The Gurupi Belt, northern Brazil: Lithostratigraphy, geochronology, and geodynamic evolution
Introduction
Early geochronological studies, using the conventional Rb–Sr and mineral K–Ar systems (Hurley et al., 1967, Hurley et al., 1968, Almeida et al., 1968, Cordani et al., 1968, Cordani et al., 1974, Almaraz and Cordani, 1969), defined two geochronological provinces in the NE part of Pará state and the NW part of Maranhão state, northern Brazil. Those studies showed that the rocks cropping out toward the Atlantic coastline have a Paleoproterozoic signature, whereas the rocks cropping out toward the inner parts of the continent have been affected by Neoproterozoic events (Fig. 1). These Paleoproterozoic and Neoproterozoic provinces have then been named São Luís Craton and Gurupi Belt, respectively (Almeida et al., 1976), and the boundary between these two provinces has been defined as being the Tentugal shear zone (Hasui et al., 1984).
Several regional-scale mapping programs have been carried out over 20 years in this region, each accompanied by lithostratigraphic proposals and geotectonic models (Costa et al., 1977, Abreu et al., 1980, Hasui et al., 1984, Pastana, 1995, Costa, 2000, Almeida, 2000), always based on the available Rb–Sr and K–Ar data. Discussions concerning the concept, geographic distribution, inferred age, and tectonic setting of some lithostratigraphic units, in addition to geotectonic models, with propositions favoring either a mono- or poly-phase evolution for the belt, fomented the debate throughout the past two decades. Correlations between the São Luís and West African cratons, and between the Gurupi Belt and the Pan-African belts that border the West African Craton, have been made as well (Torquato and Cordani, 1981, Lesquer et al., 1984, Abreu and Lesquer, 1985).
Recent studies, using more robust geochronological systems (single zircon Pb evaporation and whole rock Sm–Nd), along with new field and petrographic information, have brought out interesting constraints with implications for the lithostratigraphy and developing geotectonic models. Palheta (2001) has demonstrated that several granitoid plutons, previously grouped as a Neoproterozoic collisional suite (Costa, 2000), actually belong to at least three different generations, showing Paleoproterozoic and Neoproterozoic crystallization ages, and Paleoproterozoic to Archean model ages. Klein and Moura (2001) have defined Paleoproterozoic ages (2148–2160 Ma) for metavolcanic rocks from the major metavolcano–sedimentary sequence of the belt, and Klein and Moura (2003) have determined a Paleoproterozoic minimum age (2135 Ma) for amphibolite-facies gneisses that have previously been considered as having Archean ages (Pastana, 1995) by correlation with compositionally similar rocks of the Amazonian Craton. Therefore, these studies anticipate a more complex evolutionary history for the Gurupi Belt.
Despite these advances, the age of some units is still not constrained by robust methods, and field relationships between different units are only rarely observed, hindering a better establishment of the relative stratigraphy; this makes geochronology an indispensable tool. Furthermore, the relative contributions of crustal and mantle sources in the genesis of the various rock units is known only for a few granite bodies, hindering the discussion of the crustal evolution of the belt. For instance, were all the igneous rocks of the Gurupi Belt produced by reworking of a pre-existing crust, as suggested from the Sm–Nd data presented by Palheta (2001), or are juvenile components also present? What is the age of the Tentugal shear zone? Does the Tentugal shear zone represent a suture between the Gurupi Belt and the São Luís Craton? What is the meaning of the Neoproterozoic Rb–Sr and K–Ar ages? What was the paleogeography of the belt; that is, in what kind of tectonic setting have the distinct units formed? Since most of the rocks in the Gurupi Belt are Paleoproterozoic, what were the relationships of these rocks with the Paleoproterozoic and dominantly juvenile (Klein et al., 2005) rocks of the São Luís Craton?
It is the intention of this paper to address some of these questions. We present a review of the main geological attributes of the Gurupi Belt, including lithology, metamorphism, structure, limits, previous geochronology, and geotectonic models. We provide new field information, geochronological data on zircon (Pb evaporation, U–Pb), and Sm–Nd isotope compositions of metavolcanic, metaplutonic, and metasedimentary rocks. As such, all igneous and metaigneous, and most of the metasedimentary units of the belt have now at least one geochronological age based on zircon, which better constrains the regional lithostratigraphy. Furthermore, accretion and/or reworking and possible geological settings are assessed by a combination of zircon geochronology, Sm–Nd isotopes, and rock association. Unresolved problems that need further investigation are highlighted as well.
