Petrogenesis of Huichizi granite and hosted amphibolite lens: implications for early Paleozoic evolution of the North Qinling orogenic belt

The genesis of Early Paleozoic granite and its relationship with multi-stage metamorphism in the North Qinling Tectonic Belt (NQTB) are heavily debated and limit our understanding on the evolution of the Qinling Orogenic Belt. In this paper, we conducted systematic studies of petrology, geochemistry and zircon chronology on the Huichizi granite and a hosted composite lenticle (composed of amphibolite and felsic gneiss) in the NQTB. The results show that the composite lenticle recorded three metamorphic ages at 496–492 Ma, 452 Ma and 404–400 Ma, and metamorphic zircons from the amphibolite display flat heavy rare earth elements (REE) patterns without visible Eu anomalies. Both the metamorphic ages framework and zircon REE patterns are consistent with those from ultrahigh-pressure (UHP) metamorphic rocks in the NQTB. The Huichizi granite shows adakitic signatures with high Sr (391–741 ppm), low Y (3.99–6.87 ppm) contents and high Sr/Y ratios (60.6–173). It is enriched in light REE and large-ion lithophile elements (Rb, Sr, Ba, Pb), and depleted in high-field-strength elements (Nb, Ta, Ti). The high Th (7.47–12.9 ppm) content and Th/Ta (8.10–31.5) ratios, but low Ce/Pb (1.67–3.24) and Nb/La (0.14–0.35) ratios are consistent with those of island arc granitoids. The granite is further characterized by low MgO, Cr, Ni contents, low K 2 O/Na 2 O ratios, and slightly depleted Sr-Nd isotopic compositions that resemble those of eclogites in the NQTB, indicating derivation from partial melting of Neoproterozoic mafic rocks during crustal thickening. The granite has a protolith age of 438 ± 5 Ma, at least 50 Ma later than the peak UHP metamorphism and underwent granulite-facies metamorphism/anatexis at ca. 400 Ma. Integrating the above results with existing studies, we propose that the northward subduction of the Shangdan Oceanic crust triggered partial melting of Neoproterozoic mafic rocks that had accumulated beneath the North Qinling Terrane. The ascending magma entrapped the exhumed UHP rocks of the Qinling Complex and emplaced at 438Ma. At ca. 400Ma, during the regional tectonic regime transition from compression to extension, the Huichizi granite and entrapped composite lenticle underwent medium-pressure granulite-facies metamorphism and anatexis together.