Geochemical and isotope evidence for mantle-derived source rock of high-K calc-alkaline I-type granites, Pernambuco–Alagoas Domain, northeastern Brazil

Abstract

We report whole-rock major, trace, mineral, and isotope chemistries of the Jacare dos Homens, and Santo Antonio granitic rocks from the Aguas Belas–Caninde batholith, Pernambuco–Alagoas Domain of the Borborema Province, northeastern Brazil. These rocks exhibit low-angle foliation, suggesting emplacement under a regional strain field, associated with transpressive deformation tied to the onset of the Brasiliano orogeny. Both rocks are dominantly made up of K-feldspar, quartz, and plagioclase with biotite as the main mafic phase and minor hornblende. The Jacare dos Homens orthogneiss displays magnesian, slightly peraluminous, and alkali-calcic character, while the Santo Antonio granite is magnesian, metaluminous to slightly moderately peraluminous and alkali-calcic to alkalic. These granitic rocks are enriched in alkalis (up to 10.28 wt%), mg#, Rb, Ba, Sr, Th, and LREE, and depleted in HREE and HFSE, with distinct Nb, Ta, and Ti depletion. Decreasing MgO, CaO, TiO2, Na2O, Fe2O3T, P2O5, Al2O3, Sr, Zr, and Hf with increasing SiO2 are consistent with fractional crystallization of biotite, amphibole, plagioclase, apatite, zircon, titanite, and Fe–Ti oxides. U–Pb zircon dating of the Jacare dos Homens and Santo Antonio rocks yielded concordia ages of 642.4 ± 3 Ma and 636 ± 4 Ma, respectively. The isotope data for the Jacare dos Homens orthogneiss yielded an Nd-model age of 1.21 Ga with slightly negative eNd(642 Ma) of − 1.58, initial 87Sr/86Sr ratios of 0.7068, and δ18O (zircon) value of + 7.0‰ VSMOW. The isotope data for Santo Antonio pluton yielded Nd-model ages between 0.96 and 1.07 Ga, with eNd(636 Ma) from + 1.17 to − 0.67, initial 87Sr/86Sr ratios from 0.7048 to 0.7056, and δ18O (zircon) value of + 5.0 to 5.9‰ VSMOW. Altogether, the petrological, geochemical, and isotopic data are typical of I-type granites. We propose that the studied granitic rocks were derived from mantle-derived medium- to high-K basaltic lower crust of Stenian/Tonian (Jacare dos Homens) or Tonian (Santo Antonio) ages and that partial melting was triggered by the uplift of asthenosphere and underplating of lithospheric mantle. The migration and buoyancy-driven ascent of the magma to their present level were probably favored by the Jacare dos Homens shear zone.