Magmatic evolution of the Shira Volcanics, Mt Kilimanjaro, Tanzania

Abstract

Mt Kilimanjaro, Africa's highest mountain (5895m), is a large, young (<1.6Ma) stratovolcano at the southern end of the East African Rift, in northern Tanzania. Consisting of three distinct volcanic centres, Shira, Mawenzi and Kibo, Shira contains the highest proportion of mafic rocks. Shira samples are strongly silica under-saturated rocks, ranging from picro-basalt, to nephelinite and hawaiite (Mg numbers (Mg #) ranging from 77.2-35.5). Phenocrysts constitute up to 55% of some samples, and include aluminous augite (often containing abundant fluid and/or melt inclusions), olivine (Fo92-Fo49), plagioclase (An75-An42), nepheline (Ne77-Ne68), magnesiochromite and ulvöspinel. Groups identified on the basis of phenocryst assemblages and textures correlate with location. East Shira Hill samples contain olivine and clinopyroxene phenocrysts + microphenocrysts of plagioclase (Group 1), or plagioclase and clinopyroxene phenocrysts + microphenocrysts of olivine (Group 2). Samples with high Mg #'s contain abundant cumulate clinopyroxene and olivine (Fo92-Fo85). Group 3 samples (Shira Ridge) contain nepheline phenocrysts and Group 4 samples (Platzkegel) have distinct intergranular textures. Chondrite normalised REE patterns are steep, with light REE-enrichment up to 400x chondrite. Spider diagrams, normalised to OIB for primitive Shira samples have strong K depletions and Pb enrichments. The source of the Shira volcanic rocks is most likely an amphibole-bearing spinel lherzolite, in which amphibole remains residual. Similarities in spider diagram patterns and trace element ratios suggest a source similar to average OIB. The Shira volcanic centre is a polygenetic volcano, in which multiple small volume, low degree (4-10%) partial melts from a metasomatised subcontinental lithospheric mantle follow pre-existing structural weaknesses, before ponding in the lithosphere. Evolution of these small volume melts is dominated by shallow fractional crystallisation of clinopyroxene, olivine±spinel, with plagioclase also fractionating from Group 4 (Platzkegel) samples. A magma mixing origin is suggested for some samples and supported by complex zonation patterns in major and trace element chemistry of clinopyroxene phenocrysts as well as linear mixing arrays. The Shira volcanic centre has since ceased activity, and collapsed to form the present day Shira Ridge and caldera before being overlain by various Kibo and parasitic lavas to the east and northwest of the Shira region