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Zircon U-Pb Ages and Sr-Nd-Hf Isotopic Characteristics of the Huichizi Granitic Complex in the North Qinling Orogenic Belt and Their Geological Significance

Youwei Chen; Ruizhong Hu; Xianwu Bi; Shaohua Dong; Yue Xu; Ting Zhou
State Key Laboratory of Ore Deposit Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China

The Huichizi granite complex is the largest Paleozoic I-type intrusion located in the North Qinling orogenic belt (NQB). In this study, we present systematic geochemical element data, zircon U–Pb ages, Lu–Hf isotopic data, and Sr–Nd isotopic data for the Huichizi granites. In terms of mineral and chemical compositions, these granites are biotite monzonitic and alkali-feldspar granites, both of which are characterized by high SiO2 and total alkali contents and low MgO, TiO2, and FeOt contents. These granites are weakly peraluminous (A/CNK values are 1–1.06 for biotite monzonitic granites and 1.04–1.09 for alkali-feldspar granites) and possess the geochemical characteristics of adakitic rocks, e.g., high Sr contents (319–633 ppm), Sr/Y ratios (18.5–174 ppm), and (La/Yb)N ratios (17.6–57 ppm) and low MgO (0.04–0.83 wt.%), Y (3.0–17.2 ppm), and heavy rare-earth element (HREE) contents. This indicates that these rocks were most likely derived from the partial melting of a thickened lower crust. In situ zircon U–Pb dating of these granites yields early Caledonian ages (437 Ma for biotite monzonitic granites and 424 Ma for alkali-feldspar granites), indicating that the Huichizi granitic complex is the product of multi-periodic magmatism. The positive but varying zircon εHf(t) values (+0.6 to +8.5) suggest that this thickened lower crust was mainly juvenile, i.e., accreted from depleted mantle during the Neo–Mesoproterozoic period, but involved the ancient recycled crust. Biotite monzonitic granites formed during crust thickening at the extrusion stage, whereas the alkali granites formed during crust thickening at the extension stage (post extrusion). The Huichizi granite complex witnessed the process of extrusion to extension because of the collision between the NCB and the Qinling microcontinent in the Caledonian.

Keywords:U-Pb age, Sr-Nd-Hf isotope, Huichizi granitic complex, adakitic granite, North Qinling orogen

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