Farid Chemale

Sequence stratigraphy of continental Triassic strata of Southernmost Brazil: a contribution to Southwestern Gondwana palaeogeography and palaeoclimate

Abstract The continental Triassic succession of southernmost Brazil comprises two second-order depositional sequences—the Sanga do Cabral (Early Triassic) and the Santa Maria (Middle to Late Triassic) supersequences. The first one includes ephemeral, low-sinuosity fluvial deposits developed on a low gradient plain. Based on fossil tetrapods, especially procolophonids, an Upper Induan age is estimated for this sequence. Facies association of the Santa Maria Supersequence indicates low-sinuosity fluvial rivers, deltas and lakes. This supersequence can be further subdivided into three third-order sequences (age provided by palaeovertebrate biostratigraphic data) as follows: Santa Maria 1 (Ladinian), Santa Maria 2 (Carnian to Early Norian) and Santa Maria 3 (probably Raethian or Early Jurassic) sequences. The Gondwanides paroxysms I and II in the Sierra de la Ventana–Cape Fold Belt are directly related to the development of both supersequences. The source area of the Sanga do Cabral Supersequence was located to the south. It consisted of an uplifted peripheral bulge situated landward of the retro-foreland system, from where older sedimentary rocks were eroded. The source area of the Santa Maria Supersequence was also positioned southwards and related to the uplifted Sul-Rio-Grandense and Uruguayan shields. The Santa Maria Supersequence stratigraphic architecture is comparable to the Triassic rift basins of Western Argentina. Diagenesis, facies and palaeontology of the studied succession suggest a dominantly semiarid climate during the Triassic.

Juvenile accretion at 750–700 Ma in southern Brazil

A revista Economic Geology nao autoriza a publicacao de seus artigos em repositorios institucionais.

U-Pb and Sm-Nd geochronology of the neoproterozoic granitic-gneissic Dom Feliciano belt, Southern Brazil

The Brasiliano Cycle in southern Brazil and Uruguay is represented by three major NE-SW trending geotectonic units: the Vila Nova belt, Tijucas belt and Dom Feliciano belt. The Vila Nova belt is located in western part of Rio Grande do Sul State; its evolution took place between 900 and 700 Ma and it corresponds to one of the few areas with juvenile accretion during the Neoproterozoic in Brazil. The Tijucas belt, situated between the Vila Nova and Dom Feliciano belts, consists of a rift-related Mesoproterozoic (?) volcano-sedimentary sequence which was strongly deformed during the Brasiliano cycle. The Dom Feliciano belt is located along the eastern coast of southern Brazil and Uruguay and is a typical granite-gneiss-migmatite terrane. This belt is a key area for understanding West Gondwana assembly during Neoproterozoic and Early Paleozoic times, because of its direct connection to the Gariep and Damara belts in southern Africa. The present study defines the main tectonic phases of the ca. 600 Ma Dom Feliciano event in the Sao Feliciano belt. U-Pb zircon data for flat-lying gneisses yield ages between 610 ± 5 Ma and 616 ± 2 Ma, which we believe correspond to the approximate age of thrusting. The strike-slip deformation (main transcurrent phase) is well dated by U-Pb zircon ages for the syn-transcurrent granites (Arroio Moinho Granite, 595 ± 1 Ma; Encruzilhada do Sul Granite, 594 ± 5 Ma). These results indicate a relatively rapid evolution, from about 620 Ma (upper limit for the age of the gneiss) to 594 Ma (syn-trancurrent granites), for the known thrust related and strike-slip related tectonic phases of the Dom Feliciano belt. Sm-Nd results can be considered in three major groups. The first group (I) includes Brasiliano gneisses, granitoids, and one anorthosite with TDM ages of ca. 2.0 Ga and very negative eeNd(600) values. They may represent either direct melting of Transamazonian (Paleoproterozoic) basement or extensive contamination with older material of Paleoproterozoic to Archean age. The second group (II) includes granitoids and gneisses with TDM model ages from 1.31 to 1.41 Ga. The third group (III) comprises samples with TDM ages between 1.58 to 1.75 Ga. For groups II and III it is clear these rocks or their protoliths represent pre-Brasiliano continental crust. Unlike Group I rocks, groups II and III granites and gneisses may also contain a small fraction of a juvenile Brasiliano material. However, we have not yet found any sample from the Dom Feliciano belt with a Neoproterozoic TDM age and positive eNd value at 600 Ma that could be considered largely juvenile. Based on results from the Vila Nova belt, in which the main orogenic process developed between 753 and 704 Ma, we conclude that the Vila Nova belt was stable for over 100 Ma before the Dom Feliciano event reached its peak. It is probable that the collage of terranes in the Dom Feliciano belt and the region comprised by the Tijucas and Vila Nova belts were assembled during the Dom Feliciano event (ca. 600 Ma).

