赣北梅岭岩体花岗闪长岩的形成时代及其地质意义

Formation Age of the Meiling Granodiorite in Northern Jiangxi and Its Geological Significance

为确定赣北梅岭岩体的形成时代和岩石成因以及构造环境,对梅岭新元古代花岗岩进行了岩石地球化学分析和锆石U-Pb定年。结果得到3组锆石U-Pb年龄(806.4±8.3)Ma、(810.3±9.7)Ma、(813.2±7.8)Ma,峰值为~810 Ma,与九岭岩体的形成时代均为新元古代。梅岭花岗岩中的暗色矿物主要为黑云母,高硅(SiO2为69%~70%)、高钾(K2 O/Na2 O为1.05~1.70),属高钾钙碱性系列;铝饱和指数A/CNK平均值为1.33,A/NK平均值为1.76,显示典型的过铝质花岗岩特征;球粒陨石标准化稀土元素配分模式图为右倾型,轻稀土富集,LaN/YbN平均值为5.93,Eu负异常(δEu平均值为0.53);富集大离子亲石元素Rb、Th、U、Pb,亏损高场强元素Ba、Sr、Nb、Ti、Ta,显示壳源型花岗岩特征,表明梅岭花岗岩为高钾钙碱性过铝质S型花岗闪长岩,其地球化学特征与九岭新元古代花岗岩一致。年代学资料与地球化学特征表明,梅岭新元古代花岗闪长岩应形成于扬子板块和华夏板块碰撞造山作用的同碰撞至后碰撞阶段。

The petrogeochemistry and zircon U-Pb dating of the Neoproterozoic Meiling granitoid pluton in northern Jiangxi have been carried out in this paper in order to determine its formation age and to discuss its petrogenesis and tectonic setting.The results show that three zircon U-Pb ages of(806.4±8.3)Ma,(810.3±9.7)Ma,and(813.2±7.8)Ma for the Meiling granodiorite are consistent with the chronological data of the Jiuling batholith.Biotite is the main mafic minerals in the rock.The granodiorites have high contents of SiO2(69%-70%)and high K2 O/Na2 O values(1.05-1.70),the average A/CNK value of 1.33 and average A/NK value of 1.76,suggesting that they belong to the high-K calc-alkaline peraluminous granodiorite.They are enriched in LREE with average LaN/YbN value of 5.93 and negative Eu anomaly(averageδEu value of 0.53)in the chondrite-normalized REE distribution patterns.In the primitive mantle-normalized trace element spidergrams,they are enriched in large ion lithophile elements of Rb,Th,U,and Pb and depleted in high fieldstrength elements of Ba,Sr,Nb,Ti,and Ta,indicating characteristics of crust-sourced granitoid.This means that geochemical characteristics of the Meiling pluton,consisting of high-K calc-alkaline peraluminous S-type granodiorite,are consistent with those of the Neoproterozoic S-type granite in the Jiuling batholith.In combination of the chronological data and geochemical characteristics,we infer that the Meiling Neoproterozoic granodiorite could be formed in stages of the syncollision and/or post-collision between the Yangtze and Cathaysia blocks.