西昆仑奇台达坂二长花岗岩及其暗色微粒包体对稀有金属伟晶岩形成的指示意义  

Indicative significance of the Qitaidaban monzogranite and mafic microgranular enclaves to the formation of rare metal pegmatites in West Kunlun

作　　者：孔会磊 李文渊[2] 任广利[2] 李侃[2] 赵晓健[2] 张江伟[2] KONG HuiLei;LI WenYuan;REN GuangLi;LI Kan;ZHAO XiaoJian;ZHANG JiangWei(Chinese Academy of Geological Sciences,Beijing 100037,China;MNR Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits,Xi an Center of Geological Survey,CGS,Xi an 710054,China)

机构地区：[1]中国地质科学院,北京100037 [2]自然资源部岩浆作用成矿与找矿重点实验室,中国地质调查局西安地质调查中心,西安710054

出　　处：《岩石学报》2023年第7期2063-2084,共22页Acta Petrologica Sinica

基　　金：国家重点研发计划项目(2019YFC0605201);第二次青藏高原综合科学考察研究专题(2019QZKK0806);陕西省自然科学基础研究计划项目(2021JQ-331);中国地质调查局地质调查项目(DD20190143、DD20230032、DD20230285)联合资助.

摘　　要：晚三叠世钙碱性I型花岗岩在西昆仑造山带广泛分布,岩体中常见有大量暗色微粒包体(MMEs)。西昆仑地区中生代花岗伟晶岩十分发育,是我国重要的伟晶岩型锂铍成矿带。近年来,大红柳滩一带伟晶岩型锂矿找矿取得重大突破,而奇台达坂花岗岩体与大红柳滩一带伟晶岩型锂矿在空间上关系密切。本文选择西昆仑大红柳滩一带奇台达坂黑云二长花岗岩及其暗色微粒包体进行系统的岩相学、岩石地球化学、锆石U-Pb年代学及Sr-Nd-Hf同位素研究,以准确厘定其形成时代,探讨其岩石成因、构造环境及其与稀有金属伟晶岩成矿之间的成因联系。LA-ICP-MS锆石U-Pb测试表明,暗色包体形成于212.9±1.6Ma,寄主黑云二长花岗岩形成于213.7±2.0Ma,形成时代为晚三叠世,这些年龄资料为该地区岩浆混合作用的存在提供了有力证据。地球化学特征显示,奇台达坂黑云二长花岗岩及暗色包体均属弱过铝质高钾钙碱性I型花岗岩类,暗色包体SiO_(2)含量较寄主二长花岗岩低,二者均富集大离子亲石元素(Rb、Th、U、K),亏损高场强元素(如Nb、P、Ti),并具有相似的Sr-Nd-Hf同位素组成。黑云母二长花岗岩的(^(87)Sr/^(86)Sr)i值为0.70789~0.70839,ε_(Nd)(t)为-5.42~-4.02,εHf(t)值为-4.8~4.8,二阶段Hf模式年龄(t_(DM2))为909~1516Ma;暗色包体的(^(87)Sr/^(86)Sr)i值为0.70833~0.71011,ε_(Nd)(t)为-6.36~-5.05,εHf(t)值为-4.7~1.8,二阶段Hf模式年龄(t_(DM2))为1097~1509Ma。据此本文认为奇台达坂黑云二长花岗岩及暗色包体是富集岩石圈地幔部分熔融形成的基性岩浆与古老下地壳角闪岩熔融形成的长英质岩浆混合作用的产物。综合西昆仑地区已有年代学资料和区域构造演化特征,奇台达坂黑云二长花岗岩及暗色包体形成于古特提斯洋后碰撞伸展阶段,表明至少在晚三叠世(213Ma)古特提斯洋就已经闭合并进入后碰撞伸展阶段,这为西昆仑古特提斯构造岩�Late Triassic calc-alkaline I-type granites with abundant mafic microgranular enclaves(MMEs)are widespread in the West Kunlun Orogenic Belt,meanwhile,large quantities of Mesozoic granitic pegmatites also develop in this area,which makes an important pegmatite-type Li-Be metallogenic belt in China.In recent years,a great breakthrough in mineral resource prospecting has been made in Dahongliutan area,especially in the Qitaidaban pluton which is closely related in space with the pegmatite-type lithium deposits in this area.In this paper,we present a systematic research result from petrological,petrogeochemical,zircon U-Pb chronological and Sr-Nd-Hf isotopic studies on the biotite monzogranite and the MMEs in Qitaidaban,West Kunlun,to accurately determine their formation age,petrogenesis characteristics and tectonic settings,and thus to disclose their genetic relationship with rare metal pegmatite mineralization.The LA-ICP-MS U-Pb ages of zircons indicate that the mafic microgranular enclaves and the host biotite monzogranite are crystallized at 212.9±1.6Ma and 213.7±2.0Ma,respectively,i.e.,in the Late Triassic,which provides strong evidence for the existence of magma mixing in this area.Whole-rock geochemical data indicate that the biotite monzogranite and the MMEs belong to weak peraluminous high-K calc-alkaline I-type granites,and the content of SiO_(2)in MMEs is lower than that of the host monzogranite.Furthermore,they are enriched in LILE(Rb,Th,U,K),relatively depleting in HFSE(such as Nb,P,Ti),and have similar Sr-Nd-Hf isotopic compositions.The biotite monzogranite has(^(87)Sr/^(86)Sr)i values of 0.70789~0.70839,ε_(Nd)(t)values of-5.42~-4.02,εHf(t)values of-4.8~4.8,and Hf isotope two-stage model age(t_(DM2))of 909~1516Ma;while the enclaves have(^(87)Sr/^(86)Sr)i values of 0.70833~0.71011,ε_(Nd)(t)values of-6.36~-5.05,εHf(t)values of-4.7~1.8,and Hf isotope two-stage model age(t_(DM2))of 1097~1509Ma,respectively.It is considered that they are the products of the mixing of a basic magma formed by partial mel

关 键 词：地球化学 锆石LA-ICP-MS U-Pb测年 二长花岗岩 暗色包体 稀有金属伟晶岩 奇台达坂 古特提斯洋 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学] P618.7