Transitional tholeiitic basalts in the Tertiary Bana volcano-plutonic complex, Cameroon Line

摘要

The Bana transitional tholeiitic basalts occurring in a Tertiary volcano-plutonic complex of the Cameroon Line, Central Africa are plagioclase-bearing and olivine-free. K/Ar dating on separated plagioclases of the transitional tholeiitic basalts yields an Oligocene age of 30.1 ± 1.2 Ma. Their clinopyroxene compositions are marked by iron enrichment and calcium depletion in the Wo-En-Fs system. The whole-rock major element compositions are characterized by Mg# ～ 36-48, normative quartz and hypersthene. The youngest alkali basalts from the same igneous complex have higher Mg# ～ 56-66. These two groups of basalt have trace element characteristics of within-plate basalt with Zr/Nb ratios of 3.7-4.5 and 7.5-9.2 respectively, and different LILE/HFSE and LREE/HREE ratios. The overall trace element characteristics suggest that the transitional tholeiitic basalts of the Bana complex were derived by high degrees of partial melting in the upper mantle at shallow depths whereas younger alkali basalts in the complex were probably produced by a small degree of melting of the same source at slightly greater depths. The transitional tholeiitic character of these basalts suggests a significant lithospheric extension and mantle upwelling below the Cameroon Line in the Oligocene.