Petrological characteristics of some cretaceous igneous rocks, in Southwest of Gboko, Southern Benue Trough, Nigeria

Detailed field geological mapping, outcrop petrography and thin-section microscopy are employed to delineate and describe igneous rocks, and to infer the tectono-magmatic and geochemical constraints affecting their emplacement in the Tse-Agberagba area, Southwest of Gboko, Southern Benue Trough.  Magmatic activity, coupled with folding and deformation were the effects of the Santonian tectonic episode in the area. Petrological (outcrop and thin section) data on these rocks indicate that they are mainly dioritic and doleritic to basaltic on hand specimens. Thin-section microscopy reveals that the dioritic rocks are generally medium-grained and mesocratic, characterized by randomly oriented laths of plagioclase and hornblende. The doleritic rocks are generally melanocratic, fine-grained and porphyritic, and are characterized by randomly oriented microlites of plagioclase of labradorite composition, with olivine and augite in a groundmass of aphanitic plagioclase, olivine and nepheline. The rocks are characterized by sub-ophitic to intersertal relationships between plagioclase and clinopyroxene which develops calcite as an alteration product. The mineralogical assemblages in these rocks suggest crystallization from an alkali-enriched magma, in a divergent tectono-magmatic setting.   Key words: Southern Benue Trough, magmatism, dioritic, doleritic, alkali-enriched magma, divergent tectonic setting.