Abstract

Deerite, a typical mineral of Fe-rich metacherts metamorphosed under blueschist conditions, is not rare, but known occurrences have up to now been restricted mainly to the Tethyan collisional zone and the Western Cordillera of North America. We describe a first occurrence in the high-pressure nappes of the Escambray Massif, Cuba, in the assemblage deerite + Mg-Al-poor riebeckite + magnetite + quartz ± garnet ± phengite ± aegirine. This assemblage typically forms during exhumation and accompanies late, stress-free annealing of the quartz matrix. Mg-Al-poor riebeckite overgrows older, large, oriented crystals of glaucophane, ferroglaucophane and Mg-Al-rich riebeckite (‘crossite’) during deerite formation. Early-formed hematite was largely replaced by magnetite. Deerite is very close to ideal composition, attaining >99% Fe2+12 Fe63+ Si12O40(OH)5, allowing direct application of the experimentally determined P-TO2 stability field (Lattard and Le Breton, 1994). In combination with oxygen-isotope thermometry on magnetite-quartz, the crystallization conditions of the deerite-bearing assemblage can be constrained to ~470°C, >15 kbar, and an oxygen fugacity restricted closely to the quartz-fayalite-magnetite buffer (ƒO2 ≈ 10−23 bar). Thus, the late-stage P-T path of the metacherts mirrors a steep P-T gradient of 10°C/km or less, requiring subduction of this part of the Antillean Island Arc to be still active during exhumation of the Escambray nappes.

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