The Effect of Reaction Overstep on Garnet Microtextures in Metapelitic Rocks of the Ilesha Schist Belt, SW Nigeria

• Published in: Journal of petrology Vol. 44; no. 6; pp. 967 - 994
• Main Authors: ZEH, A., HOLNESS, M. B.
• Format: Journal Article
• Language: English
• Published: Oxford Oxford University Press 01.06.2003; Oxford Publishing Limited (England)
• Subjects: garnet microtexture; metapelite; Nigeria; pseudosection; reaction overstep; THERMOCALC
• ISSN: 0022-3530; 1460-2415; 1460-2415
• DOI: 10.1093/petrology/44.6.967

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–muscovite–chlorite–magnetite and (C) sillimanite–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, biotite and ilmenite. P–T pseudosections calculated in the system CaO–Na2O–K2O–TiO2–MnO–FeO–MgO–Al2O3 –SiO2 –H2O ± O2 suggest peak metamorphism at 590 ± 20°C at 5 ± 0·5 kbar, followed by 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.