Whole-rock geochemical and zircon Hf–O isotopic constraints on the origin of granitoids and their mafic enclaves from the Triassic Mishuling pluton in West Qinling, central China

Abstract

The origin of mafic microgranular enclaves (MMEs) in granitoids bears significant information on their petrogenesis. An integrated study including petrology, mineralogy and geochemistry was carried out for monzogranites and the hosted MMEs as well as biotite granites from the Mishuling pluton in the West Qinling, central China. LA-ICPMS zircon U–Pb dating yields similar emplacement ages of 212 ± 2 Ma to 217 ± 3 Ma for the monzogranites, biotite granites and MMEs. All of them display similar whole-rock Sr–Nd–Hf–O isotope and in-situ zircon Hf–O isotope compositions, suggesting their origination from common crustal sources, most likely meta-igneous rocks.MMEs have igneous textures, and contain acicular apatite and amphibole. Plagioclase with distinct core-rim compositions and clinopyroxene with replacement texture by amphibole have also been observed in MMEs. These features suggest that MMEs were formed by mixing of an intermediate magma with a felsic magma. Biotite granites are weakly peraluminous, and have high zircon δ18O values of 7.6–8.8‰ and low zircon saturation temperatures, indicating their derivation from partial melting of crustal rocks at relatively low temperatures. Compared with biotite granites, monzogranites are metaluminous, have high zircon saturation temperatures, and contain amphibole, reminiscent of typical I-type granites. However, they also show high in-situ zircon δ18O values of 7.9–9.2‰. These geochemical features indicate that monzogranites were derived from crustal rocks at elevated temperatures. The sources themselves have experienced low-temperature interaction with surface waters. Therefore, partial melting of crustal rocks at different temperatures plays an important role in dictating the geochemical diversity of the resulted granitoids.