u-pb and sm-nd geochronology of the neoproterozoic granitic-gneissic dom feliciano belt, southern...

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U-Pb and Sm-Nd Geochrotiology of the Neoproterozoic Granitic-Gneissic Dom Feliciano Belt, Southern Brazil

MARLY BABINSKI’, FARID CHEMALE JR.2, W.R. VAN SCHMUS3, LEO AFtiNEO HARTMANN and LUIZ CARLOS DA SILVA4

1. Instituto de Geocihcias, Universidade de Sao Paulo, Cx. Postal 11348, CEP 05522-970, Sb Paulo, SP, Brazil. E-mail: [email protected]

2. Instituto de GeociCncias, Universidade Federal do Rio Grande do Sul, Cx. Postal 15001, CEP 91501-970 Porto Alegre, RS, Brazil

3. Department of Geology, University of Kansas, Lawrence, KS, 66045 USA 4. CPRM/Brazilian Geological Survey, Rua Banco da Provincia, 105, CEP 90840-030 Porto Alegre,

RS, Brazil

(Received June 1996; accepted April 1997)

Abstract -The Brasiliano Cycle in southern Brazil and Uruguay is represented by three major NE-SW trending geotectonic units: the Vila Nova belt, Tijucas belt and Dom Feliciano belt. The Vila Nova belt is located in western part of Rio Grande do Sul State; its evolution took place between 900 and 700 Ma and it corresponds to one of the few areas with juvenile accretion during the Neoproterozoic in Brazil. The Tijucas belt, situated between the Vila Nova and Dom Feliciano belts, consists of a rift-related Mesoproterozoic (?) volcano-sedimentary sequence which was strongly deformed during the Brasiliano cycle. The Dom Feliciano belt is located along the eastern coast of southern Brazil and Uruguay and is a typical granite-gneiss-migmatite terrane. This belt is a key area for understanding West Gondwana assembly during Neoproterozoic and Early Paleozoic times, because of its direct connection to the Gariep and Damara belts in southern Africa.

The present study defines the main tectonic phases of the ca. 600 Ma Dom Feliciano event in the Sb Feliciano belt. U-Pb zircon data for flat-lying gneisses yield ages between 610 i 5 Ma and 616 + 2 Ma, which we believe correspond to the approximate age of thrusting. The strike-slip deformation (main transcurrent phase) is well dated by U-Pb zircon ages for the syn-transcurrent granites (Arroio Moinho Granite, 595 f 1 Ma; Encruzilhada do Sul Granite, 594 2 5 Ma). These results indicate a relatively rapid evolution, from about 620 Ma (upper limit for the age of the gneiss) to 594 Ma (syn-trancurrent granites), for the known thrust related and strike-slip related tectonic phases of the Dom Feliciano belt.

Sm-Nd results can be considered in three major groups. The first group (I) includes Brasiliano gneisses, granitoids, and one anorthosite with To, ages of ca. 2.0 Ga and very negative z&600) values. They may represent either direct melting of Transama- zonian (Paleoproterozoic) basement or extensive contamination with older material of Paleoproterozoic to Archean age. The second group (II) includes granitoids and gneisses with To, model ages from I .3 I to 1.4 I Ga. The third group (III) comprises samples with To, ages between 1.58 to 1.75 Ga. For groups II and III it is clear these rocks or their protoliths represent pre-Brasiliano continental crust. Unlike Group I rocks, groups II and 111 granites and gneisses may also contain a small fraction of a juvenile Brasil- iano material. However, we have not yet found any sample from the Dom Feliciano belt with a Neoproterozoic To, age and positive a,+, value at 600 Ma that could be considered largely juvenile.

Based on results from the Vila Nova belt, in which the main erogenic process developed between 753 and 704 Ma, we conclude that the Vila Nova belt was stable for over 100 Ma before the Dom Feliciano event reached its peak. It is probable that the collage of terranes in the Dom Feliciano belt and the region comprised by the lijucas and Vila Nova belts were assembled during the Dom Feliciano event (ca. 600 Ma). 0 1997 Elsevier Science Ltd

Resumo - 0 ciclo Brasiliano no sul do Brasil e Uruguai t representdo por tres unidades geotectonicas principais, corn orienta$io NE-SW Cinturb Vila Nova, Cinturao Tijucas e Cintur8o Dom Feliciano. 0 Cinturiio Vila Nova localiza-se na parte oeste do Estado do Rio Grande do Sul; sua evolu@o ocorreu no interval0 entre 900 e 700 Ma e corresponde a uma das poucas areas, no Brasil, corn acres@o de material juvenil neoproterozoico. 0 Cinturao Tijucas esta situado entre OS cinturoes Vila Nova e Dom Feliciano e consiste de uma sequ&ncia vulcano-sedimentar mesoproteroz6ica (?) que foi fortemente deformada durante o ciclo Brasiliano. 0 Cinturb Dom Feliciano, localizado ao longo da costa sudeste do Brasil e Uruguai, B urn tipico terreno granitico- gn&issico-migmatftico. Este cinturb t uma area-chave para compreender a colagem do Gondwana Ocidental durante o Neopro- teroz6ico e Paleozdico Inferior, ja que possui uma conexb direta corn OS cintur8es Gariep e Damara no sul da Africa.

0 presente trabalho define as principais fases tectonicas do evento Dom Feliciano (ca. 600 Ma) no Cinturb Dom Feliciano. Dados U-Pb em zirc&es de gnaisses corn folia@o de baixo ;ingulo fomeceram idades entre 610 2 5 Mae 616 + 2 Ma, as quais correspon- dem a idade da tect6nica tangential. A deforma@o direcional (principal fase transcorrente) foi datada atraves de idades U-Pb em zircoes de granitos sin-transcorrentes (Granito Arroio Moinho, 595 f I Ma; Granito Encruzilhada do Sul, 594 * 5 Ma). Estes resultados indicam uma evolu@o relativamente rapida, de 620 Ma (limite superior para a idade dos gnaisses) a 595 Ma (granitos sin-trancorrentes). para as fases tectonicas conhecidas (tangential e direcional) no Cintur8o Dom Feliciano.

OS dados Sm-Nd podem ser divididos em tr& grupos principais. 0 primeiro (I) inclui gnaisses, granit6ides e urn anortosito brasil- ianos corn idade modelo To, de ca. 2,0 Ga e valores muito negativos de ~~~(600). Eles podem ter sido gerados a partir da fusao direta do embasamento transamazdnico (paleoproteroz6ico) ou terem sofrido uma intensa contamina@io corn material mais amigo de idade paleoproterozdica a arqueana. 0 Segundo grupo (II) inclui granitoides e gnaisses corn idades modelo To, entm I ,3 1 e I .41 Ga. O.terceiro grupo (III) d constituido por amostras corn idades modelo To, entre I ,58 e I ,75 Ga. OS prot6htos das rochas dos grupos II e 111. claramente, representam crosta continental pm-brasiliana. Porem, difetentemente das rochas do grupo 1, as rochas dos grupos

263

264 M. BABINSKI et uf.

II e III podem comer uma pequena proporciio de material juvenil de idade brasiliana. Entretanto. mio foram encontradas rochas do

Cinturao Dom Feliciano que apresentassem idades modelo To, neoproterozoicas e valores posrttvos de +,,(600).

