The petrology and mineral chemistry of the Shira ring complex, Northern Nigeria

Abstract

The Shira complex belongs to the anorogenic province of Niger and Nigeria. It is dominated by peralkaline syenites and granites and has an estimated area of 152 sq.km. Apart from a related intrusion of granite porphyry at Zigau to the southwest, the Shira complex can be divided into three centres. Centre 1 is dominated by the Shira quartz syenite and this has been intruded by a large cone sheet of Birji granite and by microgranite dykes. These peralkaline syenites and granites are believed to represent successive intrusions from a progressively differentiating magma chamber. The Birji granite has two facies distinguished by the habit of the arfvedsonite and both facies exhibit layering. Centre 2 is composed of the Andaburi granite and a small intrusion of Amdulayi syenite. Centre 3 consists of the very poorly exposed Eldewo aluminous biotite granite.

The Zigau granite porphyry contains phenocrysts of alkali feldspar, quartz, fayalite and ferrohedenbergite in a quartzofeldspathic groundmass. The Shira quartz syenite consists of microperthite, quartz, ferrorichterite-arfvedsonite and aenigmatite. The Birji granite contains microperthite, quartz, arfvedsonite and a little aegirine, and the dykes related to it have a similar mineralogy but are more mafic-rich. The Andaburi granite consists of microperthite, quartz, ferrorichterite-arfvedsonite and aegirine. The Amdulayi syenite has three facies which collectively contain microperthite, quartz, ferrowinchite- arfvedsonite, aenigmatite and aegirine . The Eldewo granite contains microperthite, quartz and biotite.

Several minerals have been discovered which are either new to the province or are recorded for the first time in Nigeria, namely, aegirine-hedenbergite, chevkinite and narsarsukite. In the Zigau granite porphyry, fayalite has a composition of Fa93-100 and the pyroxene is ferrohedenbergite with sodic margins. Amphiboles from the Shira complex exhibit a compositional trend from ferrorichterite or ferrowinchite to arfvedsonite. In the syenitic rocks there are two periods of alkali pyroxene growth – early aegirine-hedenbergite and later aegirine, separated by a period of anphibole and aenigmatite crystallisation. Aenigmatite occurs in the syenitic rocks, which have the highest Ti contents, but it shows limited compositional range. Biotite composition in the aluminous granite is close to the theoretical annite end-member.

Whole rock geochemical data suggest that there is a progression from the syenites to the highly fractionated granites. With regard to the occurrence and characteristics of the peralkaline and aluminous granites in particular, it is concluded that the peralkaline syenites and granites from Shira are the result of fractional crystallisation from a basaltic parent, whereas the aluminous rocks are the result of partial melting in the continental crust.