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Precambrian Res., 42:213-227. The available geochronological data for the Amazonian Craton permit delineation of its main age provinces and their respective ...
Precambrian Research, 42 {1989) 213-227

213

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

A REVIEW OF THE GEOCHRONOLOGY OF THE AMAZONIAN CRATON" TECTONIC IMPLICATIONS WILSON TEIXEIRA, COLOMBO CELSO GAETA TASSINARI, U M B E R T O G I U S E P P E CORDANI and KOJI KAWASHITA Geoscience Institute, University of S?to Paulo, P.O. Box 20889, Sfto Paulo (Brazil) (Received November 1, 1987; revision accepted January 1, 1988 )

Abstract Teixeira, W., Tassinari, C.C.G., Cordani, U.G. and Kawashita, K., 1989. A review of the geochronology of the Amazonian Craton: tectonic implications. Precambrian Res., 42:213-227 The available geochronological data for the Amazonian Craton permit delineation of its main age provinces and their respective tectonic development. Demarcation of the boundaries of each age province is primarily based on RbSr whole-rock isochrons, supported by K-Ar determinations on minerals. Some U-Pb results on zircons, Pb-Pb whole-rock isochrons, and Sm-Nd model ages complement these data in key areas. At the center of the Amazonian Craton lies the Archean age, Central Amazonian Province bounded in its northern part by the Maroni-Itacaiunas mobile belt, which developed during early Proterozoic times. The western and southern margins of the Archean core are bordered by three tectonic provinces; ( 1 ) the Rio Negro-Juruena magmatic arc, (2) the Rondonian mobile belt, and (3) the Sunsas mobile belt. Each belt borders the western margin of the preceding belt, and each records a successively younger middle Proterozoic orogeny. Within the Central Amazonian Province, -2.7 Ga age determinations have been obtained from the Serra dos Carajds area. Although the Maroni-Itacaiunas belt includes several reworked Archean nuclei, rock units formed during the 2.2-1.9 Ga Trans-AmazonianOrogeny predominate. The isotopic evidence suggests that a mantle-derived component constituted a significant portion of the crust accreted at that time. The Rio Negro-Juruena Province is of Mid-Proterozoic age (1.75-1.50 Ga), and includes almost entirely mantlederived material, juxtaposed with the pre-existent continental mass along a series of successive magmatic arcs. The Rondonian and Sunsas provinces both exhibit older sialic basements (> 1.5 Ga). The former is characterized by important tectonomagrnatic events in the 1.45-1.25 Ga interval, whereas the latter records tectonic reworking at 1.10.9 Ga. Finally, the Amazonian Craton is bordered to the east by the late Proterozoic Paraguai-Araguaia (or AraguaiaTocantins) fold belt.

Introduction

Over the last five years, much progress has been made towards geochronological characterization of the Amazonian Craton, thanks to the efforts of IGCP Project 204 (1983-87), and of the following institutions involved in this research: the Geochronological Research Center 0301-9268/89/$03.50

of the University of S~o Paulo (Brazil), the Isotopic Geochemistry Laboratory of the University of Pardi (Brazil), the Department of Earth Sciences of the University of Oxford (U.K.), the Z.W.O. Laboratorium voor Isotopen - Geologie of Amsterdam (Netherlands), and the Universities of New Hampshire, Kansas and Princeton (U.S.A.).

© 1989 Elsevier Science Publishers B.V.

