Paraguá terrane, SW Amazonian craton

18 Pages·2014·5.41 MB·Spanish

DOI: 10.5327/Z2317-4889201400030006
ARTICLE
Geology, petrology, U-Pb (shrimp)
geochronology of the Morrinhos granite –
Paraguá terrane, SW Amazonian craton:
implications for the magmatic evolution of
the San Ignácio orogeny
Geologia, petrologia, geocronologia U-Pb (shrimp) do Granito
Morrinhos –terreno Paraguá, SW do Cráton Amazônico: implicações
sobre a evolução magmática da orogenia san ignácio
Ohana França1,4,6*, Amarildo Salina Ruiz1,2,4,6, Maria Zélia Aguiar de Sousa1,3,4,6,
Maria Elisa Fróes Batata4, Jean-Michel Lafon5,6
ABSTRACT: Morrinhos granite is a batholith body that is slightly RESUMO: O Granito Morrinhos é um corpo batolítico levemente
elongated in the NNW direction and approximately 1,140 km2 long; it is alongado segundo a direção NNW, de aproximadamente 1.140 km2,
located in the municipality of Vila Bela da Santíssima Trindade of the state localizado no município de Vila Bela da Santíssima Trindade, estado
of Mato Grosso, Brazil, in the Paraguá Terrane, Rondonian-San Ignácio de Mato Grosso. Situa-se no Terreno Paraguá, Província Rondoniana-
Province, in the SW portion of the Amazonian Craton. This intrusion -San Ignácio, na porção SW do Cráton Amazônico. Essa intrusão exibe
displays a compositional variation from tonalite to monzogranite, has uma variação composicional entre tonalito a monzogranito, textura ine-
a medium to coarse inequigranular texture and is locally porphyritic; quigranular média a grossa, localmente, porfirítica, tendo biotita como
biotite is the predominant mafic in one of the facies, and hornblende is máfico predominante em uma das fácies e hornblenda na outra, ambas
predominant in the other, with both metamorphosed into the greenschist metamorfizadas na fácies xisto verde. As rochas estudadas caracterizam
facies. The studied rocks characterize an intermediate to acidic sequence uma sequência intermediária a ácida formada por um magmatismo
that was formed by a subalkaline magmatism; the series is alkali-calcic to subalcali no, do tipo álcali-cálcico, metaluminoso a levemente peralu-
metaluminous to slightly peraluminous, and the rocks evolved through minoso evoluído por meio de mecanismos de cristalização fracionada.
fractioned crystallization mechanisms. The structural data show two Dados estruturais exibem registros de duas fases deformacionais, repre-
deformation phases represented by penetrative foliation (S) and open sentadas pela foliação penetrativa (S) e dobras abertas (D) ambas, pro-
1 1 2
folds (D), and both phases were most likely related to the San Ignácio vavelmente, relacionadas à Orogenia San Ignácio. A investigação geocro-
2
Orogeny. The geochronological (U-Pb SHRIMP) and isotopic (Sm-Nd) nológica (U-Pb SHRIMP) e geoquímica isotópica (Sm-Nd) dessas rochas
investigations of these rocks indicated a crystallization age of 1350 ± 12 Ma, indicaram, respectivamente, idade de cristalização 1.350 ± 12  Ma, T
DM
T of approximately 1.77 Ga and εNd with a negative value of -2.57, em torno de 1,77 Ga e valor negativo para εNd de -2,57, sugerindo
DM (1.35) (1,35)
suggesting that their generation was related to a partial melting process of a uma geração relacionada com processo de fusão parcial de uma crosta
Paleoproterozoic (Statherian) continental crust. The results herein indicate continental paleoproterozoica (estateriana). Os resultados aqui obtidos
that the Morrinhos granite was generated in a continental magmatic arc indicam que o Granito Morrinhos foi gerado em arco magmático con-
in a late- to post-orogenic stage of the San Ignácio Orogeny, and it can be tinental, em estágio tardi a pós-orogênico, da Orogenia San Ignácio e
recognized as belonging to the Pensamiento Intrusive Suite. permite reconhecê-lo como pertencente à Suíte Intrusiva Pensamiento.
