Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial I, 125-129
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
1.38-1.30 Ga A-type granites related to the evolution of the
Rondonian-San Ignacio orogenic system, SW Amazonian
Craton, Brazil: a geochemical overview
Granitos do tipo-A (1,38-1,30 Ga) relacionados com a
evolução do sistema orogênico Rondoniano-San Ignacio, SW
Craton Amazônico, Brasil: uma revisão geoquímica
W. B. Leite Júnior1*, B. L. Payolla2, J. S. Bettencourt3, C. A. Tavares Dias1
Artigo Curto
Short Article
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: Two main groups of A-type or ferroan granitoids (1.381.30 Ga) related to the development of the Rondonian-San Ignacio
Province (1.56-1.30 Ga) are recognized in the Rondônia Tin
Province: alkalic metaluminous granitoids and alkali-calcic
metaluminous granitoids with or without associated peraluminous
granitoids. The first group includes the granitoids of the Teotônio
Intrusive Suite, and the second one includes the granitoids of the
Santo Antonio, Alto Candeias, and São Lourenço-Caripunas intrusive
suite. These suites are interpreted to be related to episodic
extensional regime in a within-plate tectonic setting. We believe that
the alkalic metaluminous granitoids were formed by fractional
crystallization of ferro-basalt parent magma, whereas the alkalicalcic metaluminous granitoids were crystallized from anatectic
melts derived from quartzo-feldspathic sources in response to mafic
underplating.
Keywords: Geochemistry, A-type granite, SW Amazonian Craton,
Rondonian Tin Province.
Resumo: Dois grupos principais de granitóides ferrosos ou tipo-A com
idades entre 1,38-1,30 Ga e relacionados com o desenvolvimento da
Província Rondoniana-San Ignacio (1,56-1,30 Ga) são reconhecidos na
Província Estanífera de Rondônia: granitóides metaluminosos alcálicos
e granitóides metaluminosos álcali-cálcicos com ou sem granitóides
peraluminosos associados. Os granitóides do primeiro grupo são
incluídos na Suíte Intrusiva Teotônio e os do segundo grupo ocorrem
nas suítes intrusivas Santo Antônio, Alto Candeias e São LourençoCaripunas. Essas suítes são interpretadas como relacionadas a regimes
extensionais episódicos em ambiente tectônico intraplaca. Nós
entendemos que os granitóides metaluminosos alcálicos foram
formados por cristalização fracionada a partir de magma original de
composição ferro-basáltica, enquanto que os granitóides
metaluminosos álcali-cálcicos foram cristalizados a partir de magmas
anatécticos derivados da fusão de fontes de composição quartzofeldspática em resposta a magma máfico underplating.
Palavras-chave: Geoquímica, Granito Tipo-A, SW do Cráton
Amazônico, Província Estanífera de Rondônia.
1
Instituto de Geociências e Ciências Exatas - Universidade Estadual Paulista
(UNESP). Avenida 24A, 1515, CEP 13506-900, Rio Claro, São Paulo, Brasil.
2
Eletrobras Eletronorte. SCN, Quadra 6, Conj. A, Bloco C, CEP 70.716-901,
Brasília, Distrito Federal, Brasil.
3
Instituto de Geociências, Universidade de São Paulo (USP). Rua do Lago,
562, CEP 05508-900, Cidade Universitária, São Paulo, São Paulo, Brasil.
*
Corresponding author / Autor correspondente: [email protected]
1. Introduction
Proterozoic anorogenic granites in the Rondônia Tin
Province (RTP), northern Brazil, were recognized in the
late 1960’s and subsequently characterized as rapakivi
granites. Bettencourt et al. (1999) identified seven rapakivi
suites, which were temporarily correlated to the
development of three tectonic provinces in the
southwestern border of the Amazonian craton (Tassinari &
Macambira, 1999): Rio Negro-Juruena (1.80-1.55 Ga),
Rondonian-San Ignácio (1.50-1.30 Ga), and SunsásAguapeí tectonic provinces (1.25-1.00 Ga).
