Zoned Ca-amphibole as a new marker of the Alpine metamorphic evolution of phyllites from the Jubrique unit, Alpujarride Complex, Betic Cordillera, Spain

摘要

Zoned Ca-amphibole from metaclastic rocks and synfolial quartz-rich veins of the Jubrique area (Alpujarride complex, Betic Cordillera, Spain), and their significance in the Alpine metamorphic evolution are described here for the first time. Typical Al-rich metapelites from this area show assemblages with white mica and chlorite, with sporadic kyanite and chloritoid. Nevertheless, some Ca-richer phyllites, fine-grained quartzites and quartz veins, show assemblages consisting of Ca-amphibole, plagioclase, epidote, titanite, chlorite and quartz. Ca-amphibole, appearing in large radial grains as well as in smaller subhedral crystals, displays a zonation with actinolite cores, overgrown by magnesiohornblende (and edenite) and minor tschermakite (and pargasite or hastingsite). The zoning suggests growth during prograde metamorphism. Retrogressive rims are variably developed in the several lithotypes. General zonation is well described in terms of the tschermakite, the edenite and the Fe~(3+)Al_(-1), the MgFe_(-1)~(2+), the NaK_(-1) and the OHCl_(-1) exchanges. Chemical parameters (IVAl, A(Na+K), BNa) indicate an evolution from low- to medium-grade temperature and pressure. The Al-inamphibole thermobarometer provides a range of prograde temperatures between ~300℃ (for cores) and ~600℃ (for rims), for pressures from ~1 to ~6 kbar, respectively. Despite the uncertainty resulting from the fact that the thermobarometer used was calibrated for metabasites, amphibole defines a prograde pressure-temperature path typical of Barrovian-type metamorphism.