Corn base nos dados obtidos no CinturHo Vila Nova, onde o process0 orogenetico principal ocorreu entrr 7.53 e 704 Ma, C possivel

concluir que o Cinturao Vila Nova estava estdvel por mais de 100 Ma antes que o evento Dom Feliciano atingisse o seu pica. fi

prowivel que a colagem dos terrenos do Cinturb Dom Feliciano e da regilo compreendida pelos cinturoes Tijucas e Vila Nova

ocorreu durante o evento Dom Feliciano (ca. 600 Ma).

INTRODUCTION GEOLOGICAL SETTING

The NE-SW trending Dom Feliciano belt is exposed along the eastern coast of southern Brazil and Uruguay (Fig. I),

comprising a typical granite-gneiss-migmatite terrane. This belt, ca. 800 km long and 150 km wide, is considered one of the key areas for understanding West Gondwana

assembly during Neoproterozoic to Early Paleozoic times,

because it is the direct connection to the Gariep and

Damara Belts in southern Africa. Several studies have been

developed in the Dom Feliciano belt, focusing on its litho-

logic, structural, geochemical and geocronological aspects

(e.g., Silva and Dias, 198 1; Basei, 1985; Frantz and Remus

1986, Fragoso-Cesar et al., 1986; Fragoso-Cesar, 1991; Mantovani et al., 1987; Phillip et al. 1993; Fernandes et al., 1992), and these have been used to support different tec-

tonic models, ranging from a juvenile Brasiliano magmatic

arc to Paleo- to Mesoproterozoic crustal material reworked

during the Brasiliano orogeny.

The purposes of this work are: (a) to present better con-

straints on the chronostratigraphy of the Dom Feliciano

belt using U-Pb zircon analyses on some gneissic.

migmatitic and granitic rocks, and (b) to identify the sources of the rocks using Sm-Nd isotopic data.

~.._~ r-- -- 54”W

BRAZIL

The study area is part of the Mantiqueira province (MP;

Fig. 1) which consists of Archean to Early Paleozoic units

formed during at least three main erogenic cycles, locally called Jequit (Archean), Transamazonian (Paleoprotero-

zoic; ca. 2.1 Ga) and Brasiliano (Neoproterozoic to Early

Paleozoic; 0.6-0.5 Ga) (Schobbenhaus et ul., 1984;

Almeida and Hasui, 1984). The Mantiqueira province was strongly affected by Brasiliano tectonic activity, which

resulted mainly in NE-SW trending structures, multiple

granitic intrusions, and reworking of Archean to Transama-

zonian granite-gneiss terranes and some Mesoproterozoic

sequences.

The Dam Feliciano belt is part of the southern segment

of the Mantiqueira Province (Almeida and Hasui, 1984)

and represents a predominantly granitic-gneissic-

migmatitic terrane in Uruguay and southern Brazil ( Rio

Grande do Sul, RS, and Santa Catarina, SC, states; Fig. 1).

To the east it is covered by Mesozoic and younger units. To

the west this belt is limited by the NE-SW trending Tijucas

belt (Fig. lb), which mainly consists of Mesoproterozoic

volcano-sedimentary rocks that were deformed at greenschist to lower amphibolite facies conditions during the

I PORT0 ALEGRE ,e /

FLORlANdPOLlS

- Fig. 3

150 km

30”s

PHANEROZOIC COVER

MOLASSIC BASINS

DOM FELICIANO BELT

VILA NOVA BELT

TIJLJCAS BELT

PALEOPROTEROZOlC TERRANES

Fig. I. Geological sketch map of the southern part of the Mantiqueira province (after Chemale et ul.. 1995)

U-Pb and Sm-Nd Geochronology of the Neoproterozoic Granitic-Gneissic Dom Feliciano Belt, Southern Brazil 265

Brasiliano Cycle. In the Tijucas Belt granite-gneissic rocks transpressive deformation which gave rise to NE-SW

belonging to Paleoproterozoic to Archean basement are trending, oblique to strike-slip, shear zones. Such transcur-

also exposed. The contact between these belts is tectonic, rent shear zones are mostly sinistral in the Dom Feliciano

and it is well exposed in the Porto Belo region as the belt (Fernandes er al., 1992), but in some places, as in the

km-wide Major Gercino transcurrent zone (Bitencourt, Porto Belo region, a dextral displacement has been recog-

1997), a large NE-SW trending strike-slip shear zone. nized (Bitencourt, 1997).

The tectonic framework of Precambrian to Early Paleo-

zoic units in southern Brazil (Fig. 1) includes: (a) Paleo-

proterozoic granulite complexes (the Santa Maria Chico

complex in Rio Grande do Sul and the Santa Catarina com-

plex in Santa Catarina); (b) a Paleoproterozoic granite- greeenstone greenstone terrane in Uruguay; (c) the Vila

Nova belt, which is represented by tonalitic to granodior-

itic gneisses and volcano-sedimentary rocks, a mafic-

ultramafic unit, and foliated granites formed between 750

and 700 Ma (Chemale et al., 1995; Babinski et al., 1996); and (d) the late-erogenic to post-erogenic plutonic-

volcanic-sedimentary Seival association, which is related

to the 600 Ma Dom Feliciano event.

Structural studies in the Dom Feliciano belt (Fragoso- Cesar, 1991; Fernandes et al., 1992) defined two main

structural events, each one consisting of two or more

phases. Event 1 is related to SW-NE verging thrusts that,

after strong crustal shortening, evolved to Event 2, a

Geology of the Dom Feliciuno Belt

The Dom Feliciano belt consists mainly of syn-thrust

granites and migmatitic gneisses, strike-slip related granites, and late to post-tectonic granites, all formed between

650 Ma and 500 Ma (Basei, 1985; Chemale et al., 1997).

Sm-Nd results for composite samples (Mantovani et al., 1987) and Pb-Pb zircon dating in this belt (Chemale et al.,

1994b) indicate crustal reworking. The following lithos-

tratigrafic units for the Dom Feliciano belt have been

described (Figs. 2 and 3): Chart& gneisses, Capivarita

anorthosite, syn-tangential Pinheiro Machado and Cam-

borid complexes, Piquiri syenite, strike-slip related gran-

ites, late to post-tectonic granites, post-tectonic Passo da

Fabiana gabbros, and post-tectonic Asperezas rhyolite.

