辽东黑沟地区古元古代碱长花岗岩年代学、地球化学、Hf同位素特征及其对辽吉造山带构造演化的制约

Chronology, geochemistry and zircon Hf isotopes of the Paleoproterozoic alkali feldspar granite from the Heigou area in the eastern Liaoning Province:constraints on the tectonic evolution of the Liao-Ji orogenic belt

华北克拉通东部造山带内古元古代中期(2.1~2.2Ga)岩浆记录了造山带早期重要的地壳演化信息,对探讨辽吉造山带古元古代地质演化过程具有十分重要的意义。本文选择了辽东黑沟地区古元古代碱长花岗岩进行岩石学、地球化学、锆石U-Pb年代学和Hf同位素研究。黑沟碱长花岗岩LA-ICP-MS锆石U-Pb测年结果为2186.5±6.5Ma,形成时代为古元古代。岩石为碱性、弱过铝质A型花岗岩,岩石富硅、碱,贫钙、镁、铝,极度富钠贫钾,富集Hf、Zr、Nb、Y等高场强元素,亏损Rb、K、Ba等大离子亲石元素,具有较高的(K2O+Na2O)/Al2O3、FeOT/MgO、Ga/Al比值,负的Eu异常,稀土元素配分曲线呈右倾海鸥型配分模式。锆石饱和温度介于857~884℃之间,同时具有非常低Sr高Yb特征,指示岩石形成于低压高温的伸展构造背景。锆石εHf(t)值为-0.96^+4.58,二阶段模式年龄tDM2=2476~2821Ma,明显大于花岗岩的成岩时代,结合岩石地球化学特征指示黑沟碱长花岗岩岩浆源于太古宙TTG片麻岩在地壳浅部伸展条件下的部分熔融。综合区域已有的岩石学、地球化学和同位素特征等资料,辽吉地区在古元古代早期可能处于大陆弧后盆地构造环境,黑沟碱长花岗岩是弧后盆地伸展环境下的产物。

The middle Paleoproterozoic(2.1~2.2 Ga) magma in the eastern orogenic belt of the north China craton recorded important crustal evolution information in the early stage of the orogenic belt, which is of great significance for understanding the Paleoproterozoic geological evolution in the Liaoning-Jilin orogenic belt. In this paper, we have studied petrology, geochemistry, zircon U-Pb chronology and Hf isotope characteristics of Paleoproterozoic alkali feldspar granite in the Heigou area, eastern Liaoning province. The zircon U-Pb dating result of the granite by LA-ICP-MS was 2186.5±6.5 Ma, and the formation age was Paleoproterozoic. The rocks belong to the alkaline and weakly peraluminous A-type granites. The rocks are high in Si and rich in alkali, poor in Ca, Mg and Al, extremely rich in sodium and poor in potassium. They are also rich in the HFSEs(such as Hf, Zr, Nb, Y), deficient in the LILEs(such as Rb, K, Ba), have high(K2O+Na2O)/Al2O3, FeOT/MgO, Ga/Al ratios, and negative Eu anomaly. The rare earth elements distribution curve shows the right-leaning seagull pattern. The saturation temperature of zircon ranges is between 857 ℃ and 884 ℃, and has a very low Sr and high Yb characteristic, indicating that the rock was formed in the extensional tectonic background of low pressure and high temperature. εHf(t) values of zircons in the granites range from-0.96^+4.58, and two-stage Hf model ages(tDM2) are 2476~2821 Ma, which are significantly older than the zircon U-Pb ages of the granites. Combined with lithogeochemical characteristics, it indicates that the Paleoproterozoic granites were derived from partial melting of Archean TTG gneisses under an extensional environment at shallow crustal level. Based on the data of petrology, geochemistry and isotopic characteristics, the Liaoning-Jilin region was likely to be in the tectonic environment of continental back-arc basin in the early Paleoproterozoic, and the Heigou alkali feldspar granite is the product of the extensional environment of back-arc basin.