Reinhardt A. Fuck

Brazilian structural provinces: An introduction

Abstract The territory of Brazil coincides almost entirely with the South American Platform, the crystalline core of the continent. Its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the Phanerozoic. Sedimentary rocks with almost horizontal bedding cover this crystalline basement. This latter shows ages as old as Early Precambrian, although ages between 500 and 1000 m.y. are conspicuously frequent. The cratonic areas became consolidated more than 1700 m.y. ago, whereas the fold belts formed essentially between 500 and 1700 m.y. ago. The sedimentary cover accumulated from the Early Silurian in three large intracratonic basins, until the platform became completely stabilized. A Late Jurassic to Early Cretaceous reactivation, caused by the break-up of the ancient Gondwana continent, created another basin sequence chiefly along the Atlantic continental margin. Based on the nature of the crystalline basement rocks and the sedimentary cover, ten structural provinces can be distinguished: (1) Rio Branco Province, in the north of the country, occupied by the Guyana Shield and still only poorly known, with an important fold belt developed during the so-called Transamazonian Cycle (2000 ± 200m.y.) with high-grade metamorphic rocks and only slight influence of later events. (2) Tapajos Province, in central Brazil, corresponding to the Amazonas or GuaporeCraton, showing chiefly Precambrian crystalline rocks and behaving as an cratonic area during the Phanerozoic. (3) Sa˜o Francisco Province, located on the Atlantic Shield, with its basement covered by rocks of different ages, chiefly affected by the Brasiliano Cycle (between 1000 and 500 m.y.) and constituting another cratonic area. (4) Tocantins Province, between the Amazonas and Sa˜o Francisco Cratons, with the oldest rocks in its centre (ages over 2600 m.y.), and at the eastern and western borders metamorphic sequences of various fold belts, and almost no Phanerozoic deposits. (5) Mantiqueira Province, located along the southern part of the Atlantic coast, affected chiefly by the Brasiliano folding cycle. (6) Borborema Province, in the northeast Brazilian fold belt, affected by the Brasiliano Cycle in a very complex way, and with important faulted zones. It was reactivated in Phanerozoic times, when also sedimentary covers accumulated. (7) Amazonas Province, represented by the Amazonas sedimentary basin (syneclise), and subdivided into four parts by three important arcs. (8) Parnaiba Province, coinciding with the Piauf — Maranha˜o syneclise, and filled with a rather thick sedimentary sequence. (9) ParanaProvince, the sedimentary basin of southern Brazil, in which the well-known Late Paleozoic glaciation features are found, and which possesses an extensive cover of basaltic rocks of Late Jurassic to Early Cretaceous age. (10) Coastal Province and Continental Margin, the youngest structural unit, developed during the separation of the continent and represented by rift-valleys and coastal basins filled with Mesozoic-Cenozoic deposits of various kinds.

Neoproterozoic crustal accretion in central Brazil

Recent geochronological studies have demonstrated the existence of several Neoproterozoic orthogneiss and metavolcanic are 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 87 Sr/ 87 Sr ratios vary from ∼0.7024 to ∼0.7042 and initial ϵ Nd 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 are 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.

Granites and the geodynamic history of the neoproterozoic Brası́lia belt, Central Brazil: a review

Abstract Recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the Neoproterozoic Brasilia Belt, in Central Brazil. This is an orogenic belt developed in response to the convergence between the Amazon, Sao Francisco–Congo and Parana continental blocks. The presence of Neoproterozoic juvenile arc rocks and syn-collisional peraluminous granites challenged previous intracontinental evolution models for the belt. The granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt. The main field and isotopic characteristics of four granite suites associated with the Brasilia Belt are reviewed: (i) 1.77–1.58 Ga old rift related A-type granite intrusions, (ii) ca. 0.8–0.7 syn-collisional granitoids, (iii) arc metatonalites and metagranodiorites (ca. 0.9 to 0.63 Ga), and (iv) bimodal post-orogenic suite ranging in age from ca. 0.59 to 0.48 Ga. These rocks suggest that during most of the Neoproterozoic the western margin of the Sao Francisco continent faced a large oceanic basin, where subduction and oceanic lithosphere consumption started at ca. 0.9 Ga, roughly coeval with the initial stages of the break up of Rodinia. Final ocean closure happened at ca. 0.63–0.60 Ga with crustal thickening, uplift and erosion. Post-orogenic extension-related magmatism took place between ca. 0.6 and 0.5 Ga and was partially contemporaneous with the deposition of the Paraguay and Tucavaca sedimentary successions, resulting from the rifting event related to the break up of Laurentia from southwestern Gondwana.