The Ghana gneisses correspond to paragneisses and

orthogneisses metamorphosed in upper amphibolite facies

N 53"OO 52"OO

Phanerozoic cover

Piquiri Syenite

Encruzilhada do Sul suite

Sin-transcurrent granites

Pinheiro Machado Complex

Porongos group

Capivarita anorthosite

Char3 gneisses

Dorsal do CanguGu shear zone

Strike-slip shear zone

Sample location

Highway

Town/City 20 km I_.-. 1

Fig. 2. Geological map of the Dom Feliciano belt in Rio Grande do Sul (after Chemale et al., 1995) with sample locations. A = Piquiri Syenite (RS-2), B = Encruzilhada do Sul Suite (RS-4) C = Capivarita Anorthosite (RS-S), D = Pinheiro Machado Complex (RS-7). E = Pinheiro Machado Complex (RS-9), F = CapIo do Leao Granite (RS-12), G = Arroio Moinho Granite (RS-13). SB = Santana da Boa Vista, PM = Pinheiro Machado, PG = Pantano Grande, PT = Pelotas, CO = Cordilheira Syn-trancurrent Granite, AM = Arroio Moinho Syn-trancurrent Granite

https://www.researchgate.net/publication/249520254_Juvenile_accretion_at_750700_Ma_in_Southern_Brazil?el=1_x_8&enrichId=rgreq-6e7d1809-23d2-4c54-bc3f-66bd5baf4389&enrichSource=Y292ZXJQYWdlOzIyMzYwMTI3MjtBUzoyMzMzNjU5MjQyMTY4MzJAMTQzMjY1MDE4MTAxNw==

266 M. BABINSKI et ul.

49”30’ 48”30’ N

A 40 KM 1 1

1-j CENOZOIC SEDIMENTS

r----7 PAWNA, BASlN

mj L/j-l-E TO POST-OROGENIC

ITAJAi (IB) AND CAMP0

ALEGRE (CAB) BASINS

j+ PEDRAS GRANDES SUITE

PAULOLOPES

m VALSUNGANA GRANITE

[=I CAMBORId COMPLEX

Em BRUSQUE GROUP

‘iYz=Ij PALEOPROTEROZOIC STA.

CATARINA GRANULITE

COMPLEX

STRIKE-SLIP SHEAR

/ ZONE (SZ)

~ MGSZ = MAJOR GERCINO SZ

/sz = FERMBi) .SZ 1 1 0 Sample location

0 _. City

Fig. 3. Geological map of the Santa Catarina schield (after Chemale el al., 199.5) with sample locations. H = Pedras Grandes Suite (SC- 42), I = Paulo Lopes monzogranite (SC-43). J = Guabimba leucogranite (SC-52), Camborili Complex (SC-54-I).

conditions; they are exposed in the northern part of the area

shown in Figure 2. Paragneisses are represented by alumi- nous, talc-silicate and quartzo-feldspathic gneisses with

lenses of marbles and amphibolite. Banded granodioritic to tonalitic gneisses comprise the orthogneisses (Frantz et ul.,

1984). Although there are no radiometric ages for the

Ghana gneisses, field relationships indicate that they are

the oldest stratigraphic unit of the Dom Feliciano belt (Frantz et al., 1984, Fernandes et al., 1990).

The Capivarita anorthosite (Formoso, 1972) is a 170 km2 massive body within the Encruzilhada do Sul suite (Fig. 2). It has intercalations of banded amphibolite

and some xenoliths of talc-silicate rocks (Ghana gneisses) as well as intrusions of Brasiliano granite. Its mineralogy is plagioclase-labradorite (> 90 %) with accessory quartz, sphene, and K-feldspar. It also formed in the upper amphibolite facies with greenschist facies retrograde metamorphism. However, there are no data available for the crystallization age of the anorthosite.

The Pinheiro Machado and Camboriu complexes are

exposed in Rio Grande do Sul (Fig. 2) and Santa Catarina

(Fig. 3) respectively; they have been interpreted as Brasil-

iano units with a strong component of older rocks (Che- male et cd., 1994b). These complexes are associations of

gneiss, migmatite, and high-K talc-alkaline granite. The

gneiss and migmatite show a flat-lying foliation that is

mostly folded or cut by younger strike-slip structures. According to Fernandes et al. (1992) and Fragoso-Cesar

(1991), this flat-lying fabric formed by a W- to NW-verging thrusting event. These structures formed around 600 Ma, during the Dom Feliciano event (Chemale

et al., 1994a). In some places there are small blocks of pre- Brasiliano tonalitic-trondhjemitic-granodioritic gneiss

(Basei, 1985) and supracrustal rocks such as talc-silicates, BIFs, quartzites, and amphibolites (Silva and Dias, 1981).

Several NE-elongated granitic bodies displaying deformation related to the younger strike-slip deformation occur along the Dom Feliciano belt; they are commonly referred

U-Pb and Sm-Nd Geochronology of the Neoproterozoic Granitic-Gneissic Dom Feliciano Belt. Southern Brazil 267

to as syn-transcurrent or strike-slip related granites

(Fragoso-Cesar, 1991, Fernandes et al., 1992, Phillip ef al., 1993). These granites intrude the Pinheiro Machado and

Camboriu complexes, and they display both magmatic-

flow and solid-state foliation oriented at N40”E and N-S,

with a sub-horizontal to horizontal stretching lineation.

These bodies may be divided into a high-K talc-alkaline

metaluminous suite (e.g. Monte Bonito, Arroio Moinho,

QuitCria and Arroio Francisquinho metagranites) and a

talc-alkaline to subalkaline peraluminous suite (Cordil-

heira leucogranites). These rocks have a direct relationship

with the development of strike-slip megashear zones such as the Dorsal de Canguqu (Fernandes et al., 1990) and

Major Gercino shear zones (Bitencourt, 1997).

The Piquiri syenite is a hook-shaped, homogeneous, 130 km2 batholith (Jost et al., 1985) (Fig. 2). It consists of syenite with large K-feldspar phenocrysts (C 5 %),

orthoclase, hornblende, and Ca-clinopyroxene; quartz,

microcline, plagioclase, biotite, sphene, ilmenite, apatite

and zircon occur as accessory minerals. A characteristic

igneous banding is present in this unit. There is no radio-

metric age for the syenite, but it may be late- to post-

transcurrent with respect to the Dom Feliciano event.

Two of the most expressive granitic suites are the

Encruzilhada do Sul suite in Rio Grande do Sul (Fig. 2) and

the Pedras Grandes suite in Santa Catarina (Fig. 3). These

granitic suites occur as large, irregular bodies intruded into

the Pinheiro Machado and Camborid complexes. Petro-

graphically their composition varies from syenite to monzogranite with a dominant syenogranitic facies. In Santa

Catarina, the Pedras Grandes suite includes diverse mag-

matism, including deformed pre- and syn- strike-slip tec-

tonic bodies in addition to the dominant late to post-

tectonic granites. U-Pb zircon dating of weakly foliated late-tectonic granite yields an age of ca. 600 Ma (Basei, 1985).

The Passo da Fabiana gabbros are undeformed, mafic to ultramafic bodies varying from thousands of square kilom-

eters to few square kilometers in area (Fragoso-Cesar et al., 1986) which are exposed near the cities of Pinheiro Mach-

ado and Dom Feliciano. A post-erogenic rhyolitic dyke

swarm and isolated dykes of the Asperezas rhyolites cut units of the,Dom Feliciano Belt (Fragoso-Cesar, 1991).

