The Gurupi Belt, northern Brazil: Lithostratigraphy, geochronology, and geodynamic evolution

Abstract

The Gurupi Belt is located in northern Brazil on the southern margin of the São Luís Craton, which is dominated by juvenile calc-alkaline rocks formed in intra-oceanic island arcs between 2240 and 2150 Ma. The Gurupi Belt consists of: (i) small lenses of an Archean metatonalite of 2594 Ma; (ii) calc-alkaline/TTG tonalites and gneisses of 2147–2168 Ma and juvenile Nd isotope signature, formed in intra-oceanic arc setting; (iii) an I-type monzogranite that intruded by ca. 2159 Ma, formed by the reworking of the oceanic arcs; (iv) a metavolcano–sedimentary succession with calc-alkaline volcanic rocks of 2148–2160 Ma and juvenile Nd isotope signature formed in arc systems; (v) several peraluminous, muscovite-bearing, collision-type granites of 2070–2100 Ma with Nd isotopes indicating variable reworking (partial melting and/or erosion) of Paleoproterozoic and Archean crust; (vi) a sub-greenschist- to greenschist-facies supracrustal sequence of unknown age (tentatively considered to be older than 2159 Ma); (vii) an amphibolite-facies metasedimentary sequence in which the youngest detrital zircon has age of 1100 Ma, and Nd isotopes and sedimentological evidence indicating Archean, Paleoproterozoic, Mesoproterozoic, and Neoproterozoic sources; (viii) a deformed (gneissic) nepheline syenite of 732 Ma formed by mixing of mantle and crustal sources; (ix) a peraluminous, muscovite-bearing post-tectonic granite of 549 Ma. Combined geological and geochronological information indicate that the Paleoproterozoic rocks are part of a Trans-Amazonian/Eburnean orogen initiated by an accretionary phase (2240–2150 Ma), better represented in the adjacent São Luís Craton, and terminated by a collisional event (2100–2080 Ma) that amalgamated juvenile and reworked Paleoproterozoic terrains and an Archean terrain existing to the south. This landmass amalgamated in the Paleoproterozoic broke up before 732 Ma, forming a rift, as suggested by the emplacement of the nepheline syenite pluton. The rift received detritus coming from Archean, Paleoproterozoic, Mesoproterozoic, and Neoproterozoic sources. We infer that the rift evolved to an oceanic basin and that the closure of this basin occurred at the end of the Neoproterozoic (580–550 Ma), as part of the Brasiliano/Pan-African cycle of orogenies.

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.