The Mara Rosa Arch in the Tocantins Province: further evidence for Neoproterozoic crustal accretion in Central Brazil

The Mara Rosa volcano-sedimentary sequence consists of several NNE-trending belts of metavolcanic (metabasalts to metarhyolites) and metasedimentary rocks (micaschists, quartzites, cherts), exposed over large areas of the Tocantins Province, a Neoproterozoic (Brasiliano) orogenic region in the central part of Brazil. The supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post-orogenic granitic and dioritic bodies. These volcano-sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite-greenstone terrain of the Archaean basement. In this paper we present geochronological and isotopic data for rocks of the metavolcanic-metaplutonic rock association. Samples of a felsic metavolcanic rock from the Posse gold mine, and a metatonalite yielded UPb zircon ages of 862±8 Ma and 856−7+13 Ma, respectively. These are interpreted as crystallization ages of the igneous protoliths. Titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632±4 Ma. SmNd isotopes for these rocks indicate primitive compositions, with ϵNd(T) of +4.6 and +3.7 and TDM model ages of ∼1.0 Ga. A syn-tectonic dioritic intrusion has an UPb crystallization age of 630±6 Ma, and ϵNd(t) of +1.9 and TDM of ∼1.0 Ga. Additional SmNd analyses on metasedimentary rocks and post-orogenic granites of the Mara Rosa region yield TDM ages between ∼1.2 and 1.0 Ga. The isotopic data, combined with preliminary trace element results, suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system, off the coast of the Sao Francisco-Congo continent at around 860 Ma. Recrystallization and deformational ages of ∼630 Ma may represent the timing of final ocean closure and continental collision. The rock associations, structural pattern, geochronological and isotopic characteristics of the rocks from the Mara Rosa region are very similar to those of Neoproterozoic arc terrains exposed ∼ 300 km to the southwest, in the Arenspolis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.

U-Pb zircon geochronology of Precambrian tin-bearing continental-type acid magmatism in central Brazil

Abstract U-Pb geochronological data for alkali-rich granitic and associated rhyolitic rocks from two different tin-bearing sub-provinces of the Goias tin province, in central Brazil, reveal two distinct episodes of mid-Proterozoic continental acid magmatism at ca. 1770 Ma and 1600 Ma ago. U-Pb zircon data for the Sucuri and Soledade granites of the eastern Rio Parana Sub-Province (RPS) define upper intercept ages of 1767±10 Ma and 1769±2 Ma, respectively. These ages are indistinguishable from a U-Pb age of a rhyolite from the basal portion of the Arai Group (1771±2 Ma). These results attest to the contemporaneity of the intrusion of the granites and the crystallization of the rhyolitic lavas in the Rio Parana Sub-Province. U-Pb isotopic results from zircons from the Serra da Mesa granite in the western Rio Tocantins Sub-Province (RTS) indicate that the granitic igneous activity in that area is ca. 150 Ma younger than that in the RPS. The inheritance pattern observed in the zircons from the Arai rhyolite suggests that the sialic basement in the RPS region may include an early Proterozoic (ca. 2.2 Ga) component. Inheritance of possibly Archaean age was also found in zircons from the Sucuri granite. Lower intercept age of 658 Ma for the Soledade granite zircons indicate Pb loss during the late Proterozoic Brasiliano (Pan-African) thermo-tectonic event (650-550 Ma). Similar associations of tin-bearing, alkali-rich granites with rhyolites and succeeding continental sediments are known in other parts of the Precambrian shield of Brazil. This indicates that extensive areas of this shield experienced extensional tectonics in intracontinental settings between ca. 1700 and 1900 Ma ago.

Influence of basement structures on the evolution of the major sedimentary basins of Brazil: A case of tectonic heritage

Abstract The present study deals with the correlation between the geotectonic features of the basement and the internal structure, shape, geologic evolution, etc. of the major sedimentary basins of the South American platform. The Parana, Parnaiba and Amazonas basins occupy an area of the order of 3.6 × 10 6 km 2 , and their sedimentary cratonic sequences were deposited from Silurian to Triassic times. Subsidence rates are estimated around 15 m/Ma in the main depocenters. A geologic study was carried out along the basement features in the surroundings of such basins in order to identify the major structural, geotectonic and geochronological discontinuities. The extension of these basement characteristics towards the interior of the basins are examined. Basement core samples from deep wells were investigated through petrological and geochronological analyses, and pre-existing geophysical and structural maps of those basins were taken again into consideration. It can be concluded that many of the identified basement discontinuities display a direct influence on the depositional history of the basins as well as on their internal subdivisions and external outlines. Basement structures generated during the late Precambrian Brasiliano Cycle turned out to be particularly important. The Parana and Parnaiba basins are considered to represent cratonic basins, located on rigid lithosphere, tectonically stabilized in the latest Precambrian/early Palaeozoic, and their subsidence is attributed to the establishement of some initial rifted grabens. The Amazonas basin is more complex and includes three large sub-basins with distinct evolutions, each located on a different tectonic segment of the basement.

Geology of the northern Borborema Province, NE Brazil and its correlation with Nigeria, NW Africa

Abstract The Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks >2.0 Ga; in NE Brazil, 2.6–3.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic (c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Seridó Group of NE Brazil (<0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceará Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating north–south continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaíba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi–4°50 Lineament in Hoggar, may represent a cryptic suture.

