A new occurrence of hydrous andradite within pyroclastic rocks of the Sierra De Bahoruco, Dominican Republic

Abstract

Titanium-bearing hydrous andradite (up to 3.31 wt% TiO2) was identified in intensely altered pyroclastic rocks in the Sierra de Bahoruco, Dominican Republic. It forms small isolated crystals, fracture infillings, and large polycrystalline aggregates, associated with clinochlore and calcite. Other minerals in the secondary assemblage, formed through hydrothermal or supergenic processes, include prehnite, pumpellyite, and natrolite. This assemblage resulted from metasomatism under low-temperature zeolite facies conditions, requiring high-pH fluids enriched in Ca and Fe. The calcium needed for garnet crystallization likely came from primary calcic plagioclases, diopside, and surrounding limestones. Geothermometry on coexisting chlorite indicates that andradite formed at around 160°C under oxidizing conditions. This hydrous titanian garnet represents one of the latest stages of calcium metasomatism in the region.