Abstract

The Fobur gabbros are situated in Jos East Local Government Area of Jos Plateau, Central Nigeria. They occur as minor intrusions within the migmatites, Jos biotite granites and Neil biotite granite. They are distinguished into quartz-olivine gabbro and normal gabbro. Mineralogically, they are composed of essentially plagioclases majorly, hornblende, with subordinate amounts of biotite, olivine, clinopyroxenes and titanaugite with little quartz, iron oxides and apatite occurring as accessory minerals. A total of 10 rock samples were analyzed for major, trace and rare earth elements using ICPMS. Geochemical features indicate that these gabbroic rocks are tholeiitic with weak alkaline affiliation and were formed within a continental tectonic setting (within plates). On the TAS classification diagrams, the rocks expressed variations in their composition ranging from gabbro – gabbroicdiorite – syenodiorite. Major oxide trends show that they are product of fractional crystallization with partial melting playing an important role in their petrogenesis. Their trace element geochemistry revealed enrichment in incompatible elements (LILE) relative to High Field Strength Elements (HFSE) as displayed in EMORB, MORB and OIB normalized spiderdiagrams suggesting that the magma which gave rise to the gabbros most likely came from the astenospheric mantle source rich in incompatible elements.

Key words: Fobur, crystal fractionation, gabbros, monzodiorites-syenodiorite, quartz olivine gabbro.