Geochemical characteristics and new eruption ages of ruby-related basalts from southeast Kenya

Abstract

Two ruby-related basaltic fields were recently discovered in the southeast region of Kenya, exposed in the Nguu and Ngulai Hills vicinities. These fields host abundant deep-seated xenoliths, including corundum-bearing granulites. The basalts are alkaline affinity having compositions from foidrite to basanite. The Ngulai basalts have a wider range of SiO2 (38.2 wt.%–44.8 wt.%) covering those of the Nguu basalts (38.7 wt.%–42.3 wt.%). This overlapping behavior also holds for other major oxides and trace elements, e.g., Al2O3, Na2O, K2O, Cr, Ni, Rb and Ga. The overall OIB-like incompatible patterns with strong K depletion and slight spike of Ti enrichment signatures imply low degrees of partial melting of the upper mantle region source induced under a mantle plume-related process. The K-depletion signature indicates a residual K-bearing phase still retained in the source domain. Chondrite-normalized REE patterns exhibiting strong LREE enrichment without Eu anomalies suggest that plagioclase fractionation is insignificant. New 40Ar/39Ar ages indicate eruption events occurred during the Pleistocene times, which are around 2 Ma for the Ngulai basalts and 0.9 to 1.6 Ma for the Nguu basalts. Clinopyroxene-basalt thermobarometric calculations yield the equilibrium P-T ranges of ∼8-29 kbar and 1 200–1 450 °C.