Petrogenesis of ruby-bearing xenolith from Simba and Emali basaltic terranes, Kenya

Abstract

Abundant ruby-bearing granulite xenoliths have been found in association with ultramafic xenoliths in the northern Chyulu Volcanic Province, within two small basaltic terranes of the Nguu Hills and the Ngulai Hills, located in the vicinity of Simba town, SE Kenya. The host basalts belong to an alkali affinity ranging in chemical composition from foidite to tephite/basanite. The chemical characteristics suggest that the magmas are derived from partial melting of an amphibole- and/or phlogophite-bearing spinel lherzolite in the upper mantle source region that involves the interaction between pre-existing zones of lithospheric weak zones, caused by the evolution of the Mozambic Belt (~500-800 Ma), and slightly upwelling of mantle plume. The observed xenoliths, e.g. peridotite, pyroxenite and granulite, bear equivalent basaltic composition. However, complex reactions, as well as plastic deformation, combined with geochemical characteristics of these xenoliths may lead to progressive transformation of a layering mafic (gabbroic/anorthositic) cumulate protolith including corundum-bearing formation and other related rocks under a condition of granulite facies. Based on geothermobarometric constraints, these rocks appear to have undertaken high P-T ranges of ~750-1500 ℃ and 5-23 kb equivalent to depths of the lower crust (~20 km) across the Moho (~44 km) down to the upper mantle (~75 km), corresponding to where the host magmas generated (~30-90 km). The corundum-related basalt eruptions had intermittently occurred during late Pliocene (2.1±0.09 Ma) to middle Pleistocene (0.83±0.03 Ma) times.