The late stages of evolution of the Kwandonkaya A-type granite complex, Nigeria, as deduced from mafic minerals

In the Kwandonkaya Complex, an A-type metaluminous to peraluminous granite complex in northern Nigeria, the presence of: Fe2+-rich ferromagnesian phas es (fayalite, hedenbergite and amphibole) at the initial and annite at fina l stages of crystallisation indicates relatively reduced melts throughout ( less than or equal to QFM). Annite and associated species in the biotite gr anites provide the best indication as to the nature of volatile loss, albit isation and greisen formation. From the mica chemistry, it is inferred that degassing was accompanied by p referential loss of CI in the roof zones and margins of the plutonic rocks, with the resulting enrichment of F and inferred Li in mica from the drusy facies. During albitisation, the mica composition was rock-buffered with re spect to major constituents like Fe. However, the F+/-(LI, REE, Y, Nb, Ta, Sn) contents were enhanced during albitisation to produce F-rich mica assoc iated with disseminated-type topaz-columbite-cassiterite mineralisation. Gr eisen formation was accompanied by the buildup of Si, Al, Ti, F and possibl y Li, which is in agreement with enhanced normative quartz and corundum, an d increases in modal mica, quartz, topaz and fluorite. The mafic minerals a nd their alteration assemblages indicate that volatile loss, incipient subs olidus modifications, albitisation and greisen formation were associated wi th increases in f(HF) and f(H2O). Both oxidation and preferential CI loss p romoted the deposition of cassiterite at post-magmatic stages. (C) 2000 Els evier Science Limited. All rights reserved.