Post-Brasiliano (Pan-African) high-K granitic magmatism in Central Brazil: the role of late Precambrian-early Palaeozoic extension

In western Goias, Brazil, the emplacement of large, high-K postorogenic granites and associated small gabbro-dioritic intrusions, followed immediately after the last deformational events of the Brasiliano-Pan-African orogeny at ∼600 Ma. Well-fitted whole-rock RbSr isochrons indicate ages which suggest two discrete intrusive events: the older between ∼588 and 560 Ma and the younger between ∼508 and 485 Ma. The older granites display general petrographic and geochemical characteristics of highly differentiated calc-alkaline I-type granitoids, whereas the younger intrusions are more alkaline, similar to A-type granites. Initial 87Sr86Sr ratios vary from ∼0.703 to 0.710 and initial Nd isotope ratios yield ϵNd(T) values in the range between −4.0 and +3.0. There are no major differences in initial isotopic compositions between the two granite groups, suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the ∼940-640 Ma basement arc-type metatonalites-metagranodiorites and associated arc metavolcanics. The major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust, with younger melts being produced by the refusion of anhydrous, depleted crustal sources left after the extraction of previous batches of more hydrated calc-alkaline magmas. The heat input required to promote extensive remelting of the continental crust was, most likely, provided by mantle-derived mafic magmas that invaded and probably underplated the crust, during uplift and extension. The two intrusive events are bimodal in nature and are interpreted as shallow-level extension-related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the Proterozoic and early Palaeozoic, the older at ∼600 Ma and the younger between ∼550 and 510 Ma. The onset of the granite magmatism at ∼590 Ma, shortly post-dating the Brasiliano orogeny (∼600 Ma), is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic Paraguay Belt in Brazil and its correlative in Bolivia, the Tucavaca Belt, which probably correspond to rift deposits related to the break up of Laurentia from Gondwana at the end of the Proterozoic and beginning of the Palaeozoic. Granites of the younger group cut the deformational structures of the Paraguay Belt metasediments and pre-date, by between ∼40 and 20 Ma, the initial stages of subsidence and sedimentation of the Parana Basin.