KEYWORDS: Morrinhos Granite; Pensamiento Intrusive Suite; PALAVRAS-CHAVE: Granito Morrinhos; Suíte Intrusiva
San Ignácio Orogeny. Pensamiento; Orogenia San Ignácio.
1Graduate Program in Geosciences, Instituto de Ciências Exatas e da Terra, Universidade Federal de Mato Grosso – UFMT. Cuiabá (MT), Brasil. Emails: [email protected];
[email protected]; [email protected]
2Department of General Geology, Instituto de Ciências Exatas e da Terra, Universidade Federal de Mato Grosso – UFMT, Cuiabá (MT), Brasil.
3Department of Mineral Resources, Instituto de Ciências Exatas e da Terra, Universidade Federal de Mato Grosso – UFMT, Cuiabá (MT), Brasil.
4Research Group in Crustal and Tectonic Evolution, Guaporé, Universidade Federal de Mato Grosso – UFMT, Cuiabá (MT), Brasil. E-mail: [email protected]
5Laboratory of Isotope Geology, Institute of Geosciences, Universidade Federal do Pará – UFPA, Belém (PA), Brasil. E-mail: [email protected]
6National Institute of Science and Geosciences Technology of the Amazon – GEOCIAM, Belém (PA), Brasil.
*Corresponding author
Manuscrito ID 30069. Recebido em: 17/12/2013. Aprovado em: 01/09/2014.
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Morrinhos granite – magmatism associated with the San Ignácio orogeny at the Paraguá terrane
INTRODUCTION through successive volcanic arc accretions, oceanic basin
closure and microcontinent-continent collision. The
The Pensamiento Granitoid Complex/Pensamiento province consists of the Jauru (1.78 to 1.42 Ga), Paraguá
Intrusive Suite (PIS), which is located in the southwest- (1.74 to 1.32 Ga) and Rio Alegre (1.51 to 1.38 Ga)
ern portion of the Amazonian Craton, represents a sig- terranes and the Alto Guaporé Belt (1.42 to 1.34 Ga).
nificant and voluminous acid plutonic magmatism of The PT was initially called the Paraguá Craton by
Ectasian age and is classified as syn- to post-kinematic Litherland et al. (1986) and described by Boger et al.
according to its placement in relation to the San Ignácio (2005) as an allochthonous crustal fragment that was
Orogeny (Litherland et al. 1986; Ruiz 2005; Ruiz et al. accreted to the edge of the Amazonian Proto-Craton
2009; Matos et al. 2009; Bettencourt et al. 2010). during the Mesoproterozoic or Neoproterozoic. According
The PIS is included in the Paraguá Terrane (PT), which to the latter, the collision and agglutination of this ter-
extends from western Bolivia to the far west of Brazil, rane to the Amazonian Craton occurred during the Sunsás
and is described as an allochthonous crustal fragment Orogeny (1.0 to 0.9 Ga). Ruiz et al. (2011) interpreted
that was added to the margin of the Amazonian Proto- the PT as a crustal fragment (1.8 to 1.6 Ga) reworked
Craton from the Mesoproterozoic to the Neoproterozoic, into two orogenic cycles: the San Ignácio Orogeny (1.4
during the San Ignácio Orogeny (1.40 to 1.28 Ga) and to 1.3 Ga) and Sunsás Orogeny (1.0 to 0.9 Ga). This ter-
inserted in the Rondonian-San Ignácio Province. rane hosts Paleoproterozoic units from prior to the San
The Morrinhos Granite (MG) is named after the Ignácio Orogeny; these units are the Lomas Manechis
homonymous community located near the Vila Santa Granulitic Complex, San Ignácio Schist Group and
Clara do Monte Cristo, Ponta do Aterro, in the munici- Chiquitania Gneissic Complex/Serra do Baú Intrusive
pality of Vila Bela da Santíssima Trindade of southwest- Suite and Mesoproterozoic intrusives of the Pensamiento
ern Mato Grosso State, along the Brazil-Bolivia bor- Granitoid Complex/PIS (Litherland et al. 1986; Matos
der, where the rocks that compose the PT occur. The et al. 2009; Bettencourt et al. 2010; Jesus et al. 2010;
MG is intrusive in orthogneisses of the Chiquitania Ruiz et al. 2011; Ruiz et al. 2012; Nalon et al. 2013).