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IGCP Project 204 was designed to solve some of the controversial aspects of the evolution of the Amazonian Craton, and to evaluate the preexisting available geological data through an integrated review. Strong geological interchange, including some field excursions in selected areas, detailed geochronological control; petrological, structural and geochemical studies, were incorporated in the aims of the program. Additionally, attention was also drawn to the use of available geophysical and paleomagnetic data. Over the course of Project 204, scientific interchange has been assured between scientists from developing countries in South America (Brazil, Venezuela, Colombia, Chile, Bolivia, Guyana, Surinam, French Guiana) and scientists from Europe and the U.S.A. Detailed geological knowledge of the Precambrian units within the Amazonian Craton is very patchy. Notwithstanding the great progress made in understanding the geology in the past decade, reconnaissance studies still predominate. More detailed studies are available only for areas where mineral exploration exists. The Amazonian Craton is host to important deposits of Fe, Au, A1, Sn, Cu, diamond, REEs and Mn. The Amazonian Craton (Fig. 1 ) is made up of the Guapore and Guiana shields. Its basement rocks are part of the South American platform and are partially overlain by the Amazon sedimentary basin. As described by Cordani and Brito Neves (1982), it is a very large unit acting as a stable foreland in late Proterozoic times for the Paraguai-Araguaia marginal fold belt (Araguaia-Tocatins belt of Herz et al., 1989). The central part of the craton is characterized by extensive sequences of mildly deformed and marginally metamorphosed volcanic and sedimentary units which were deposited after the 2.2-1.9 Ga Trans-Amazonian Orogeny. About half of the craton is covered by these middle Proterozoic intracratonic basins, which

7o°

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Central Brazil Shield

~

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J ~

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Fig. 1. The Amazonian Craton. Heavy line is the western edge of the Paraguai-Araguaia marginal belt. Key: 1 Guiana Shield; 2 - Guapor~ Shield, which belongs to the Central Brazil Shield.

are intruded by mafic and granitoid rocks. The felsic magmatic rocks of the middle Proterozoic are particularly extensive and well exposed. The region around Carajas ridge (Fig. 2) is underlain by Archean granite-greenstone terranes and high-grade metamorphic rocks, but to the north (Venezuela, Guyana and Brazil) the basement is predominantly Trans-Amazonian in age with some other Archean fragments. Recently, debate on the geotectonic evolution of this craton has focused on whether the craton is a large Archean platform, partially reworked and reactivated during the Proterozoic (e.g., Amaral, 1974; Moltalv~o and Bezerra, 1985; Santos and Loguercio, 1984; Jones, 1985; Hasui and Almeida, 1985) or whether its evolution is punctuated by episodic crustal accretion during the Proterozoic (e.g., Cordani et al., 1979; Gibbs and Barron, 1983; Gaudette and Olszewski, 1985; Teixeira et al., 1986). A review of the available radiometric data of the Amazonian Craton and their implications with respect to its tectonic evolution are the subject of this paper.

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, 2.5 Ga ); 8 - t r a n s i t i o n zone b e t w e e n belts; 9 - a p p r o x i m a t e c o n t a c t s b e t w e e n belts; 10 - n a t i o n a l boundaries. A - Amap~i Territory; R R - R o r a i m a Territory.

216

R e v i e w of Geochronology With respect to the Amazonian Craton, ~3000 age determinations are available by various methodologies, and many of these results have already been published. Most of these data are Rb-Sr and P b - P b whole-rock analyses and K-Ar mineral dates, but some U - P b analyses were performed on zircons. In addition, a few S m - N d age determinations have been carried out in recent years, some of them on sediments from the Amazon basin. This isotopic database allows the Amazonian Craton to be partitioned into geochronological provinces, each of which is characterized by a unique geodynamic event during a period of the geological time (e.g., KrSner, 1981; Moorbath and Taylor, 1981). In this paper we define a geochronological (or radiometric) province as a region, typified by a coherent age pattern over all of its areal extent. For example, the ages of units belonging to a single province could record the evolution of a mobile belt, active during a single tectonothermal episode. A province could also exhibit unique structural trends, complex deformation, faulting, and episodes of granitization and migmatization which can be interpreted either as juvenile accretion or crustal reworking (see, e.g., Cordani et al., 1979). Our review of the geochronology will therefore be presented according to the prior defined geochronological provinces, as follows: (1) the geographic/geologic distribution of the radiometric age patterns for each province; (2) the ages found for the basement rock units and the associated metamorphosed supracrustal sequences; (3) the ages for the overlying volcanic and sedimentary sequences; and (4) the time intervals for orogenic granite and anorogenic basic to alkaline intrusive magmatism. The Amazonian Craton (Fig. 2) can be divided into an ancient nucleus, the Central Amazonian Province, which is surrounded by early to middle Proterozoic provinces: Maroni-

