Petrography Of Some Alfered Intrusive Rocks From The Lower Benue Trough, Nigeria

Abstract

Some intrusive rocks of doleritic to dioritic compositions horn the Ejekwe-Wanikande and Okigwe-Ishiagu districts in the

Lower Benue Trough have been studied using thin section microscopy and X-ray (powder) difiaction method. The rocks have

intmded pre-Santonian sedimentary rocks which are predominantly calcareous shales. The exposures of these intrusive rocks

generally have widths ranging from about 40m to 228 m. s

The intrusive rocks are generally melanocratic to mesocratic (dark.to-light grey) and medium-to coarse-grained. They

commonly contain well crystallized calcite which often appear as amygdales or cavity-fillings. Randomly oriented crystals of

plagioclase feldspars with intersertal to ophitic clinopyroxene showing high degree of alteration are recognizable. The alteration

assemblages in the dolerites consist of calcite, tremolite, chlorite (thuringite), epidote, analcime, serpentine, pyrite, with relict

primary constituents of oligoclase and diopside. The diorites contain the same alteration assemblages as the dolerites with

exception of serpentine, but with apatite and relict primary constituents including andesine and diopside.

The assemblage: tremolite, epidote, chlorite and analcime(a diagenetic zeolite) represents a metamorphic alteration still at the

lower part of the greenschist facies. The alteration assemblages are most probably the result of fluid/rock interaction