Sample Locations and Description

Samples for geochronological studies were collected

from several different localities (see Tables 1 and 2, Fig-

ures 2 and 3). We chose rocks from the main stratigraphic

units of the Dom Feliciano belt in order to get a better approach to the chronostratigraphy as well as some addi- tional information on regional crustal and mantle evolution. Details of their descriptions and locations are given in Appendix 1.

ANALYTICAL PROCEDURES

Isotopic analyses were done in the Isotope Geochemis- try Lab, University of Kansas. Zircons were dissolved and

Pb and U were separated using procedures modified after

Krogh (1973, 1982) and Parrish (1987). All samples were

total-spiked with a mixed 2osPb-2’“U tracer solution. Iso-

topic ratios were usually measured in dynamic mode using

a VG Sector multi-collector mass spectrometer equipped

with a Daly detector for large samples; this mode allows

real-time internal calibration of the Daly gain during runs.

We used single-collector mode with the Daly detector for

smaller samples. Pb samples were analyzed on single Re

filaments using silica gel and H,PO,; measured isotopic

compositions were corrected for average mass discrimina-

tion of 0.12 + 0.05 percent per mass unit for Faraday data and 0.25 percent per mass unit for Daly data. These correc-

tions were determined by analyses of NBS SRM-982 (equal-atom Pb), and monitored by analyses of NBS SRM-

983 (radiogenic Pb). Uranium was loaded on a single Re

filament with H,PO, and a layer of colloidal carbon and

analyzed as U+. Uranium fractionation was monitored by

analyses of NBS SRM U-500 and averaged 0.1 percent per

mass unit for Faraday data and 0.2 percent per mass unit

for Daly data. Uncertainties in U/Pb ratios due to uncer-

tainties in fractionation and mass spectrometry for typical

analyses are 2 0.5%; in some instances weak signals cause

uncertainties to range up to k 2%. Radiogenic 208Pb, 207Pb

and 206Pb were calculated by correcting for modern blank

Pb and for nonradiogenic original Pb corresponding to Sta-

cey and Kramers (1975) model Pb for the approximate age

of the sample. Data were corrected for average blanks of

5-10 pg total Pb and 5-10 pg total U. Uncertainties in

radiogenic Pb ratios are typically f 0.1% unless the samples had an unusually low 206Pb/204Pb ratio, in which case

uncertainties in the common Pb correction could cause

greater uncertainties. Decay constants used were 0.155125 x 10e9 yeai’ for 238U and 0.98485 x 10m9 year-’ for 235U

(Steiger and Jager, 1977). Zircon data were regressed using the ISOPLOT program of Ludwig (1993). Model 1 regres-

sions were accepted if probabilities of fit were better than 30%; Model 2 regressions were accepted if probabilities of

fit were less than 30%. Uncertainties in concordia intercept ages are given at the 2-sigma confidence level.

For Sm/Nd analyses, whole-rock powders were dis-

solved and REE were extracted using the general method

of Patchett and Ruiz (1987). Isotopic compositions were measured with a VG Sector multi-collector mass spectrom-

eter. Sm was loaded with H,PO, on a single Ta filament

and typically analyzed as Sm+ in static multicollector mode. Nd was loaded with H,PO, on a single Re filament

having a thin layer of AGW-50 resin beads and analyzed as Nd+ using dynamic mode. We collected 100 ratios with a

1V 144Nd beam, which typically yields internal precision

of 10 ppm. External precision based on repeated analyses of our internal standard was f 30 ppm (2 o) during the course of these analyses; all ‘43Nd/‘44Nd ratios were adjusted for instrumental bias as determined by measure- ments of our internal Nd standard for periodic adjustment of collector positions; on this basis our analyses of La Jolla Nd average 0.511860 rt- 0.000020 (2 0). During the course of these analyses Nd blanks were about 150 pg, with corresponding Sm blanks of 50 pg. Correction for blanks was

M. BABINSKI er al.

Table I U/Pb zircon data ofgramte-gneissic samples from Rio Grande do Sul and Santa Catarina States

Concentration? Observed Ratios’ Atomic Ratios’ Age? SIX Pb u XK’Pb, Z”“Pb, ZX>Pb, *““Pb/ X’,Pb, LWPb, “‘“pb, ZO-iPb, ?“7Pb,

Fraction’ (mg) (ppm) (ppm) ‘“JPb ?“7Pb Z’,XPb z ‘H” L 3.5” “‘“Pb *inu L ‘5U ‘I,“Pb

RS-4 Encruzilhada do Sul Granite NM(-2) E. I. C 0.023 23.95 228.6 4.183.7 15.5628 5.01 197 0.09646 0.81812 0 06151 594 607 657 M(-2) E, VPY, C 0.013 45.99 482.4 1.523.2 14.5.5.57 4.99962 0.08872 0.73118 0.05977 548 557 595 M(-l)a E, VPY, C 0.026 44.34 469.2 14,559.j 16.5224 5.19012 0.0876X 0.72364 0.05986 542 553 599 M(-l)b E, YPY, C 0.013 60.64 670. I I ,939.9 14.9287 5.94810 0.08617 0.71106 0.05985 533 54s 598 M(0) E, UPY, F 0.024 74.24 855.3 24.749.6 16.5965 7.29340 0.08432 0.69638 0.05990 522 537 600

RS-13-H Arroio Moinho Granite NM(- I )a E,l/PY, C 0.132 39 26 384.3 22.195.5 15.7179 7.49441 0.09920 0.86494 0.06324 610 633 716 NM(-1 )b E,VPY,C 0.033 22.98 235.6 4.713.7 16.0062 7.23893 0.09467 0.78220 0.05993 583 587 601 M(- l)a E, VPY,C 0.099 43.76 452.6 12,343. I 16.4704 7.3 1922 0.09386 0.77318 0.05975 578 582 594 M(-l)b E, PY, F 0.041 35.31 373.1 5.546.5 16.1097 6.00774 0.08991 0.74148 0.0598 I 555 563 597 M(0) E, UPY,C 0.033 53.91 550.0 798.6 12.8982 6.39558 0.09103 0.75055 0.05980 562 569 596 M( I ) E, VPYC 0.027 33.10 348.4 1.085.1 13.8093 5.89054 0.09020 0.74085 0.05957 557 563 588

RS-7B Pinheiro Machado Complex NM(-2)el, E, l/Y, C 0.098 35.16 344.9 16,175.4 16.3415 7.92264 0.09987 0.83363 0.06054 614 616 623 NM(-2)n I, En, UPY, C 0.042 47.24 469.0 20,169.l 16.4700 8.00798 0.09879 0.822 I3 0.06036 607 609 616 NM(-2)n2. En, 1. C 0.037 32.49 324.4 13,029.O 16.3327 X.37956 0.09867 0.82306 0.06050 607 610 621 M(-2) E, I, C 0.073 38.72 383.5 12.593-S 16.1790 9.04878 0.10023 0.84207 0.06094 616 620 637 M(0) E, VPY, C 0.054 3.5.08 350.7 4,726.0 I5 6143 9.59831 0.09969 0.84135 0.06121 613 620 647