Itacaiunas, Rio Negro-Juruena, Rondonian and Sunsas (e.g., Cordani et al., 1979; Teixeira et al., 1986). Central Amazonian Province

This domain is characterized by cratonic conditions existing since the early Proterozoic, with widespread occurrences of undeformed and unmetamorphosed volcanic and sedimentary sequences, 1.9-1.4 Ga in age (Fig. 2), overlying Archean to early Proterozoic basement. In the Serra dos Carajds area in the southeastern section of the Central Amazonian Province (Fig. 2), the basement complex is composed of Archean granite-greenstone terranes and high-grade metamorphic rocks, which include orthogneiss, migmatites, granites, granodiorites, adamellites, diorites and tonalites, with minor occurrences of metasediments and metabasic and ultrabasic igneous rocks, such as that of the Salobo Formation (Hirata et al., 1982). The unconformably overlying supracrustal rocks correspond to the Gr~o Pard Group, which forms a broad synclinorium with a N N W - S S E trending axis. This sequence contains greenschist-grade basaltic to rhyolitic metavolcanic units at the base, overlain by banded iron formation, and topped by an alluvial sedimentary sequence. Rb-Sr Archean ages were obtained on the granite-greenstone terranes (2750 Ma) and on the Salobo Formation {2700 Ma) (Tassinari et al., 1982 ). U - P b zircon analyses from saprolitic alteration of rhyolites in the Gr~o Pard Group indicate that the volcanism and iron deposition occurred 2758 + 39 Ma ago. The (Wirth et al., 1986) U - P b results are in agreement with the Rb-Sr age of 2687 + 54 Ma for the metabasalts of the Gr~o Pard Group (Gibbs et al., 1986; O1szewski et al., 1989). Rb-Sr measurements of 2640+40 Ma (I.R.--0.7009) and 2564+64 Ma (I.R.-0.70288) for the Rio Maria granitoids, and of 2480±40 Ma (I.R.--0.7072) for the gneisses cropping out along the Itacaiunas river, near

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Caraj~s, were reported by Montalv~o et al. (1984) and Pinto Gastal et al. (1987). The K Ar mica and amphibole regional cooling age pattern for most of the Central Amazonian Province span the 3.3-1.75 Ga time interval (Gomes et al., 1975; Tassinari et al., 1982 ). The unmetamorphosed platform sequence is comprised of sedimentary deposits overlying acid to intermediate volcanics. The former are related to the Roraima Group, which occurs extensively in northern Brazil, Venezuela and Guyana (RR in Fig. 2), and to the Gorotire, Rio Fresco and Beneficente groups, which occur in the southern part of the Amazonian Craton just west of the Serra dos Carajas region. These sediments are exposed mainly as 'mesas', with gentle dips which steepen in proximity to block faults or intrusions, and include orthoquartzites, arkoses, conglomerates, minor shales and jaspilitic tufts. These tufts have yielded Rb-Sr ages of 1.75-1.66 Ga (Priem et al., 1973; Gaudette and Olszewski, 1985). The volcanic sequences, referred to as the Cuchivero Group in Venezuela (Mendoza, 1972; Rios, 1972 ), the Uatuma Group in Brazil (Santos, 1984), and the Dalbana Formation in Suriname (Kroonenburg and deVletter, 1988), yield Rb-Sr whole-rock dates ranging from 1.9 to 1.8 Ga (Priem et al., 1971; Basei and Teixeira, 1975; Gaudette et al., 1978). An extensive suite of middle Proterozoic dikes and sills of continental tholeiite type intrude the sedimentary and volcanic formations and, together with the available geochronological data for the volcanic formations, help to constrain the depositional age of the Roraima Group. Rb-Sr ages ranging from 1.81 to 1.66 Ga have been reported for these basic intrusives (e.g., McDougall, 1968; Basei and Teixeira, 1975; Ascanio and Lopez, 1984). Recently, Ar4°-Ar 39 measurements carried out on diabase dikes in southern Guayana have suggested a minimum age of 1.84-1.80 Ga for the Roraima Group there (Onstott et al., 1984).

