华北地块南缘黑山-木龙沟地区中酸性岩的锆石U-Pb年龄、岩石化学和Sr-Nd-Hf同位素研究

Zircon U-Pb age, geochemistry and Sr-Nd-Hf isotopic geochemistry of the intermediate-acid rocks from the Heishan-Mulonggou area in the southern margin of North China Block

华北地块南缘黑山-木龙沟地区晚中生代中酸性岩的LA-ICPMS锆石U-Pb定年结果表明,黑山下斜辉石黑云母闪长岩和木龙沟花岗闪长斑岩的成岩年龄分别为154±2Ma(2σ,N=16)和151±1Ma(2σ,N=15)。该中酸性岩的A/CNK=0.75～1.04,Na2O+K2O=6.15%～8.42%,K2O/Na2O=0.77～2.29,属于准铝质-弱过铝质和高钾钙碱性/钾玄岩系列。在稀土和微量元素上,富集LREE和Rb、Ba、K、Pb、Sr等大离子亲石元素,亏损HREE和Zr、Hf、Ta、Nb、P、Ti等高场强元素,具有弱的负铕异常(δEu=0.85～0.99)。在同位素上,全岩的ISr为0.708676～0.711480,εNd(t)变化于-22～-16.1之间,tDM2为2.25～2.72Ga;锆石的εHf(t)为-27.8～-18.4,二阶段模式年龄tDM2为2.37～2.96Ga。岩石地球化学和Sr-Nd-Hf同位素组成特征显示,这些中酸性岩的源区物质由古老的壳源物质和年轻组分组成,其中以壳源物质为主,可能为太古宙太华群。岩相学和区域地质特征显示,年轻组分可能为幔源组分,岩体可能是由底侵的幔源岩浆与其诱发部分熔融的壳源岩浆混合形成的。

Zircon U-Pb LA-ICPMS dating for the Xiaxie and Mulonggou granitoid plutons from the southern margin of the North China block yield 154 ±2Ma （2σ, N = 16） and 151 ＋ 1Ma （2σ, N = 15）, respectively. These granitoids are metaluminous or weakly peraluminous and high-K calc-alkaline or shoshonite series with A/CNK =0. 75 - 1. 04, Na20 ＋ K2O =6. 15% - 8.42% , K2O/Na2O =0. 77 -2.29. They are riched in LILE, e.g. LREE, Rb, Ba, K, Pb, Sr and depleted in HFSE, e. g. HREE, Zr, Hf, Ta, Nb, P, Ti with weak negative abnormal of Eu （δEu = 0. 85 - 0. 99）. Whole rocks Sr-Nd compositions of the granitoids are characterized by Isr =0. 708676 -0. 711480 and εNd（t） = -22 -- 16. 1 with Nd model ages of 2. 37Ga to 2. 96Ga. The granitoids show a large variation in zircon Hf compositions with εHf（t） = -27. 8- - 18.4 and two-stage model ages of 2. 37Ga to 2. 96Ga. Geochemistry and Sr-Nd-Hf isotopic compositions of the granitoids suggest that they are derived from an ancient continental crust such as the Archaean Taihua Group, but the zircon Hf compositions and the marie magmatic enclaves in the granitoids may imply that juvenile component contribute to their formation. Acording to the petrography and regional geological features, the juvenile component are mainly mantle related material. Therefore, the formation of these granitoids could be interpreted as mixing/mingling of an underplated mantle-derived magma and its induced crustal-melted magma in a deep crust.