Geochemistry, zircon U–Pb geochronology and Hf isotopes of granitic rocks in the Xitieshan area, North Qaidam, Northwest China: Implications for Neoproterozoic geodynamic evolutions of North Qaidam

Abstract

Neoproterozoic granitic rocks in the Dakendaban Group are widely distributed throughout the Xitieshan area as the old basement in North Qaidam, NW China. The granitic rocks are composed predominantly of K-feldspar, plagioclase, quartz, muscovite and biotite, with subordinate zircon, garnet, titanite and sillimanite. Two granitic rock samples yielded ages of 930±6Ma and 918±6Ma using LA-ICPMS zircon U–Pb dating, which are interpreted as the protolith formation age of the granitic rocks, and reflecting an important Neoproterozoic magmatic event in the Xitieshan area.Geochemical data suggest that the granitic rocks are characterized by high SiO2, K2O, and CaO/Na2O ratios (0.48–1.04), with LREE enrichment and strong Eu negative anomalies (Eu/Eu*=0.44–0.51), and negative anomalies of Nb, Ta, Ti, Zr, and Hf. These rocks have consistent Nd isotope compositions with ɛNd(t) values ranging from −4.83 to −4.27, while the sample 2011ZJ18-3 yielded the negative ɛHf(t) values ranging from −6.7 to −1.3, and tDM2(Hf) model ages ranging from 1.9 to 2.2Ga. These geochemical and zircon Lu–Hf isotopic features suggest that the granitic rocks in the Xitieshan area belong to the S-type granite, which were mainly derived from the partial melting of the Paleoproterozoic continental crust material (1.9–2.2Ga) that were predominantly composed of metagreywackes and subordinate metapelitic sources. Based on the regional tectonic reconstruction, we propose that the studied rocks occurred as syn-orogeny on an active continental margin in a compressional environment, which resulted from the Neoproterozoic arc-continent collision and subsequently continent–continent collision, leading to the significant continental growth. The Early Neoproterozoic tectonothermal event in North Qaidam shares many affinities with that of the Tarim block and South China block (Yangtze block) that were associated with the final consolidation of block at the Jinning orogeny, showing an agreement with the formation of Rodinia. The Early Neoproterozoic S-type granitic magmatism in NW China, including the Qaidam and adjacent blocks, is an important episode of continental crust reworking.