Petrogenesis and tectonic implications of Indosinian granitoids from Western Qinling Orogen, China: Products of magma-mixing and fractionation

The Western Qinling Orogen (WQO) is characterized by voluminous distribution of Indosinian granitoids, the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny. Here we investigate a group of intrusions termed “Five Golden Flowers” based on petrological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic studies on the granitoids and their mafic microgranular enclaves (MMEs). Our results show that these intrusions are genetically divided into two types, namely, magma-mixing and highly fractionated. The Jiaochangba, Lujing, Zhongchuan, and Luchuba granitoids are biotite monzogranites (220
0.8 Ma to 217
2.6 Ma) with abundant coeval MMEs (220
1.1 Ma to 217
2.7 Ma). The rocks contain moderate to high SiO2, high MgO, Rb, Sr, Ba, and Th contents, but low TiO2, P2O5, and Sc values, A/CNK of <1.1, and a range of εHf(t) values of 11.7 to þ2.23 with corresponding TDM2 values of 1967–1228 Ma. The MMEs possess K-feldspar megacrysts, abundant acicular apatites, and show lopsided textures. They have lower SiO2, Al2O3, and Th contents, but higher MgO, TiO2, and Sc, with εHf(t) values of 18.0 to þ3.18 and TDM1 of 849–720 Ma. The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle. We suggest that these host granitoids were produced by partial melting of late- Paleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas. The Baijiazhuang pluton is dominantly composed of leucogranite (muscovite granite and twomica monzogranite, 216
1.5 Ma) without MMEs. The rocks are peraluminous with high A/CNK (1.06–1.27). Compared with the other four granitoids, the Baijiazhuang leucogranite shows higher SiO2 content, markedly lower concentrations of TiO2, MgO, Al2O3, CaO, and Fe2O3 T, and lower LREE/HREE and (La/Yb)N values. These leucogranites are also rich in Rb, Th, and U, and display marked depletions in Ba, Sr, Ti, and Eu, indicating that they experienced significant fractionation. Zircon εHf(t) values (10.2 to 3.27) and TDM2 (1868–1424 Ma), as well as the Nb/Ta and K2O/Na2O values are similar to the other four granitoids, indicating that they are likely to have been derived from a similar source; with sediments playing only a minor role in the magma generation. The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling. Based on the above features, we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures, within magma chambers at mid-crust depths. We propose that the granitoid suite was formed in the transitional setting from synto post-collision during the collisional orogeny between the SCB and NCB, following break-off of the subducted South China Block lithosphere during 220–216 Ma.