In a recent review, Bettencourt et al. (2010a)
characterized the Rondonian-San Ignacio province (RSIP;
1.56-1.30 Ga) in the southwestern margin of the
Amazonian craton as an orogenic system, comprising an
older complex accretionary orogen (1.56-1.34 Ga), and a
terminal microcontinent-continent collisional orogen
(1.34-1.32 Ga). In the RTP (northern sector of the RSIP)
the Santo Antônio (SAIS; ~ 1.4 Ga), Teotônio (TIS; ~ 1.38
Ga), Alto Candeias (ACIS; 1.34-1.33 Ga) and São
Lourenço-Caripunas (SLCIS; 1.31-1.30 Ga) intrusive
suites are interpreted by Bettencourt et al. (2010a) as Atype granites related to the evolution of the RSIP. The
SAIS and TIS are related to the rift stage (1.50-1.38 Ga),
whereas the ACIS and SLCIS are related to the late to
post-collisional stage (1.34-1.30 Ga).
The aim of this paper is to reveal the main
lithogeochemical characteristics of the 1.38-1.30 Ga Atype granites related to the evolution of the Rondonian-San
Ignacio orogenic system in the Rondônia tin Province.
2. Regional setting
Precambrian geology of the north-central Rondônia
includes geological units with ages varying from 1.76 to
0.97 Ga. The older units are the Jamari Complex (1.761.73 Ga) that is composed of calc-alkaline tonalitic and
dioritic
gneisses,
paleoproterozoic
metavulcanosedimentary (Mutum-Paraná and Igarapé Lourdes
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formations: ~ 1.75 Ga) and metasedimentary (Quatro
Cachoeiras Suite: 1.67-1.59 Ga) sequences, and rapakivi
granites, charnockites and gabbros of the Serra da
Providência Intrusive Suite (1.60-1.53 Ga) (Quadros &
Rizzoto, 2007). These units are included in the Rio NegroJuruena province (1.78-1.55 Ga) that is interpreted as a
juvenile accretionary belt (Tassinari et al., 1996), with a
terminal arc-continent collision at the time interval 1.671.63 Ga, and post-collisional magmatism from 1.60 to 1.53
Ga (Scandolara, 2006). Younger units are the A-type
granites and related rocks of the Rio Crespo (RCIS; ~1.50
Ga), TIS (~1.38 Ga), SAIS (~1.37 Ga), ACIS (1.35-1.33
Ga), and SLCIS (1.31-1.30 Ga) (Bettencourt et al., 1999,
2010b; Payolla et al., 2012), which are interpreted to be
related to the evolution of the Rondonian-San Ignacio
province (1.56-1.30 Ga; Fig. 1) (Bettencourt et al., 2010a).
The youngest units are the mafic dikes and sills of the
Nova Floresta Formation (1.20 Ga), rapakivi granites and
minor syenites of the Santa Clara (1.08-1.07 Ga) and
Younger Granites of Rondônia (0.99-0.97 Ga) intrusive
suites, and continental sediments of the Palmeiral
Formation (<1.03 Ga) (Bettencourt et al., 1999; Quadros
& Rizzotto, 2007), which are interpreted to be related to
the evolution of the Sunsás-Aguapei province (1.20-0.95
Ga) (Teixeira et al., 2010).
3. Geology and petrography
The TIS and SAIS crops out in the northwestern sector of
the RTP (Fig. 1). Bettencourt et al. (2010b) reported more
precise ages for TIS (1.38-1.37 Ga) and SAIS (~ 1.37 Ga)
intrusive suites that are more consistent to the field
relations, where rocks of former suite host dykes of the
later ones.
The TIS were described by Payolla (1994) in the
Teotônio cataract area. Major units are massive coarsegrained ferrohedenbergite alkali-feldspar granite, banded
medium-grained ferrohedenbergite alkali-feldspar granite,
and pink coarse- to medium-grained quartz alkali-feldspar
syenite with less common alkali-feldspar granite and
syenogranite. The coarse grained and banded granites are
cut by NE dipping, up to 2 meters wide, tabular bodies of
fine– to medium-grained fayalite-ferrohedenbergite alkalifeldspar syenites, as well as synplutonic dykes of diorite,
monzodiorite, and monzonite. The parallel arrangement of
the tabular bodies and dykes defines a large scale banding
in outcrops at Teotônio cataract. Late pink, fine-grained
subsolvus monzogranites of the SAIS occur as SW dipping
dykes cutting through the above rock types.