RS-YA Pinheiro Machado Complex M(-l)E,I.C 0.016 38.45 383.8 3,786.7 15.23.51 14.34754 0.10300 0.88667 0.06244 632 645 68’) M(0) E, UPY, C 0.030 47 78 485.6 7.592.4 15 5754 14.62234 0.10150 0.87673 0.06265 623 630 696

X-54-I Camhoriti Complex NM(-I) E. PY, F 0.033 71.22 322.4 840.9 6 0406 7.44304 0.19472 4.062 I4 0.15110 1147 1647 7361 M(- I) E. VPY, C 0.018 40 77 231.1 488.Y 6 7504 -1.45813 0.14793 2.50320 0 12273 X89 1273 IYY6 M(- I) E, PY, Met 0.034 74.62 333 I 313.3 5.5874 4.23723 0. I7592 3.37314 0. I3907 1045 1498 2216 M(0) E/El, UPYSF 0.015 48.12 208.8 932.0 6.3356 5.17580 0.19920 3.99860 0 14559 1171 1634 2295 M(0) E, PY. Met 0.022 61.65 431.0 2.424.2 8.5976 8.11794 0 13416 2.06327 0 11154 812 1137 182.5 M(2) E. PY, Met 0.027 93.13 709.8 761.5 6.7934 5.68171 0.11335 2.04634 0.13093 692 1131 2111

Notes for Table: 1: NM = nonmagnetic, M = magnetic, numbers in parentheses indicate side tilt used on Frantz separator at I .S amp power; E = hand-picked euhedral

3: I grains; El = hand-picked elongate grains; En = hand-picked euhedral needle grains; I = Colorless; PY = Pale yellow; C = internally clear; F = frosted; SF = slightly frosted; Met = metamict.

2: Total U and Pb concentrations corrected for analytIcal blank. 3: Not corrected for blank or non-radiogenic Pb. 4: Radiogenic Pb corrected for blank and initial Pb; U corrected for blank. 5: Ages given in Ma using decay constants recommended by Steiger and Jiger (1977).

insignificant for Nd isotopic composition and generally

insignificant for Sm/Nd concentrations and ratios. Sm/Nd

ratios are correct to within + OS%, based on analytical

uncertainties.

Extrapolations to T,, for plutonic rocks can have large

uncertantainties due to possible REE fractionation during magma genesis if they are much younger than their source

terrains, as is the case for ca. 550-600 Ma Brasiliano granites intruding and presumably derived from Archean to

Mesoproterozoic (?) basement in parts of the Dom Feli-

ciano belt. As a result, we prefer to use 600 Ma as the main reference time for Nd compositions in the past, since we can extrapolate accurately to that time without worrying about possible fractionation of Sm/Nd during either magma genesis or high grade metamorphism. Because

most felsic samples have similar Sm/Nd ratios (Table 2), this approach gives a quick estimate of the age of the crustal sources for volcanic and plutonic rocks, the protoliths of

gneisses, or the provenance of sediments. To, results following the model of DePaolo (1981) are also presented in Table 2, but allowances must be made for absolute uncertainties (as opposed to relative precision) of f 0.2 Ga or more due to both analytical and geological factors.

U-PB ZIRCON RESULTS

Six zircon samples from Rio Grande do Sul and one

sample from Santa Catarina were analyzed (Table 1).

RS-4 Encruzilhadu do Sul Granite. We analysed five

zircon fractions from sample RS-4, the Encruzilhada do

Sul granite (Table 1). Four fractions yielded good results

which define a normal discordia with an upper intercept of

594 + 5 Ma (Fig. 4). We interpret the upper intercept from the chord defined by the four colinear fractions as the crys-

tallization age of this granite. The one fraction plotting to the right of this upper intercept indicates the presence of xenocrystic zircons that were not completely avoided during preparation of this fraction. This fraction cannot be used to define an age of this inherited component because it was also probably affected by Pb-loss, so no meaningful chord can be drawn through it.

RS-13-11: Syn-transcurrent Arroio Moinho Granite. We analysed 6 zircon fractions (Table 1) from RS- 13-11, the monzogranitic phase of the Arroio Moinho granite. Five fractions yielded good analyses which scatter along a normal discordia (Fig. 5). Fractions M(i), M(O), M(-l)a,


Table 2. Sm-Nd isotopic data from Dom Feliciano Belt whole-rock samples

Sample # Sample Type

(KU)

Rio Grande do Sul Samples RS-2 Piquiri syenite RS-4 Syenogranite RS-5 Anorthosite RS-7A Diorite gneiss RS-7B Granodiorite gneiss RS-7C Syenogranite RS-9A Migmatitic gneiss RS-9B Syenogranite RS-12 Metaluminous granite RS- 13-B Monzogranite

Santa Catarina Samples SC-42-l Granodiorite SC-42-B Porphyritic leucogranite SC-43 Porphyritic monzogranite SC-52 Leucogranite SC-54-I Migmatitic gneiss

Nd Sm 147Sm 143Nd r2o %‘I I %d 2.1 T(DM)”

ppm ppm 144Nd 144Nd (0) (0.6 Ca) Ga

91.28 14.32 0.09487 0.511678 + 8 -18.7 -10.9 1.75 183.26 29.26 0.09652 0.511449 + 8 -23.2 -15.5 2.08

2.02 0.32 0.09434 0.5 11480 *II -22.6 -14.8 2.00 33.06 6.65 0.12165 0.511831 i 9 -15.7 -10.0 2.01 29.60 5.54 0.11320 0.511928 + 7 -13.9 -7.5 1.69 32.86 4.76 0.08753 0.511906 + 8 -14.3 -5.9 1.37 7.44 1.62 0.13144 0.511914 +I2 -14.1 -9.1 2.09

24.14 3.39 0.0848 1 0.511853 + 7 -15.3 -6.7 1.41 45.03 12.53 0.16827 0.512340 * 9 -5.8 -3.6 2.37 45.06 6.87 0.09217 0.511995 +8 -12.5 -4.5 1.31

58.77 9.98 0.10264 0.512032 2 8 -11.8 -4.6 1.39 18.87 4.79 0.15340 0.512184 + 8 -8.9 -5.6 2.18 58.69 9.60 0.09893 0.511853 & 9 -15.3 -7.8 1.58 45.73 6.38 0.08436 0.511142 + 9 -29.2 -20.6 2.25 57.10 6.54 0.0692 1 0.511001 ~8 -31.9 -22.2 2.16

1. Calculated assuming ““Nd/‘“Nd today = 0.512638 with data normalized to ‘4nNd/‘44Nd = 0.72190. sNd(0) = ((‘43Nd/‘44Nd[sample, now]/0.512638) - I) x 104.

2. ENd = ((‘4’Nd/““‘Nd[sample, 600 Ma]/‘43Nd/‘“Nd[CHUR. 600 Ma]) - I) x IO“. 3. Based on UlPb zircon ages of Dom Feliciano Event. 4. Calculated following model of DePaolo (198 I).

and M(-1)b define an upper intercept of 595 f 1 Ma if

forced through the origin (an age of 593 +- 3 Ma is obtained

if not forced through the origin, but it has a negative inter-

cept). This chord includes two analyses of the same frac-

tion, M(-1), and we believe it represents the true

crystallization age for this rock. If NM(-1)b is included with the other four analyses, an upper intercept age of 604

f 9 Ma is obtained; this chord is slightly influenced by the

second analysis of the least magnetic fraction, NM(-1)b.