Metamorphic Complex, Serra do Baú Intrusive Suite, The Pensamiento Granitoid Complex in Bolivian
which is on the eastern edge of the PT. territory was divided by Litherland et al. (1986) into
This granite was preliminarily described by França syn- to late-kinematic granites (1.37 to 1.35 Ga;
et al. (2013) and is now being discussed in detail in Matos et  al. 2009) and late- to post-kinematic gran-
the present study, which consists of data obtained from ites (1.34 Ga; Matos et al. 2009), and represents the
systematic geological mapping on a 1:300,000 scale, main magmatic product related to the San Ignácio
petrographic characterizations, and geochemical, geo- Orogen (Ruiz 2005; Ruiz et al. 2009; Matos et al. 2009;
chronological (U-Pb/SHRIMP) and isotopic (Sm-Nd) Bettencourt et al. 2010).
analyses that were used to define the age of placement Table 1 presents the geochronological data recorded
of the intrusion, magma petrogenesis and tectonic set- in research from recent years, which includes units related
ting where it was likely generated. The results will con- to the San Ignácio Orogeny, that form the Pensamiento
tribute to the knowledge of the magmatic and defor- Granitoid Complex in Bolivian territory and are correlated
mational evolution related to the San Ignácio Orogeny with the PIS in Brazil.
in Brazilian territory.
FIELD AND PETROGRAPHIC ASPECTS
REGIONAL GEOLOGICAL
BACKGROUND The MG highlighted in this work is characterized as an
elongated intrusion of approximately 1,140 km² with its
The Amazonian Craton, which is located on the South major axis oriented in the NNW direction. It has tectonic
American Platform, represents one of the main Precambrian contact with the Lomas Manechis Granulitic Complex and
geotectonic entities of the world and is an outcropping, it is covered, at its ends, by unconsolidated Quaternary
mainly in the Guianas and Central Brazilian shields, that sediments from the Guaporé Formation (Fig. 2).
is partially covered by Quaternary sediments. The rocks of the MG form hills (Fig. 3A) and rocky
According to Bettencourt et al. (2010; Fig. 1), the pavements which outcrop mainly in recessed areas,
Rondonian-San Ignácio Province is an orogen established wetlands or bays. Occasionally, the MG exhibits relic
416
Brazilian Journal of Geology, 44(3): 415-432, September 2014
Ohana França et al.
Brazil 66º 64º 62º 60º
Porto
Velho
FP FP FP
RNJP (R)
FP
10º 10º
Rio Negro-Juruena
Rio Crespo
Province (RNJP)
FP Intrusive Suite
RNJP (R)
Alto ? Nova Brasilândia
FP
Guaporé Terrane
?
12º Belt RNJP (R) 12º
? R
SI
P
Bou 58º
0 40 80 km n
A RNJP (R)dary Brazil
PSI ?
Amazonian
80º 60º Craton 14º 14º
RNJ VT MI MI 0º Rio Negro Front Terreno Paraguá TJearurarnuCea?cO?hrooegierinnha
?
Basin
Amazon Bolivia BAeglutapei ? ?
San ? ?
SS RO Javier PSSanI t(aR )CatalinaP S SItraignt Zone OHSreaolnegtneana 16º
San ?
Ignácio Sunsás Belt Brazil
B Rio Alegre
Terrane
Phanerozoic San Diablo Front
Undifferemtiated Sedimentary Covers PSI (R)
Late-Mesoproterozoic
Post Sunsás Units
Sunsás units 18º
RT
GS/AS Sunsás Group/ Aguapeí Group Nova Brasilândia Terrane
S.I. Sunsás/Guapé RT Rincon del Tigre FP Palmeiral Formation
Paleo-mesoproterozoico
Rondonian-San Ignacio Lithologic and Tectonic Units
Granitoide Pensamiento Complex/ Alto Guaporé Belt
Suíte Intrusiva Pensamiento Tectonic Boundaries
(1 Morrinhos, 2 Tarumã, 3 Lajes Rio Alegre Terrane Inferred Tectonic Boundaries
4 Amparo, 5 Três Reis, 6 Fronteira
7 Guaporei e 8 Passagem) Jauru Terrane Rondonian-San Ignacio Province (RSIP) Boundaries
Basement Rocks
Pre-San Ignácio Basement Rocks (PSI), [reworked-PSI (R)]
Rio Negro-Juruena Province (RNJP), [reworked-RNJP (R)]
Figure 1. (A) Tectonic map of the Rondonian-San Ignácio Province, southwestern Amazonian Craton. (B) Main
provinces of the Amazonian Craton. Extracted and modified from Bettencourt et al. (2010).
