Umberto G. Cordani

Tectonic evolution of South America during the late proterozoic

Abstract This paper discusses the evolution and the Late Proterozoic assembly of the South American continent (i.e. the West Gondwana) under the light of global tectonics. The northern portion of the continent—the Amazonian craton—is a “Pre-Brasiliano” domain; it may be a fragment derived from Laurentia. This domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent (“Brasiliano domain”). The Brasiliano domain had a more complex history and composition during the Brasiliano cycle (Middle Proterozoic to Early Paleozoic). In the Brasiliano domain, several Late Proterozoic continental plates have been identified (Sao Luiz-West Africa, Congo-Kasai/Sao Francisco, Rio de La Plata, Arequipa-Antofalla) which acted as forelands to the Brasiliano orogenic belts that surround and amalgamate these plates. Smaller fragments of Pre-Brasiliano continental lithosphere are common in the interior of the Brasiliano orogenic belts and played diversified roles (microplates, microcontinents, “structural highs”). Some of them have acted as true backlands (hinterlands) flanked by arc magmatism. Two principal types of Brasiliano orogenic belts are recognized between the continental plates: (a) marginal orogenies to the Late Proterozoic plates, with pelitic-carbonatic rocks and discrete volcanism, which were thrust by collision onto the margin of the plates, and which seem to represent final products of previous passive margin sequences; (b) distal orogenics (internal belts within the continental masses, sometimes a branching system of orogenics), that contain varied clastic and minor carbonate sedimentary rocks, accompanied by important bimodal volcanism and calc-alkaline plutonism. For these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized. The amalgamation of the continent during the Late Proterozoic, i.e. the formation of the Brasiliano structures, indicates the existence of practically synchronous multilateral stresses within lithospheric plates. This can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere (analogous to modern continental collisions). Two large and important tectonic zones may be candidates to such a scenario: the Hijaz-Mozambique zone (site of connection of East and West Gondwana) and the Transbrasiliano lineament, which cuts across South America and extends to North Africa following the zone of connection between Pre-Brasiliano and Brasiliano domains. Other small ocean-openings and -closings inside the various Brasiliano areas (Dom Feliciano-Gariep, for instance) were minor events and complementary to the other two major tectonic zones mentioned above.

A review of the geochronology of the Amazonian Craton: Tectonic implications

Abstract 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 Rb-Sr 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 Carajas area. Although the Maroni-Itacaiunas belt includes several reworked Archean nuclei, rock units formed during the 2.2-1.9 Ga Trans-Amazonian Orogeny 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 mantle-derived 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 tectonomagmatic events in the 1.45-1.25 Ga interval, whereas the latter records tectonic reworking at 1.1-0.9 Ga. Finally, the Amazonian Craton is bordered to the east by the late Proterozoic Paraguai-Araguaia (or Araguaia-Tocantins) fold belt.

African, southern Indian and South American cratons were not part of the Rodinia supercontinent: evidence from field relationships and geochronology

We discuss the question whether the late Mesoproterozoic and early Neoproterozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago. First, there is little evidence for the production of significant volumes of ~1.4–1.0 Ga (Kibaran or Grenvillian age) continental crust in the Mozambique belt (MB) of East Africa, except, perhaps, in parts of northern Mozambique. This is also valid for most terranes related to West Gondwana, which are made up of basement rocks older than Mesoproterozoic, reworked in the Brasiliano/Pan-African orogenic cycle. This crust cannot be conclusively related to either magmatic accretion processes on the active margin of Rodinia or continental collision leading to amalgamation of the supercontinent. So far, no 1.4–1.0 Ga rocks have been identified in Madagascar. Secondly, there is no conclusive evidence for a ~1.0 Ga high-grade metamorphic event in the MB, although such metamorphism has been recorded in the presumed continuation of the MB in East Antarctica. In South America, even the Sunsas mobile belt, which is correlated with the Grenville belt of North America, does not include high-grade metamorphic rocks. All terranes with Mesoproterozoic ages seem to have evolved within extensional, aulacogen-type structures, and their compressional deformation, where observed, is normally much younger and is related to amalgamation of Gondwana. This is also valid for the Trans-Saharan and West Congo belts of West Africa. Third, there is also no evidence for post-1000 Ma sedimentary sequences that were deposited on the passive margin(s) of Rodinia. In contrast, the MB of East Africa and Madagascar is characterized by extensive structural reworking and metamorphic overprinting of Archaean rocks, particularly in Tanzania and Madagascar, and these rocks either constitute marginal parts of cratonic domains or represent crustal blocks (terranes or microcontinents?) of unknown derivation. This is also the case for most terranes included in the Borborema/Trans-Saharan belt of northeastern Brazil and west-central Africa, as well as those of the Central Goo´as Massif in central Brazil and the Mantiqueira province of eastern and southeastern Brazil. Furthermore, there is evidence for extensive granitoid magmatism in the period ~840 to <600 Ma whose predominant calcalkaline chemistry suggests subduction-related active margin processes during the assembly of the supercontinent Gondwana. The location of the main Neoproterozoic magmatic arcs suggests that a large oceanic domain separated the core of Rodinia, namely Laurentia plus Amazonia, Baltica and West Africa, from several continental masses and fragments now in the southern

