Geochemistry of the granitic rocks from the Kwandonkaya Complex, northern Nigeria

Abstract

The Kwandonkaya Complex forms one of the most prominent and typical ring structures in the Centre of Jos, Plateau State, Nigeria. Lithologically, it is composed of amphibole and biotite granites emplaced within Archean basement gneisses and migmatites. The chemical and mineralogical characteristics of this complex suggest that it was derived from crustal source materials. Trace elements are high in Rb, Li, F, Sn, Nb, W and low in Sr and Ba compared to the world average for rocks of similar composition indicating a high degree of fractionation. However, the K Rb ratios and petrographic evidence indicate that recrystallization may have redistributed the elements after the initial solidification of the rocks particularly in the biotite granites; and that the distribution of tin may have been initially controlled by crystal-silicate melt equilibra. Tin probably preferentially entered the aqueous phase during the late stages of solidification. It was the interaction between this phase and early formed minerals that led to greisenization and tin-tungsten mineralization. Although total rock chemical composition show no marked trends the composition of the biotites and hornblende define a trend in the compositional triangle Fe 3+Fe 2+Mg. When the biotites are related to the experimentally studied ternary system KFe 3 3+AlSi 3O 12(H−1)KFe 3 2+AlSi 3O 10(OH) 2KMg 3AlSi 3O 10(OH) 2 and coupled with the estimated position of biotite solid solutions for different oxygen buffers, the trend suggests that oxygen fugacities in the magmas during biotite formation were slightly higher than those defined by the Fe 2SiO 4SiO 2Fe 3O 4 buffer. The ratio of ferric to ferrous iron in pairs of the minerals from the Kwandonkaya Complex is the same for both hornblende and biotite and suggests crystallization in a closed but buffered environment.