Geochronology, geochemistry, and tectonic implications of early Neoproterozoic granitic rocks from the eastern Beishan Orogenic Belt, southern Central Asian Orogenic Belt

Abstract

The distribution of Precambrian rocks and microcontinents is a key to understanding the architecture of the Beishan Orogenic Belt, southern Central Asian Orogenic Belt (CAOB). We present zircon U-Pb-Hf isotopic and whole-rock geochemical data for early Neoproterozoic granitic rocks from the eastern Shuangyingshan–Huaniushan Terrane in the Beishan Orogenic Belt. The zircon ages show that the granitic gneiss, the mylonitized granite, and the mylonitized monzonite formed at 895 Ma, 894 Ma, and 884 Ma, respectively. These ages confirm that the Shuangyingshan–Huaniushan Terrane is a microcontinental block and that Precambrian magmatic rocks are not restricted to the western Gubaoquan area. Both the gneiss and granite contain Al-rich muscovite, high SiO2 (69.59–78.10 wt%) contents, and show peraluminous features (molar Al2O3/(CaO + Na2O + K2O) values = 1.03–1.28), negative correlations of Th and Y with respect to Rb, typical features of S-type granites. The monzonite contains hornblende, and relatively low SiO2 (62.11–64.10 wt%), high CaO (5.33–5.41 wt%), Na2O (5.88–6.02 wt%), V and Cr contents, and low K2O/Na2O, Rb/Sr and Rb/Ba ratios, consistent with I-type granites. These studied granitic rocks are calc-alkaline to shoshonitic, and all exhibit similar LREE-enriched trace-element patterns, pronounced negative Eu anomalies, positive Rb, Th, U and K anomalies, negative Ba, Nb, Ta, Sr, P and Ti anomalies, evolved zircon εHf(t) values of −6.25 to + 1.09, and two-stage Hf model ages of 2.16–1.70 Ga. These characteristics suggest that they have mainly crustal sources, similar to coeval reported granitic rocks in the Beishan Orogenic Belt, and indicating an older crustal reworking event. The Precambrian basement of the Beishan Orogenic Belt shows evidence of Mesoproterozoic crustal growth and early Neoproterozoic crustal reworking, with zircon εHf(t) values plotting within the crustal evolution trend defined by 2.1–1.3 Ga crustal material, similar to the Yili, Central Tianshan and Northern Alxa blocks, and indicating an affinity with those blocks. Geochronological data from these blocks and terranes suggest that the southern CAOB contains an extensive assemblage of early Neoproterozoic magmatic rocks that are related to the assembly of Rodinia and can be trace over 2000 km as an east–west-trending belt.