Petrogenesis and implications for tin mineralization of the Beidashan granitic pluton, southern Great Xing'an Range, NE China: Constraints from whole-rock and accessory mineral geochemistry

Abstract

The Beidashan pluton, consisting of quartz monzonite in the northeast and biotite granite in the southwest, is a representative granitic intrusion in the southern Great Xing'an Range. To investigate its petrogenesis and relationship with tin mineralization, whole-rock and accessory minerals (zircon and apatite) geochemical analyses were carried out. The quartz monzonite mainly belongs to high-K calc-alkaline strongly peraluminous granite, and the biotite granite is high-K calc-alkaline weakly peraluminous granite. Zircon U-Pb dating indicates a younger age for the biotite granite (138 Ma) than the quartz monzonite (140 Ma). The zircon Hf and apatite Nd isotopic compositions of the quartz monzonite (εHf(t) = 5.2 to 9.1 and εNd(t) = −1.1 to 1.3) and those of the biotite granite (εHf(t) = 5.6 to 8.0 and εNd(t) = −0.9 to 1.0) are indistinguishable, implying that they were derived from the same sources. Modeling results using apatite Nd isotope data show that they were from a mixing source of the Xilinhot Group and mantle-derived basaltic melt. Whole-rock and apatite trace elements reveal that the granitic magma mainly experienced fractional crystallization of amphibole, biotite, plagioclase, K-feldspar, apatite, and monazite from the quartz monzonite to the biotite granite. Mineralogical, whole-rock geochemical, and apatite trace element data indicate that the Beidashan pluton has an I-type granite affinity. A crystal mush model was proposed accounting for the composition gap between these two granites (e.g., SiO2, Al2O3, and Ba), in which the biotite granite represents the highly fractionated interstitial melt extracted from the crystal mush system whereas the quartz monzonite is the residual crystal mush consisting of the cumulate crystals and trapped interstitial melt. The melt was extracted when the crystallinity of the crystal mush reached about 33–37%, and the residual crystal mush trapped about 38–46% interstitial melt. Both Ce anomalies in zircon and Eu anomalies in apatite imply reducing conditions. The calculated values of oxygen fugacity for the Beidashan pluton range from −19.3 to −15.7, and most of them are lower than the fayalite-magnetite-quartz (FMQ) oxygen buffer. Apatite in the Beidashan pluton has high F (2.35–3.61%) and low Cl (0–0.41%) concentrations, which are similar to those of Sn-W deposits worldwide. The Sn contents of apatite increase from the quartz monzonite (mean 0.30 ppm) to the biotite granite (mean 1.09 ppm), suggesting that magma evolution is important for the enrichment of Sn in the melt.