Petrographic Study of Basaltic Rocks of the Kerang Twin Volcanoes Within the Panyam Volcanic Line, Jos Plateau, Central Nigeria.

Abstract

The Kerang twin volcanoes are located at Kerang town and its environs in Mangu
Local Government Area, Jos Plateau, Central Nigeria. The study area covers an
aerial extent of 40 km² and is bounded between Latitudes 9o20ʹ02ʺ and 9o22ʹ05ʺN
and Longitudes 9o08ʹ02ʺ and 9o14ʹ02ʺE, part of Pankshin Sheet 190 (NW). The
Kerang twin volcanoes, as the name implies, are clusters of two (2) large volcanoes
which are major landmarks in the southeastern Jos Plateau Volcanic Province,
Central Nigeria. The traverse mapping method was used to map the area. Fifteen
samples from the field were subjected to petrographic analysis using the polarized
petrological microscope in the Department of Geology University of Jos, Jos, Nigeria.
The nature, size, and physical characteristics as well as the outcrop pattern of the
Kerang twin volcanoes were determined. Field findings showed that the Kerang twin
volcanoes extruded the pre-existing porphyroblastic gneisses complex, and the Pan-
African porphyritic biotite granite rocks. The volcanic rocks collected for this
research project are essentially basaltic rocks which are products of cinder and
composite volcanoes. Nodules of orthopyroxenes and crystals were observed both
in hand specimen and microscopically in the basaltic rocks of the study area.
Mineralogically, these rocks from the Kerang twin volcanoes, which are essentially
basaltic, are composed of majorly: phenocrysts of clinopyroxenes, titan augites,
olivines, orthopyroxenes, and labradorites embedded in finer groundmass and less
commonly ilmenites and iron oxides. Some of the olivine crystals exhibit overgrowth
features and interlocking grains (grains of olivine and clinopyroxene) which are a
result of reaction between the olivine crystal and the magma which is presumably
more silica-rich than the one the olivine crystal is in equilibrium with. Microcrystals
of clinopyroxenes were observed surrounding the periphery of the orthopyroxenes
(reaction rim). This suggests that the orthopyroxenes were formed from greater
depth first before the clinopyroxenes which is typical of a solid solution enstatite
and ferrosilite series