Igneous pyrometamorphism in the Potiguar Basin, Northeastern Brazil

Abstract

In the Potiguar Basin (NE Brazil), cretaceous rocks (sandstones, siltstones, shales, limestones) are intruded by Paleogene to Neogene basic bodies. As a result, were formed buchites, pyrometamorphic rocks indicating very low pressures and very high temperatures. Field descriptions permitted distinguishing light buchites (LB) and dark buchites (DB), which were investigated throughout petrographic, electron microprobe and X-ray diffraction studies. LBs contain abundant clasts of quartz surrounded by radial tridymite needles, besides phenocrysts of sanidine and clinopyroxene included in a vitreous groundmass. DBs have mainly microcrystals of Fe-cordierite (sekaninaite), mullite, armalcolite, ilmenite and spinel, dispersed within a black cryptocrystalline matrix. Chemically, LBs are richer in SiO2 (~76.7%) and K2O (~5.7%) and poorer in Al2O3 (~12.8%) when compared to DBs (respectively ~51.5, ~0.2 and ~42.7%). Based on phase diagrams published in the literature, the habit of crystals (acicular, elongated sometimes hollow crystals) and the significant content of glassy material we consider that the liquid formed by melting at ~1100 - 1150ºC of sedimentary material cooled quickly at very high temperatures and pressures below 1 kbar. The results obtained are relevant in petrological terms, and may also have economic implications since a large number of basic bodies intrude rocks with hydrocarbon reservoirs.