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

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 epsilon Hf(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.