黄陵野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成

Zircon U-Pb Age and Hf Isotope Composition from Yemadong Mafic Dikes in the Huangling Area

采用激光剥蚀-等离子质谱(LA-ICP-MS)分析技术测定野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成,以探讨黄陵地区TTG片麻岩原岩的形成及变质时间、是否存在比崆岭群更古老的地壳等问题。野马洞辉绿岩脉(1850 Ma)侵入TTG片麻岩,并从TTG片麻岩中捕获了大量捕掳晶锆石。捕掳晶锆石岩浆结晶核部的U-Pb年龄分别为2842 Ma、2900 Ma和2949 Ma,指示TTG花岗岩体为复式岩体,其至少经历了2949 Ma、2900 Ma和2842 Ma三期岩浆作用。捕掳晶锆石变质边部的U-Pb年龄为2557 Ma,指示TTG花岗岩体转变为TTG片麻岩,是"水月寺运动"及其构造热事件共同作用的结果,其变形变质的时间为2557～2511 Ma。捕掳晶锆石的εHf(t)为-9.85～0.89、平均值为-4.07,亏损地幔模式年龄TDM为3.6～3.2 Ga,指示黄陵地区存在比崆岭群(3.2 Ga)更古老的陆壳。

By LA-ICP-MS methods,the U-Pb dating and Lu-Hf isotopic analysis were carried out for zircons from Yemadong mafic dikes in Huangling area,we can ascertain the forming and metamorphic age of TTG granites,and discuss whether there were some crustal remnants older than Kongling Group in the Huangling area.The Yemadong doleritic dikes intruded in the TTG gneisses,and captured many xenocrystic zircons from the country rocks.The xenocrystic zircons show the core-rim structure,the cores have magmatic oscillatory zoning,and the rims are not zoning,The thirty individual U-Pb spot analyses were done on magmatic cores of 30 xenocrystic zircon grains,and yielded three ages of 2949 Ma,2900 Ma and 2842 Ma,these indicate that the TTG granites is a complex intrusive which experiencing at least three stages of magmatism including 2949 Ma,2900 Ma and 2842 Ma.One U-Pb spot analysis was done on metamorphic rim of zircon grain,and yielded a concordant age of 2557 Ma,which is responsible for the timing of metamorphism.This provides the first demonstration for the occurrence of tectono-thermal event at about 2557 Ma,which has been connected to the Shuiyuesi movement.In the same time,the TTG granite has been metamorphism into the TTG gneisses during the tectono-thermal event resulted from the Shuiyuesi movement in the Huangling area.The xenocrystic zircons have the negative εHf（t） values of-9.85～0.89 with the average values of-4.07,and the Hf model age of 3.6～3.2 Ga,these reveals the existence of old crustal relicts with the age of 3.6～3.2 Ga rocks,which is older than the Kongling Group（3.2 Ga） in Huangling area.