Chronology and petrogenesis of the Hejiazhuang granitoid pluton and its constraints on the Early Triassic tectonic evolution of the South Qinling Belt

The Hejiazhuang pluton is located in the South Qinling Tectonic Belt (SQTB) in the north side of the Mianxian-Lueyang Suture Zone, and consists dominantly of granodiorites. LA-ICP-MS zircon U-Pb dating and Lu-Hf isotopic analyses reveal that these granodiorites of the Hejiazhaung pluton emplaced at ～248 Ma, and show a large variation in zircon ?Hf(t) values from ?4.8 to 8.8. These granodiorite samples are attributed to high-K to mid-K calc-alkaline series, and characterized by high SiO₂ (66.6%–70.0%), Al₂O₃ (15.04%–16.10%) and Na₂O (3.74%–4.33%) concentrations, with high Mg# (54.2–61.7). All samples have high Sr (627–751 ppm), Cr (55–373 ppm) and Ni (17.2–182 ppm), but low Y (5.42–8.41 ppm) and Yb (0.59–0.74 ppm) concentrations with high Sr/Y ratios (84.90–120.66). They also display highly fractionated REE patterns with (La/Yb)_N ratios of 18.9–34.0 and positive Eu anomalies (δEu=1.10–2.22) in the chondrite-normalized REE patterns. In the primitive mantle normalized spidergrams, these samples exhibit enrichment in LILEs but depletion in Nb, Ta, P and Ti. These geochemical features indicate that the granodioritic magma of the Hejiazhuang pluton was derived from the partial melting of hybrid sources comprising the subducted oceanic slab and sediments, and the melts were polluted by the mantle wedge materials during their ascent. The emplacement ages and petrogenesis of the Hejiazhuang pluton prove that the initial subduction of the Mianlue oceanic crust occurred at ～248 Ma ago, and the SQTB was still under subduction tectonic setting in the Early Triassic.