Unravelling the hidden evidences of magma mixing processes via combination of in situ Sr isotopes and trace elements analyses on plagioclase crystals

Abstract

Granites from the ~580 Ma oxidized A-type Salto Rapakivi Granite (Itu Granitic Province, SE Brazil) bear abundant felsic microgranular enclaves (FMEs) and scarce cm-sized mafic microgranular enclaves (MMEs) with whole-rock Sr and Nd isotope signatures identical to those of their host rapakivi granites. In order to unravel the sources involved in the formation of both enclave types we conducted in situ trace elements and Sr isotopic analyses on plagioclase megacrysts from representative rocks of rapakivi granites, porphyry granites and both microgranular enclave varieties. Plagioclase from host rapakivi granites have 87Sr/86Sr ~0.7068, and are relatively more radiogenic than those from the enclaves. Plagioclase xenocrysts from the FMEs typically register a rimward decrease in 87Sr/86Sr ratios, with core 87Sr/86Sr up to 0.7069, whereas rims are usually in equilibrium with whole rock (~0.7060–0.7063). Crystals with U-shaped Sr isotopes profiles (less radiogenic cores with rimward increase in 87Sr/86Sr ratios) are rare, as are crystals with strongly radiogenic signatures observed in partially digested xenoliths found within the enclaves (87Sr/86Sr ~0.709–0.710). Plagioclase from MMEs show more primitive signatures (87Sr/86Sr 0.7055–0.7058). Crystals from the MMEs are distinguished by higher Sr (~1000 ppm) and lower Eu (< 0.5 ppm) and light rare earth elements (LREE, 6–12 ppm) compared to crystals from FMEs, porphyry and granite, which have Sr = 200–600 ppm, and LREE = 20–80 ppm. Our results allowed us to infer that FMEs were formed by mixing between a resident melt similar to that of rapakivi granites with a less radiogenic melt similar to that which originated the MMEs. Therefore the combined use of Sr isotopes and trace element contents in plagioclase crystals from A-type granites might shed light on the sources of rock varieties that have similar whole-rock isotope signatures.