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Morrinhos granite – magmatism associated with the San Ignácio orogeny at the Paraguá terrane
Table 1. Geochronological data from the granitoids of the Pensamiento Granitoid Complex/PIS
References U-Pb Pb-Pb Rb-Sr Sm-Nd
LithologicUnits K-Ar (Ma)
Age (Ma) Age (Ma) Age (Ma) Sr87/Sr86 T (Ga) εNd(0) εNd(t) Age (Ma)
(DM)
Litherland et al. (B) 1326 ± 19
Padre Eterno _ _ _ _ _ _ _
(1986) (M) 1268 ± 20
Litherland et al.
Orabayaya _ _ (RT) 1283 ± 33 0.7058 _ _ _
(1986)
Litherland et al.
_ _ (RT) 1325 ± 45 0.7044 _ _
(1986)
Piso Firme
Matos et al.
_ _ _ _ 1.59 -7.99 2.32 _
c (2009)
matiMa) Litherland et al.
ne40 (1986) _ _ (RT) 1391 ± 70 0.7004 _ _ _
ost-Kids (13 Diamantina Ma(2to0s0 e9t) al. (Z) 1340 ± 20 _ _ _ 1.65 – 1.92 -16.6 – -8.51 -1.25 – 0.38
e-to Panitoi Lithe(r1l9a8n6d) et al. _ _ _ _ _ _ (B) 1296 ± 18
atGr San Cristobal
L Matos et al.
_ _ _ _ 1.58 – 1.59 -6.28 – -5.87 2.63 – 2.75
(2009)
Matos et al.
Porvenir _ _ _ _ 1.74 -6.89 1.48 _
(2009)
Geraldes
Lajes (Z) 1310 ± 34 _ _ _ 1.69 – 0.0 _
(2000)
Jesus et al.
Passagem (Z) 1284 ± 20 _ _ _ 1.60 -10.56 -7 _
(2010)
Litherland et al. (B) 1380 ± 19
Florida _ _ _ _ _ _ _
(1986) (B) 1244 ± 27
Litherland et al.
cMa) (1986) _ _ (RT) 1375 ± 80 0.7052 _ _ _ _
Late-Kinematis(1373 – 1347 SaLna MJuanrttaín MMaa((22ttoo00ss00 ee99tt)) aall.. ((ZZ)) 11334773 ±± 2210 __ __ __ 1.871 .–6 82.04 -19.-67 –.4 -317.8 - 4.291 .7–8 -2.94 _
Syn-to Granitoid FGruoanptoerireaí RuNiza e(l2to 0an1l .e3 (t2) a0l1. 2) __ ((ZZ))1 1331343 ± ±2 .45 __ __ 1._76 -21_.42 -1_4 __
Comunicação
Tarumã Verbal Matos (Z)1377 ± 5 _ _ _ 1.9 -19.68 -4.11 _
J. B.
(B) biotite. (M) muscovite. (Z) zircon and (RT) whole rock. *granitic bodies of Bolivian territory. **granitic bodies of Brazilian territory.
magmatic flow structures in a N10E direction, with a The HBTF predominates in area size; however, it can-
width of approximately 1 m as well as dikes of granitic, not be individualized.
fine-grained composition and enclaves. Macroscopically, the HBTF (Fig. 3C) is characterized
This unit consists of leucocratic to mesocratic locally by mesocratic to leucocratic rocks (M: 35 to 25%) that are
porphyritic rocks with an M index between 15 and 35%, light to dark gray, and consists of quartz, plagioclase, alkali
colors ranging from light gray to dark gray and pinkish feldspar (orthoclase and microcline), biotite and hornblende.
