Abstract
Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon UPb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415±2.6Ma; internal zone-413.5±2.5Ma), in line with a pyrite PbPb isochron age (413±22Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon (~413Ma) εHf(t) (−1.53−+3.24), low εNd(t) values (−2.4), and old Hf and Nd model ages (tDM2=1.5–1.19Ga, T2DM=1.35Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from −24.81 to −13.34 of log fO2 and 570°C to 793°C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (−3.9 to −2.8) and T2DM (1.47–1.37Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of magma. However, the heterogeneous Nd isotopic composition in individual pegmatites imply an incorporation of wallrock-derived melt with less radiogenic Nd.
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