The effect of reaction overstep on garnet microtextures in metapelitic rocks of the Ilesha Schist Belt, SW Nigeria

Abstract

Garnet-bearing assemblages of K-rich and K-poor metapelites from the Ilesha Schist belt, SW Nigeria, are investigated. K-rich samples contain the assemblages (A) garnet-staurolite-muscovite-chlorite-magnetite, (B) andalusite-garnet-staurolite-muscovile-chlorite-magnetite and (C) sillimanite and andalusite-garnet-muscovite-chlorite-magnetite. K-poor samples contain the assemblages (D) garnet-staurolite-cordierite-chlorite and (E) garnet-cordierite-chlorite ± staurolite. All assemblages contain quartz, plagioclase, biolite and ilmenite. P-T pseudosections calculated in the system retrogression to 550°C at 3·0 kbar, in agreement with field evidence, domain assemblages, mineral compositions, modes and geothermobarometry. The absence of compositional zonation shows that garnet in all investigated rocks nucleated and grew at constant P-T-X in equilibrium with associated minerals on the thin-section scale. However, the garnet-in reaction did not begin until the establishment of a significant temperature overstep of ∼80°C, with consequent rapid dehydration of chlorite- and muscovite-bearing assemblages resulting in interface-controlled garnet growth under locally hydrostatic pressure conditions. Garnet in rock types A and B formed with a characteristic pattern of biotite and quartz inclusions-a 'house of cards' texture. Additionally, clusters and aggregates of unzoned garnet grains formed in rock types A and D. Garnet resorption in rock types A-C resulted from prograde staurolite- and retrograde andalusite-forming reactions.