Granitoid Petrogenesis and Tectonic Implications of the Late Triassic Baoji Pluton, North Qinling Orogen, China: Zircon U-Pb Ages and Geochemical and Sr-Nd-Pb-Hf Isotopic Compositions

The Qinling orogenic belt in central China resulted from the final collision of the North China and South China blocks in the Triassic. Early Mesozoic granitoids are widespread in the Qinling orogen. Their genesis can provide key constraints on the tectonic evolution of this orogenic belt. The Late Triassic Baoji pluton, located in the North Qinling unit, consists of granodiorites, monzogranites, quartz monzodiorites, and K-feldspar granites. U-Pb zircon dating via the laser ablation ICP-MS technique yields ages of 217.4 +/- 1.4, 212.3 +/- 1.8, 212.6 +/- 1.6, and 195.1 +/- 2.0 Ma for granodiorites, monzogranites, quartz monzodiorites, and K-feldspar granites, respectively. The granitoids have high-K calc-alkaline, metaluminous to weakly peraluminous composition(s). Silica contents increase and Mg, Fe, Ti, Ca, and P contents decrease from quartz monzodiorites through granodiorites to monzogranites and K-feldspar granites. Combining our findings with evidence from whole-rock Sr-Nd and zircon Hf isotopes, we suggest that the granodiorites formed by magma-mixing processes involving metasomatized lithospheric mantle and Mesoproterozoic crustal material. The monzogranites record the contribution of an asthenospheric mantle source. The quartz monzodiorites were derived from a metasomatized lithospheric mantle source. Partial melting of crustal lithologies generated the K-feldspar granites. It is argued that the Baoji pluton formed in a late-collision setting.