华北克拉通南缘小秦岭地区花岗质片麻岩年代学和地球化学特征及其地质意义

小秦岭地区位于华北中部造山带南段,是研究华北克拉通南缘演化的关键地区,对其研究不仅有助于深入了解华北克拉通古元古代构造演化特征,同时对于华北克拉通东西陆块碰撞时间的厘定也具有重要意义。通过对小秦岭北侧大湖金钼矿床围岩(花岗质片麻岩)的SIMS锆石U-Pb定年分析,获得其岩浆锆石谐和年龄为(2 455±59) Ma、变质锆石谐和年龄为(1 821±31) Ma。花岗质片麻岩具有高SiO₂、富碱和低Al₂O₃、CaO、MgO、P₂O₅ 的特征,同时富集LILEs(Rb、Th、K)、亏损 HFSEs(Nb)和Sr、P、Ti等元素。结合A/CNK-A/NK图解和Ga、Rb等微量元素特征分析,认为其原岩为A型花岗岩,具有非造山A1型花岗岩特征;大湖金钼矿床A型花岗岩在古元古代早期起源于基性岩浆(提供热源)底侵作用下的中—新太古代结晶基底物质的部分熔融,为陆内裂谷环境,并在古元古代晚期受到华北克拉通东部和西部陆块碰撞造山后伸展作用的影响。

Chronology and Geochemical Characteristics of Granitic Gneiss in Xiaoqinling District on the Southern Margin of North China Craton and Its Significance

The Xiaoqinling district, located in the southern end of the Trans-North China Orogen, is important for constraining the Proterozoic evolution of the southern margin of the North China Craton (NCC), and the collision timing of the eastern and western NCC blocks. SIMS zircon dating of the granitic gneiss from the Dahu Au-Mo deposit in the Xiaoqinling district yielded a concordant age of (2,455±59) Ma (magmatic zircons) and(1,821±31) Ma (metamorphic zircons). Whole-rock geochemistry shows that the granitic gneiss has high contents of SiO₂, total-alkali, and low Al₂O₃, CaO, MgO, and P₂O₅. Additionally, the rocks show enrichment in LILEs (Rb, Th, K) and depletion in HFSEs (Nb), Sr, P, and Ti. Combined with the A/CNK-A/NK discrimination and the Ga-Rb compositional features, we conclude that the protolith of the granitic gneiss is A1-type granites formed in a non-collisional environment. This implies that the A-type granites in the Dahu Au-Mo deposit were dominantly derived from partial melting of the Meso-Neoarchean crystalline basement caused by the early Paleoproterozoic mafic magma underplating in an intracontinental rift setting. In the late Paleoproterozoic, the region may have been affected by post-orogenic extension of the eastern and western NCC blocks.