新疆吉木乃县恰其海A型花岗岩的地球化学特征、年代学及构造意义

Geochemical Characteristics,Geochronology and Tectonic Implications of A-type Granite from Qiaqihai Intrusion in Jeminay County,Xinjiang

新疆西准噶尔北部发育大量碱性花岗岩,其形成时代为早二叠世,岩石类型为钾长花岗岩和石英钾长正长岩。恰其海岩体的形成年龄为280 Ma,SiO2为66.83%～68.44%,高铝(14.46%～15.41%);富含碱质,里特曼指数(δ)均大于3.3,介于4.47～4.61之间;富钾贫钠,K2O/Na2O为1.10～1.25;铁高而镁低,FeOt/MgO为4.06～7.85。其稀土配分模式右倾,轻重稀土分馏明显,具有中等-强烈铕负异常。地球化学特征表明:岩石形成于造山晚期伸展体制的构造环境中,应为后碰撞演化的晚期阶段,早于东准噶尔后碰撞的时间(二叠纪末)。构造图解判别岩体为A2型花岗岩,可能受到与俯冲有关的流体交代的影响,为探索区域晚古生代地壳演化提供了约束。

Large-size alkali granites are distributed in the northern part of western Junggar,Xinjiang,belonging to early Permian.Rock types include moyite and quartz syenite.The Qiaqihai intrusion is one of these granites.The SiO2 content of Qiaqihai intrusion ranges from 66.83% to 68.44%,and the intrusion is also characterized by high content of Al（14.46%～15.41%） and alkali,rich K,depleted Na,and low MgO.Rittmann values vary between 4.47 and 4.61,whereas K2O/Na2O values between 1.10 and 1.25,and FeOt/MgO values between 4.06 and 7.85.The REE patterns are characterized by LREE enrichment and medium to strong negative Eu anomalies with obvi-ous fractionation between LREE and HREE.These geochemical characteristics suggest that the Qiaqihai intrusion formed in a post-collisional extensional tectonic environment and occurred earlier than the collision time of East Junggar.The trace element features suggest that this intrusion belongs to A2-type granite and was possibly influ-enced by subduction-related fluid metasomatism.A-type granite can provide important constraints for late Paleozoic crustal evolution in this region.