Maroni-Itacaiunas Province

This domain is very widespread (Figs. 2 and 3), extending into French Guiana, Suriname, Guyana and parts of Brazil (Amap~, Roraima, and north of Carajas) and Venezuela (Bolivar state and Amazonas federal territory), and contains a vast expanse of early Proterozoic crust (Gibbs and Olszewski, 1982; Gibbs and Barron, 1983; Teixeira et al., 1985; DeVletter, 1984). The province shows a strong structural coherence, although towards the southeast in Brazil and northwest in Venezuela, its contacts with the Central Amazonian Province are poorly constrained owing to limited geologic information and lack of geochronological data. This tectonic entity can be subdivided into; (1) granulitic and gneissoid terranes, and (2) early Proterozoic ( 2.25-1.9 Ga ) granite-greenstone terranes (see fig. 5 of Cohen and Gibbs, 1989). Some of the granulite terranes have Archean protoliths, such as the Imataca Complex in Venezuela, and the Cupixi and Tumucumagne areas in Amap~i, Brazil (A in Fig. 2), and some of them do not, such as the Central Guiana Granulite Belt of Gibbs and Barron (1983). The Archean basement, which occurs as scattered nuclei within the Proterozoic terranes, is made up of high-grade polymetamorphic rocks, partially reworked during the Trans-Amazonian Orogeny. The geochronological studies reveal a subordinate simatic component involved in the reworking of these remnants (e.g., Priem et al., 1978), whereas the granite-greenstone terranes appear to have originated through mantle-derivation processes during early Proterozoic time (Gibbs and Barron, 1983; Gruau et al., 1985; Teixeira et al., 1985; DeVletter, 1984). Montgomery (1979) and Montgomery and Hurley (1978) reported 3.4 Ga U-Pb wholerock ages for the Imataca Complex of Venezuela. More recently, Rb-Sr geochronological studies (Lima et al., 1982) of the tonalitic and

218

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Fig. 3. Geologic outline of the Maroni-Itacaiunas mobile belt. Key: 1 - Undifferentiated basement rocks of the Central Amazonian cratonic province ( 2 . 2 5 - 1 . 9 G a ) ; 2 - Platform covers ( 1 . 9 - 1 . 7 G a ) , sandstones of Roraima type and acidintermediate volcanics; 3 - plutonic rocks associated with volcanism; 4 - structural trends, mainly gneisses, migmatites and granitoids; 5 - pre-existing granulitic nuclei (3.0-2.9 Ga and > 2 . 2 G a ) ; 6 - supracrustal rocks including greenstone belts ( 2 . 2 5 - 2 . 1 G a ) ; 7 - Phanerozoic sediments.

trondhjemitic rocks from the Amapa Territory (Brazil) in the Cupixi area yielded an age of 2860+60 Ga (I.R.=0.7025), while the granulitic rocks gave a 2450+ 74 Ga (I.R.=0.7063) Rb-Sr isochron age. The Maroni-Itacaiunas belt accounts for a large portion of the Amazonian Craton supracrustal rocks identified as metavolcanic and metasedimentary sequences (Fig. 3 ). These include the Carichapo-Pastora Group in Venezuela, the Barama-Mazaruni Group in Guyana, the Orapu and Bonidoro groups in French Guiana and possibly part of the Vila Nova Group in Brazil, all of which are metamorphosed from lower greenschist to amphibolite

facies, and intensely deformed. The nature, composition and evolution of these rocks have led some authors to consider them as early Proterozoic analog of Archean greenstone belts (e.g., Gibbs, 1980). The radiometric data available on these supracrustal units and the granites intruding them include U-Pb, Rb-Sr, Sm-Nd, Pb-Pb and KAr analyses. Representative studies have been carried out in Venezuela, French Guiana, Guyana, and in the Brazilian Roraima and Amap~i territories (Figs. 2 and 3). The geochronological concordancy of these units across the entire domain of the Maroni-Itacaiunas Province is remarkable. This is illustrated by the recent re-

