华北克拉通胶东地区～2.7Ga TTG岩石的成因及地质意义

Petrogenesis of ca. 2.7Ga TTG rocks in the Jiaodong terrane,North China craton and its geological implications

胶东是华北克拉通保存～2.7Ga岩石的主要地区之一。通过地质调查和锆石SHRIMP测年,我们在胶东5个地方识别出2.7Ga的岩石,这些岩石包括片麻状英云闪长岩、片麻状花岗闪长岩以及片麻状高硅奥长花岗岩。SHRIMP测年结果显示,除样品QX12126外,其他样品中的锆石大多具有清晰的岩浆环带,207Pb/206Pb加权平均年龄为2718～2743Ma,指示这些样品的原岩结晶年龄为～2.7Ga。样品QX12126中锆石大多遭受后期改造,核部获得的最老的207Pb/206Pb年龄（2710±11Ma）应最接近原岩的结晶年龄。根据地球化学结果,胶东～2.7Ga岩石可以分为两组。组1以高的（La/Yb）N、Sr/Y,低的Yb、Y为特征,并表现出无Eu异常或正Eu异常;组2则具有低的（La/Yb）N和Sr/Y,高Yb、Y的特征,并具有负的Eu异常。这些地球化学特征指示组1可能来自以榴辉岩或斜长石很少的石榴角闪岩为残留的源区,而组2源区残留可能为几乎不含石榴石的角闪岩。结合放射性同位素数据,我们认为组1可能起源于加厚的中太古代玄武质下地壳,榴辉岩下地壳发生拆沉作用,上涌的热的软流圈加热浅部残留的地壳物质,形成高HREE（组2）的TTG岩石。胶东与霍邱地区新太古代早期岩石成因、更早期地质记录存在相似性,两者可能为同一古老陆块。出露～2.7Ga岩石的地区在华北分布分散,不同地区～2.7Ga岩石的成因可能存在差异,在探讨华北克拉通新太古代早期构造演化过程时应充分考虑这些因素。

Ca. 2. 7Ga magmatic rocks are well preserved in the Jiaodong terrane （eastern Shandong）. Based on detailed field investigations and SHRIMP zircon dating, we find the early Neoarchean （ca. 2.7Ga） rocks in five new outcrops. They are composed of gneissic tonalite, gneissic granodiorite and gneissic high-Si trondhjemite. Although zircons from sample QX12126 underwent intensive recrystallization, the oldest 207Tpb/206pb age of 2710Ma obtained from a core is considered to be interpreted as its formation age. Zircons from the other four samples show clear oscillatory zoning and give weighted mean 207pb/206Pb ages ranging from 2718Ma to 2743Ma, which are interpreted as the intrusive time of the magmatic rocks. Based on geochemical data, the ca. 2. 7Ga rocks from Jiaodong can be subdivided into two groups. Group 1 is characterized by high （La/Yb） N and Sr/Y, low MgO, Yb and Y, with no negative or positive Eu anomalies, while rocks from Group 2 show low （La/Yb） N and St/Y, high MgO, Yb and Y, with negative Eu anomalies. These indicate that rocks from Group 1 may be derived from a source with a residue of eclogite or garnet amphibolite which contains little or no plagioclase, while rocks from Group 2 may be generated from an amphibolite source with little or no garnet in the residue at a relatively shallow depth. Combined with the published radioactive isotopic data, delamination of eclogitic lower crust is the key to interpret the petrogenesis of these two types of TI＇G rocks. Partial melting of eclogitic lower crust produces low HREE type of TFG. Then, the dense lower crust founders into the mantle, resulting in upwelling of the asthenosphere into the space, which is previously occupied by the delaminated lower crust. The high HREE rocks may be generated from the melting of the bottom of thinned continental crust, which was heated by hot, uprising asthenosphere. Some similarities between Jiaodong and Huoqiu indicate these two areas may previously belong to the same ancient block. The areas exposed ca. 2.7Ga rocks are distributed dispersion, and the ca. 2. 7Ga rocks from different areas may record different petrogenetic processes. These should be considered when discussing the tectonic evolution of the North China craton.