The Geshere syenite-peralkaline granite pluton: a key to understanding the anorogenic Nigerian Younger Granites and analogues elsewhere

摘要

The Geshere complex, one of the few intrusive centers among the Nigerian Younger Granites that has not yet been characterized, contains an association of syenite and peralkaline granite. The suite shows a classic sequence of rocks related by fractional crystallization of a parental magma, leading to the pseudoternary minimum in the granite system. The syenite is hypersolvus and contains a ferro-richteritic amphibole as the dominant femic mineral, whereas the most evolved granite is subsolvus and contains annite as the dominant femic mineral. Textural evidence points to the crystallization of these minerals from an interstitial femic melt present at the solidus of these rocks, in which magnesium is a trace constituent. Such a late melt appears because of the low thermal stability of these extremely Fe-enriched minerals and the massive removal of alkali feldspar. Calcite inclusions in the primary amphibole and annite, locally defining an emulsion texture, are considered to represent globules of a carbonate melt present at the source.