Crystallization conditions of the Carmo stock, NE Brazil: Implications for magmatic epidote-bearing granitoids petrogenesis

摘要

The Carmo stock is part of a 650-620 Ma magmatic-epidote bearing metaluminous calc-alkalic magma association intruded within the Cachoeirinha-Salgueiro domain (CSD, Northeastern Brazil). The pluton consists mainly of granodiorites and monzogranites, with abundant mafic microgranular enclaves (MMEs). Textural and chemical evidence indicate that epidote is magmatic, while titanite was partially re-equilibrated during sub-solidus processes. Amphibole in granitoids is hastingsite, and varies in composition from edenite to pargasite in the MMEs. Biotite exhibits similar values of Al-IV (2.3-2.5 apfu) in the MMEs and host rocks, but shows lower Fe/(Fe + Mg) ratios in the former than in the host granitoids (0.53-0.56 and 0.56-0.60, respectively). Pressures between 6 and 7 kbar and temperatures between 656 and 735 degrees C were estimated from the Al-in-hornblende geobarometer and the amphibole-plagioclase geothermometer, which are consistent with presence of magmatic epidote. The zircon saturation thermometer provided higher temperatures, between 749 and 776 degrees C, and are interpreted as near-liquidus temperatures. Estimative of fO(2) indicates moderately oxidizing crystallization conditions slightly below the NNO buffer (Delta NNO from -1.2 to -0.4). The initial water content estimated for these magmas suggests that they were highly hydrated (similar to 5-6 wt%), consistent with presence of early-crystallized hornblende. The chemical composition of ferromagnesian minerals indicates that the rocks of Carmo stock crystallized from I-type calc-alkalic magma and suggests interaction between juvenile and crustal components. Based on these new data, in combination with previously published data, we suggest that magmatic epidote-bearing granitoids from CSD were likely derived from partial melting of mantle-like sources along with recycled terrigenous sediment.