gray, and a texture that is inequigranular medium to The BGMF (Fig. 3D) is characterized by rocks that are pink-
coarse. The main composition of these lithotypes con- ish gray with a lower color index (M: 15 to 20%), with only
sists of quartz, plagioclase, alkali feldspar, hornblende biotite as an essential mafic mineral.
and biotite. The porphyritic samples are characterized Under petrographic microscope, the HBTF rocks have
by the presence of phenocrysts of plagioclase and alkali a xenomorphic to hypidiomorphic inequigranular texture
feldspars (orthoclase and microcline). The MG rocks are (Fig. 4A) that is medium to coarse and sometimes por-
classified from tonalite to monzogranite; based on field phyritic, whereas the BGMF is predominantly xenomor-
and mineralogical features, the rocks are divided into two phic (Fig. 5A). A large amount of hornblende and bio-
petrographic facies: hornblende-biotite tonalite (HBTF) tite aggregates characterize the HBTF (Fig. 4B), whereas
and biotite granodiorite to monzogranite (BGMF). biotite is the only mafic mineral present in the BGMF,
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Ohana França et al.
60º10’0”W 60º0’0”W 59º0’50”W
B Al
arb Stratigraphy Units
ad GF Al
o
m R Alluvium
15º50’0”S Conceição Strea 75 GiverFBarbado River GF MG GF Gomalina River GF 15º50’0”S MPoernrisnahmoGMsiun GaGeprnaotnroéGi It FFneotTrraumrsuaivTmteiGo ãSn uGirtaenite
GF LMGC
MG
Lomas Manechis Granulitic Complex
V. São Simão GF
16º0’0”S Bolívia Conceição Stream Mato Grosso V. Mor7 r7i88nhos MG 75Monte MCrGisto StrAealm Al GFabraB TG 16º0’0”S SSSFaar1omm FnopptilllS(eieeGahr unFFseaCCtaii24roc 21nuZBAorun ((sUeS)H m-IyPnV-dbfNeir/loSrdlragH)egrdRaeIpMhyP)
Al R od TG 60º0’0”W 55º0’0”W 50º0’0”W
FG FFGGFFGG V. SMaonnFttGae C ClraiMrsatG odo FG886003 Al GF revi S10º0’0”S RAonmdaôznoinaas Pará Tocantis 10º0’0”S
16º10’0”S LMGC FG FG FG FG FGFG FG LMGC GF 16º10’0”15º0’0”S MaCtuoi aGbraosso Goiás 15º0’0”S
0 2,5 5 10Km LMGC LMGC Mato Grosso do Sul
60º0’0”W 55º0’0”W 50º0’0”W
60º10’0”W 60º0’0”W 59º0’50”W
Figure 2. Geological map of the study area on a 1:300,000 scale.
often as aggregates. The primary accessory minerals are grains of quartz. Alteration processes, such as saussuri-
titanite, apatite, zircon, rutile, allanite and opaques, and tization, sericitization and argillization, are observed in
the hydrothermal alteration products, which are associated the described facies and are shown in the locally altered
with low-grade metamorphism of the greenschist facies, fibrous muscovite crystals.
are represented by sericite, epidote, clay minerals, chlorite, Alkali feldspars are represented by orthoclase and
muscovite, titanite and opaques. microcline and occur as subhedral to anhedral tabular
The plagioclase in both facies occurs as anhedral to crystals; in the BGMF, they also appear as phenocrysts.
subhedral tabular crystals with albite, Carlsbad and peri- Both facies show Carlsbad twinning or combined (albite +
cline twinning, which may be combined with apatite, bio- pericline) twinning, and are partially sericitized or argil-
tite, quartz and opaques inclusions. Based on the Michel- lized. In general, they appear perthitic, both in venules
Levy statistical method, the plagioclase is classified from or grains/droplets, and may exhibit graphic intergrowths
andesine in the less differentiated tonalites to albite in as well as inclusions of apatite, biotite and opaques.
