Zoned Ca-amphiboles and related P-T evolution in metabasites from the classical Barrovian metamorphic zones in Scotland

摘要

Metabasites in the Dalradian Argyll and Southern Highland Groups experienced multiphase deformation and coeval Barrovian-type prograde and retrograde metamorphism during the Caledonian Grampian orogeny. Metamorphic Ca-amphiboles crystallized with plagioclase, epidote, chlorite and quartz, and sometimes with garnet and clinopyroxene. The minor changes in metabasite mineral assemblages and systematic mineral-chemical variations in Ca-amphibole confirm the classical concept of increasing metamorphic grade in metapelitic Chlorite to Sillimanite zones. In the Chlorite zone, high-Ti brown amphibole enclosed by green Ca-amphibole is interpreted as a magmatic relic. In the Chlorite, Biotite, Garnet and Andalusite zones, Ca-amphibole displays zonation with actinolite in cores and magnesio-hornblende to tschermakite in rims. Poor amphibole zonations occur in the Kyanite and Sillimanite zones. Ca-amphibole zonations are best described in terms of ~(IV)A1, ~(VI)A1 and Ti which semiquantitatively monitor temperature and pressure changes. Maximal ~(IV)A1 in amphibole increase with metamorphic grade. Empirically calibrated amphibole equilibria enabled us to reconstruct coherent prograde P-T paths and maximal P and T from the mineral zones; with T_(max) of 680 + - 50 deg C in the Sillimanite zone and P_(max) of 8 + - 1.2 kbar in the Kyanite zone. Prograde P-T paths indicate that elevated geothermal gradients should be considered even at the beginning of the Grampian Barrovian metamorphism.