Two stage mantle-derived granitic rocks and the onset of the Brasiliano orogeny: Evidence from Sr, Nd, and O isotopes

Abstract

The elongate Monteirópolis batholith (270km2) is composed of alkali feldspar granite to granodiorite, it is part of the Águas Belas–Canindé composite batholith and it intruded rocks of the Pernambuco–Alagoas Domain, northeastern Brazil. This batholith is bounded by the NNE–SSW-trending Jacaré dos Homens transpressional shear zone on its southwestern margin, and displays low-angle foliation, coeval to the development of a regional flat-lying foliation. Microgranular dioritic enclaves and amphibole-rich clots are abundant. The mineralogy of this pluton comprises biotite and amphibole as major accessory phases, and titanite and magmatic epidote as trace minerals. Major and trace element chemistry shows high SiO2, total alkalis, Ba and Sr, low Fe# and Nb contents, all of these conferring a high-K calc-alkaline character. The rocks are enriched in LREE and LILE and depleted in HFSE, and show fractionated chondrite-normalized REE patterns with Eu/Eu*=0.67 to 1.25. Chondrite-normalized spidergrams show marked negative Nb–Ta and Ti anomalies, typical of subduction-related magmas. U–Pb SHRIMP zircon data yielded a crystallization age of 626±4Ma. Regional structures and U–Pb geochronological data for the Jacaré dos Homens transpressional shear zone suggest that shearing was initiated at ca. 640Ma. Dilatational movements along this shear zone opened space for magma emplacement. The rocks in this batholith are characterized by slightly negative to slightly positive εNd values (−0.78 to +1.06), average Nd-model age of 1.0Ga, low initial 87Sr/86Sr(626 Ma) values of 0.7050 to 0.7052, and low δ18O values (zircon) of +5.00 to +5.94‰ V-SMOW. A possible protolith, Tonian mantle-derived rocks in the lower continental crust, could have been partially melted by underplating of mantle-derived mafic magma during collision of the São Francisco Craton and the Pernambuco–Alagoas Domain during onset of the Brasiliano orogeny.