Early Cretaceous granitic rocks from the southern Jiaodong Peninsula, eastern China: implications for lithospheric extension

Zircon U–Pb ages, major element and trace element compositions, and Sr, Nd, and Pb isotopic compositions for late Mesozoic granites from the southern Jiaodong Peninsula (eastern China) were determined. Ages for the Wulianshan, Xiaozhushan, and Dazhushan plutons are 119.1–122.3, 114.2, and 108.9 Ma, respectively. Major and trace element characteristics show that these granitic rocks belong to alkaline, A-type granites formed in an extensional setting. Trace element compositions show strong, variable negative anomalies in Ba, K, P and Ti, and positive anomalies in Rb, Th, U, Pb, Ce, Zr, and Hf, which are typical characteristics of A-type granites. Variable Sr and Nd isotopic compositions, ⁸⁷Sr/⁸⁶Sr(i) = 0.70540–0.7071 and <i>ε</i>Nd(<i>t</i>) = −14.5 to −20.9. Whole-rock Pb isotopic compositions have the following ranges, (²⁰⁶Pb/²⁰⁴Pb)<i>t</i> = 15.707–16.561, (²⁰⁷Pb/²⁰⁴Pb)<i>t</i> = 15.376–16.462, and (²⁰⁸Pb/²⁰⁴Pb)<i>t</i> = 36.324 to 37.064. Isotopic modelling indicates an origin that lies between mantle tapped by Cenozoic basalts around the Tan-Lu megafault and lower continental crust (LCC), and which can be explained by mixing of 11–18% mantle and 82–89% LCC. Based on new and compiled data, we suggest that the southern Jiaodong Peninsula, as well as the Laoshan area, was in a regional extensional setting of an orogenic belt during 106–126 Ma. The granitic rocks may be the result of late Mesozoic lithospheric thinning and decratonization (i.e. late Mesozoic craton destruction event occurring throughout eastern China).

本研究测定了中国东部胶东半岛南部晚中生代花岗岩的锆石U-Pb年龄、主量元素与微量元素组成，以及Sr、Nd、Pb同位素组成。五莲山、小珠山及大珠山岩体的形成年龄分别为119.1~122.3 Ma、114.2 Ma及108.9 Ma。主量与微量元素特征显示，此类花岗岩属于形成于伸展环境的碱性A型（A-type）花岗岩。微量元素组成表现出Ba、K、P、Ti显著且多变的负异常，以及Rb、Th、U、Pb、Ce、Zr、Hf正异常，这是A型花岗岩的典型地球化学特征。Sr、Nd同位素组成存在显著变化，初始锶同位素比值(⁸⁷Sr/⁸⁶Sr)i为0.70540~0.7071，εNd(t)值为-14.5~-20.9。全岩Pb同位素组成的变化范围如下：(²⁰⁶Pb/²⁰⁴Pb)t为15.707~16.561，(²⁰⁷Pb/²⁰⁴Pb)t为15.376~16.462，(²⁰⁸Pb/²⁰⁴Pb)t为36.324~37.064。同位素模拟结果表明，此类花岗岩的源区介于郯庐巨型断裂带（Tan-Lu megafault）周边新生代玄武岩所起源的地幔与下陆壳（lower continental crust, LCC）之间，其成因可通过11%~18%的地幔物质与82%~89%的下陆壳物质混合得到合理解释。结合本次实测的新数据与已有的汇编资料，本研究认为，在106~126 Ma期间，胶东半岛南部及崂山地区均处于造山带的区域伸展环境之中。此类花岗岩的形成可能与晚中生代岩石圈减薄及克拉通破坏（decratonization）密切相关，即对应于发生于中国东部的晚中生代克拉通破坏事件。