蒙古国东南部巨斑状二长花岗岩地球化学特征与岩石成因机制探讨

Discussion on the Geochemical Characteristics and Mmechanism of Rock Formation of the Giant Phenocryst Adamellite in Southeast Mongolia

巨斑状二长花岗岩是蒙古洋板块早古生代晚期重要的岩浆记录,其花岗岩年龄和地球化学特征对于揭示蒙古洋板块该时期的构造格局与演化历史有重要意义。对蒙古国东南部出露的巨斑状二长花岗岩进行锆石的LA-ICP-MS法定年,获得锆石U-Pb年龄为(454.3±3.8)Ma,表明该二长花岗岩形成于晚奥陶世。花岗岩的元素地球化学研究结果表明:该花岗岩属高钾钙碱性I型花岗岩;花岗岩具高硅特征,其SiO2质量分数为69.82%～75.16%,A12O3质量分数为12.91%～14.49%,Na2O/K2O为0.60～0.77,过铝质,其铝饱和指数为1.11～1.22,Mg#指数为39～44;稀土元素总量为(43.47～158.11)×10-6,稀土元素分布型式为具有明显负铕异常的"燕型"曲线(δEu=0.26～0.49);在原始地幔标准化的微量元素蛛网图中,Rb、Th、U、K、Zr、Hf和轻稀土元素(如La、Ce、Nd和Sm等)富集,Ba、Sr、P和Ti等元素强烈亏损,Nb和Ta具有中等—弱亏损特点;143 Nd/144 Nd初始比值较高(0.512 449～0.512 507),平均为0.512 477;εNd(t)为0.2～1.3,均为正值。主量、微量元素和同位素特征表明,该巨斑花岗岩形成于同碰撞向后造山构造体制转换过渡的伸展大地构造环境背景之下。伸展构造和幔源基性岩浆的底侵,很可能是形成花岗岩的2个最主要的动力机制。

The giant phenocryst adamellite is an important magmatism record of the Mongol Ocean plate in the late period of the Early Paleozoic Era.Their formation age and geochemical characteristics are significant to reveal the tectonic framework and evolution history of the Mongol Ocean plate at that period.The paper presents the zircon U-Pb age of the giant phenocryst adamellite in Southeast Mongolia by LA-ICP MS method.The U-Pb age of zircon of adamellite given is（454.3±3.8） Ma,belonging to the Late Ordovician.The geochemical data of granite show its high-K calc-alkaline nature.The granite is also characterized with high silicon,with SiO2 being 69.82%-75.16%,Al2O3 12.91%-14.49%,and Na2O/K2O 0.60%-0.77%,peraluminous,with aluminum saturation index being in the range of 1.11-1.22,and Mg# index of 39-44.The total content of REE is 43.47×10-6-158.11×10-6,δEu=0.26-0.49.The REE distribution pattern of adamellite is demonstrated by a significant negative Eu anomaly and a ＂swallow-type＂ shape.The spider diagrams of trace elements standardized by primitive mantle show apparent enrichment of Rb,Th,U,K,Zr,Hf,strong depletion of Ba,Sr,P,Ti and medium to slight depletion of Nb and Ta.The initial 143Nd/144Nd values are high（0.512 449-0.512 507）,and the average value is 0.512 477.The εNd（t） values are all positive（0.2-1.3）.The characters of major elements,trace elements,and isotopes show that the giant phenocryst adamellite in the Southeast Mongolia was formed at an extensional geotectonic environment during the transitional period from syn-collision to post-orogenic tectonic systems.The extensional tectonics and the underplating of mantle-derived basic magmas are suggested to be the main dynamic mechanism to form the granite.Since the adamellite is the important magma record of the Mongol Ocean plate subduction in the early Paleozonic,the present research is helpful to reveal the tectonic evolution and the characters in the Palaeo-Mongolia Ocean plate area.