Post-Brasiliano (Pan-African) high-K granitic magmatism in Central Brazil: the role of late Precambrian-early Palaeozoic extension

Abstract In western Goias, Brazil, the emplacement of large, high-K postorogenic granites and associated small gabbro-dioritic intrusions, followed immediately after the last deformational events of the Brasiliano-Pan-African orogeny at ∼600 Ma. Well-fitted whole-rock RbSr isochrons indicate ages which suggest two discrete intrusive events: the older between ∼588 and 560 Ma and the younger between ∼508 and 485 Ma. The older granites display general petrographic and geochemical characteristics of highly differentiated calc-alkaline I-type granitoids, whereas the younger intrusions are more alkaline, similar to A-type granites. Initial 87 Sr 86 Sr ratios vary from ∼0.703 to 0.710 and initial Nd isotope ratios yield ϵNd(T) values in the range between −4.0 and +3.0. There are no major differences in initial isotopic compositions between the two granite groups, suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the ∼940-640 Ma basement arc-type metatonalites-metagranodiorites and associated arc metavolcanics. The major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust, with younger melts being produced by the refusion of anhydrous, depleted crustal sources left after the extraction of previous batches of more hydrated calc-alkaline magmas. The heat input required to promote extensive remelting of the continental crust was, most likely, provided by mantle-derived mafic magmas that invaded and probably underplated the crust, during uplift and extension. The two intrusive events are bimodal in nature and are interpreted as shallow-level extension-related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the Proterozoic and early Palaeozoic, the older at ∼600 Ma and the younger between ∼550 and 510 Ma. The onset of the granite magmatism at ∼590 Ma, shortly post-dating the Brasiliano orogeny (∼600 Ma), is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic Paraguay Belt in Brazil and its correlative in Bolivia, the Tucavaca Belt, which probably correspond to rift deposits related to the break up of Laurentia from Gondwana at the end of the Proterozoic and beginning of the Palaeozoic. Granites of the younger group cut the deformational structures of the Paraguay Belt metasediments and pre-date, by between ∼40 and 20 Ma, the initial stages of subsidence and sedimentation of the Parana Basin.

The Brasiliano collage in South America: a review

Analysis of recent geological and geochronological data from the basement of the South American platform indicates that the Brasiliano orogenic collage took place in four distinct pulses: a) Early Cryogenian (ca. 800 - 740Ma); b) Late Cryogenian-Early Ediacaran (ca. 660 - 610 Ma); c) Early-Middle Ediacaran (c. 590 - 560 Ma); and d) Late Cambrian (520 - 500 Ma). The first three pulses are well represented in most Neoproterozoic structural provinces in West Gondwana. The youngest orogenic phase/pulse, however, is only seen in Argentina (Pampean Orogeny) and Brazil, in eastern Rio de Janeiro State (Buzios Orogeny). The period between ca. 750 and 500 Ma is comparable to that reported for the amalgamation of various continental fragments in East (Arabian-Nubian, Mozambique, Kuunga) and North Gondwana (Cadomian). However, important differences in the nature and ages of events are recognized, which can be expected in view of the magnitude of Gondwana agglutination and the diversity of paleogeographic and tectonic scenarios. West Gondwana shows an interesting peculiarity: lithologically and tectonically diversified Tonian terranes underlie Brasiliano orogenic buildups. They were strongly reworked during most of the orogenic pulses. The Tonian terranes (1000 - 900 Ma) and their relation with Rodinia or with the processes of Gondwana fusion remains an open question. Indications of their presence in East Gondwana are still poorly documented.

THE SIGNIFICANCE OF THE TRANSBRASILIANO-KANDI TECTONIC CORRIDOR FOR THE AMALGAMATION OF WEST GONDWANA.

The assembly of West Gondwana was completed by the end of the Precambrian, when the Amazonian, West African, Sao Francisco-Congo, Kalahari and Rio de la Plata cratons, as well as the Saharan metacraton and the Parnaiba, Paranapanema and Luiz Alves cratonic fragments were united by means of the Brasiliano-Pan African orogeny, a geotectonic process that was active from the late Neoproterozoic to the early Paleozoic, related to the closure of a large oceanic domain, the Goias-Pharusian Ocean. Several accretionary complexes and possible microcontinents were trapped within the Brasiliano-Pan African mobile belts, and they have been accommodated within a few hundred kilometers of the Transbrasiliano-Kandi tectonic corridor. The supercontinent was already formed at about 600 Ma, as indicated by the existence of a very large Ediacaran epicontinental sea covering large areas of -west-central Brazil and southern Uruguay along the margins of the Amazonian and Rio de la Plata cratons, demonstrating the connection of both cratonic units at that time and making the idea of a collisional suture closing a supposed Clymene Ocean unsustainable. In the Cambrian, a major plate reorganization occurred, being responsible for the initiation of subduction of the oceanic lithosphere along an open and unconfined Pacific Ocean. The resulting Pampean Orogeny correlates nicely in time with the Saldania, Ross, and Tasmanian belts along the southern Gondwana margin. Simultaneously, extensional-type post-tectonic episodes occurred repeatedly along the Transbrasiliano-Kandi tectonic corridor.