Geochronology, petrogenesis and tectonic significance of Huashugou granitoids in North Qilian, NW China

Abstract

North Qilian Orogenic Belt (NQOB) is a typical early Paleozoic orogenic belt in China. However, the tectonic setting of NQOB remains controversial, especially the post-subduction processes. In this study, we report whole-rock geochemistry, zircon U–Pb ages, and Hf–Nd isotopic compositions of Huashugou granitoids in the western segment of the NQOB, NW China. LA-ICP-MS U–Pb dating of zircon from Huashugou granitoids indicate magmatic crystallization ages of ca. 420 Ma, with late-Paleoproterozoic captured zircons of ca. 1700 Ma. The Huashugou granitoids show enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Th and U), depletion of heavy rare earth elements and high-field-strength elements (e.g., Nb, Ta and Ti), negative Eu anomalies, and A/CNK < 1, showing I-type granite features. The zircon grains from these rocks display negative to positive εHf(t) values (−2.0 to 6.2) and slightly negative εNd(t) values (−5.2 to −1.7). The two-stage zircon Hf model ages range from 1010 to 1533 Ma, overlapping with the two-stage Nd model ages of 1302–1559 Ma. These features could reflect the source magma of the Huashugou granitoids was derived from partial melting of crustal components of the Paleoproterozoic–Mesoproterozoic basement with input of mantle-derived material. The data obtained in this study, in conjunction with those from previous studies, suggest the transition of the tectonic regime of NQOB from compression to extension probably occurred before 420 Ma.