Neoproterozoic crustal accretion in central Brazil

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Neoproterozoic crustal accretion in central Brazil Márcio Martins Pimentel, Reinhardt Adolfo Fuck Instituto de Geosciéncias, Universidade de Brasilia Brasilia, DF 70910, Brasil ABSTRACT Recent geochronological studies have demonstrated the existence of several Neoproterozoic orthogneiss and metavolcanic arc suites (ca. 900 to 600 Ma) in the southern part of the Tocantins province, a major Neoproterozoic orogenic area in central Brazil, exposed between the Amazon and Sao Francisco cratons. Nd and Sr isotopic characteristics of these metamorphic suites are primitive: initial 8 7 Sr/ 8 6 Sr ratios vary from -0.7024 to -0.7042 and initial eNd values are positive, varying between +0.2 and +6.9. The most primitive initial Sr and Nd isotopic compositions are very similar to model depleted-mantle compositions at the time of formation of the original magmas. The mantle-like characteristics of these arc suites in western Goias contradict previous ensialic evolution models for the Tocantins province and reveal that crustal accretion processes were important in large areas of central Brazil during Neoproterozoic time. INTRODUCTION In recent years, several studies have highlighted the debate on the relative importance of crustal accretion vs. crustal reworking processes in the development of Pan-African-Brasiliano (Neoproterozoic) orogenic areas (Duyverman et al., 1982; Harris et al., 1984; Sultan et al., 1990).

These studies, carried out mainly in the PanAfrican belts of Africa, have important implications for the investigation of plate-tectonic processes during the Proterozoic (Dixon and Golombek, 1988; Kroner et al., 1990; Pallister et al., 1990), as well as for the reconstruction of Neoproterozoic continental displacements

(e.g., Hoffman, 1991; Murphy and Nance, 1991). Large areas of the Precambrian shield of Brazil are underlain by Neoproterozoic orogenic belts, developed during the so-called Brasiliano orogenic cycle (Fig. 1). Thus, the Brazilian Shield represents one of the best places to study 4 5° 00' W

GEOLOGY, v. 20, p. 375-379, April 1992

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late Precambrian tectonic processes (Cordani et al., 1988). In general, however, studies on the Brasiliano orogens are still insufficient to provide a comprehensive picture of crust formation and reworking processes in their development. The Tocantins province (Almeida et al., 1981) in central Brazil is a major Neoproterozoic orogenic zone developed between the Amazon and Sao Francisco cratons (Fig. 1). Because of the apparent lack of calc-alkaline igneous activity associated with the evolution of the province during the Neoproterozoic, some authors have suggested ensialic evolution models (Marini et al., 1984). However, recent detailed field work combined with Rb-Sr, Sm-Nd, and U-Pb geochronological studies have detected the presence of several Neoproterozoic (-900-600 Ma) metavolcanic and metaplutonic assemblages in previously poorly known areas in western and southern Goias, central Brazil (Pimentel, 1990; Pimentel et al., 1991a, 1991b; Table 1). Geochronological data alone, however, are not sufficient to discriminate between crustal accretion and reworking processes. This study uses initial Sr and Nd isotopic compositions of selected Neoproterozoic rock units from western Goias to assess the nature of the original magmas, as well as to investigate processes of crust formation and reworking in the development of these terranes. REGIONAL GEOLOGIC SETTING The Tocantins province is a Neoproterozoic orogenic area that consists, from west to east, of the following main zones (Fig. 1). 1. The Paraguai belt comprises mainly Neoproterozoic to Eocambrian sedimentary and metasedimentary rocks deposited and deformed along the eastern and southeastern margin of the Amazon craton. The depositional environment was a shallow marine platform with a submarine fan system, adjacent to the craton ("nonmetamorphic" zone in Fig. 1) and a deeper ocean basin to the east (metamorphic zone in Fig. 1; Alvarenga, 1990). Structural vergence is toward the Amazon craton. 2. The Goias Median massif represents a complex and still poorly understood association of crystalline rocks of various ages. Lack of sufficiently detailed field and geochronological studies hampers a comprehensive understanding of its constitution and tectonic significance. It has been interpreted either as a structural high of the old sialic basement underlying adjacent Neoproterozoic supracrustal belts (Marini et al., 1984) or as a complex collage of crustal segments, including pre-Brasiliano terranes (Archean and Paleoproterozoic), as well as Neoproterozoic areas (Fuck et al., 1987). We studied recently dated Neoproterozoic metavolcanic and metaplutonic associations in the southern part of the massif (area shown in Fig. 1). 376

TABLE 1. SUMMARY OF GEOCHRONOLOGICAL DATA OF ORTHOGNEISS AND METAVOLCANIC UNITS OF WESTERN GOIAS, BRAZIL

Unit

Arenöpolis gneiss

U-Pb zircon age* (Ma)

Rb-Sr isochron age (Ma)

("sr/'srlj K-Ar amphibole age (Ma)

899 ± 7

818 ± 571

0.7042

-

-

Sanclerländia gneiss

-

940 ± 150*

0.7024

Matrinxa gneiss

-

895 ± 290*

0.7025

-

arenöpolis metarhyolite

929 ± 8

933 ± 60*

0.7035

859 ± 43§

Jaupaci metarhyolite

764 + 14

587 ± 45*

0.7052

-

-

643 ± 19*

0.7032

-

Fazenda Nova metarhyolite

-

608 ± 48*

0.7032

Ban Jardim metabasalt

-

Jaupaci subvolcanic granite

* + § #

-

-

-

712 ± 53"

Pimentel et al. (1991a). Pimentel (1990). Hasui and Almeida (1970). Fragcmeni and Costa (1976).

