Zircon U-Pb-Hf characteristics and its geological implication of the gneissic granite in the northern Alxa

The ages, nature, and distribution of Precambrian geological bodies are the important basis for the division of tectonic units. In the tectonic division, although some tectonic boundaries have no evidence of ophiolitic mélange, they have dividing significance due to the different geological evolution sequences of tectonic units on both sides. The Yagan Fault, located on the northern Alxa in the middle section of the southern Central Asian Orogenic Belt, is one of such tectonic boundaries. In this paper, a gneissic granite was newly discovered in the Dalunwusu area on the north side of Yagan Fault, and its zircon geochronology, geochemical characteristics, and zircon Lu-Hf isotope characteristics were employed to discuss the age, genesis, and tectonic setting. The results show that it was formed at 934 Ma, belonging to the high-K calc-alkaline-shoshonite series, peraluminous I-type granite, mainly derived from the late Paleoproterozoic crustal remelting (ε_Hf(t) = -2.2 ~ + 0.3, T_DM² = 1.79 ~ 1.92 Ga). It is similar to the Hf isotopic characteristics (ε_Hf(t) = -3.5 ~ + 1.0, T_DM² = 1.69 ~ 1.96 Ga) of the same period zircons in the Huobuhar area, the southern side of the Yagan Fault. Based on the previous data, it is suggested that the Neoproterozoic crystalline basement of the South Mongolia Microcontinent may extend to the Dalunwusu area in the north of the Yagan Fault. The basement has similar zircon geochronology, geochemistry, and zircon Lu-Hf isotope characteristics to the Neoproterozoic basement in the northern Beishan. Integrating with previous data, it suggests that the Heiyingshan-Hanshan block in Beishan has a similar Precambrian evolution process to the northern Alxa. It further indicates that the Yagan Fault may have similar tectonic implications to the Hongshishan-Xiaohuangshan ophiolite mélange belt. Similar magmatic events on both sides of the Yagan Fault, however, indicate that the current location of the Yagan Fault may not have the significance of dividing the Neoproterozoic crystalline basement. Combined with geophysical data, it is preliminarily believed that the location of the Yagan Fault may be at the north of the Hongguerjiwula Mountain.