithos. 11:311-323.
Carneiro, M.A.; Teixeira, W.; Nalini Júnior, H.A.;
Bilal, E.; Oliveira, A.H. de; Carvalho Júnior I.M.
de 1996a. Archean ultramafic-mafic magmatism
in the Southern São Francisco Craton (Campo
Belo Complex): Preliminary petrographic and
geochemical results. In: SYMPOSIUM ON
ARCHEAN TERRANES OF THE SOUTH
AMERICAN PLATAFORM, Brasília. Anais...,
SBG. 1996. p. 32-33.
Carneiro, M.A.; Teixeira, W.; Noce, C.M.; Fernandes
R.A. 1996b. Archean growth processes in the
Quadrilátero Ferrífero: A geochronological U-Pb
and Sm-Nd approach to the Rio das Velhas Event
(2780 - 2700 Ma). In: SYMPOSIUM ON
ARCHEAN TERRANES OF THE SOUTH
AMERICAN PLATAFORM. Brasília. Anais...,
SBG 1996. p. 59-60.
Carneiro, M.A.; Nalini Júnior, H.A.; Bilal, E.;
Carvalho Júnior, I.M. de; Teixeira, W.; Moutte,
J. 1997a. A seqüência Acamadada de Ribeirão
dos Motas, Minas Gerais, Brasil. In: X
SEMANA
DE
GEOQUÍMICA,
IV
CONGRESSO DE GEOQUÍMICA DOS
PAÍSES DE LÍNGUA PORTUGUESA. Braga.
Actas ...,1997.p. 31-33.
Carneiro, M.A.; Teixeira, W.; Carvalho Júnior, I.M.
de; Oliveira, A.H. de; Fernandes, R.A. 1997b.
Archean Sm/Nd isochron age from the Ribeirão
dos Motas layered rocks sequence, Southern São
Francisco Craton, Brazil. In: SOUTHAMERICAN SYMPOSIUM ON ISOTOPE
GEOLOGY. Campos do Jordão. Extended
Abstracts..., SBG,1997. p. 63-64.
Carneiro, M.A.; Carvalho Júnior, I.M. de; Fernandes,
R.A.; Teixeira, W. 1997c. Geologia de parte do
segmento crustal situado entre as cidades de
Oliveira, Campo Belo e Santana do Jacaré, MG.
In: SIMPÓSIO DE GEOLOGIA DE MINAS
GERAIS, IX, Ouro Preto, Anais...,
18
Geo.br 1 (2002) 1-20
Carneiro.
SBG/NMG, 1997. p. 37-38.
Carneiro, M.A.; Carvalho Júnior, I.M. de; Teixeira,
W.
1998a.
Petrologia,
Geoquímica
e
Geocronologia dos Diques Máficos do Complexo
Metamórfico Bonfim Setentrional (Quadrilátero
Ferrífero) e suas Implicações na Evolução
Crustal do Craton do São Francisco Meridional.
Rev. Bras. Geo. 28(1):29-44.
Carneiro, M.A.; Carvalho Júnior, I.M. de &;
Teixeira, W. 1998b. Ensialic tectonic setting of
the Archean Rio das Velhas greenstone belt: Nd
and Pb isotopic evidence from the Bonfim
metamorphic complex, quadrilátero ferrífero,
Brasil. Rev. Bras. Geo. 28(2):189-200.
Carneiro, M.A.; Grossi Sad, J.H.; Carvalho Júnior,
I.M. de; Nalini Júnior, H.A.; Lima, J.F.; Perim,
J.A.; Teixeira, W. 1999. Ribeirão dos Motas
Layered Sequence: Geochemistry and Possible
Komatiitic Affinity In: SIMPÓSIO DE
GEOLOGIA DE MINAS GERAIS, IX., Brasília.
Resumos..., SBG/NMG, 1999. p. 55.
Carvalho Júnior, I.M. de 2001. Petrologia e
geoquímica de rochas máficas e ultramáficas
arqueanas do Cráton São Francisco Meridional:
A seqüência acamadada Ribeirão dos Motas.
Departamento de Geologia da Escola de Minas
da Universidade Federal de Ouro Preto.
Dissertação de mestrado. 96p.
Carvalho Júnior, I.M. de; Carneiro, M,A.; Suíta, M.T.
de F.; Teixeira, W. 1997. Anfibólio intercúmulos
na Seqüência Acamadada de Ribeirão dos Motas:
um modelo petrogenético para sua origem. In:
SIMPÓSIO DE GEOLOGIA DE MINAS
GERAIS, IX, Ouro Preto. Anais..., SBG/NMG,
1997. p.38-39.
Carvalho Júnior, I.M. de; Carneiro, M.A.; Nalini
Júnior, H.A.; Teixeira, W. 1998. Seriam as
rochas da Seqüência Acamadada de Ribeirão dos
Motas
o
correspondente
plutônico
do
magmatismo ultramáfico do Greenstone Belt Rio
das Velhas?. In: CONGRESSO BRASILEIRO
DE GEOLOGIA, XL. Belo Horizonte.
Resumos..., SBG, 1998. p. 68.
Carvalho Júnior, I.M. de & Carneiro, M.A. 1999.
Seqüência Acamadada de Ribeirão dos Motas:
Uma ocorrência de rochas ultramáficas
plutônicas no Cráton São Francisco Meridional.
In: SIMPÓSIO DE GEOLOGIA DE MINAS
ISSN1519-5708
GERAIS e Centro Oeste, X, Brasília. Resumos...,
SBG/NMG, 1999. p. 6.
Carvalho Júnior, I.M. de & Carneiro, M.A. 1999.
Modelo petrogenético para os anfibólios ígneos
Intercúmulos da Seqüência Acamadada de
Ribeirão dos Motas. Revista da Pesquisa & Pós
Graduação. 1:26-32
Fernandes, R.A. & Carneiro, M.A. 2000. Evolução