Potassium-argon dates of basaltic rocks from Southern Brazil

Abstract Potassium-argon ages are reported for 20 basalts (including a few core samples) and 15 diabases from the Parana basin of Southern Brazil. Histograms of both basalt and diabase ages exhibit strong peaks close to 120 m.y. Thus the principal Brazilian volcanism was Mid-Lower Cretaceous and not Jurassic as earlier supposed. While the few dates younger than 120 m.y. are likely due to argon loss, there is a real “tail” of higher ages which shows that there was precursory volcanic activity 147 m.y. ago, in the Upper Jurassic, and perhaps earlier. The span of at least 28 m.y. for the volcanism is similar to that which has been found for South Africa. The basaltic rocks of Brazil differ, however, both in age and potassium content from the older diabase rocks of Southeast Africa, Antarctica and Tasmania which have been dated thus far. Auxiliary studies included a careful evaluation of the analytical precision in our techniques and control experiments in dating series of spaced samples from narrow diabase dikes which represent examples of “instantaneous” cooling. The latter experiments show that whole-rock dating of diabase from Brazil is occasionally susceptible to discrepancies of up to 8 per cent even when fairly rigid thin-section criteria have been exercised in the selection of the samples for dating. An argon diffusion experiment on diabase material gave results characteristic of feldspar. Acid etching was found not to be effective in “rectifying” discordant feldspar separates. Auxiliary studies by electron microprobe showed that a diabase sample, in which only plagioclase feldspar would be noted in cursory visual examination, actually contains feldspars ranging from plagioclase through potash oligoclase and anorthoclase to sodic sanidine, where inconspicuous weathering of the last might be important in creating discrepancies of the type noted.

Potassium-argon ages of alkaline rocks from southern Brazil*

Abstract We present potassium-argon ages for 66 samples (micas, feldspars, fine-grained whole rocks and amphiboles in descending frequency) from 21 alkaline rock localities in southern Brazil. A general geological description is given for each locality. Analytical precision is about one per cent for potassium and two per cent or better for radiogenic argon determinations. It is possible to classify many samples as concordant (within 4 m.y.) or discordant with a preferred age for their localities. On this basis, 41 samples are found concordant and 8 samples, discordant. Four examples of anomalously “old” alkali feldspar were encountered, exhibiting a hitherto unreported reversal of the orthodox mica-feldspar discrepancy in K Ar dating. Pyroxene ages were erratic and have been discarded. Studies with the electron microprobe indicate that the potassium content of pyroxene is much lower than that often reported, the difference being contamination. With some exceptions, the alkaline occurrences fall into two age groups, one Early Cretaceous (122–133 m.y.) and the other Late Cretaceous to Early Tertiary (51–82 m.y.). The older group coincides, in age, with the basaltic vulcanism of the Parana Basin, but the younger group, to which belongs the major part of Brazilian alkaline occurrences, cannot be associated in age with these basalts. The intrusions with older ages are clustered in southern Sao Paulo State, and the younger group, with one exception, lies to the north and east of the older group. At some localities, like Pocos de Caldas, the alkaline magmatism apparently took place at several different stages. There is no close correlation in time between Brazilian and African alkaline intrusions the latter being generally older, but there seem to exist similar age patterns (including coincidence between the vulcanism and the oldest alkaline rocks) in both continents.

