Petrology of peridotite nodules from Ndonyuo Olnchoro, samburu district, central Kenya

Abstract

Sodic alkaline volcanic rocks were extruded repeatedly in enormous volumes from Miocene time onward in Kenya, eastern Uganda and northern Tanzania. The volcanic cone of Ndonyuo Olnchoro in central Kenya consists of Recent olivine melanephelinite agglutinates and lava that contain many ejected ultramafic nodules. The nodules are divisible into two groups. The first group contains rocks consisting of magnesian olivine, orthopyroxene, and small amounts of chromian spinel and clinopyroxene. Harzburgite and lherzolite comprise this group and account for more than 95% of the peridotite nodules by volume. The second group of ultramafic nodules consists of websterite consisting of orthopyroxene, clinopyroxene and small amounts of chromian spinel. This group comprises less than 5% of the peridotite nodules by volume. These two groups of nodules are of different origins. The pressure and temperature of formation of the first group are estimated as 1200–1350°C 36–38 kbar. Their MgO σFeO ratios fall within the limit of variation of nodules from kimberlite and are higher than those for oceanic lherzolites. Mineralogical and chemical characters of the phases in the first group indicate more affinities with those in mantle-derived rocks than with peridotites derived by accumulation from a basaltic melt. The nodules were originally garnet peridotites and were transported upward into a lower pressure regime presumably by mantle convection. The pressure and temperature of formation of the second group of nodules are estimated as 1090° C 16 kbar. They were produced by crystallization from an alkaline magma in relatively deep levels of the continental crust. Both groups of nodules were transported to the surface when olivine melanephelinite was erupted.