Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In this study, we report new U-Pb zircon ages and amphibole and biotite 40Ar-39Ar cooling ages for igneous and metaigneous rocks from the eastern portion of the Borborema Province (NE Brazil). We combine these results with preliminary thermobarometric estimates and existing data to place constraints on the timing and duration of the Brasiliano-Pan-African Orogeny in northeastern Brazil and on the subsequent late- to post-orogenic history. The Alcantil orthogneiss contains a complex zircon population, but the youngest concordant analyses yielded an age of 632 ± 17 Ma (2σ). These grains have low Th/U ratios (<0.1), and the age of 632 Ma is therefore interpreted as dating the metamorphic event responsible for the acquisition of the gneissic fabric. In the Caruaru biotite amphibole orthogneiss, concordant analyses provided an age of 618 ± 4 Ma (2σ), which we relate to emplacement of the protolith of the Caruaru orthogneiss. Together with available U-Pb data from the literature, the 632-600 Ma age range is regarded as dating development of a regional flat-lying foliation coeval with high-grade metamorphism (upper amphibolite facies conditions). The 586 ± 3 Ma age (2σ) of the Panelas pluton is similar to those of other granitic plutons associated with a voluminous plutonism of high-K calc-alkaline to shoshonitic nature that occurred at 590-580 Ma. This plutonism slightly predated or was synchronous with development of large strike-slip shear zones. Amphibole grains yielded 40Ar-39Ar ages ranging from 584 ± 4 Ma to about 530 Ma, suggesting that regional temperatures remained near the closing temperature of this mineral (≈500 °C) for a long time. Most 40Ar-39Ar biotite ages range from 552 to 533 Ma, but two samples yielded plateau ages of 519 ± 5 Ma and 509 ± 5 Ma. The inferred rather slow cooling of the studied region possibly reflects low rates of unroofing, allowing persistence of a protracted thermal anomaly caused by the massive injection of magmas at the end of the Neoproterozoic.
ISSN: 0895-9811

hal-00745698
https://hal.archives-ouvertes.fr/hal-00745698
DOI : 10.1016/j.jsames.2012.06.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Deformation of the continental lithosphere is traditionally modeled assuming that heat production in the subcontinental lithospheric mantle is negligible. Although this may be appropriate for highly depleted Archean lithosphere, studies of mantle xenoliths and lithospheric-derived mafic rocks suggest that heat productions of up to 0.4 ;μW/m3 may be attained in the mantle of Phanerozoic and Neoproterozoic regions due to modal metasomatism. To investigate the effect of a local enrichment in heat-producing elements within the lithospheric mantle on the continental deformation, we performed a series of 2D thermo-mechanical models simulating the deformation of a lithospheric plate 1000 km long, which contains in its central part a 200 km wide and 20 km-thick mantle layer with a heat production of 0.05;–0.25 ;μW/m3 between 40 and 80 km depth, leading to a local enhancement of the surface heat flow by 1;–5 mW/m2. Compression models show that a 20 km-thick layer with a heat production as low as 0.05 ;μW/m3 within the shallow lithospheric mantle leads to strain localization in both the enriched mantle and the overlying crust. Strain localization depends exponentially on the temperature increase in the lithosphere section, which is controlled by the volume of metasomatized mantle and the intensity of the metasomatic enrichment in heat-producing elements. A heat production of 0.25 μW/m3 between 60 and 80 km depth results, for instance, in strains within and above the metasomatized mantle 5 times higher than in the surrounding lithosphere. Strain localization also depends on the location of the heat-producing domain and on the plate rheological structure; deeper heat-producing domains and weaker plates leading to stronger localization. These results provide an explanation for strain localization in intraplate environments, leading to the formation of orogenic belts situated hundreds to thousands of kilometers away from known plate boundaries, as observed in the Neoproterozoic belts of Gondwana and in the present-day deformation of the Asian plate.
ISSN: 0012-821X

hal-00411386
https://hal.archives-ouvertes.fr/hal-00411386
DOI : 10.1016/j.epsl.2008.07.040