The SAIS is composed of two main granitic types:
seriate to locally porphyritic biotite monzogranite and
syenogranite with sparse rapakivi and anti-rapakivi
textures and medium grained equigranular biotite
monzongranite. Distinctive rock types of smaller areal
extent include fine-grained hornblende-biotite quartzmonzonite, dyke-like bodies of hybrid rocks
(monzogranite,
quartz
monzonite,
and
quartz
monzodiorite), and synplutonic monzodioritic to dioritic
and diabase dikes.
The ACIS crops out mainly in the Alto Candeias
massif in the southern part of the RTP. The massif is ovalshaped with a general WNW-ESE trending area (up to
10.000 km2), and is partially covered by Nova Floresta and
Palmeiral formations (Fig. 1). The internal structure of the
massif is poorly known as well as the contacts with the
basement rocks. According to Scandolara (2006) the
granites of the northern boundary were affected by ductile
shear zone, whereas only sparse ductile shear bands are
observed internally. Two major types of rock associations
are identified: granitic and charnockitic associations. The
granitic association (~ 1.34 Ga; Bettencourt et al., 1999) is
dominant in area and includes pinkish-gray and gray
medium- to coarse-grained porphyritic and pyterlitic
syenogranite, monzogranite and quartz-monzonite, with
alkali-feldspar phenocrysts (up to 5 cm long), sometimes
mantled by plagioclase, and hornblende and biotite as the
main mafic minerals. Pinkish-gray medium- to finegrained equigranular biotite (± hornblende) granites and
aplites are observed in various places, mainly as dykes.
The charnockitic association (~ 1.35 Ga; Payolla et al.,
2012) occurs apparently as elongated body in the
southeastern margin of the massif, as well as isolated areas
in the granitic domain. Greenish-gray and reddish-brown
medium- to coarse-grained porphyritic charnockites and
quartz-monzonitic charnockites are the dominant
petrographic varieties. The associated rocks are mediumto
coarse-grained,
slightly
porphyritic
quartzmonzodioritic charnockite and fine- to medium-grained
slightly porphyritic dioritic charnockite.
The SLCIS crops out in a northeasterly direction
homonymous massif (up to 3.000 km2) along the left bank
of the Madeira River in the northwestern part of the RTP
(Fig. 1). The São Lourenço-Caripunas massif is composed
of plutonic, subvolcanic and volcanic rocks mainly of
granitic composition (syenogranites, alkali-feldspar
granites and rhyolites). Pink fine- to medium-grained
equigranular granites are apparently dominant in the
Caripunas region, besides pink porphyritic granite,
pyterlite, and minor wiborgite (Leite Júnior et al., 2011).
They contain biotite and hornblende as the main mafic
minerals, but clinopyroxene and fayalite occur in some
even-grained varieties. In the São Lourenço area, pink
porphyritic hornblende-biotite syenogranites and biotitehornblende quartz-syenites, and pink to cream evengrained biotite granites are common, besides brown
rhyolite porphyry, and minor gabbro (Leite Júnior et al.,
2013).
4. Geochemistry
Major elements were analyzed by XRF at the São Paulo
State University in Rio Claro, São Paulo, Brazil, while the
trace-elements and REE were determined by ICP-MS at
Acme Analytical Laboratories Ltd. in Vancouver, Canada,
both using a lithium borate fusion method.
The granites of all suites are mainly metaluminous to
slightly peraluminous rocks (A/CNK= 0.87-1.04),
excepting some high-SiO2 granites (>75.0 wt %) of the
1.38-1.30 Ga A-type granites, Rondônia, Brazil
SLCIS that are peraluminous in character (A/CNK1.10).
The K2O/Na2O ratios vary from 1.1 to 3.0 revealing a
distinct potassic affinity for the granites.
