Reavaliaçao da evoluçao tectono-magmática do Graben de Jaibaras (nordeste do Brasil)




The Jaibaras Trough (GJ), located in the Northeast Brazil, represents the most important, prominent and extensive exposure of Early Phanerozoic sedimentary sequences in a wide Brasiliano-age area, including Borborema Province and West Africa (mainly Nigerian Shield). The main basin feature is delineated by major brittle normal faults, resulted from reactivation of mylonite shear zones inside the so-called Transbrasiliano Lineament. The Jaibaras rift evolution is not only partially constrained by the stratigraphic record of the basin in fill, which is characterized by basal fault-scarp-related paraconglomerates followed laterally and vertically by fluvial-lacustrine sandstones which are interbeded with reddish shale, but also by the petrological signatura rift-related magmatic activity. Magmatic evolutive aspects of GJ comprises basically four temporal and spatially separated igneous steps. The earlier phase involved the Coreaú dike swa rm, which is Vendian in age, representing the initial tectonic pulse of the rift opening. With the continued breakup, reactivation of deeper shear zones gave rise to the emplacement of Mucambo Pluton during Early Cambrian times, preceding the main rift infill. The basin sedimentation was accomppanied by huge volume of volcanism, most of it occurring as flood basalts, dikes and sills in shape. The Meruoca Pluton is the last basin-related igneous manifestation during Upper Cambrian. Sequential stratigraphic interactions between sediments and magmatic events are perfectly constrained at the expense of thermal-structural features such as contact aureoles, faulted contacts, abuting and cross-cuting. Local and regional stress-field during GJ formation were obtained by using the configuration of magmatic bodies and their interplays with host-rocks. The physical-chemical characteristics of igneous activity also allows us to discuss the simultaneous thermal crustal-mantelic response due to magmagenesis and the emplacement processes. Initially, during the GJ nucleation, a NW-SE extensional pulse is recorded by the intrusion of the Coreaú Dike Swa rm, following hydraulic fracturing model. Following the same extensional stress-field, the studied plutons were emplaced by using nort h west-trending preexisting basement structures in areas of releasing bends where their feeder dikes were intruded. We hypothesize that two distinctive thermo-mechanical models are able to produce substantial heat flow (magma as consequence) within the JT and surrounding rift-host lineament. The model 1 invo l ves the peeling of the lower part of continental lithosphere from crust, followed by lithospheric thinning while hot upwelling asthenospheric replace the original cold lithospheric material. Decompression in the ascending mantle leads to partial melting and bimodal magmatism (crust and mantle) take place; the second model consists of underplating of mafic material probably in a stretched crust-mantle boundary. The changing from the model 1 to model 2 seems to be a progressive process.