胶东玲珑花岗岩的地球化学、U-Pb年代学、Lu-Hf同位素及地质意义

Geochemistry,U-Pb dating,Lu-Hf isotopic analysis and geological significance of Linglong granite in Jiaodong Peninsula

通过玲珑花岗岩的岩石地球化学、锆石LA-ICP-MS U-Pb年龄和Lu-Hf同位素组成的测试和研究,讨论了玲珑花岗岩的成因、源区和构造背景。研究表明,玲珑花岗岩SiO2的质量分数为68.12%～75.4%,K2O的质量分数为3.7%～5.17%(平均为4.39%),A/CNK为0.72～1.04,属于富硅、高钾钙碱性、准铝质至弱过铝质系列花岗岩,其富集K、Ba、Rb、Sr等大离子亲石元素和Th、U等活泼的不相容元素,相对亏损Zr、Ti、Nb等高场强元素。岩石的高Sr含量(wSr>770×10-6)、低Y含量(wY<7.64×10-6)以及没有明显的负铕异常(δEu=0.85～1.84),与埃达克岩类似。花岗岩锆石εHf(t)=-18.06～-23.85,第二阶段Hf模式年龄TDM2=2358～2710Ma;T-εHf(t)图解和t-(176 Hf/177 Hf)图解显示岩浆源区为2.5Ga±的下地壳,Hf-Rb-Ta、Th/Hf-Ta/Hf判别图解和高的wLa/wNb比值(>2)显示岩石具活动大陆边缘火山弧花岗岩特征。花岗岩锆石LA-ICP-MS U-Pb加权平均年龄为(158.53±0.79)Ma(MSWD=0.13),属晚侏罗世。综合分析认为,燕山早期太平洋板块俯冲引起大陆弧伸展和岩石圈减薄,幔源岩浆底侵提供热动力,古老的镁铁质下地壳物质部分熔融形成玲珑花岗岩。

Based on the studies of geochemistry, zircon LA-ICP-MS U-Pb dating and the test of the Lu- Hf isotopic composition, this paper mainly discusses the petrogenesis, magma source, tectonic setting of the Linglong granite. Eleven granite samples have high content of silica （the mass fraction of SiO2 is 68.12%～75.4%, that of K2O is 3.7%～5.17%, and that of A/CNK is 0.72～1.04）, are rich in LILE （such as K, Ba, Rb, Sr） and more mobile incompatible elements （such as Th, U）, and relatively absence of HFSE （such as Zr, Ti, Nb）. They have the geochemical characteristics similar to adakites with high Sr （the mass fraction of Sr is 〉770×10^-6）, low Y （the mass fraction of Y is 〈7. 64×10^-6 ） , and no obvious Eu negative anomaly （δEu=0.85～1.84）. They have zircon eHf（t） values of -18. 06～ - 23. 85 and Hf model ages of 2358～2710 Ma. The magma source area is the lower crust with age 2. 5 Ga±shown by t-εHf （t） and t-^176Hf/^177Hf diagrams. The geochemical characteristics of Hf-Rb-Ta and Th/Hf-Ta/Hf diagrams as well as high La/Nb （〉2） are similar to the characteristics of Andean-type active continental margin volcanic rc magma. The LA-ICP-MS zircon U-Pb dating of the granite yields Late Jurassic age （the weighted average age is 158.53±0.79 Ma, MSWD=0. 13）. It is suggested that during the early Yanshan, the subduetion of Pacific plate leaded to the extension of the continental arc and the lithospheric thinning, directly resulting in the mantle magma underplating which provided thermal power and the old and mafic lower crust partly melting to form the Linglong granite.