柴北缘西段落凤坡南榴辉岩的冷却史:锆石和金红石年代学及微量元素温度计制约  

The cooling history of the eclogite from the southern part of the Luofengpo in the western section of the Qaidam Block:Constraints from zircon and rutile geochronological and trace element thermometers

作　　者：于泽睿 孟繁聪[2] 张建新[2] 陈松永[2] 申婷婷 李小伟[1] YU ZeRui;MENG FanCong;ZHANG JianXin;CHEN SongYong;SHEN TingTing;LI XiaoWei(School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China;Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2024年第12期3830-3846,共17页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42072049);中国地质调查局项目(DD20230340、DD20242125)联合资助。

摘　　要：金红石是一种重要的战略矿产资源,榴辉岩型金红石矿床是其重要来源之一。柴北缘西段鱼卡-落凤坡高压-超高压变质地体是榴辉岩型金红石的远景区之一,但该类型金红石的保存条件仍不清楚。本文选择在落凤坡以南沙梁沟地区新发现的榴辉岩透镜体,对榴辉岩和榴闪岩进行了LA-ICP-MS锆石和金红石U-Pb定年及微量元素分析。榴闪岩中锆石给出了435±2Ma(MSWD=0.26)的加权平均年龄,锆石内部结构和较为平坦的重稀土配分曲线((Lu/Gd)_(N)=2.08~8.56)以及无明显的Eu负异常(δEu=0.59~0.81)表明该年龄代表榴辉岩相变质时间,锆石Ti含量温度计给出了643±27℃的峰期变质温度。榴辉岩中金红石给出了410±8Ma的回归下交点年龄,这代表了榴辉岩从峰期变质时代冷却到金红石封闭温度约570℃的时代,对应的金红石Zr温度计计算结果给出618±1℃的峰期变质温度。由此可以得出榴辉岩在折返过程中的冷却速率为2~3℃/Myr,低于鱼卡榴辉岩的冷却速率(~4℃/Myr)。榴辉岩规模,矿体品位等证据显示落凤坡南榴辉岩比鱼卡榴辉岩中的金红石退变强,形成钛铁矿或榍石。这表明了较缓慢冷却不利于榴辉岩中金红石的保存。Rutile is an important strategic mineral resource,and eclogite-type rutile deposits are one of its significant sources,the high-pressure to ultra-high-pressure metamorphic terrane at the western section of the northern margin of the Qaidam Basin,including the Yuka-Luofengpo area,is one of the prospective areas for eclogite-type rutile,but the preservation conditions of this type of rutile are still unclear.This paper selects newly discovered eclogite lenses in the Shalianggou area,located in southern part of Luofengpo.We analyzed zircon and rutile U-Pb dating and trace elements using LA-ICP-MS of eclogite and garnet amphibolite.The study shows that zircons from the garnet amphibolite yielded a weighted mean age of 435±2Ma(MSWD=0.26),and the internal structure of the zircon along with a relatively flat heavy rare earth element distribution curve(Lu_(N)/Gd_(N)=2.08-8.56)and no significant Eu anomaly(δEu=0.59-0.81)suggest that this age represents the metamorphic time of the eclogite facies,with the corresponding calculation results of T Ti-in-Zircon thermometry is 643±27℃.Rutile from the eclogite yielded a regression lower intersection age of 410±8Ma,which represents that eclogite cooled from the peak metamorphic conditions to the rutile closure temperature of about 570℃.The corresponding T Zr-in-rutile thermometery calculation results in a peak metamorphic temperature of 618±1℃.from which a cooling rate of 2~3℃/Myr during the exhumation process of the eclogite can be derived,lower than the cooling rate of the Yuka eclogite(~4℃/Myr).Evidence from the size of the eclogite and the tenor of the ore shows that the rutile in the eclogite of the southern part of Luofengpo has experienced stronger retrogression than that in the Yuka eclogite,formed ilmenite and titanite.This indicating that slower cooling is detriment to the preservation of rutile in eclogite.

关 键 词：榴辉岩 金红石 锆石 U-PB定年 柴达木北缘 

分 类 号：P578.47[天文地球—矿物学] P578.941[天文地球—地质学] P588.348P597.3