The first analysis of that fraction, NM(-l)a, shows a sig-

nificant inherited component, and we believe it is likely

that fraction NM(-1)b also contains a much smaller inherited component which causes this regression to yield an

age slightly greater than the true crystallization age of 595 Ma.

I I i I I ;

0.104 -

Rs4

0.76 0.88

207pb/235u

Fig. 4. Concordia Diagram for Encruzilhada do Sul rocks (RS-4). Chord shown is a regression forced through the origin (t = 0) and four co-linear points. The fifth point to the right of the upper end of discordia probably includes an inherited component.

RS-7B and RS-9A: migmatitic gneisses of the Pin- heiro Machado Complex. We analysed zircons from

RS-7B, a migmatitic gneiss from the Pinheiro Machado

complex. Five analyses (Table 1) cluster near concordia at

about 620 Ma (Fig. 6a). They all show small effects due to

both inheritance and Pb-loss, so that we could not define a unique discordia. The single fraction with the least appar-

ent Pb inheritance, NM(-2)nl, yields a Pb/Pb age of 616 f.

2 Ma, while two other fractions NM(-2)n2 and NM(-2)el

(shaded, Fig. 6a) are consistent with an average age of 623

+ 2 Ma (regression forced through zero). Two other frac-

tions, M(0) and M(2), show significantly more influence of

an inherited component. In any case, since both Pb-loss

Fs 13-11

I T=595+1Ma 1 i

0.080 I 1 4 J I / I I I I I I

0.64 0.76 0.88

207pb/235u

Fig. 5. Concordia diagram for syn-trancurrent Arroio Moinho granite (RS-13-U). The 595 * 1 Ma discordia shown is for the four fractions least likely to have any inheritance. Fraction NM(-1)b was excluded from this regression because fraction NM(-1)a definitely shows inheritance, and NM(-1)b may also have a slight amount of inheritance; if NM(-1)b is included with the other four analyses, an upper intercept age of 604 f 9 Ma is obtained.

270 M. BABINSKI et al.

_~_ 0.80 0.84 0.88

207pb/235U

I __~_

I 650

"1 FLS-7B

,I I

t RS-9A /a 4

640 /

0.097 1 /: ’ 1 .~

0.82 0.84 0.86 0.88

207pb/235U

Fig. 6. (a) Concordia diagram for migmatitic gneiss RS-7B from Pinheiro Machado Complex. (b) Concordia Diagram for-

migmatitic gneiss RS-9A from Pinheiro Machado Complex.

Regression line shown is a chord from the origin (t = 0) through the two shaded points. See text for discussion.

and inheritance effects are small, there is little doubt that

this rock is late Neoproterozoic.

We also analysed two fractions of RS9A, a migmatitic

gneiss in outcrop RS-9 that is equivalent to RS7B. The zircons from RS-9A show greater effects of inheritance (Fig. 6b); if it is assumed that the inheritance in RS-7B is

the same as that in RS-BA, a biotite-rich phase of the migmatitic gneiss, then we can use the combined data to set better limits on the age of these samples. A maximum age of 616k 2 Ma is given by the data from RS-7B fraction NM(-2)n 1, whereas a minimum age of 610 + 5 Ma is given by the lower intercept of a chord through RS7B fraction NM(-2)el and RS-9A fraction M(- 1). The 616 Ma age assumes that fraction NM(-2)nl from RS-7B has experi-

enced Pb-loss but has no inherited component. The 6 10 Ma age assumes that data for fraction NM(-2)nl are still

slightly influenced by an inherited component, but that fraction NM(-2)el from RS-7B has experienced less Pb-loss. At present we cannot choose between these alter-

natives, and we prefer to set the interval between 610 2

5 Ma and 6 16* 2 Ma as the best estimate for the age of this

unit (6 13 + 6 Ma).

SC-54-I: Migmatitic gneiss of Camboriri Complex. We analysed 6 fractions horn SC-54-1, migmatitic gneiss

of the Camborili complex (Table 1). Fractions M(O)a.

M(O)b, M(-1)a and M(-l)b define a chord with an upper

intercept apparent age of 2736 f 90 Ma and a lower inter-

cept apparent age of 544 +- 45 Ma (Fig. 7). These data

clearly demonstrate that this gneiss is part of the old base-

ment. However, the age of that basement is not well defined, since Nd data from this locality indicate a Sm-Nd T model age of only 2.16 Ga (Table 2 and see further

dz%ssion). The lower intercept age cannot be interpreted

as the age of metamorphism because of the possible occur-

rence of indeterminate Pb-loss mechanisms (combined epi- sodic Pb-loss during the Brasiliano orogeny and diffusive

Pb-loss at other times). Fraction NM(-1) plots slightly

below the upper end of the chord, which may reflect a slightly greater degree of recent Pb-loss. Fraction M(2) plots significantly below the lower end of chord, which

probably reflects greater recent Pb-loss. It is not clear at

this time if this rock was an Archean gneiss that experi-

enced Sm-Nd fractionation during the Brasiliano orogeny,

a Transamazonian (Paleoproterozoic) rock (as defined by the Sm-Nd model age) that also contains some Archean

detrital zircons, or a Transamazonian rock in which the dis-

cordia was rotated to an older upper intercept by greater

Pb-loss in the fractions with greater inheritance.

SM-ND RESULTS

Capivarita Anorthosite (RS-5). The whole-rock sam-

ple yields a To, age of 2.00 Ga (Table 2), which is inter-

preted as the age of formation of the anorthosite from a

depleted-mantle source. We do not believe that this age

represents an inherited Nd signature from older crust,

___. 1800 ,

,A/: 0.301

I SC-54-I ,/ 1

// I L

3 1400

al ! /

g 0.221 / 7 , r

g .:~..~~~

1 2 3 4 5

207Pbl235U

Fig. 7. Concordia diagram for gneiss from Camborili Complex (SC-54-l). The two discordia shown represent regressions through fractions which define a minimum age of610 c 5 Ma and a maximum age of 616 + 2 Ma. See text for further discussion.

U-Pb and Sm-Nd Geochronology of the Neoproterozoic Granitic-Gneissic Dom Feliciano Belt. Southern Brazil 27 1

because it is not feasible to produce an anorthosite magma

from pre-existing continental crust.

Pinheiro Machado and Camboriri Complexes. We analysed whole-rock samples RS-7A, RS-7B, RS-7C, RS-9A, and RS-9B from the Pinheiro Machado complex in

Rio Grande do Sul (D, E, F, G, and H, respectively in Fig.

