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
