Preliminary Study of the Geology and Structural Trends of Lower Proterozoic Basement Rocks in Ogbomoso, SW Nigeria

Abstract

The rock distribution of Ogbomoso Town studied and mapped revealed that the study area has five rock types. The study area shows that the terrain is underlain by a migmatite – gneiss - quartzite complex. The relationship suggests an evolved granitic melt from a migmatite with a metasediment precursor. The rocks are generally low lying except for the porphyritic granite gneiss that shares the same relief with rocks at Oyo town. Outcrops encountered were studied for their mineralogical and structural characteristics. Twelve hand specimen samples for all rock type as well as four thin section slides cut for the granite and Porphyroclastic gneisses show the presence of quartz, potash feldspar, plagioclase, biotite and muscovite (especially from quartzofeldsparthic veins) as the main mineral phases. A high amphibolites facies is implied. Myrmekite, dissolution rim texture and vein anti-perthite were observed from thin section slides while the gneissose rocks generally reveal a range in fabric from idiomorphic to hyp-idiomorphic grains that are medium to coarse in size.The structural pattern from remotely sensed images was quite revealing as faults that were not conspicuous in the field probably due to vegetation were inferred. Lineaments trace which appear generally as curvilinear reveal sinistral NE/SW trend wrench fault. Measurements taken for the joints observed in the field describe a bimodal set. Two joint sets that trend NE/SW and NW/SE were observed to intersect at c. 60⁰. The terrain which does not appear to be highly jointed is suspected to be faulted at the southern and north eastern parts of the town.

Key words: Migmatite-gneiss, granitization, myrmekite, crenulation and lineament)