8) and samples SC-52 and SC-54-I from the Camboriu

complex in Santa Catarina (N, 0, respectively in Fig. 9); all

samples display the same flat-lying fabric. These seven

samples cluster in three groups. The samples with the old-

est To, ages (7A: 2.01 Ga, 9A: 2.09 Ga, SC-52: 2.25 Ga,

and SC-54-I: 2.16 Ga) apparently represent the oldest pro-

tolith for rocks in these outcrops. However, since zircons from 9A are near the lower end of discordia at ca. 600 Ma,

this gneiss either represents an early Brasiliano granitoid derived from Transamazonian basement or Transamazo- nian basement gneiss in which zircons were nearly com-

pletely reset during the Brasiliano orogeny. The typical

igneous morphology of the zircons from 9A suggests that the former alternative is the best interpretation.

The two Brasiliano metagranite samples (RS-7C and 9B) have less negative e&600) and significantly younger

To, ages than those of the two gneiss samples (RS-‘IA and

9A) from the respective outcrops, even though their ~~~(0) values are similar (Fig. 8). In this case the interpretation is

I

1 ? 3

T GO Fig. 8. ~~~(600) diagram for rocks from Rio Grande do Sul in the Dom Feliciano Belt: A = RS-2; B = RS-4; C = RS-5; D = RS-7A; E = RS-7B; F = RS-7C; G = RS-9A; H = RS-9B; I = RS-12; J = RS- 13-B. Dashed lines represent extrapolations of Nd evolution prior to 600 Ma assuming no Sm/Nd fractionation during the Bra- siliano orogeny.

0 1

T @a)

3

Fig. 9. ~~~(600) diagram for rocks from Santa Catarina in the Dom Feliciano Belt: L = SC-42-l; K = SC-42-B; M = SC-43; N = SC-52; 0 = SC-54-l. Dashed lines represent extrapolations of Nd evolution prior to 600 Ma assuming no Sm/Nd fractionation during the Brasiliano orogeny.

more ambiguous. If it is assumed no REE fractionation

occurred during magma genesis, then the protolith for the

metagranites could have been Mesoproterozoic basement.

However, 1.2 to 1.6 Ga basement has not been recognized

in this region, so other possibilities must be considered.

The most likely explanation is that magmas for 7C and 9B came from protolith with an age similar to that for 7A and

9A, but with slightly higher 147Sm/144Nd ratios (more

mafic, deeper equivalents?). At 600 Ma the ~~~(600) for these protoliths would have been slightly less negative than for the exposed gneisses, 7A and 9A. During magma gen-

esis significant REE fractionation then occurred, producing distinctly lower ‘47Sm/‘44Nd ratios and resulting in fortui-

tously similar ~~~(0) values for the granites (F, H; Fig. 8) and their host gneisses (D, G; Fig. 8). A second alternative

is that the magma for 7C and 9B was derived from a source

nearly identical to that for 9A and 7A, but that it was mixed

with a small amount of fluid (magma’?) from a more juve-

nile source. In both cases the To, ages of ca. 1.4 Ga would be artifacts of the chemical changes that occurred during magma genesis.

The migmatitic gneiss sample, RS-7B, could represent a rock intermediate between the extremes of the gneisses and granites, or it could have had a more complex history. In any case, the zircons from this sample also indicate that it, like RS-9A, represents granitoid derived from Transama- zonian basement.


Syn-tramcurrent and late- to post-tectonic granites. We also recognized three major groups in these rocks.

Three samples show the oldest To, ages (RS-4: 2.08 Ga,

RS- 12: 2.37 Ga, and SC-42-11: 2.18 Ga) that may represent

the oldest protolith of the syn- to late- strike-slip related

granites. The only sample with U/Pb zircon data, the

Encruzilhada do Sul Granite (RS-4), contains evidence of inheritance of older Proterozoic zircon (Fig. 4). As we

mentioned previously, the Encruzilhada do Sul granite has

a high 87Sr/86Sr initial ratio and a metaluminous character.

In addition, a Transamazonian model age (TDM) and very negative Ed,, values (Figs. 8a, 8b) indicate that these Brasil-

iano granites were derived from Paleoproterozoic base-

ment.

A second group represents samples with To, ages of ca.

1.3-l .4 Ga (RS- 13-B and SC-42-I) that may either corre-

spond to remelting of older Transamazonian material simi-

lar to that for the first group, but with significant REE

fractionation occuring during magma genesis, or to mixing

between a Transmazonian basement source and a small amount of fluid from a more juvenile source. Two zircon

fractions of the RS-13-B show Pb-inheritance (Fig. 5), but the data are insufficient to define the age of the protolith.

A third group includes two samples with intermediate

T or,,, values (RS-2: 1.75 Ga, and SC-43: 1.58 Ga), similar

to the sample RS-7B. Like RS-7B, they may have had a

more complex history; no zircon data are available for this

group.

DISCUSSION AND CONCLUSIONS

The present study allows us to define precisely the main

tectonic phases of the ca. 600 Ma Dom Feliciano event

which affected the rocks in the Dom Feliciano belt. U-Pb

zircon data for flat-lying gneisses yield ages of 6 10 f 5 Ma and 616 + 2 Ma, which we believe corresponds to the

approximate age of thrusting. A whole-rock Rb/Sr isochron

age of 783+29 Ma for outcrop RS-7 (Soliani Jr., 1986) may

represent a mixed age. In this case, the characterization of

a ca. 800 Ma magmatic arc for this region, as interpreted by Fragoso-Cesar (1991) and Fernandes et al. (1992), cannot be supported by our U-Pb zircon data.

The strike-slip deformation (main transcurrent phase) is well dated by the U-Pb zircon age of 595 f 1 Ma for the syn-transcurrent Arroio Moinho Granite, which shows par-

allel state-solid and magmatic flow foliation. Because the Encruzilhada do Sul granite has a similar zircon age (594 + 5 Ma), we assume that this granite is also a syn- transcurrent granite and was probably formed in the tran- stensional enviroment along the main strike-slip Canguqu Shear zone, as concluded by Fragoso-Cesar (1991) based on field work.

These results point out to a relatively rapid evolution, from 623 Ma (gneiss RS-7B) to 594 Ma (syn-transcurrent granite, RS1311), for the known tectonic phases (thrust related and strike-slip related) of the Dom Feliciano belt.

The Sm-Nd results can be considered in three major

groups. The first of these (I) includes Brasiliano gneisses, granitoids, and one anorthosite with To, ages of 2.0 Ga

(RS-4, RS-5, RS-9A, RS-12, SC-42-11, SC-52, and

SC-54-I) and very negative EN, (600) values. The gneisses

and granitoids may represent direct melting of a Transama-

zonian (Paleoproterozoic) basement, which was generated

during the most important erogenic cycle that preceeded the Brasiliano orogeny in southern Brazil

The second group (II) includes granitoids and gneisses wtth To, model ages from 1.31 to 1.41 Ga (RS-7C, RS-OB, RS-I3-II, SC-42-I). The third group (III) com-

prises samples with To, ages between 1.58 and 1.75 Ga

(RS-2, RS-7B, SC-43). For both cases it is clear that these

rocks or their protoliths represent pre-Brasiliano continen-

tal crust. However, unlike the case for Group I, Groups II

and III may contain a small fraction of a juvenile Brasil-

iano material, although we have not yet found any sample

from the Dom Feliciano belt with a Brasiliano T,, age and

positive ~~~ value at 600 Ma.