3. The Uruafu and Brasilia belts are complexly deformed supracrustal belts formed along the western flank of the Sao Francisco craton. Structural vergence is toward the cratonic area (Marini et al., 1984). A metarhyolite interlayered with mica schists of the Araxa Group in the southern part of the Uruagu belt has been dated by the U-Pb zircon method at 794 ±10 Ma for the crystallization age of the protolith (Pimentel et al., 1991b). Terranes of oceanic affinity such as those forming an ophiolitic melange - 1 5 0 km long in the western part of the Uruafu belt probably represent vestiges of a Neoproterozoic oceanic basin (Drake, 1980; Strieder and Nilson, 1991). GEOLOGY AND GEOCHRONOLOGY OF WESTERN GOIAS Western Goias is characterized by a metamorphic basement composed largely of volcanosedimentary arc assemblages and orthogneiss terranes whose ages were unknown until recently. These metamorphic rock units are juxtaposed along major north-northwest and northnortheast strike-slip mylonitic zones, so that stratigraphic interrelationships are difficult to establish (Pimentel and Fuck, 1987a; Fig. 2). Extensive outcrop areas of orthogneisses are called the Arenopolis, Matrinxa, and Sanclerlandia gneisses, after nearby towns (Fig. 2). They are mainly gray, medium-grained, hornblende- and biotite-bearing gneisses ranging in composition from gabbro-dioritic to granodioritic (Simoes, 1984; Pimentel, 1985; Amaro, 1989). Preliminary U-Pb and Rb-Sr geochronological results indicate Neoproterozoic ages (ca. 940 to 895 Ma; see Table 1). The trace element compositions of these calcic to calc-alkalic orthogneisses show features that are typical of calc-alkalic volcanic arc granitoids (Pimentel, 1990).

Several volcano-sedimentary assemblages that consist largely of metavolcanic and associated subvolcanic rocks ranging in composition from tholeiitic basalts to ^O-rich rhyolites are known in the area. These suites are, from west to east, the Bom Jardim, Arenopolis, Ipora and Jaupaci sequences (Fig. 2). They were metamorphosed at pressure and temperature conditions in the greenschist-amphibolite facies transition (Seer, 1985; Pimentel, 1985; Amaro, 1989). Previous studies have demonstrated the petrographic and geochemical similarities between the Neoproterozoic metavolcanic suites in western Goias and modern volcanic arc associations (Seer, 1985; Pimentel and Fuck, 1987b; Amaro, 1989). In the westernmost volcanic suites (Bom Jardim and Arenopolis; Fig. 2), rocks with intermediate compositions (andesites and dacites) are abundant, whereas the younger Jaupaci volcanic association has bimodal character: abundant K20-rich rhyolite and subordinate tholeiitic metabasalt. Metabasalts in the Arenopolis suite are geochemically similar to low-K^O basalts of modern immature island-arc associations (Pimentel and Fuck, 1987b). Strongly deformed and metamorphosed mafic to ultramafic rocks forming linear swarms of bodies to the northnorthwest are found in the Arenopolis, Ipora, and Israelandia areas (Fig. 2). In the Arenopolis sequence, they are spatially associated with metabasalt, metachert, marble, and mica schist and are interpreted as fragments of disrupted ophiolitic complexes (Pimentel, 1985). Geochronological results by the U-Pb and Rb-Sr methods on several metavolcanic units from western Goias indicate ages ranging from ca. 930 to 587 Ma (Table 1). These results contradict studies that compared these units to "greenstone belts," assigning Archean ages to them (e.g., Bernasconi, 1983). Resetting of Rb-Sr whole-rock isotopic sysGEOLOGY, April 1992

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Figure 2. Geologie sketch map of western Goias. See Figure 1 for location.

tems in mylonitic rocks at ca. 600 Ma (Fazenda Nova and Jaupaci metarhyolites; Table 1), as well as U-Pb sphene ages of ca. 600 Ma in metavolcanic rocks and orthogneisses from the Arenópolis area, indicate a last, major deformation and recrystallization event in the area (Pimentel et al., 1991a). This event was probably related to the peak of metamorphism and deformation of the Brasiliano orogeny, which promoted widespread resetting of the K-Ar isotopie system on a regional scale in central Brazil (Marini et al., 1984). In the central part of the Arenópolis volcanosedimentary assemblage, a narrow (