the monzogranites. The plagioclase often shows normal Quartz is found in the interstitial anhedral crystals as
zoning where the most calcic core is slightly altered in the subgrains or recrystallized aggregates, and it is also found
HBTF, whereas two generations of plagioclase are distin- with a vermicular habit intergrown with plagioclase and
guished in the BGMF (Fig. 5B): a primary generation alkali feldspar. The intracrystalline quartz deformation is
represented by heavily altered crystals and a post-mag- marked by undulatory extinction, lamellae and deforma-
matic generation represented clear recrystallized crys- tion bands and subgrains.
tals of albitic composition. The plagioclase exhibits a Biotite occurs in subhedral lamellae, blades and plate-
myrmekitic texture at the interface with alkali feldspars, lets with light-brown to dark-brown pleochroism, and
and it is arranged as small crystals associated with tiny it can be isolated, formed into aggregates or included
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Morrinhos granite – magmatism associated with the San Ignácio orogeny at the Paraguá terrane
A B
C D
Figure 3. Field and petrographic aspect of the MG. (A) Geomorphological aspect of floodplain occurrences. (B) oriented
mineralogical fabric detail. (C) HBTF macroscopic aspect with plagioclase phenocrysts in an inequigranular matrix.
(D) BGMF macroscopic aspect of the xenomorphic inequigranular texture with biotite as the only essential mafic.
in feldspars. The biotite is usually associated with the Apatite is a primary accessory mineral that occurs in a
hornblende, titanite and opaques in the HBTF rocks, prismatic or acicular habit, and it is indiscriminately included
and it shows a partial alteration into chlorite, musco- in feldspars and mafic minerals. Zircon shows typical bipyra-
vite (Fig. 4D) and opaque minerals. Biotite occasionally midal prismatic habits, is sometimes subhedral and is most
includes rutile needles characterizing a sagenitic texture often associated with mafics, where it develops pleochroic
and euhedral to subhedral zircon crystals that forming halos. Rutile occurs with an acicular habit, it is included
pleochroic halos in the biotite. in biotite and forms a sagenitic texture (Fig. 5D). Allanite
Amphibole is represented by the magmatic hornblende is a primary accessory appearing in tiny metamictic grains
and only found in the HBTF. Amphibole occurs in anhe- of yellow color (Fig. 5E), and it is associated with epidote.
dral grains and subhedral crystals with olive-green to brown Opaques can be primary minerals that occur in isolated
pleochroism and forms mafic aggregates; it subordinately euhedral/subhedral crystals or secondary minerals of bio-
exhibits sector twinning, drop-like quartz texture and apa- tite, hornblende and titanite alterations.
tite inclusions. Amphibole is partially observed as altered Epidote consists of microgranular aggregate associated
into chlorite, biotite and opaque minerals. with plagioclase alterations. The clay minerals are products
Titanite is represented by two different generations: one of the alteration of feldspars and form a thin blurred mass
that is formed by anhedral to subhedral crystals of rhombo- with tiny blades of sericite that are difficult to distinguish
hedral habit with edging of opaque minerals, characterizing under the microscope. Chlorite occurs with fibrous to
a coronitic texture (Fig. 5C), and another that is defined by fibro-radiated habit as a biotite and hornblende alteration.
poikilitic, orange, anhedral (Fig. 4C) grains and associated The secondary biotite is derived from hornblende and is
with mafic minerals. found associated with chlorite.
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Ohana França et al.
A
2 mm
B C
2 mm 2 mm
D
0,2 mm
Figure 4. Photomicrographs of rocks from the MG hornblende-biotite tonalite facies showing the following:
(A) inequigranular xenomorphic to hypidiomorphic texture formed by plagioclase, alkali feldspar and mafic
aggregate, consisting of biotite with drop-like quartz texture and opaque minerals. (B) association with
subhedral hornblende, biotite blades and opaque minerals, all with drop-like quartz texture. (C) mafic aggregate
formed by hornblende, biotite/alteration chlorite, poikiloblast of titanite and opaque minerals. (D) partially
chloritized biotite and fibro-radiated aggregate of secondary muscovite. Parallel polarizers are to the left, and
cross-polarizers are to the right.
