青海南山黑马河岩体岩浆混合成因的岩石地球化学和年代学证据及其构造意义

Magma Mixing of the Heimahe Pluton in the Qinghai Nanshan Tectonic Zone: Evidence from Petrology, Geochemistry and Geochronology, and Its Tectonic Implications

黑马河岩体位于西秦岭北缘青海南山构造带内,岩体中发育大量闪长质包体,但对其成因却有一定的争议.从岩石学、矿物学、地球化学和年代学等方面对黑马河岩体花岗闪长岩及其中的闪长质包体进行了详细研究.包体的野外产出状态、形态、结构构造和矿物学特征均显示出,它们是基性岩浆注入到中酸性岩浆中快速冷凝结晶的产物.在主量元素协变图解中,花岗闪长岩和闪长质包体显示出壳幔岩浆混合作用的趋势.另外,两者稀土总量和相似的稀土元素配分模式及Eu负异常程度,也显示二者具有岩浆混合的特征.LA-ICPMS锆石U-Pb定年结果显示,花岗闪长岩形成于246 Ma,闪长质包体形成于245 Ma,两者具有几乎一致的结晶年龄,排除了包体为源区难熔残余或围岩捕掳体的可能性,也排除了基性岩浆在花岗质岩浆固结后再侵入的可能性.结合区域地质资料,认为黑马河岩体形成于西秦岭北缘有限小洋盆向南的俯冲阶段,青海南山构造带与天峻南山一带具有相同的构造岩浆演化历史.

The Heimahe pluton is located in the Qinghai Nanshan tectonic belt,northern margin of the West Qinling orogeny.Mafic microgranular enclaves(MMEs) are widely distributed in the Heimahe pluton. However, the petrogenesis of the MMEs is an issue of debate. We present the petrology,mineral chemistry, rock geochemistry and geochronology of the granodiorite and MMEs from the pluton in this paper. The outcrop conditions,shapes, structural and mineral characters of the MMEs reflect that they were products of rapid crystallization when mafic magma injected into the intermediate-acid magma. In the binary diagram of major elements, the granodiorites and MMEs show a mixing trend between mantle-and crust-derived magmas. Besides,their total REE contents and similar REE patterns and Eu anomalies also support the magma mixing model. LA-ICPMS zircon U-Pb dating results show that the granodiorite formed at 246 Ma, and the MMEs formed at 245 Ma. The same crystallization age suggests that the MMEs were not restites or host-rock xenoliths. Combined with regional geology, we suggest that the Heimahe pluton formed by the southward subduction of the limited oceanic basin in the northern margin of the West Qinling, and the Qinghai Nanshan tectonic belt experienced the same evolutionary process with the Tianjun Nanshan belt.