219

suits reported from supracrustal units in French Guiana (Table I). All of the ages are very concordant and apparently record a formation, metamorphism, intrusive plutonism and regional cooling, within a 200 Ma period of time extending from 2.1 to 1.9 Ga. However, U - P b zircon geochronological studies carried out in Venezuela have suggested the occurrence of two different igneous thermal episodes during the early Proterozoic: one extending from ~ 2.2 to 2.05 Ga ago and the other extending from 2.0 to 1.8 Ga ago (Klar, 1979). The earlier episode is consistent with 2.25 Ga U - P b zircon ages reported in neighboring Guyana, from plutonic clasts within the Barama-Mazaruni (Gibbs, 1980; Gibbs and Olszewski, 1982). We have defined the Maroni-Itacaiunas belt to include the Central Guyana Granulitic Belt (CGGB), which in the Roraima territory of Brazil yields a Rb-Sr whole-rock isochron age of 1908 + 48 Ga, and an initial STSr/S6Sr value of 0.7005 (Lima et al., 1986). The high-grade metamorphic rocks of the Fallawatra Group (the northeastern extension of the CGGB into Suriname, have also been dated (Priem et al., 1978), with Rb-Sr results in the 2.2-2.0 Ga range. Othman et al. (1984) reported 2.3 Ga N d - S m model ages on granulites from the Bakhuis mountain (Suriname) and 2.2 Ga N d - S m TABLE I Synthesis of isotopic data on Precambrian units from French Guiana Units

Rocks

Isochron ages (Ga)

Isotopic parameters

L'lle de Cayenne

Migmatites, granulites Volcanosedimentary sequences Syntectonic granites Latetectonic granites

Rb-Sr=2.0

I.R.=0.7018

S m - N d - - 2.1 Rb-Sr = 2.0

end----+2.1 I.R. = 0.7024

P b - P b = 2.1 Rb-Sr = 2.0 Rb-Sr = 1.9

u, -- 8.095 I.R. -- 0.7020 I.R. -- 0.7024

Paramac~

Guyanais type Caraibe type

Pegmatoids: K - A t and Rb -Sr ages: 1.96-1.9 Ga and 2.1-2.0 Ga. Regional K - A t cooling pattern ~ 1.9-1.8 Ga. References: Gruau et al. (1985); Teixeira et al. (1984, 1985).

model ages on the Kanuku granulites (Guyana). Further west in the southwestern part of Venezuela, U - P b and Rb-Sr radiometric analyses on medium- to high-grade gneisses and granitoids from the Amazonas territory of Venezuela (Gaudette and Olszewski, 1985) reveal slightly younger ages, consistent with regional metamorphism followed by intrusions between 1.86 and 1.76 Ga.

Rio Negro-Juruena Province In southwestern Venezuela and northwestern Brazil, the E - W trending Maroni-Itacaiunas belt is truncated by the N W - S E striking structural trends of the Rio Negro-Juruena Province (Fig. 4) (Tassinari, 1981). The latter province is composed predominantly of granitic to granodioritic rocks (gneisses, migmatites, granodiorites and tonalites), many showing gneissoid structure. In general, metamorphism attained amphibolite facies with minor occurrences of granulite facies assemblages (Tassinari, 1984; Hasui and Almeida, 1985). Although the dominant structural trend is NW-SE, a N E - S W overprinting has been observed in some areas (e.g., Lima, 1984; Lima et al., 1986). The most recently published radiometric results of the Rio Negro-Juruena Province (Table II) are from the Pico da Neblina region (P in Fig. 4), and the Aripuan~ region (A in Fig. 4). The available isotopic dates (Rb-Sr, P b Pb, U - P b methods) from the two widely dispersed regions of the Rio Negro-Juruena Province are remarkably coherent, ranging between 1.75 and 1.60 Ga (Table II) and demonstrate that the evolution of the belt took place in the middle Proterozoic (Tassinari, 1981, 1984; Gaudette and Olszewski, 1985). These rocks show Sr and Pb isotopic parameters which are consistent with the basement crust originating from magma which separated from the upper mantle no earlier than 2.0 Ga. Associated with the evolution of the Rio Negro-Juruena Province is the 1.65-1.6 Ga acid

