Alkaline Intrusive and Extrusive Rock from Areas West of the Anyim River, South-Eastern Benue Though

Abstract

Twenty-one outcrops of igneous bodies were studied in a small area (ca. 335 sq. km) of folded sediments west of Anyim River in the Lower Benue Rift. The extrusives occur mainly as basaltic to trachybasaltic tuffs and lapilli tuffs. The minor intrusives are found as basaltic and dolerite sills composed essentially of plagioclase (An,,), augite, with or without olivine. The major intrusives are dioritic small bodies considing of sodic plagioclase (An,,), biotite andor hornblende, and rarely nepheline. Common accessory minerals in these rocks are magnetite, pyrite, apatite, sphene and quartz. Calcite appears to be secondpry, and biotite, in some cases, is a pseudomorph after hornblende. The rocks are mostlyalkalic, total alkali ranges from 3.21 to 9.92% with Na,0,2.62 to 6.55% and K,O <3.76%. In an alkali-silica diagram, 80%oftheanalysespldisalkalic. These lower Benuerocks arealsoaluminous, A1 0 rangesfiom 12.96 to 19.95%. Inanalkali- ' ? alufnina diagram, the Benue rocks compare well with those of the Baikal rift, Siberia and d~ffefrr om those of the rocks from Eastern Uganda, East African Rift. The aluminous character of the Benue rocks studied suggest crustal contamination. The occurrence of alkalic suite in a divergent sift setting is considered to be a derivation fiom an upper mantle alkaline olivine-basalt magma through fractional crystallization.