Contrasting sources and P-T crystallization conditions of epidote-bearing granitic rocks, northeastern Brazil

Abstract

The 618Ma Curral de Cima tonalite and 577Ma Lourenço monzodiorite, northeastern Brazil, are magmatic epidote-bearing plutons that carry ferrohornblende, biotite, titanite, and epidote. Major, trace, and isotope chemistry suggests that the major magmas of the two plutons followed similar differentiation trends but derived from source rocks that differed in age and isotopic composition. The mineral phases of the Curral de Cima tonalite, the presence of amphibole-rich clots, and juvenile component (average εNd=-3.55) point to an I-type source for these rocks. These data and high calculated δ18O(w.r.) (10.0‰) for the tonalite and high δ18O value for a clot (9.3‰) argue that the clots are fragments of a metabasaltic source rock that has been hydrothermally altered at a low temperature. In contrast, average calculated δ18O(w.r.) for the Lourenço monzodiorite =7.8‰, 87Sr/86Sr=0.7083, εNd=-14.6, and TDM=1.92Ga. These data are compatible with a long crustal residence time of lower crust amphibolites source. Epidote in the Curral de Cima pluton crystallized close to the NNO buffer, and hornblende chemistry, due to Al reequilibration, yielded sub-solidus temperature and pressure. In contrast, in the Lourenço pluton epidote crystallized close to the HM buffer and Al-in-hornblende points to near-solidus solidification (685 °C) around 4.4Kbar. This study confirms that magmatic epidote in granitic plutons can crystallize at pressures lower than 5.5Kbar under higher fO2 as experimentally foreseen. Rapid magma transportation through hot continental crust during the peak of metamorphism in early stages of an orogenic cycle prevents epidote dissolution. Show more