Comparison of these geochronological results with

those from the Vila Nova belt, in which the main erogenic

process was from 753 to 704 Ma (Chemale et al., 1994a), allows us to conclude that the Vila Nova belt was stable for over 100 Ma before the Dom Feliciano event reached its

peak. It is probable that the collage of terranes in the Dom

Feliciano belt and the region comprised by the Tijucas and

Vila Nova belts was formed during the main tectonism of

the Dom Feliciano event (ca. 600 Ma).

Acknowledgements - The research was supported in part by fundmg

from the U.S. National Science Foundation and from CNPq in Brazil. We

wish to thank Mr. Allen Fetter and Mr. Elton Dantas for assistance during

the laboratory work. We also wish to thank Dr. Marcio Pimentel and Dr

Stephen Moorbath for very helpful comments on an earlier version of the

manuscript.

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APPENDIX

Sample Location and Description

Rio Grande do Sul State. Piquiri Syenite (RS-2) (quarry outcrop 13.2 km SW of Hwy. 290 on a road which

leaves Hwy. 290 near km 257). This outcrop consists of red

syenitic rocks with >60 percent K-feldpar (orthoclase).

Hornblende and clinopyroxene are minor components;

quartz, microcline, plagioclase, biotite, sphene, ilmenite,

apatite and zircon are accessory minerals. A primary igne-

ous foliation, which is marked by alignment of feldspars

and mafic stringers, is the most conspicuous fabric of this

syenite.

Encruzilhada do Sul Suite (RS-4) (3OO25.22’ S; 52O28.03’ W; roadcut on road RS-233 between Pantano

Grande and Encruzilhada do Sul at km 211.6, 11.5 km

from town of Encruzilhada do Sul). This outcrop consists

of subalkaline syenogranite. Mesoscopically the rocks do

not show any deformation, although we recognized some

hydrothermal alteration (epidote). Microscopically, the

quartz and feldspar crystals display undulose extinction,

indicating that this rock was subjected to some deforma-

tional process.

Capivarite Anorthosite (RS-5) (3OO18.64’; 52O24.56’; outcrop 4.7 km from road RS-233 on a road which leaves

RS-233 near km 198.9, 26.6 km from town Encruzilha do

Sul). In this outcrop the anorthosite is associated with some lenses of talc-silicate rocks and amphibolite that have a

flat-lying fabric formed in upper amphibolite conditions. We collected a sample of anorthosite (RS-5) which is com- posed of >90 % plagioclase (labradorite) with diopside,

hornblende, sphene and quartz as accessory minerals.

Pinheiro Machado Complex (RS-7) (31O37.43’ S; 53O23.13’ W; outcrop at the western entrance to town of

Pinheiro Machado). This outcrop consists of at least four granitic phases: (a) blastomylonitic diorite that occurs as

centimeter lenses (Sample RS-7A) is the oldest phase; (b) grey migmatitic granodioritic gneiss with flat-lying banding (foliation), stretched matic stringers and K-feldspar porphyroclasts (up to ca. 5 cm diameter) (RS-7B); (c) fine grained, deformed syenogranite which cuts the previous phase (RS-7C); and (d) 20 cm wide aplitic dykes oriented at N40’E and N-S, which is the youngest granitic phase. There are also meter-size xenoliths of quartzite and calc- silicate rocks.


Pinheiro Machado Complex (RS-9) (3 l”36.70’;

53O17.22’ W; roadcut on Hwy Br-293, 9.3 km from Pin-

heiro Machado). This outcrop consists of polydeformed

banded gneiss with some layers of the foliated grey,

migmatititic granodioritic gneiss similar to that at the RS-7

outcrop. There are some Encruzilhada do Sul “dykcs”

showing assimilation (forming biotite and amphibole) as

well as a migmatitic phase. We collected biotite-rich

migmatitic gneiss (sample RS-9A). a foliated metasy-

enogranite (RS-9B), and biotite-poor miglnatitic gneiss

(RS-9C). The tectonic fabric is related to the tangential

event, including folding of layering. A 2 m-thick quartz

vein is present, injected into the gneisses; it is also strongly

affected by the transcurrent fabric.

CapEo do Letio Monzogranite (RS-12) (quarry outcrop

at the Capso do Lego locality, - 3.0 km from Hwy Br-293,

83 km from Pinheiro Machado). This unit is an unde-

formed granitoid with garnet as an accessory mineral; it is

metaluminous in composition, and it cuts the Pinheiro

Machado Complex.

Arroio Moinho Granite (RS-13) (31O30.79’ S;

152~36.27’ W; outcrop in a quarry along Hwy Br-392, 5 km SE of town of CanguGu). This unit is a strike-slip

related granite. Two samples were collected from this outcrop: a pink syenogranitc facies (RS- 13-I) and a gray monzogranitic facies (RS- 13-11). The outcrop displays

magmatic-flow and solid-state planar and linear fabrics

related to the strike-slip deformation.

Santa Catarina State. Pedras Grandes Suite (SC-42)

(outcrop in quarry in TubaGo city, 400M W of km post 338 on Hwy Br-101). This outcrop consists of non-deformed

granites of this suite. We collected samples of two facies.

One is a pink, coarse-grained, equigranular to porphyritic leuco-granite with accessory biotite (SC-42-11); it is the

dominant granitic facies in the suite. The other facies is a

mesocratic, medium-grained, gray granodiorite (SC-42-I)

with abundant mafic clots. Some preferential alignment of

mafic minerals can be observed, which is interpreted as

magmatic flow structure. Some meter-size amphibolitc enclaves also occur in this unit.

Paulo Lopes monzogrunite (SC-43) (outcrop in quarry

in Paula Lopes town, along west side of Hwy Br- 101 at km

249.8). This is probably a strike-slip related body. It is characterized by megacrysts that may add up to 4&60%) of

the rock volume; the megacrysts range from idiomorphic

to rounded (porphyroclasts). The megacrysts (augen) are

flattened and stretched parallel to a well-defined subverti-

cal banding. The matrix is medium-grained, gray, and rich

in biotite that may wrap around the porphyroclasts, forn-

ing pressure shadows.

Cuabiruba metaleucogranite (SC-52) (Quarry along- side BR-101 in the Serra de Camborit’i, close to resort Bal-

nekio Camborid). This is a fine to medium-grained, two- mica peraluminous metaleucogranite that cuts migmatitic

gneisses (SC-54); both units belong to the Camborili complex.

Cumboriu’ Complex (SC-54-l) (outcrop in Cesarna

Quarry in the Balnejrio Camborili). This unit consists of

migmatitic gneisses with agmatitic structure comprised 01

fine-grained metagranite and pieces of banded gneiss. A Hat-lying fabric is recognized as the main deformational

structure.

u-pb and sm-nd geochronology of the neoproterozoic granitic-gneissic dom feliciano belt, southern...

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