Peak Metamorphic Conditions of Garnet Amphibolites Beneath the Semail Ophiolite: Implications for an Inverted Pressure Gradient

Abstract

Garnet amphibolites beneath the Semail Ophiolite formed at 1.1 ± 0.2 GPa and greenschist-facies rocks in the same metamorphic sole yielded pressure estimates of 0.45 to 0.55 GPa. Thus, in addition to the well-known inverted thermal gradient in subophiolitic metamorphic sequences, an inverted metamorphic pressure gradient of 0.3 to 2.0 MPa/m over <2000 m also exists. Temperature estimates for samples systematically collected every 10 m across a 70 m thick sequence that contains the assemblage garnet + plagioclase + amphibole (± diopside ± ilmenite ± titanite ± rutile) allowed the determination of an average inverted metamorphic temperature gradient of 1 to 4°C/m, using different thermometers. Pressures in garnet amphibolites were estimated from the presence of cm-sized kyanite, by comparison of the observed assemblage with relevant published experimental results, and by using heterogeneous equilibrium calculations. The different estimations result in peak P-T conditions of 800 ± 100°C and 1.1 ± 0.2 GPa. The inverted metamorphic pressure gradient can be explained by the continuous accretion of material formed under different P-T conditions, although there are no obvious structural breaks in outcrop. The ≤20 km thick overlying Semail Ophiolite is insufficient to produce the computed 1.1 ± 0.2 GPa lithostatic pressures. Piling of nappes or structural thinning at the base of the nappe and continuous denudation/structural thinning at rates of 0.36 to 0.45 cm per year during emplacement may explain the inverted pressure gradient.