INTRUSIVE EQUIVALENTS OF FLOOD VOLCANICS: EVIDENCE FROM PETROLOGY OF XENOLITHS IN QUATERNARY TANA BASANITES

Abstract

The Injibara Quaternary basanites enclose a variety of xenoliths spanning in composition from peridotite through pyroxenite and gabbro to granite. This study focuses on the pyroxenite, gabbro and granite xenoliths. The pyroxenite xenoliths (enstatite + diopside + olivine ± spinel ± plagioclase ± ilmenite ± paragasite) are diverse, including olivine-orthopyroxenite, olivine-clinopyroxenite and websterite. They represent a suite of crystal cumulates from basalts with tholeiitic affinity evolving by polybaric crystal fractionation processes, or alternatively they may be related to chemical diversification of parental magma. The gabbro xenoliths, containing widely varying modal proportions of plagioclase, augite, enstatite, olivine and ilmenite, appear to be fragments of cumulate plutonic rocks, fractionated from tholeiitic basalts at high-level. They often show reaction textures, with a vermicular intergrowth of smaller augite, plagioclase and a chemically complex opaque phase composition embedded in glass from orthopyroxene. The granitic xenoliths contain quartz and two distinct feldspars–a sodic plagioclase and a potassic alkali feldspar– coprecipitated from the melt; accompanying mafic minerals are hydrous biotite and muscovite. This suggests that such granite magmas crystallized under water-saturated condition (PH2O = 5 kbar).