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

Abstract

Zoned Ca-amphibole from metaclastic rocks and synfolial quartz-rich veins of the Jubrique area (Alpujárride 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 magnesiohomblende (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 Fe3+Al-1, the MgFe2+-1, 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-in-amphibole thermobarometer provides a range of prograde temperatures between ∼300°C (for cores) and ∼600°C (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.

Superposed and refolded metamorphic isograds and superposed directions of shear during late orogenic extension in the Alborán Domain, southern Spain

“…The timing and duration of the high‐ P stage is contentious, with muscovite Ar‐Ar ages ranging from circa 25 to 48 Ma [ Monié et al ., ; Platt et al ., ] in the Betics and circa 23 to 30 Ma in the Maghrebide belt and External Rif [ Michard et al ., ; Negro et al ., ]. Barrovian‐type medium‐ P / T metamorphism is likely associated with thermal relaxation following initial burial and is evidenced by an early garnet + staurolite + kyanite assemblage [ Azañón et al ., ; Manjón‐Cabeza Córdoba et al ., ] and prograde zonation in Ca‐amphiboles [ Ruiz Cruz , ; Ruiz Cruz and Sanz de Galdeano , ].…”