北山造山带东段早志留世岩浆演化特征--来自基东花岗闪长岩的地球化学和年代学约束

Early Silurian Magma Evolution in the Eastern Beishan Orogenic Belt-Geochemical and Chronological Constraints from the Jidong Granodiorite

基东花岗闪长岩体出露于内蒙古北山地区基东一带,侵入古元古代北山岩群、晚奥陶世—早志留世公婆泉组、早志留世细粒闪长岩等,被早志留世中粒二长花岗岩、早泥盆世中粒二长花岗岩等侵入。SiO2含量平均63.91%,K2O/Na2O为0.59~1.42,铝指数A/CNK为0.93~1.05,MgO#为39.01~41.14,里特曼指数σ为1.34~1.60。稀土总量ΣREE平均为208.77×10-6;δEu值变化于0.67~0.79,负铕异常明显;稀土配分曲线为LREE富集的右倾型;在球粒陨石标准化的蜘蛛网图中,富集Rb、Th、K等强不相容元素,Nb、Ta、P、Ti等元素亏损。LA-ICP-MS锆石U-Pb测年表明其形成年龄为436.65±0.94 Ma,地质时代为早志留世。锆石εHf(t)均为负值,两阶段Hf模式年龄(tDM2)介于2015~2274 Ma。地质背景、地球化学及同位素特征显示,基东花岗闪长岩体形成于早志留世俯冲环境,可能为古元古代下地壳物质再熔融的产物。

The Jidong granodiorite in Beishan area of Inner Mongolia intruded into the Paleoproterozoic Beishan Group,Late Ordovician-Early Silurian Gongpoquan Formation,Early Silurian fine-grained diorite,etc,and was intruded by Early Silurian middle-grained monzogranite,Early Devonian middle-grained monzogranite,etc.The average SiO2 content is 63.91%,K2O/Na2O is 0.59~1.42,aluminum index A/CNK is 0.93~1.05,MgO#is 39.01~41.14,Rittman indexσis 1.34~1.60.The averageΣREE is 208.77×10-6.δEu value varies from 0.67 to 0.79,and the negative Eu anomaly is obvious.The distribution curve of rare earth is right inclined with LREE enrichment.In the standardized spiderweb map of chondrites,strong incompatible elements such as Rb,Th and K are enriched,while elements such as Nb,Ta,P and Ti are depleted.LA-ICP-MS zircon U-Pb dating indicates that the formation age is 436.65±0.94 Ma,and the geological age is the Early Silurian.ZirconεHf(t)values are all negative,and the two-stage Hf model ages(TDM2)range from 2015 Ma to 2274 Ma.The geological setting,geochemistry and isotopic characteristics indicate that the Jidong granodiorite was formed in the subduction environment of the Early Silurian and may be the product of the re-melting of the Paleoproterozoic lower crust.