Petrogenesis of the early Paleozoic low-Mg and high-Mg adakitic rocks in the North Qilian orogenic belt, NW China: Implications for transition from crustal thickening to extension thinning

Abstract

The petrogenesis and geodynamic implications of the early Paleozoic adakitic rocks in the North Qilian orogenic belt remain topics of debate. In this study, petrology, zircon U–Pb age and Lu–Hf isotopic composition, whole-rock geochemistry and Sr–Nd isotopes were carried on both low-Mg and high-Mg adakitic rocks from the North Qilian orogenic belt. These results will provide significant constraints on the evolution of the North Qilian orogenic belt. In general, the low-Mg adakitic granites mainly consist of plagioclase (50–55%), alkali feldspar (20–25%), quartz (20–25%) and biotite (5%). In contrast, the high-Mg adakitic granodiorites are generally composed of alkali feldspar (30–35%), plagioclase (30–45%), quartz (20–25%) and amphibole (∼10%). The low-Mg adakitic granitoids (446–457Ma) are characterized by high SiO2 (69–72wt.%), low Mg# (39–42) and low Cr and Ni contents. However, the high-Mg adakitic granitoids (430–431Ma) have relatively lower SiO2 (65–66wt.%), higher Mg# (50–59) and higher Cr and Ni contents. The low-Mg adakitic rocks have high initial 87Sr/86Sr ratios (0.7068–0.7080), negative εNd(t) (−2.3 to −3.3) and εHf(t) values (−6.8 to 1.9) with old zircon Hf model ages (1.2–1.6Ga). However, the high-Mg adakitic rocks show lower initial 87Sr/86Sr ratios (0.7044–0.7066), higher εNd(t) (−1.8 to 3.0) and positive εHf(t) values (2.3–7.7) with younger zircon Hf model ages (0.9–1.1Ga). These results suggest that the low-Mg rocks were generated by partial melting of thickened crust, whereas the high-Mg rocks were derived from anatexis of delaminated lower crust, which subsequently interacted with mantle magma upon ascent. The data obtained in this study provide significant information about the geological and tectonic processes after the closure of the Qilian Ocean. The continent–continent collision and thickening probably occurred during 440–467Ma with formation of low-Mg adakitic rocks; and the transition of the tectonic regime from compression to extension probably occurred at ∼430Ma with formation of high-Mg adakitic rocks.