Tourmaline-bearing two-mica granites and associated mafic dikes in an intra-continental extension setting: Implications for the petrogenesis of leucogranites and crustal growth

Unlike most granites, primary muscovite-bearing peraluminous granites (MPGs) are traditionally considered as purely sediment-derived crustal melts with little to no involvement of mantle-derived materials. Consequently, they are conventionally considered insignificant in crustal growth. However, the possibility of their formation through alternative mechanisms (e.g., mantle-crust interaction) and their potential contribution to crustal growth remain uncertain. In this study, we report on a rare tourmaline-bearing two-mica granite-diabase suite in an intra-continental setting from the Shangbao area of interior South China. Zircon U-Pb dating results show that this rock suite was generated in the Late Cretaceous (93−84 Ma) and that the Shangbao tourmaline-bearing two-mica granites are among the youngest granitoids in South China. The Shangbao tourmaline-bearing two-mica granites (MPGs) contain primary muscovite and tourmaline, exhibiting high SiO2 contents (71.8−77.8 wt%) and weakly to strongly peraluminous compositions. They are enriched in Rb, Th, and U and depleted in Ba, Sr, and Ti, with pronounced negative Eu anomalies. The Shangbao MPGs exhibit distinctly higher whole-rock εNd(t) and εHf(t) (−2.6 to −0.8 and +3.0 to +4.9, respectively) and zircon εHf(t) (−4.7 to +11.0) values, along with a relatively wide range of zircon δ18O values (5.9‰−10.9‰), compared to most Paleozoic−Mesozoic granitoids in South China, but closely resembling the isotopic signatures of hybrid mafic microgranular enclaves. The associated diabase dikes are characterized by low SiO2 (48.3−51.5 wt%) and high MgO (5.9−7.0 wt%) contents, low (87Sr/86Sr)i ratios (0.7040−0.7072), positive whole-rock εNd(t) and εHf(t) values (+4.1 to +5.5 and +8.6 to +10.5, respectively), and zircon δ18O values ranging from 4.5‰ to 6.7‰, similar to oceanic island basalts (OIB). They are enriched in light rare earth elements and depleted in heavy rare earth elements with negligible Eu anomalies and low La/Nb ratios (0.8−1.0), also similar to OIB. We suggest that the Shangbao MPGs were generated by fractional crystallization of hybridized magmas formed by the mixing of metasedimentary rock-derived felsic melts and asthenosphere mantle-derived mafic melts, since they have slightly depleted Nd-Hf and a relatively wide range of zircon O-Hf isotopic compositions. The diabase dikes likely originated from the decompression melting of upwelling asthenospheric mantle given that they have trace elements and Sr-Nd-Hf-O isotopic compositions similar to OIB. Therefore, the rare tourmaline-bearing two-mica granite-diabase suite suggests that the Shangbao MPGs incorporate a mantle component in addition to a crustal sediment component. This study provides new insights into the petrogenesis of MPGs and raises the possibility that, at least in some cases, post-collisional MPGs may contribute to crustal growth.