The granites and some associated rocks (syenites,
quartz monzonites, monzodiorites and charnockites) of the
suites were evaluated on the geochemical classification
scheme for granitic rocks based on major-element
chemistry proposed by Frost et al. (2001). In a plot of
FeOtot/(FeOtot+MgO) vs SiO2 all the samples lie in the
ferroan field (Fig. 2a). In the MALI diagram (Na2O+K2OCaO vs SiO2), the TIS samples plot in the alkalic field, the
SAIS and SLCIS samples in the alkali-calcic field,
whereas the granites and charnockites of the ACIS are
alkali-calcic and calc-alkalic, respectively. Some granites
of TIS, ACIS and SLCIS showing >75 wt.% SiO2 plot in
the calc-alkalic field (Fig. 2b).
Dall’Agnoll & Oliveira (2007) proposed a distinction
between A-type granites and calc-alkaline granites based
on some oxide ratios. In a plot of CaO/(FeOt+MgO+TiO2)
vs CaO+Al2O3 all the samples fall in the A-type granites
field (Fig. 2c), while in the FeOt/(FeOt+MgO) vs
Al2O3/(K2O/Na2O) diagram, that discriminate oxidized and
reduced A-type granites, the SAIS is oxidized, ACIS is
reduced, SLCIS shows both characters, and the TIS
samples fall out of the fields, but the high
FeOt/(FeOt+MgO) ratios (>0.90) suggest a reduced nature
(Fig. 2d).
In the FeO*/MgO vs Zr+Nb+Ce+Y diagram of Whalen
et al. (1987) all the samples lie in the A-type field (Fig 2e),
127
and according to Nb-Y-3Ga diagram (Eby, 1992) the
SAIS, ACIS, and SLCIS samples plot in the A2-type
granite field, whereas TIS samples in the A1-type granite
field (Fig. 2f). All the samples when plotted in the Rb vs
Y+Nb tectonic discrimination diagram of Pearce (1996)
fall in the within-plate granite field (Fig. 2g).
The rare earth elements (REE) patterns are shown in
Figure 2h. The granites of the SAIS, ACIS and SLCIS
exhibit very similar patterns, showing low to moderate
concentrations of REE (~10 to 400 times the chondrite),
low to moderate enrichment of the light REE (LREE)
relative to the heavy REE (HREE) (LaN/YbN= 2.2-13.2),
and moderate to strong Eu anomalies (Eu/Eu*= 0.54-0.06)
(Fig. 2h), whereas the granites and syenites of the TIS are
richer in LREE contents (~200 to 2000 times the
chondrite), and show higher enrichment of the LREE over
the HREE (LaN/YbN= 9.4-31.8) and moderate Eu
anomalies (Eu/Eu*= 0.50-0.15) (Fig. 2h). No fractionation
of the middle REE (MREE) relative to the HREE is shown
by SAIS and SLCIS granites and quartz monzonite
(GdN/YbN= 0.43-1.45), while a low depletion of the HREE
relative to the MREE is observed in the TIS and ACIS
granites and syenites (GdN/YbN= 1.11-3.09). Associated
rocks as monzonites, monzodiorites (TIS and SAIS) and
charnockites (ACIS) exhibit also low to moderate
concentration of REE with respect to chondritic values
(~30 to 300 times), low to moderate enrichment of the
LREE relative to the HREE (LaN/YbN= 4.0-8.2), and none
to moderate Eu anomalies (Eu/Eu*= 1.0-0.53) (Fig. 2h).
Fig. 1. Simplified geologic map of the Rondônia Tin Province (RTP) and adjacent areas (modified after Bettencourt et al., 1999).
Fig. 1. Mapa geológico simplificado da província Estanífera de Rondônia (RTP) e áreas adjacentes (modificado de Bettencourt et al., 1999).