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Morrinhos granite – magmatism associated with the San Ignácio orogeny at the Paraguá terrane
A
2 mm
B C
1 mm 1 mm
D E
0.2 mm 2 mm
Figure 5. Photomicrographs of the rocks from the MG biotite granodiorite to monzogranite facies showing the
following: (A) xenomorphic texture and sharp distinction between quartz, clear alkali feldspar and intensely
saussuritized plagioclase and blades of partially chloritized biotite. (B) two generations of plagioclase: the
primary generation represented by intensely saussuritized crystals and the secondary generation formed by neo-
crystallized and clear crystals. (C) rhombohedral crystal of titanite with coronitic texture formed by an opaque
mineral. (D) biotite with acicular rutile inclusions characterizing a sagenitic texture. (E) aggregate of biotite
blades, opaque minerals and metamictic allanite grain. Parallel polarizers are to the left, and cross-polarizers are
to the right (A); cross polarizers in (B) and parallel polarizers in (C), (D) and (E).
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Ohana França et al.
DEFORMATIONAL ASPECTS foliation (S) with a preferential attitude at approximately
1
240/80 (Fig. 6A). The S foliation is defined by the planar
1
The conventions adopted for the organization of infor- fabric provided by the preferential arrangement of the min-
mation related to the MG deformation were as follows: (Sn) erals (micas) and prismatic minerals (plagioclase and alkali
to denote foliation, (Ln) to denote lineation, (Dn) to denote feldspar) as well as their discrete flattening (Fig. 7). The S
1
folding and (Fn) to denote the deformation phase. In each tectonic foliation is arranged according to the major length
of the symbols, the letter n is an index (n = 1, 2...) according of the batholith (NNW) and usually appears faint, although
to the related deformation phase. The attitudes of the struc- there are sites with higher intensity, indicating a heteroge-
tures are shown in azimuth notation, for example, 120/45, neous deformation distribution.
where the first value indicates the direction of the bearing The phase (F ) is defined by the formation of open
2
or strike and the second value is the dip or plunge value. folds (D ) designed by penetrative foliation (S ) with
2 1
From the analysis of the structures and tectonic textures, axial planes that have dips of approximately 80° toward
the structural elements generated in two deformational the azimuth, 350 Az (Fig. 6B). Discrete ductile shear
phases (F and F) were identified. zones (S ) are parallel to the axial plane of the D folds
1 2 2 2
The main deformation phase (F ) affects the entire and produce the local transposition of tectonic folia-
1
MG and is responsible for the development of penetrative tion (S ) (Fig. 8).
1
A N Morrinhos Granite - S1 B N Morrinhos Granite - S2
Density
+ +
16.35
14.54
12.72
10.90Maximum Density: 16,4%
9.0830.0/7.4 (pole)
7.27210.0/82.6 (plane)
5.45
3.63
1.82
Isofrequency N=38 0.00 Pole Stereogram N=12
Figure 6. Isofrequency and polar stereograms of the MG representing the measurements related to the foliation (S)
1
and shear zones (S) according to a lower-hemisphere projection. The S foliation exhibits a preferential concentration
2 1
at approximately 240/80 Az and shear zones (S), which show a maximum concentration at 350/82 Az.
2
A
2 mm
Figure 7. Photomicrograph of the MG rocks showing foliation (S ) provided by the preferential arrangement of
1
the minerals (micas) and prismatic minerals (plagioclase and alkali feldspar) as well as their discrete flattening.
Parallel polarizers are to the left, and cross-polarizers are to the right.
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Brazilian Journal of Geology, 44(3): 415-432, September 2014
Morrinhos granite – magmatism associated with the San Ignácio orogeny at the Paraguá terrane
variation diagram (La Roche 1980; Fig. 10A), and when the
Q-P diagram is used (Debon & Le Fort 1983; Fig. 10B),
they fall within the domains of the quartz-monzonites, mon-
zogranites and granites.
