Disrupted peridotites and basalts from the Neoproterozoic Araguaia belt (northern Brazil): Remnants of a poorly evolved oceanic crust?

Fault-bound slices of serpentinized peridotites and pillow basalts interbedded with the metasedimentary rocks of the Tocantins Group in the central western Araguaia belt are probably remnants of poorly developed oceanic crust dismembered into a melange. The hydrothermally altered basalts display a fairly well-defined T-MORB, whereas the serpentinite lenses appear to be derived from moderately depleted upper mantle rocks, such as harzburgites and dunites. Neither sheeted dykes nor gabbros, which characterize a normal ophiolitic sequence, have been identified. The restricted volume of mafic volcanics, their moderate REE fractionation, the LREE enrichment of the serpentinized upper mantle peridotites, and the lack of other ophiolitic lithologies suggest that no appropriate geotectonic conditions were reached for normal oceanic crust development. Instead, it appears that the rifting of a large cratonic block resulted in local mantle exhumation and the formation of a partially mantle-floored basin, with sporadic basaltic magma supply. Thus, only a thin and discontinuous oceanic crust formed, somewhat similar to the Alpine-Apennine poorly evolved oceanic basins. Basin inversion occurred during the Neoproterozoic Brasiliano tectonic event before it reached an advanced ocean stage. The sedimentary sequence underwent intense deformation and greenschist facies metamorphism and was transformed into a complex system of westward to northwestward, overthrusted, imbricated slabs with slivers of basalt and serpentinized peridotites.