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Borborema Province of northeastern Brazil is divided into three main domains: northern, central, and southern. Several U-Pb zircon ages of plutons and orthogneisses became available in the recent years in the central and northern domains, but similar results are scarce in the southern domain. This study reports U-Pb dates for single zircon grains from one orthogneiss (Jupi orthogneiss) and two plutons (Cachoeirinha syenitic pluton and Cabanas granite) south of the East Pernambuco shear zone system (EPSZ). The results provide geochronological constraints on the timing of deformation and magmatism in this part of the southern domain and allow correlations with the central domain. The Jupi orthogneiss was emplaced and deformed during development of the regional flat-lying foliation. A Pb-206/U-238 weighted apparent mean age of 606 +/- 8 Ma is interpreted as the crystallization age of the protolith of the orthogneiss and consequently the age of high-grade Brasiliano metamorphism. The NNE-trending Cachoeirinha pluton is only locally affected by strike-slip deformation, whereas the ENE-trending Cabanas granite is intensely affected by deformation related to the EPSZ. The 587 8 Ma and 573 4 Ma ages of the Cachoeirinha pluton and Cabanas granite, respectively, bracket the main period of activity of the EPSZ. Tectonomagmatic activity in the study area is similar to the age of Brasiliano events in the central domain, north of the EPSZ. In addition, xenocrystic zircons in the Jupi orthogneiss and Cabanas granite are interpreted as inherited from Paleoproterozoic source rocks, suggesting the presence of widespread reworked old crust in the southern domain, similar to the central domain. These results support the idea that the central and southern domains belonged to the same crustal block before the onset of the Brasiliano orogeny.
ISSN: 0895-9811

hal-00411394
https://hal.archives-ouvertes.fr/hal-00411394
DOI : 10.1016/j.jsames.2007.06.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Detrital zircons in metasedimentary rocks attributed to three different complexes in the Central and Pernambuco-Alagoas domains of the Borborema Province, northeastern Brazil, were dated by LA-ICP-MS. Two samples from each of the following complexes were analyzed: Sertania and Surubim, in the Central Domain, and Rio Una, in the Pernambuco-Alagoas Domain. One sample of the Sertania Complex yielded only Paleoproterozoic zircons, whereas Neoproterozoic zircons predominate in the other, with the youngest zircon indicating deposition after 642 Ma. One of the samples of the Surubim Complex yielded, again, only Paleoproterozoic concordant zircon ages. In the other sample, although Paleoproterozoic zircons predominate, ages vary from the Mesoarchean to the Neoproterozoic. The age of the youngest zircon in this latter sample indicates deposition after 850 Ma and an overgrowth on an Archean grain yielded an age of 623 +/- 6 Ma (2 sigma), interpreted as the age of peak metamorphism. The two samples of the Rio Una Complex yielded predominantly detrital zircons with ages in the ranges 2.2-2.1 Ga and 1.7-1.6 Ga. In this complex, the youngest detrital zircon suggests deposition after 917 Ma and metamorphic zircons indicate peak metamorphic conditions between 600 Ma and 617 Ma.;The results of this study, together with available data from the literature, allow the following conclusions: (a) The Sertania Complex is a Neoproterozoic sequence, and not Paleoproterozoic, as previously thought; (b) The similarity in lithological associations, detrital zircon populations and C isotope signature between the Sertania and Surubim complexes suggest that they belong to the same lithostratigraphic unit; (c) Although the Rio Una Complex can be older than the Sertania/Surubim Complex, the lack of zircons younger than 900 Ma can also result from insufficient sampling or absence of young rocks alongside the drainage system that collected detritus to this complex: (d) The regional metamorphism related to the Brasiliano Orogeny attained its climax at 630-600 Ma; (e) The age spectra of detrital zircons require their derivation from erosion of rocks formed not only during known geological events in the Borborema Province and in the nearby Sao Francisco/Congo Craton, but also from the Amazonian Craton, suggesting accumulation in an intracratonic setting within a preexisting large continent: (f) The short timespan between deposition and orogenic deformation may explain the overall medium- to high-temperature metamorphism due to maintenance, in the subsequent compressional event, of the elevated geothermal gradients produced during broad-scale lithosphere extension.
ISSN: 0301-9268

hal-00443856
https://hal.archives-ouvertes.fr/hal-00443856
DOI : 10.1016/j.precamres.2009.09.009