西昆仑大红柳滩岩体地质和地球化学特征及对岩石成因的制约

Petrogenesis of the Dahongliutan Monzogranite in Western Kunlun:Constraints from SHRIMP Zircon U-Pb Geochronology and Geochemical Characteristics

新疆西昆仑康西瓦断裂带南缘的巴颜喀拉褶断带中分布着中生代三十里营房-泉水沟岩浆岩带,该带中最典型的岩体即为大红柳滩岩体,其主要岩性为中细粒二长花岗岩。结合锆石阴极发光图像（CL）和U、Th元素特征,通过SHRIMP锆石U-Pb定年,获得大红柳滩二长花岗岩体的年龄为220±2.2～217.4±2.2 Ma,时代属晚三叠世（T3）。二长花岗岩含石榴子石和电气石,具有高硅（SiO2为71.77%～74.20%）、富碱（Na2O＋K2O为6.18%～8.02%）、富钾（K2O为3.03%～5.30%）、铝饱和指数较高（A/CNK介于1.20～1.59）的典型特点,属高钾钙碱性过铝质系列。岩石相对富集轻稀土且轻重稀土元素分馏明显（LREE/HREE为10.21～11.82,（La/Yb）N为24.06～31.65）,具有强烈的铕负异常（δEu为0.25～0.44）;明显富集Ta、Hf等高场强元素及Rb、Th、U等大离子亲石元素,而贫Ba、Sr、Ti、Nb、Zr等元素,显示经历了较高程度的分异演化（分异指数DI平均89.40%）,综合矿物组合和地球化学特征的判别表明,大红柳滩岩体属于高分异的S型花岗岩,是同碰撞背景下壳源物质部分熔融的产物。根据岩体的成因类型并结合区域构造环境演化,分析认为晚三叠世随着古特提斯洋向北消减直至最终闭合,构造应力由俯冲作用转化为碰撞挤压作用环境时形成了大红柳滩岩体,表明该区在晚三叠世已进入陆-陆碰撞造山的构造演化阶段。

Lying in the Western Kunlun area of Xinjiang,Bayankala folded fault belt in the south margin of Kangxiwa fracture zone hosts a Mesozoic granite belt extending from Sanshiliyingfang to Quanshuigou,the typical granitoids in which is the Dahongliutan pluton.The pluton is composed mainly of the medium-fine grained monzogranite.This study yielded an age of 220±2.2～217.4±2.2 Ma for the pluton using the SHRIMP zircon U-Pb dating.Combining with the cathodoluminescence（CL）images and element U and Th features of zircon,authors suggest that the pluton formed at the Late Triassic（T3）.Monzogranite contains accessory minerals such as garnet and tourmaline.The geochemical analysis shows that the pluton is characterized by high SiO2（71.77%～74.20%）,high potassium（K2O=3.03%～5.30%,Na2O＋K2O=6.18%～8.02%）,and high aluminum index A/CNK（1.20～1.59）,belonging to high-K peraluminous calcic-alkaline series.The pluton is rich in LREE,deplete in HREE（LREE/HREE =10.21～11.82）,with obviously negative Eu anomaly（δEu=0.25～0.44）.Meanwhile,the trace elements are characterized by enrichment of Ta,Hf,Rb,Th and U,and depletion of Ba,Sr,Ti,Nb,Zr,as well as with high differentiation index（DI=89.40%）.The enrichment of HFSE（Ta and Hf）and LILE（Rb,Th and U）,and depletion in Ba,Sr,Ti,Nb,Zr suggest that he pluton experienced the highly fractionated process of evolution.Mineral association and geochemical features indicate that the Dahongliutan pluton belongs to highly fractionated S-type granite,the product probably resulting from partial melting of crust material in the syn-collisional setting.Based on the genetic type of the pluton and regional tectonic setting,it can be concluded that in the Late Triassic,the Paleo-Tethys Ocean gradually evolved from subduction toward north to the complete closure,and regional tectonic stress was transformed from subduction process to collision compressional environment.Therefore,we suggest that the Dahongliutan pluton formed just after this tectonic stress transfromation,indicating that this area entered continent-continent collision orogenic regime in the Late Triassic.