北祁连构造带东端拼贴过程的探讨——金柳滩韧性剪切带锆石U-Pb年龄和Lu-Hf同位素特征  

A discussion on the amalgamation of the eastern North Qilian Orogenic Belt——Constraints from zircon U-Pb ages and Lu-Hf isotopic characteristics of the Jinliutan ductile shear zone

作　　者：朱子璇 何艳红 陈亮[1,2] 徐可心[3] ZHU Zixuan;HE Yanhong;CHEN Liang;XU Kexin(Department of Geology,Northwest University,Xi’an,710069;State Key Laboratory of Continental Dynamics,Northwest University,Xi’an,710069;The High School Affiliated to Xi’an Jiaotong University,Xi’an,710043)

机构地区：[1]西北大学地质学系,西安710069 [2]西北大学大陆动力学国家重点实验室,西安710069 [3]西安交通大学附属中学,西安710043

出　　处：《地质论评》2022年第1期47-68,共22页Geological Review

基　　金：国家自然科学基金资助项目“早元古代胶-辽-吉带岩浆作用:地球化学和年代学”(编号:41421002);国家自然科学创新群体项目“大陆构造与动力学”(编号:41102121)的成果。

摘　　要：金柳滩韧性剪切带位于北祁连构造带东端山门镇,是陇山岩群与葫芦河岩群拼贴的构造边界。通过陇山岩群的黑云角闪斜长片麻岩和金柳滩韧性剪切带内的花岗质糜棱岩LA-ICP-MS锆石U-Pb定年分析发现:陇山岩群的黑云角闪斜长片麻岩形成于461.9±7.0 Ma;金柳滩韧性剪切带内的花岗质糜棱岩获得了~460 Ma,~420 Ma和~380 Ma三组年龄。其中,~460 Ma的锆石具清晰的振荡环带、高的Th/U值、左倾型的REE模式和显著的Ce正异常以及Eu负异常的特征,表明了~460 Ma为原岩的形成年龄;~420 Ma和~380 Ma的锆石虽然具有类似岩浆成因的REE模式,但是其内部结构显示为弱环带或者无环带的特征,为变质成因锆石的特征。~420 Ma锆石的高Th/U值暗示其为超固相线变质锆石,富Th的矿物(如独居石等)已经熔融消失。~380 Ma锆石显示变化的Th含量和稳定的U含量,明显区别于~420 Ma锆石,表明其生长过程中有Th富集的矿物(如独居石)同时晶出。金柳滩韧性剪切带内糜棱岩中发育条带状构造,以及浅色糜棱岩发育条片状暗色体,记录了部分熔融作用。因此,~420 Ma和~380 Ma代表了两期部分熔融的时限。综合剪切带南侧葫芦河岩群~440 Ma的最大沉积年龄,金柳滩韧性剪切带形成时代限定于440~420 Ma。此外,陇山岩群黑云斜长角闪片麻岩锆石具有正的ε_(Hf)(t)值(10.26~15.13)和487~790 Ma的t_(DM2)年龄,记录了强烈的早古生代陆壳生长事件。花岗质糜棱岩锆石ε_(Hf)(t)值和t_(DM2)年龄分别为-4.02~4.46和1120~1660 Ma,指示其源区以中元古代或者更加古老的陆壳物质为主。因此,北侧陇山岩群和南侧葫芦河岩群均不可能是花岗质糜棱岩的原岩,暗示其在440~420 Ma金柳滩韧性剪切带形成过程中已被强烈消减,与早古生代碰撞抬升地质作用过程一致。Jinliutan ductile shear zone locates in Shanmen Town,the east end of the North Qilian Orogenic Belt,which is the tectonic boundary between Longshan Group and Huluhe Group.LA-ICP-MS zircon U-Pb dating shows biotite amphibolite plagiogneiss from Longshan Group has formed in 461.9±7.0 Ma and granitic mylonite from Jinliutan ductile shear zone has three age groups(~460 Ma,~420 Ma and~380 Ma).The~460 Ma zircons are characterized by evident oscillatory zones,high Th/U ratios and left slope REE patterns with significant Ce positive anomalies and Eu negative anomalies,indicating the formation age of their protolith.Although~420 Ma and~380 Ma zircons have high Th/U ratios and REE patterns similar to the magmatic zircons,the internal structure exhibits weak oscillatory zone or homogeneous texture,which is the characteristics of metamorphic zircons.High Th/U ratios for the~420 Ma zircons suggest suprasolidus conditions with monazites exhausted.~380 Ma zircons show variable Th content and stable U content,which is distinct from the~420 Ma zircons,indicating that Th enriched minerals(such as monazite)crystallized simultaneously during~380 Ma zircons growth.Mylonites in the Jinliutan ductile shear zone show leucosome and melanosome bands,and some leucosome mylonites remain some irregular melanosome lens,which indicates the partial melting processes.Therefore,~420 Ma and~380 Ma are two episodes of partial melting occurred on the mylonites.Considering the~440 Ma maximum sedimentary age of the Huluhe Group south to the Jinliutan ductile shear zone,the formation age of the Jinliutan ductile shear zone is limited to 440~420 Ma.In addition,the zircons of the biotite amphibole plagiogneiss from Longshan Group have positiveε_(Hf)(t)values(10.26~15.13)and t_(DM2) of 487~790 Ma,indicating extensive continental crust growth in early Paleozoic.Zirconε_(Hf)(t)values and t_(DM2) ages of the granitic mylonites are-4.02~4.46 and 1120~1660 Ma respectively,indicating their source is dominated by Mesoproterozoic or more ancient continental cr

关 键 词：北祁连东段 韧性剪切带 陇山岩群 锆石U-PB HF同位素 

分 类 号：P597.3[天文地球—地球化学] P542[天文地球—地质学]