Petrogenesis and metallogenic implications of Late Cretaceous I- and S-type granites in Dachang–Kunlunguan ore belt, southwestern South China Block

Abstract

The southwestern part of the South China Block is one of the major Sn, W and Cu metallogenic provinces related to Mesozoic granitoid suites. However, the genetic relationship between the magmatic petrogenesis and metallogeny is not well-constrained. Here we evaluate the petrogenesis of Late Cretaceous granites and their link to regional metallogeny from the Dachang–Kunlunguan ore belt in the eastern part of the province. This belt includes biotite granites formed at ca. 98–97 Ma associated with Cu–W deposit in the Kunlunguan area, and the alkali-feldspar granites emplaced at ca. 92 Ma responsible for Dachang Sn mineralization. The Kunlunguan biotite granites have low P2O5 (0.12–0.18 wt%), CIPW normative corundum contents (0.28–1.99 wt%) and ASI values (1.00 to 1.14), consistent with the features of I-type granites. In contrast, the Dachang alkali-feldspar granites are strongly peraluminous (ASI = 1.29–1.44) with high P2O5 (0.25–0.34 wt%) and CIPW normative corundum contents (3.75–4.91 wt%), comparable to S-type granites. Zircon grains from the Kunlunguan and Dachang granites show different εHf(t) values ranging from −11.0 to −1.2 and from −6.5 to −2.1, respectively. Based on the geochemical and Hf isotope features, the parent magmas of the Kunlunguan granites are inferred to have been dominantly derived from mixed melts of Paleo- to Mesoproterozoic metaigneous and metasedimentary sources with minor mantle components, whereas Dachang granites were mainly formed through partial melting of Mesoproterozoic metasedimentary rocks. The Dachang S-type granites display high values of DI and FeOT/MgO, low Tzr and ratios of La/Yb, Zr/Hf and Nb/Ta and strongly negative Sr, Ba and Eu anomalies, suggesting that they are highly fractionated granites; whereas the Kunlunguan I-type granites have witnessed less fractionation. The Dachang granites underwent fractionation of K-feldspar, plagioclase, biotite, monazite and allanite with magmatic-hydrothermal interaction indicated by Nb/Ta < 5. The Kunlunguan granites have high zircon Ce/Ce* ratios (1–400, average 70), which resemble those of typical Cu ore-bearing granitoids in Myanmar, indicating relatively high oxygen fugacity and explaining the associated formation of Cu-W deposit. The highly fractionated Dachang S-type granites show low oxygen fugacity (mostly < 40), similar with that of the typical Sn ore-bearing granitoids, which favored the Sn mineralization.