Abstract

We have conducted a detailed chemical, structural, and spectroscopic study of the alteration products of cordierite in meta-rhyolites from the Ceuta zone (Rif belt) with the aim of determining the nature of the products, the main stages of cordierite transformation and the metamorphic pressure-temperature path. In this zone, meta-rhyolites are in tectonic contact with granulites and migmatites interpreted as the feeder zones for the granitic melt. This study has revealed that N-bearing magmatic cordierite was transformed in concentric aggregates of mica with decreasing NH4 contents (tobelite, NH4-bearing white mica, and K-micas), probably through three successive processes: deuteric alteration, prograde reactions, and retrogressive reactions. The presence of N in cordierite is interpreted to be due to the transfer of the NH4, initially contained in biotite from granulites, during the melting stage. This finding suggests that magmatic cordierite can act as an important nitrogen reservoir during formation of granite melts and differentiation.

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