Petrology and Geochemistry of Granitoids of the Northern Part of Adamawa Massif, N.E Nigeria

摘要

The granitoids of the northern part of Adamawa Massif in northeastern Nigeria have been differentiated based on field and petrochemical data into the granodiorite and granites. Although there are slight mineralogical and geochemical differences between the granodiorite and the granites (e.g. Rb/Sr ratios lower in granodiorite than the granites), the two rock units have similar geochemical characteristics. The rocks are characterized by a wide range in SiO2, Calc-alkaline affinity, syn- to within-plate granite signatures, metaluminous to peraluminous composition and more K2O-rich and hypersthenes-poor comparable to fractionated I-type granitoids. The rocks display slightly fractionated to fractionated LREE, almost flat HREE patterns, with significant negative EU and Ba anomalies, Linear major element trends and progressive rise in SiO2, K2O, Rb and Rb/Sr ratios with depleting MgO, Fe2O3, CaO, TiO2, Sr and Ba consistent with removal of plagioclase during fractionation of basic melts to yield silicic magma. This linear trend is reflected in the normative mineralogy where orthoclase and quartz increase from granodiorite to the granites whereas other minerals behave in a reverse manner. Based on field and petrochemical features, the granodiorites and the granites of south Adamawa Massif are I-type, generated in a syn- to within-plate collision-related tectonic setting and genetically related to a common source by fractional crystallization dominated by the removal from the melt hornblende, plagioclase, biotite, K-feldspar and accessory phases such as apatie, epidote and zircon.