Geológica dos Terrenos Arqueanos do Cráton
São Francisco Meridional, MG: Um exemplo a
partir do Complexo Metamórfico Campo Belo.
Rev. Bras. Geoc. 30(4):671-678
Fernandes, R.A.; Carneiro, M.A.; Teixeira, W. 1997.
O metamorfismo das rochas da região de Santana
do Jacaré, MG. In: SBG/Núcleo de Minas Gerais,
SIMPÓSIO DE GEOLOGIA DE MINAS
GERAIS, 40, Belo Horizonte, Anais..., 45-46.
Irvine, T.N. 1980. Magmatic density currents and
cumulus processes. Am J. Sci. 280(A):1-58.
Maalφ, E.S. 1978. The origin of rhythmic layering.
Mineral. Mag. 42:337-345.
Naslund, H.R. & MacBirney, A.R. 1996.
Mechanisms of Formation of Igneous Layering.
In CAWTHORN, R.G. (Ed): Layered Intrusions.
Amsterdam, Elsevier, p.1-43.
Padilha, A.V. 1984. Formação Córrego dos
Boiadeiros - Uma Seqüência Komatiítica na Base
do Grupo Nova Lima- Supergrupo Rio das
Velhas- Quadrilátero Ferrífero, Minas Gerais,
Brasil. In: CONGRESSO BRASILEIRO DE
GEOLOGIA, 33. Rio de Janeiro, Anais...,1984. 6
p.2668-2678.
Paktunç, A.D. 1984. Metamorphism of the
Ultramafic Rocks of the Thompson Mine,
Thompson Nickel Belt, Northern Manitoba. Can.
Mineral. 22:77-91.
Schneider, M.E. & Eggler, D.H. 1986. Fluids in
equilibrium
with
peridotite
minerals:
Implications for Mantle metasomatism. Geochim.
et Cosmochim. Acta. 50:711-724.
Shorscher, H.D. 1992. Arcabouço petrográfico e
evolução crustal de terrenos precambrianos do
sudeste de Minas Gerais: Quadrilátero
Ferrífero, Espinhaço meridional e domínios
granito-gnáissico adjacentes. São Paulo, 2v.
Tese de Livre docência - Instituto de
19
Geo.br 1 (2002) 1-20
Carneiro.
ISSN1519-5708
Geociências / USP. 349p.
Sichel, S.E. 1983. Geologia do Arqueano da Região
de Santa Bárbara, Quadrilátero Ferrífero, Minas
Gerais. In: Simpósio De Geologia De Minas
Gerais, 2, Belo Horizonte, Anais..., 1983. p.1978.Sparks, D.W. & Parmentier, E.M. 1994.
Generation and Migration of Partial Melt beneath
Oceanic Spreading Centers. In: RYAN, M.P.
(Ed); Magmatic Systems. Reston, Academic
Press, p. 55-76.
Sparks, R.S.; Huppertt, H.E.; Koyaguchi, T.;
Hallworth, M.A. 1993. Origin of modal and
rhythmic igneous layering by sedimentation in a
convecting magma chamber. Nature 361:246249.
Streckeisen,
A.
1974.
Classification
and
Nomenclature of Plutonic Rocks. Geol.
Rundschau. 63:773-786.
Suíta, M.T. 1988. Geologia da Área Luanga com
ênfase na petrologia do Complexo BásicoUltrabásico Luanga e depósitos de cromita
associados. Instituto de Geociências da
Universidade de Brasília, Brasília, Dissertação de
mestrado, 321p.
Teixeira, W.; Carneiro, M.A.; Noce, C.M.; Machado,
N.; Sato, K.; Taylor, P.N. 1996. Pb, Sr and Nd
isotope constraints on the Archean evolution of
gneissic-granitoid complexes in the southern São
Francisco Craton. Precambrian Res. 78:151-164.
Thériault, R.D. & Fowler, A.D. 1996. Gravity driven
and in situ fractional crystallization processes in
the Centre Hill complex, Abitibi Subprovince,
Canada: Evidence from bilateral-paired cyclic
units. Lithos 39:41-55.
Tribuzio, R.; Tiepolo, M.; Vannucci, R.; Botazzi, P.
1999. Trace element distribution within olivinebearing gabbros from the Northern Apennine
ophiolites (Italy): evidence for post-cumulus
crystallization in MOR-type gabbroic rocks.
Contrib. Mineral. Petrol. 134:123-133.
Wager, L.R & Brown, G.M. 1968. Layered Igneous
Rocks. Edinburgh: Oliver and Boyd, p. 588.
Windley, B.F. 1995. The Evolving Continents. 3rd.
Ed. Chichester, John Wiley & Sons. p.526.
20

Artigo Formato- - Degeo

Transcrição

Documentos relacionados

Contents

PR-Navio Vazio F

Rómulo de Carvalho and António Gedeão, Scientist

hotel sofitel luxury hotels rio de janeiro – copacabana

MedBook - Medsky

TEGMA GESTÃO LOGÍSTICA S.A. N.I.R.E. 35.300.340.931 CNPJ

Characterization of feet temperature

SEMINÁRIO 1: Current Issues and Frontiers in Demographic

Newsletter - St. Nicholas School

Full Paper - Vienna Talk 2010

A New Area of Alkaline Rocks in Eastern Paraguay