New insights into the petrogenesis of the Bozhushan W-Sn mineralization-associated granites, Yunnan province, SW China: Evidence of microgranitoid enclaves

Abstract

The Bozhushan granites, located in the southeastern Yunnan Province W–Sn polymetallic metallogenic belt in SW China, have considerable W-Sn endowment. However, their petrogenesis remains uncertain, as both crustal and mantle origins have been proposed. Numerous microgranular enclaves (MEs) are found in the Bozhushan granites, and the origin of these MEs can give insights into the petrogenesis of these ore-forming granites. Here we present an integrated study of the major and trace elements, whole-rock Sr–Nd isotopes, and zircon Hf–O isotopes of the MEs. The MEs from the Bozhushan granites are commonly composed of monzodiorite and granodiorite, and they have lower SiO2 (51.7–66.4 wt%) and higher TiO2 (0.83–1.67 wt%), MgO (1.68–4.49 wt%), and CaO (1.74–4.26 wt%) contents than those of their hosts (SiO2 = 64.3–76.9 wt%; TiO2 = 0.4–0.67 wt%; MgO = 0.86–1.33 wt%; CaO = 2.08–3.25 wt%). Furthermore, the chemical compositions of the MEs and host granites are discontinuous and do not show linear trends in element variation diagrams, as might be expected if the MEs represented restites, thus implying disparate genesis. The observed near-linear element variation most probably reflects mixing of mafic and felsic magma end-members in various proportions. In addition, the MEs have Sr–Nd–Hf–O isotopic characteristics (87Sr/86Sri = 0.7117–0.7138, εNd = − 12.59 to − 9.26, εHf(t) = − 13.1 to − 6.1, and δ18O = 8.39‰–9.19‰) and zircon U–Pb ages (87.4 ± 1.0 Ma to 86.5 ± 0.4 Ma) that are similar to those of their host granites. We conclude that the similarity of trace-element contents and Sr–Nd isotopic compositions between the MEs and host granites was caused by diffusion and partial re-equilibration. However, if the magma temperature of the MEs was 850 °C, 950 °C, or 1050 °C, the required Zr contents for zircon saturation would be 238–854 ppm, 497–1785 ppm, or 928–3334 ppm, respectively, which are substantially higher than those measured for the bulk rocks (44–436 ppm). Furthermore, zircons from the MEs have Ti-in-zircon temperatures (Tzr; MEs, 669–884 °C; host, 790–842 °C) and Th/U ratios (MEs, 3.43–8.85; host, 2.68–11.86) that are indistinguishable from those of their host granites. Thus, on the basis of our new data, we propose that the MEs of the Bozhushan granites might have been derived from mantle-derived mafic magma. Zircons from the MEs did not crystallize directly within the MEs but rather were xenocrysts that formed during the early stage of magmatic evolution at the bottom of the granitic magma chamber and were subsequently incorporated into the MEs when mafic magma was injected into the granitic magma chamber. The Hf–O isotopes of zircons in this case cannot be used to constrain the primary composition of the magma that formed the MEs. During the Late Cretaceous, owing to lithospheric extension and asthenospheric underplating, the lower crust was partially melted to form high-temperature (790–842 °C) granitic magma in the Bozhushan area. During the formation of the W–Sn mineralization associated with the Bozhushan granites, 10%–20% underplated mafic magma mixed with granitic magma and contributed both heat and mantle-derived material. Therefore, we consider that the Bozhushan granites are clearly characteristic of A-type granites, but the W–Sn mineralization was formed as a result of high-temperature partial melting of Sn–W-rich metasedimentary sources triggered by additional heat provided by mantle-sourced mafic magma.