Silurian syn- and post-collision granitic magmatism in the western section of the North Qinling Orogen: implications for collisional orogenic processes

The Liqiao and Xianping plutons can provide crucial evidence for the collision-orogeny process of the Proto-Tethys Ocean in the western section of the North Qinling Orogen. In this study, we present petrological, zircon U-Pb geochronological, geochemical, and zircon Lu-Hf isotopic data for these plutons. Both the Liqiao and Xianping plutons are characterized as high-K, calc-alkaline, metaluminous to weakly peraluminous granites, with ages of 429 and 421 Ma, respectively. The Liqiao pluton was classified as I-type granite, displaying positive ϵ Hf ( t ) values ranging from −0.1 to +3.4, and high Mg # values from 37.86 to 48.25. We interpret this to indicate that it was generated by the partial melting of juvenile felsic lower crust, with a contribution from mantle-derived material. In contrast, the Xianping pluton exhibits lower Mg # values (20.40 to 35.11) and negative ϵ Hf ( t ) values (−18.0 to −13.9), consistent with the geochemical characteristics of highly fractionated I-type granite. This suggests that the Xianping pluton formed through the partial melting of ancient felsic crust and followed by extensive fractional crystallization. We propose that the Liqiao pluton originated in a syn-collisional setting, while the Xianping pluton formed in a post-collisional environment. Both plutons are products of the collisional orogeny between the Yangtze Block and the North Qinling Orogen, which were associated with the closure of the Wushan-Shangdan Ocean, the northern of the Proto-Tethys Ocean.