The Pb/Pb age of the Minas Supergroup carbonate rocks, Quadrilátero Ferrífero, Brazil

Abstract Pb/Pb isochron ages determined on carbonate rocks from the Minas Supergroup, Quadrilatero Ferrifero (QF), show that the deposition of the Gandarela Formation, the intermediate unit of the Minas Supergroup, took place at 2420 ± 19 Ma. The entire sedimentary sequence and underlying basement rocks have been deformed at a regional scale, and data from greenschist facies carbonate rocks from the Piracicaba Group indicate that the Pb isotopes were reset at about 2100 Ma. This age is in agreement with whole-rock Rb/Sr and U/Pb titanite ages determined by many authors for metamorphism of the surrounding Archean granite-gneiss terrane.

SHRIMP U/Pb zircon geochronology of Neoproterozoic juvenile and crustal-reworked terranes in southernmost Brazil

U/Pb zircon ages of six rock units from southernmost Brazil were obtained by means of SHRIMP II techniques. The studied rocks occur on either side of the Neoproterozoic contact between a juvenile accreted terrane (the Vila Nova Terrane) and a crustal-reworked belt (the Dom Feliciano Belt). This contact is located in the western portion of the exposed Precambrian rocks in the state of Rio Grande do Sul, Brazil. Contrasting patterns of U/Pb ages are shown by zircons hosted in metamorphosed basic and granitic rocks of the Cerro Mantiqueiras region and in the post-tectonic Lavras Granite, both located in the Vila Nova Terrane and in the syntectonic crust-derived Cacapava Granite in the Dom Feliciano Belt. Contrasting isotopic characteristics also are supported by Nd and Sr isotopes. For the Vila Nova Terrane, Nd model ages are no older than 1.3 Ga, eNd(740 Ma) values are between +2 and +6 and 87Sr/86Sr ratios are low, ∼0.704 (Babinski et al., 1995; Leite, 1997). In the Dom Feliciano Belt, in contrast, Nd mode...

Pb–Pb dating and Pb isotope geochemistry of Neoproterozoic carbonate rocks from the São Francisco basin, Brazil: implications for the mobility of Pb isotopes during tectonism and metamorphism

Abstract A U–Pb study was carried out on carbonate rocks from the Neoproterozoic Bambui Group in the southern part of the Sao Francisco basin, Brazil. Pb isotopic compositions and U and Pb concentrations were determined on more than 90 samples from different parts of the basin. These samples were found to contain four distinct types of Pb, here called Types I, II, III and IV. Type I Pb was found in samples with low Pb concentrations and relatively high U concentrations; it represents in situ growth of radiogenic Pb and has a large enough variation to define Pb/Pb isochron ages. Type II Pb is present in samples with relatively high Pb concentrations and low U concentrations; it is nonradiogenic crustal Pb that could either represent average crustal Pb at the time of deposition or, as interpreted here, at the time of deformation of these rocks. Type III Pb is also found in samples with high Pb concentrations and low U concentrations, but it apparently is radiogenic crustal Pb from the Archean or Paleoproterozoic basement that was incorporated into the carbonates during the 600 Ma Brasiliano orogeny. Type IV Pb is intermediate in composition between Type III and Type I Pb; it represents a mixture of these two types. Pb/Pb isochron ages obtained from mesoscopically undeformed carbonates containing Type I Pb range from 686±69 to 520±53 Ma; the older age is the minimum depositional age for carbonate rocks from the Sete Lagoas Formation. During the interval from 690 to 500 Ma, the Pb isotope system of carbonates from the Sao Francisco basin was disturbed, and in some areas it was totally reset. Type III Pb compositions define a straight line; Type II Pb falls on the lower end of this line, which intercepts the Stacey and Kramers (S&K) Pb growth curve at about 520 and 2100 Ma. This line is interpreted as representing a stage of crustal Pb evolution beginning about 2100 Ma and ending about 550 to 500 Ma, when variably radiogenic Pb from the basement was incorporated into these carbonates. The ages determined in this study are in agreement with most published ages for Brasiliano fold belts marginal to underlying Sao Francisco craton, showing that isotopic systems of Sao Francisco basin rocks were largely affected by Brasiliano tectonism.

The precise temporal calibration of dinosaur origins.

