Cretaceous alkaline igneous rocks from the Águas Emendadas region, Goiás, central Brazil

Abstract

The area of study is located in central Brazil, in the southern portion of Goiás state. The alkaline igneous rocks from Águas Emendadas Region comprise volcanic and pyroclastic varieties, emplaced in Phanerozoic sediments of the Paraná Basin and in Precambrian basement rocks. They were formed during a magmatic event which took place during Upper Cretaceous and belong to the Rio Verde - Iporá Igneous Province. The pyroclastic rocks were formed by processes involving fluidization and phreatomagmatic events. Exsolution of volatiles and magma mixing are also involved in their genesis. The most common pyroclastic products are breccias with fragmental, lapilli-size "matrices" and fragments of various origins (e.g. accessory, cognate, juvenile) reaching up to - metric dimensions. Armoured lapilli, "spinning droplets" and frozen droplets" of magma occur in the breccia matrix and represent different stages of explosiveness. Lavas, empted In non-explosive intervals, are ultramafic to mafic (melaleucitites, melanephelinites, leucitites, basalts and basanites), They are usually porphyritic, with phenocrysts characteristically of olivine and/or clinopyroxene. Other common mineral phases include leucite (pseudo-leucite), nepheline, kalsilite, perovskite, phlogopite, Fe-Ti oxides and, in basalts and basanites only, plagioclase. The rocks were variably altered by a combination of hydrothermal processes and weathering. Minerals resulting from these alterations include carbonate, zeolites, serpentine and hydroxides. The chemical composition of several mineral phases is reported and discussed in terms of its effect on the magmatic processes. Fractionation of olivine, clinopyroxene, spinel-group minerals and perovskite controlled the chemical composition of magmas during evolution. The whole-rock chemical data show that these rocks are divided into two groups one Mg- rich and the other Mg-poor. The Mg-rich rocks are SiO(_2-)Poor, with high contents of CaO, TiO(_2) and incompatible elements, and have chemical affinity with kamafugites (Ti-rich diopside phenocrysts and groundmass kalsilite are consistent with this). The occurrence of magma mixing is supported by the chemical data.