Petrogenesis and tectonomagmatic updates on the origin of the igneous rocks in the lower Benue rift, southeastern Nigeria

Abstract

This work presents updates on the tectonomagmatic origin of igneous rocks in the lower Benue rift based on recent available geochemical data. The igneous rocks occur as volcanic and plutonic rock series. The rock types are basalts and basaltic sills, pyroclastic rocks (tuffs and lapilli tuff), trachytes, trachybasalts, gabbros (stocks), dolerite sills, and dioritic rocks (stocks). The rocks were emplaced within folded sedimentary sequence of shales, subordinate sandstones, and siltstone and carbonate rocks of Asu-River and Eze-Aku Group. Most of the rocks are moderately to highly altered, with the pyroclastic rocks showing higher effects of alteration (LOI 3 to 22 %). Incompatible and immobile elements and less altered rocks with LOI < 3% show that the rocks were derived from mantle enriched source and possibly HIMU signatures. The high ratios of (La/Yb)N 4.40 to 31.55 in the basic rocks and 23.59 to 135.35 in the trachytic rocks show alkaline affinity with sparse tholeiites and garnets in the source region. The rocks were formed by plume uprising in a within-plate setting of intra-continental rift system similar to the South Atlantic Ocean rocks and East African rift. The magnesium number (Mg#) and differences of the spidergrams and rare earth patterns indicate that the rocks are cogenetic, and source is heterogeneous. The depletion of K, P, Th, and Rb and formation of calcites and sericites in the rocks show mobilization probably by interaction with calcareous host rocks in prevailing conditions.