Hornblende-garnet metapyroxenite beneath serpentinite in the Ballantrae complex of SW Scotland and its bearing on the depth provenance of obducted oceanic lithosphere

Abstract

ABSTRACTMinimum T and P conditions for the crystallisation of banded amphibole (tschermakitic pargasite)-bearing garnet metapyroxenites that occur beneath and within a tectonically emplaced unit of serpentinite are 900±70°C and 10–11 (possibly 14–15) kb. This indicates both a depth provenance of at least 35–45 km for the obducted oceanic lithosphre which forms part of the ophiolite assemblage of the Ballantrae complex and also a palaeoheat-flow of at least 20°C/km. The metapyroxenites are interpreted as representing the products of subsolidus recrystallisation of a mantle accumulate pyroxenite. Together with associated amphibolites and epidote-rich mylonites formed at higher levels, they constitute a thin, transported and telescoped aureole made up of tectonic pieces of successively-formed metamorphic rocks that welded on to the base of an upward-moving and progressively-cooling peridotite slab.Estimates of temperature of recrystallisation of the metapyroxenites were obtained using the garnet-pyroxene and garnet-amphibole exchange reactions as geothermometers. Estimates of pressure were obtained from assessments of the stability fields of the amphiboles, clinopyroxenes and garnets present and of the mineral assemblages in relation to published studies on the eclogite-granulite-amphibolite facies system.