According to the Total Alkali Silica (TAS) diagram
(Fig. 11A), the Irvine and Baragar (1971) dividing line
suggests that the magmatism that originated the MG
rocks is of subalkaline affinity, and its alkali-calcic char-
acter is evident from the Peacock diagram (Peacock 1931;
Fig. 11B) because of the intersection of the trends of total
alkalis and CaO versus SiO obtained for the rocks stud-
2
ied with an alkalinity index of approximately 55, which
is confirmed by the Na O+K O-CaO versus SiO trend
2 2 2
proposed by Frost et al. (2001; Fig. 11C). In the K O ver-
Figure 8. Cross-cutting relationship between the 2
foliation (S ) and ductile shear zones (S ) that are sus SiO diagram, the high K values classify this magma-
1 2 2
parallel to the D folds. tism as shoshonitic (Peccerillo & Taylor 1976; Fig. 11D).
2
Using alkalinity index (Shand 1927) and the A/CNK
versus A/NK diagram proposed by Maniar and Piccoli (1989;
GEOCHEMICAL CHARACTERIZATION Fig. 12A), the samples have been classified as metaluminous
except for two from the BGMF, which plot in the peralu-
For the geochemical study of the MG rocks, twelve minous field. In the diagram proposed by Frost et al. (2001;
samples were selected to obtain the chemical composi- Fig. 12B), which is based on the FeOt/ (FeOt+MgO) ver-
tion of the major, minor and trace elements, including sus SiO relationship and which differentiates between the
2
the rare earth elements (REE), to determine the geo- magnesium and iron granites, the samples are plotted in
chemical characterization and to identify the nature of the iron granite field.
magmatism and tectonic setting of these rocks. Initially, To characterize the tectonic setting, the R1-R2 (Fig. 13A)
the samples were treated in the Laboratory of Sample and Rb versus Y+Nb (Fig. 13B) diagrams, proposed by
Preparation of the Department of Mineral Resources Batchelor and Bowden (1985), Pearce et al. (1984) and
(DRM), Federal University of Mato Grosso (Universidade Pearce (1996), respectively, were used. The distributions of
Federal de Mato Grosso - UFMT) and then sent to Acme the analyzed samples in these diagrams suggest magmatism
Analytical Laboratories (Acmelab) in Vancouver, Canada that is compatible with the granite series generated in late-
(Table 2) for analysis by the methods Inductively Coupled to post-orogeny environments.
Plasma (ICP) and Inductively Coupled Plasma Mass The geochemical pattern obtained for these rocks,
Spectrometry (ICP-MS). which was normalized to Nakamura’s chondrite val-
The levels of SiO in the MG rocks are classified ues (Nakamura 1977; Fig. 14A), shows a fractionation
2
from intermediate to acidic, with concentrations ranging of Heavy Rare Earth Elements (HREE) relative to the
from 59.98 to 62.89% (hornblende-biotite to tonalite Light Rare Earth Elements (LREE) and (La/Yb)n ratios
facies) and 65.72 to 68.64% (biotite granodiorite to between 9.01 and 35.05. Discrete negative Eu anoma-
monzogranite facies), and with a distinct compositional lies are observed with Eu/Eu* ratios ranging from 0.25
gap between 62.89 and 65.72%, because of a possible to 0.56, which corroborate the plagioclase fractionation
lack of outcrops. The Harker diagrams (Harker 1909; hypothesis suggested by the CaO versus silica graph
Fig. 9) show trends with negative linear correlations (Fig. 9). In the multi-element distribution and K O dia-
2
between silica and TiO , MgO, (Fe O )t, P O , CaO and gram (Fig. 14B), which were normalized against Ocean
2 2 3 2 5
Sr, which reflect the fractionation of calcic plagioclase Ridge Granite (ORG) values from Pearce et al. (1984),
and primary mafic minerals, such as amphibole, biotite, these rocks are characterized by enrichment in large
titanite, Fe/Ti oxides and apatite during the magmatic ionic radius lithophile elements (LILE) relative to the
evolution. Correlations are not observed for the K O, high field strength elements (HFSE) and show a pattern
2
Na O and Al O oxides. similar to that of volcanic arc granitoids (VAG), which
2 2 3
The MG rocks are geochemically classified as tonalites, were defined by these authors, and Ta and Nb negative
granodiorites and monzogranites in the R1-R2 chemical anomalies, which support this hypothesis.
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Brazilian Journal of Geology, 44(3): 415-432, September 2014