Section snippets
Geographical extent and limits
The Gurupi Belt trends NNW/SSE; its present-day cropping area is about 160 km long and up to 50 km wide. The boundary between the Gurupi Belt and the São Luís Craton has usually been considered as being the Tentugal shear zone (Fig. 1), which is also interpreted as the suture between these two terranes (Hasui et al., 1984, Abreu and Lesquer, 1985). However, most of the rock units in the belt crop out as discontinuous erosive and tectonic windows within Phanerozoic sedimentary basins (Fig. 1),
Sampling and analytical procedures
Sampling sites are showed in Fig. 3. Zircons were dated by the single zircon Pb evaporation, U–Pb isotopic dilution (ID-TIMS), and U–Pb laser ablation (LAM-ICP-MS) methods, and whole rock samples were analyzed for their Sm–Nd isotope compositions. The Pb evaporation, ID-TIMS, and Sm–Nd analyses were conduced at the Laboratório de Geologia Isotópica (Pará-Iso) of the Universidade Federal do Pará, Belém, Brazil, using a Finnigan Mat 262 mass spectrometer, whereas the LAM-ICP-MS analyses were
Igarapé Grande Metatonalite
The metatonalite (sample EK32) is a dark gray rock without visible mesoscopic tectonic fabric. Under the microscope, quartz and plagioclase form granoblastic arrays and occur in association with brownish biotite and minor amphibole and K-feldspar. Zircon, apatite, garnet, and opaque minerals are accessory phases, whereas chlorite and white mica are retrometamorphic products. The analyzed zircon crystals are brownish, prismatic, with asymmetric and rounded pyramids. Four crystals yielded
Itapeva complex
Sample EK18A (tonalitic gneiss) has previously been dated by the Pb evaporation method (minimum age 2135 ± 4 Ma—Klein and Moura, 2003). Zircons for ID-TIMS U–Pb dating have been separated from the same concentrate used for the Pb evaporation analysis. Five abraded zircon crystals, showing two distinct morphologic patterns, have been analyzed (Fig. 5; Table 4). The first group (grains a, b, and i) consists of elongated, prismatic crystals with rounded terminations. Zircons of the second group
Sm–Nd isotope results
Sm–Nd isotope compositions have been determined in gneisses of the Itapeva Complex, supracrustal rocks of the Chega Tudo and Marajupema Formations, muscovite–granite of the Maria Suprema unit, and in the Boca Nova nepheline syenite gneiss. The isotopic results are reported in Table 6. With one exception (sample EK35), the analyzed samples have 147Sm/144Nd ratios and Sm–Nd fractionation factors [f(Sm–Nd)] within, or close to, the accepted range for felsic rocks that have not undergone chemical
Discussion
In the age versus ɛNd diagram (Fig. 9), the compositions of the rocks of the Gurupi Belt form broadly three domains. One Paleoproterozoic domain is composed of calc-alkaline/TTG juvenile rocks formed at 2150–2168 Ma. Another Paleoproterozoic domain is comprised of peraluminous granites formed at 2080–2100 Ma through the reworking of Paleoproterozoic and Archean crustal protoliths. These two domains match those formed by similar and coeval rocks of the São Luís Craton (Fig. 9), and the older one
Geodynamic evolution and concluding remarks
Information on field relationships (scarce), rock association and geochemistry, and geochronological (zircon, Rb–Sr, and K–Ar) and Nd isotope data provide some insights into the tectonic settings and geodynamic evolution of the Gurupi Belt. It has become clear that this belt is the result of a poly-phase evolution with magmatic activity taking place in at least five events: ∼2600 Ma (Neoarchean), 2167–2148 Ma (early Rhyacian), 2100–2080 Ma (late Rhyacian), 730 Ma (Cryogenian), and 550 Ma
Acknowledgements
Thomas Scheller, Valter Gama Avelar, Marco Antonio Galarza, Rosemary Brabo Monteiro, Elma Oliveira, and Roberta Florencio are greatly acknowledged for technical support during the analytical work at UFPA. Suzy Elhlou and Norman Pearson provided much help during the U–Pb analyses at Macquarie University. Comments by J.M. Lafon, B.B. Brito Neves, R.N.N. Villas, and A. Giret on an early version of the manuscript are much appreciated. We are grateful to Dr. U.G. Cordani and an anonymous reviewer
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Rhyacian and Neoproterozoic magmatic associations of the Gurupi Belt, Brazil: Implications for the tectonic evolution, and regional correlations
2020, Geoscience FrontiersCitation Excerpt :Furthermore, most of the exposed Gurupi Belt comprises reworked portions of the Rhyacian terrane (present-day São Luís cratonic fragment). Therefore, our data do not support neither the Neoproterozoic oceanization indicated by Klein et al. (2005a), nor the deep continental subduction suggested by Ganade de Araújo et al. (2014) for the Gurupi Belt region. However, metamorphism and deformation are present at variable degrees in igneous and sedimentary rocks formed in the Neoproterozoic (Klein et al., 2005a; Tavares et al., 2017).
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2020, TectonophysicsCitation Excerpt :Another cratonic unit is found in the north of the study area, the São Luís craton (SLC), which evolution is also related to Transamazonian events (Klein et al., 2005b). It consists on a small fragment of the West Africa craton that was left in the SAP after Pangea breakup (Klein and Moura, 2008), and is now limited by the Brasiliano Gurupi belt at the south; this limit is defined by the Tentugal shear zone (TSZ) (Klein et al., 2005a; Klein and Moura, 2008). The Borborema Province (BP) is a Neoproterozoic structural province (e.g., Almeida et al., 1981) formed due to the accretionary processes that resulted in West Gondwana (Brasiliano cycle), through the convergence of the Congo-São Francisco and West Africa cratons (Brito Neves et al., 2000; Cordani et al., 2000).
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Present address: All-Russian Geological Research Institute (VSEGEI), Sredny Prospect, 74, Saint Petersburg 199106, Russia.