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-,, 1.5 Ga. The basement complexes of these provinces include migmatites and granitic to granodioritic rocks, mainly metamorphosed to amphibolite facies (Litherland, 1981; Jones, 1985; Hasui and Almeida, 1985; Leite, 1987 ). The high initial ratios suggest a dominantly ensialic character to the basement rocks within the Rondonian belt (Table III). Its main metamorphic phase occurred between 1.45 and 1.25 Ga ago. Further west, the 'Sunsas Orogenic Cycle' has been reported as being the result of erosion of older rock units nearby, deposition, subsequent deformation and metamorphism, followed by an episode of granitic plutonism (Litherland, 1986). The age of this orogeny is defined by 1.10.9 Ga Rb-Sr and K-Ar ages on both syntectonic and late-tectonic intrusive granites (Teixeira and Tassinari, 1984; Litherland et al., 1986; Priem et al., 1987). Low-grade supracrustal sequences and undeformed sedimentary deposits of the Palmeiral, Uopione and Pacaas Novos formations are situated within the Rondonian Province (Fig. 5 ). Their ages are estimated to lie between 1.4 and 1.1 Ga, based on K-Ar and Rb-Sr age determinations carried out on rocks intrusive into these formations. Moreover, two periods of anorogenic granitic activity along the Rondon-

222

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Fig. 5. Geologic outline of the Rondonian and S u n s a s mobile belts and their m a i n r a d i o m e t r i c r e s u l t s (in G a ) (modified f r o m J o n e s , 1985; H a s u i a n d A l m e i d a , 1985; L i t h e r l a n d , 1986). Key: 1 undifferentiated gneissic and migmatitic b a s e m e n t ; 2 - h i g h - g r a d e m e t a m o r p h i c c o m p l e x e s ( L M = L o m a s M a n e c h e s area); 3 - a s s o c i a t e d g r a n i t i c rocks; 4 - S a n Ignacio C o m e m o r a c i o n s c h i s t belts; 5 - Vibosi, A g u a p e i , S u n s a s groups; 6 - Rondonia granites; 7 - v o l c a n i c - p l u t o n i c C a r i p u n a s suite; 8 - s e d i m e n t a r y s e q u e n c e s ( P = P a l m e i r a l ); 9 - b a s a l t lavas; 10 - C i r i q u i q u i a n o r t h o s i t e ; 11 - P h a n e r o z o i c cover; 12 s t r u c t u r a l t r e n d s ; 13 - m a j o r faults; 14 - S u n s a s orogenic domain; 15 - p r e - R o n d o n i a n age nuclei ( I = I t u x i a r e a ) . A = Abun~; Ar = A r i q u e m e s ; D = D i a m a n t i n a .

ian and Sunsas belts are defined: 1.25 Ga and 1.0-0.95 Ga (Teixeira and Tassinari, 1984).

Granitic plutonism Granitic plutonism is widespread throughout the entire Amazonian Craton. The limited geologic information available precludes a systematic discussion about their genesis in each of the geochronological provinces. Nevertheless, con-

sidering the characteristics of the granite formed in different periods or different tectonic settings, a tentative subdivision of them into five groups has been made (e.g., Snelling and McConnel, 1969; Santos, 1982; Dall'Agnol et al., 1986, 1987; Tassinari et al., 1984; DeVletter, 1984): Archean granitoids; Trans-Amazonian age granitoids; middle Proterozoic (1.8-1.4 Ga) anorogenic granites of the Central Amazonian Province; middle Proterozoic (1.7-1.4 or 1.2

223 TABLE III

Main Rb-Sr radiometric results on rocks from Rondonian Province Location a

Rocks

Isochron ages (Ga)

Isotope parameters

Ituxi (I) Abun~ (A)

Granulites Granite-

1515_+28 1520 _+24

I.R.=0.707 I.R. = 0.705

1440_+21

I.R.=0.705

1375-+80

I.R.=0.7052

1391-+70 1960

I.R.=0.7004 I.R. = 0.703

Ariquemes {Ar) Diamantina (D)

Lomas Maneches

gneisses Granitegneisses Syntectonic granites Granulites

aSee Fig. 5K - A t cooling pattern ~ 1.35-1.40 Ga and 1.10-0.95 Ga. References: Teixeira and Tassinari (1984); Litherland et al., ( 1986 ); Carneiro (1985).

