Bulk rock and mineral chemistries and ascent rates of high-K calc-alkalic epidote-bearing magmas, Northeastern Brazil

Abstract

A manifestation of the Pan-African-Brasiliano orogeny (700–550Ma) in northeastern Brazil was the emplacement of widespread Neoproterozoic granitoids in diverse tectonic terranes. Among these plutons are the magmatic epidote-bearing Conceição das Creoulas, Caldeirão Encantado, Murici, and Boqueirão plutons, located close to the boundary between the Alto Pajeú and Cachoeirinha–Salgueiro terranes. The plutons are high-K calc-alkalic granodiorites to monzogranites, with tabular K-feldspar megacrysts.Pistacite [atomic Fe+3/(Fe3++Al)] in epidote in these granitoids ranges from 21 to 27%. High oxygen fugacity (log fO2 −19 to −13) and the preservation of epidote suggest that the magma was oxidized. Al-in-hornblende barometry indicates hornblende solidification between 6 and 8kbar, at 620 to 780°C according to the hornblende–plagioclase thermometer. Zircon saturation thermometry attests to a near-liquidus temperature range from 794 to 853°C.Partial corrosion of magmatic epidote in these four plutons occurred during an interval of no more than 10–30years, which corresponds to maximum magma ascent rates of 650–1000m/year. Diking, associated with regional shearing, probably facilitated rapid transport of granitic magma through hot continental crust at peak metamorphism, and permitted survival of epidote that was out of equilibrium at the low pressure of final emplacement.Similarities between mineralogical composition, chemistry, and isotopic compositions (εNd(0.60Ga) between −2 and −5,TDM from 1.2 to 1.3Ga, δ18O values>10‰, V-SMOW) of these four plutons and Neoproterozoic magmatic epidote-bearing plutons elsewhere in northeastern Brazil, argue for similar metabasaltic/mafic sources that had previously experienced low-temperature alteration.