Late Jurassic bimodal magmatism in the northern sea-floor remnant of the Rocas Verdes basin, southern Patagonian Andes

Magmatic and detrital zircon ages from the Rocas Verdes basin, a tectonically juxtaposed remnant of sea floor in the Magallanes fold and thrust belt (southern Patagonia, South America), indicates that a rifting phase of the Rocas Verdes basin occurred between 152 and 142 Ma, and was accompanied by bimodal magmatism. A dacite dyke cross-cutting pillow-basalt successions and a plagiogranite dyke in mixed mafic–felsic terranes of the basal Sarmiento Ophiolite Complex contain 150 Ma zircon crystals, indicating that mafic submarine volcanism had started prior to or during the Late Jurassic, 10–15 Ma earlier than previously thought. The silicic pyroclastic rocks of the Tobífera Formation, with two samples dated at 148 and 142 Ma, were heralded by synrift sedimentation along fault-bounded grabens within Palaeozoic metasediments. No evidence for an active volcanic arc during the early formation of the Rocas Verdes basin was detected in detrital zircon grains of the lower sedimentary member of the Tobífera Formation. A minimum of 25 Ma of continuous sedimentation in the Rocas Verdes basin is suggested by detrital zircon grains in the upper member of the Zapata Formation. The Rocas Verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution.

Geochronology of Proterozoic basement inliers in the Colombian Andes: tectonic history of remnants of a fragmented Grenville belt

Abstract Basement inliers of high-grade metamorphic rocks within the eastern Colombian Andes record a Grenvillian history. Among them, the Garzón Complex and the Dibulla, Bucaramanga and Jojoncito gneisses were studied using different geochronological methods to produce better correlations in the context of the reconstruction of the Grenville belt and of the supercontinent of Rodinia. The dynamic evolution of all of these units includes a final collisional event with exhumation of high-grade rocks. Such a tectonic history bears strong similarities with the Grenville Province in Canada and seems to confirm that these domains took part in the aggregation of Rodinia. Mesoproterozoic U-Pb zircon ages indicate heritage from magmatic protoliths, and the Sm-Nd model ages, as well as the εNd values, suggest derivation from an evolved continental domain, such as the Amazonian craton, with some mixing with juvenile Neoproterozoic material. When these continental fragments are correlated with similar terrains in Mexico and the Central Andes, a large crustal fragment is implied; very probably it made up the southern portion of the Grenville belt within Rodinia, which was disrupted when Laurentia separated from Gondwana forming the Iapetus Ocean, leaving behind cratonic fragments that were later accreted to the South American Platform.

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.

Geochronological Systematics on Basement Rocks from the Río Negro-Juruena Province (Amazonian Craton) and Tectonic Implications

The Rio Negro-Juruena Province (RNJP) occupies a large portion of the western part of the Amazonian Craton and is a zone of complex granitization and migmatization. Regional metamorphism, in general, occurred in the upper amphibolite facies. The granites and gneisses of the RNJP yield Rb-Sr and Pb-Pb whole-rock isochron dates ranging from 1.8 Ga to 1.55 Ga, with initial 87Sr/86Sr ratios of ∼ 0.703 and a single-stage model μ1 value of ∼ 8.1. In order to improve the geochronological control, SHRIMP U-Pb zircon ages, conventional U-Pb zircon ages, and additional Pb-Pb whole-rock isochron ages were determined for samples of granitoids and gneisses from the Papuri-Uaupes and Guaviare-Orinoco rivers areas (northern part of the province) and Jamari-Machado rivers and Pontes de Lacerda areas (southern part). The granitoids from the northern part of the province yield conventional U-Pb zircon ages of 1709 ± 17 Ma and 1521 ± 31 Ma, and SHRIMP U-Pb concordant zircon results of 1800 ± 18 Ma. Samples of gneissic rocks...

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.