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W. Leite Júnior et al. / Comunicações Geológicas (2014) 101, Especial I, 125-129
Fig. 2. TIS, SAIS, ACIS and SLCIS granites and associated rocks compositions plotted on granite classification diagrams of Frost et al. (2001) (a and b),
Dall’Agnoll & Oliveira (2007) (c and d); Whalen et al. (1987) (e), Eby (1992) (f), Pearce (1996) (g), and chondrite-normalized rare earth elements
composition (Boynton, 1984) (h). TIS= green triangles; SAIS= purple crosses; ACIS= red circles; SLCIS= blue X. Gray shadow in a) and b): A-type
fields (Frost et al., 2001).
Fig. 2. Composições de granitos e rochas associadas das SIT, SISA, SIAC e SISLC plotadas nos diagramas de classificação para granitos de Frost et al.
(2001) (a e b), Dall’Agnol & Oliviera (2007) (c e d), Whalen et al. (1987) (e), Eby (1992) (f), Pearce (1996) (g), e composições dos elementos de terras
raras normalizados para o valor do condrito (Boyton, 1984) (h). Triângulos verdes= SIT; cruzes roxas= SISA; círculos vermelhos= SIAC; X azuis=
SISLC. Em cinza nas figuras a e b, campos dos granitos tipo A (Frost et al., 2001).
5. Discussion
Two main varieties of A-type or ferroan granitoids
related to the development of the RSIP are recognized in
the RTP, according to the Frost & Frost (2011)
classification: alkalic metaluminous granitoids and alkalicalcic metaluminous granitoids with or without
peraluminous granitoids. The first one includes the
granitoids of the TIS, and the second one includes the
granitoids of the SAIS, ACIS, and SLCIS.
The alkalic metaluminous granitoids of the TIS are
mainly alkali-feldspar granites, although the rocks
associated with them cover a wide range of silica content.
Diorite is the most silica-poor rock of this suite and it
grades to monzodiorite, monzonite, syenite, alkali
feldspar syenite, and quartz-alkali feldspar syenite. The
granites with highest silica contents (> 73 wt. %) show an
alkali-calcic composition.
The alkali-calcic metaluminous granitoids and
associated rocks of the SAIS, ACIS, and SLCIS show
also a range of silica contents. Basic rock (diabase or
gabbro) is the most silica-poor rock of the SAIS and
SLCIS, indicating a bimodal character to these suites,
whereas ferroan calc-alkalic charnockites, including the
most silica-poor dioritic charnockites, are the associated
rocks in the ACIS. High silica (> 75 wt. %) granites of
the ACIS and SLCIS show, respectively, calc-alkalic and
calc-alkalic peraluminous compositions. The latter ones
are spatially related to the greisen-type tin deposits.
These suites show timing of crystallization (1.38-1.30
Ga) and tectonic setting consistent with an anorogenic
period and intraplate setting (TIS and SAIS: 1.38-1.37
Ga), and post-collision (ACIS: 1.35-1.34 Ga) and postorogenic (SLCIS: 1.31-1.30 Ga) periods and intraplate
setting related to the development of the Rondonian-San
Ignacio orogeny (1.37-1.34 Ga) (Bettencourt et al.,
2010a; Rizzotto et al., 2013).
These suites represent multiphase granitoid plutons.
Some of them are bimodal in character, or include
intermediate rocks as minor facies, synplutonic dykes,
and enclaves and only one is clearly related to
subvolcanic rocks and primary tin deposit. We believe
that the alkalic metaluminous granitoids were formed by
fractional crystallization of ferro-basalt parent magma,
whereas the alkali-calcic metaluminous granitoids were
crystallized from anatectic melts derived from quartzofeldspathic sources in response to mafic underplating.
6. Conclusion
Alkalic metaluminous granitoids and alkali-calcic
metaluminous granitoids are the main varieties of A-type
or ferroan granitoids in the RTP that are timing related to
the RSIP. They constitute multiphase granitoid plutons that
show bimodal character or include rocks of intermediate
composition. Only one pluton is clearly related to
subvolcanic rocks and primary tin deposit. The alkalic
metaluminous granitoids were formed by fractional
crystallization of ferro-basalt parent magma, whereas the
alkali-calcic metaluminous granitoids were crystallized
from anatectic melts derived from quartzo-feldspathic
sources in response to mafic underplating.
1.38-1.30 Ga A-type granites, Rondônia, Brazil
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