! TIME i (6o)

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MARONI ITACAIUNAS

RIO NEGRO JURUENA

RONDONIAN

SUNSAS

Q8-

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1.2

zonian, Rio Negro-Juruena and Rondonian provinces are contemporaneous with orogenic activity in the younger, neighboring province. The good correlation between the ages of the intrusive granites and those of the volcanic rocks is illustrated in Fig. 7, with characteristic periods throughout the geological/tectonic evolution of the craton. Based on this histogram the main plutonic-volcanic activity took place during the period 1.85-1.6 Ga within the Central Amazonian Province under progressively more stable tectonic conditions. The basic (and alkalic) magmatism within the Amazonian Craton shows similar tectonic association with the geochronological provinces, as described earlier for the granitic plutonism. The Archean granitoids of the Amap~i Territory (Brazil) are the best preserved and studied. In general, they are sodic granitoids older than 2.6 Ga. In the Rio Maria area, south of Caraj~is, the granodiorite rocks are slightly

1.4-

1.6-

AGES (Go)

1.8

0.9-

INTRUSIVE GRANITES

VOLCANIC ROCKS

1.0 DISTRIBLITION OF INTRUSIVE GRANITES WITHIN THE GEOCHRONOLOGICAL PROVINCES, BASED ON THEIR AGES

1.1

Fig. 6. Age distribution of intrusive granites within each

geochronological province. 1.3-

Ga) anorogenic granites of the Rio Negro-Juruena Province; and middle to late Proterozoic (1.40-0.9 Ga) anorogenic granites of the Rio Negro-Juruena, Rondonian and Sunsas provinces. Figure 6 portrays the age distribution of the granites within each of the provinces, illustrating the coherent relationship with the proposed evolution of the craton. Each province is characterized by its own contemporaneous plutonism and no granites older than the main orogenic phase of each province are identified. Additionally, the periods of anorogenic granitic plutonism characteristic for the Central Ama-

1.41~5-

~

1.7 1.8-

1.9HISTOGRAM OF R b - S r AGES ON ASSOCIATED GRANITES AND VOLCANIC ROCKS

Fig. 7. Histogram of R b - S r whole rock isochron ages on granites and associated volcanic rocks (Brazilian territory). Lengths of lines are proportional to the number of age determinations.

224

younger but still Archean (Pinto Gastal et al., 1987). The Trans-Amazonian Orogeny granitoids are widespread in the Central Amazonian Province (Venezuela and Guyana areas, Roraima territory and Carajas region) and especially within the Maroni-Itacaiunas Province. In general, the radiometric ages are in the 2.01.9 Ga interval (Snelling and McConnel, 1969; Teixeira et al., 1985; DeVletter, 1984). The middle Proterozoic granitoids are very widespread, forming one of the largest anorogenic magmatic provinces in the world (Fig. 8). They are concentrated in the Central Amazonian and the Rio Negro-Juruena provinces. Radiometric ages lie between 1.75 and 1.4 Ga for

the Central Amazonian Province (Santos and Neto, 1982; Dall'Agnoll, 1987; Macambira et al., 1987) and slightly younger (1.65-1.0 Ga) for the Rio Negro-Juruena Province (Tassinari, 1984). The emplacement of these complexes was largely controlled by deep fractures and they seem to be formed in a stable cratonic setting, in association with successive reactivations during Proterozoic times. Finally, the middle to late Proterozoic anorogenic granites are closely related to the Rio Negro-Juruena and Rondonian tectonic evolutions. The main magmatic phases can be defined between 1.45 and 1.3 Ga, 1.5 and 1.2 Ga, and 1.05 and 0.9 Ga (Fig. 6) (Teixeira and Tassinari, 1984; Litherland, 1986). The later ones ,o

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