Significance Many hypotheses have been put forth to explain the origin and early radiation of dinosaurs, but poor age constraints for constituent fossil assemblages make these scenarios difficult to test. Using precise radioisotopic ages, we demonstrate that the temporal gap between assemblages containing only dinosaur precursors and those with the first dinosaurs was 5–10 million years shorter than previously thought. Thus, these data suggest that the origin of dinosaurs was a relatively rapid evolutionary event. Combined with our synthesis of paleoecological data, we demonstrate there was little compositional difference between the dinosaur precursor assemblages and the earliest dinosaur assemblages, and thus, the initial appearance of dinosaurs was not associated with a fundamental shift in ecosystem composition, as classically stated. Dinosaurs have been major components of ecosystems for over 200 million years. Although different macroevolutionary scenarios exist to explain the Triassic origin and subsequent rise to dominance of dinosaurs and their closest relatives (dinosauromorphs), all lack critical support from a precise biostratigraphically independent temporal framework. The absence of robust geochronologic age control for comparing alternative scenarios makes it impossible to determine if observed faunal differences vary across time, space, or a combination of both. To better constrain the origin of dinosaurs, we produced radioisotopic ages for the Argentinian Chañares Formation, which preserves a quintessential assemblage of dinosaurian precursors (early dinosauromorphs) just before the first dinosaurs. Our new high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U–Pb zircon ages reveal that the assemblage is early Carnian (early Late Triassic), 5- to 10-Ma younger than previously thought. Combined with other geochronologic data from the same basin, we constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs. After their origin, dinosaurs only gradually dominated mid- to high-latitude terrestrial ecosystems millions of years later, closer to the Triassic–Jurassic boundary.

Tectonic setting and U/Pb zircon dating of the plutonic Socorro Complex in the Transpressive Rio Paraíba do Sul Shear Belt, SE Brazil

The Precambrian Rio Paraiba do Sul Shear Belt comprises a 200-km-wide anastomosing network of NE-SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic-migmatitic, and granitoid terrains as well as low- to medium-grade metavolcano-sedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west-northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen-parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike-slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right-lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono-metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calc-alkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones.

Evolução tectônica do Cinturão Dom Feliciano no Sul do Brasil: relações geológicas e geocronologia U-Pb

The Dom Feliciano Belt is an important Neoproterozoic to Cambrian orogenic complex, extending from eastern Uruguay to southern Brazil. It comprises a collage of oceanic domains and continental fragments developed between 900 and 540 Ma between the Rio de La Plata, Congo and Kalahari cratons. The integration of field and structural data with recent isotopic results has introduced new insights on the sources of the magmatism and sedimentary processes. This paper presents a review of the geochronological results combined with stratigraphic, structural and geochemical data. The evolution of the Dom Feliciano Belt involved three orogenic events known as the Passinho (0.89 - 0.86 Ga), Sao Gabriel (0.77 - 0.68 Ga) and Dom Feliciano (0.65 - 0.54 Ga). The first two events involved the closure of the Charrua Ocean generating an intra-oceanic arc (Passinho) and, subsequently, an active continental margin arc (Sao Gabriel). This ocean separated the continental areas represented by the Rio de la Plata Craton and the Nico Perez continental microplate. Closure of the Adamastor ocean resulted in an important collisional event between the Nico Perez Microplate/Rio de La Plata Craton and Kalahari and Congo cratons between 650 and 620 Ma, involving high T/intermediate P metamorphism. At this time of crustal thickening, the partition of the deformation controled the final evolution of the belt with important escape tectonics, responsible for nucleating crustal-scale transcurrent shear zones. These structures were deep and promoted the rise of mafic magmas, which, associated with high regional thermal gradient, lead to an important event of crustal reworking, responsible for the formation of the Pelotas Batholith. The orogenic collapse is represented by late magmatism of Pelotas Batholith and deposition of upper section of the Camaqua Basin.

U-Pb zircon in situ dating with LA-MC-ICP-MS using a mixed detector configuration

The LA-MC-ICP-MS method applied to U-Pb in situ dating is still rapidly evolving due to improvements in both lasers and ICP-MS. To test the validity and reproducibility of the method, 5 different zircon samples, including the standard Temora-2, ranging in age between 2.2 Ga and 246 Ma, were dated using both LA-MC-ICP-MS and SHRIMP. The selected zircons were dated by SHRIMP and, after gentle polishing, the laser spot was driven to the same site or on the same zircon phase with a 213 nm laser microprobe coupled to a multi-collector mixed system. The data were collected with a routine spot size of 25 μm and, in some cases, of 15 and 40 μm. A careful cross-calibration using a diluted U-Th-Pb solution to calculate the Faraday reading to counting rate conversion factors and the highly suitable GJ-1 standard zircon for external calibrations were of paramount importance for obtaining reliable results. All age results were concordant within the experimental errors. The assigned age errors using the LA-MC-ICP-MS technique were, in most cases, higher than those obtained by SHRIMP, but if we are not faced with a high resolution stratigraphy, the laser technique has certain advantages.