Geochemical constraints on the source nature and melting conditions of Triassic granites from South Qinling in central China

The Qinling orogen marks the continental collision between the North China Block and the South China Block in the Triassic, but the Triassic collisional orogeny is only prominent in the South Qinling zone with the occurrence of voluminous granites. In order to understand the petrogenesis of these granites, a combined study of petrology, geochronology and geochemistry was carried out for them from outcrops at Guangtoushan (GTS) and Huayang (HY) in South Qinling. The results indicate that the studied granites were derived from partial melting of metasedimentary rocks rather than metaigneous rocks as suggested by previous studies. LA-ICPMS zircon U-Pb dating yields concordant ages of 207 ± 3 Ma to 212 ± 4 Ma for their magma emplacement. The GTS granites are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.15 and K2O/Na2O ratios of 0.5 to 1.32 and show high whole-rock δ18O values of 9.52 to 11.20‰. The HY granites are peraluminous with high A/CNK values of 1.04 to 1.14 and K2O/Na2O ratios of 1.01 to 2.82, and have high zircon δ18O values of 10.71 to 11.43‰ and whole-rock δ18O values of 12.42-13.77‰. On the other hand, the GTS granites exhibit low Rb/Sr ratios but high Sr/Ba ratios, variable Eu anomalies (Eu/Eu⁎ = 0.29-1.05), in association with low zircon saturation temperatures. In contrast, the HY granites have high Rb/Sr ratios but low Rb/Ba ratios, conspicuous negative Eu anomalies (Eu/Eu⁎ = 0.52-0.77) and relatively high zircon saturation temperatures. An integrated interpretation of all these results is that the GTS granites were produced by fluid-present partial melting of a mixture source of metapelite and metagreywacke at low temperatures, whereas the HY granites were generated by fluid-absent partial melting of a metagreywacke at relatively higher temperatures. Furthermore, the two granite plutons show variably high whole-rock (87Sr/86Sr)i ratios of 0.7047 to 0.7115, negative whole-rock εNd (t) values of − 9.8 to − 6.8 and zircon εHf (t) values of − 13.1 to − 2.0, with two-stage Nd-Hf model ages of Mesoproterozoic to Paleoproterozoic. Their εNd (t) values are comparable with those for sedimentary rocks in the southern margin of the South Qinling zone and the northern margin of the South China Block, suggesting a genetic link between them. Therefore, the GTS and HY granites originated from the sedimentary rocks rather than igneous rocks, and thus belong to S-type rather than I-type. It is inferred that these sedimentary rocks were subducted to the lower crustal depth during the Late Triassic collisional orogeny in the Qinling orogen for partial melting to produce the S-type granites.