Origin of atoll garnet in schists from the Alpujárride Complex (Central zone of the Betic Cordillera, Spain): Implications on the P-T evolution

Abstract

Atoll-shaped and normal garnets from schists of the Betic Cordillera (Spain) were studied by electron microprobe, scanning electron microscopy and microstructural electron backscattered scanning diffraction (EBSD). Medium-grade schists contain a textural variety of atoll garnets, characterized by the presence of muscovite, annite, and quartz as main “core” phases. Zoning patterns, EBSD analyses and themobarometric data indicate that the micaceous intergrowths formed in most cases through breakdown of a first garnet generation, with orientation of micas being controlled by garnet (c*mica//[111]*Grt and c*mica//[110]*Grt as main crystallographic relationships). Rings formed from multiple nucleation and coalescence, with orientation being controlled, in some cases, by the relics of the initial garnet and more generally by mica orientation. P-T estimates indicate that the first stage of garnet growth occurred at relatively high P (9–12 kbar/500–550°C) whereas the second metamorphic stage occurred at lower P and slightly higher T conditions (5–7 kbar/500–600°C).