柴北缘超高压变质带变质蛇绿岩组合及其地质意义  

Petrological Study of the Meta-ophiolite from the North Qaidam UHP Metamorphic Belt and Its Geological Implications

作　　者：王佳兴 蔺梦 刘泽宇 王琳 张贵宾[1] WANG Jiaxing;LIN Meng;LIU Zeyu;WANG Lin;ZHANG Guibin(Key Laboratory of Orogenic Belts and Crustal Evolution,MOE,School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]造山带与地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京100871

出　　处：《西北地质》2020年第4期1-10,共10页Northwestern Geology

基　　金：国家自然科学基金“尼泊尔东北部Arun Valley榴辉岩岩石学研究”项目(41972056);优秀青年科学基金“变质岩石学”项目(41622202);重大研究计划“中喜马拉雅(中国—尼泊尔—不丹)榴辉岩带的岩石学特征、PTt轨迹及其大地构造意义”项目(91755206)。

摘　　要：柴北缘超高压变质带为典型的早古生代大陆型俯冲、碰撞造山带,在都兰地区保存有2条蛇绿岩剖面,其代表了陆壳俯冲之前的大洋俯冲残片。笔者选取野马滩和沙柳河2条蛇绿岩剖面,对其岩石学、地球化学和年代学研究进展进行综述,并结合整个柴北缘超高压变质带的研究进展,探讨这些变质蛇绿岩组合的地质意义。野马滩变质蛇绿岩剖面由出露于片麻岩中的榴辉岩、石榴辉石岩、蛇纹石化橄榄岩组成。榴辉岩中保存有K-cymrite及绿辉石中出溶白云母的超高压变质作用证据,峰期变质作用条件为P=2.9~3.2 GPa,T=631~687℃。榴辉岩的地球化学数据表明其原岩类似于N-MORB和E-MORB,锆石U Pb定年显示变质时代约为445~457 Ma。沙柳河变质蛇绿岩剖面由3类榴辉岩、方辉橄榄岩及异剥钙榴岩组成。榴辉岩中保存有柯石英,峰期变质作用条件为P=2.7~3.4 GPa,T=610~725℃。榴辉岩的地球化学数据表明其原岩包括了堆晶辉长岩和E-MORB;而方辉橄榄岩属于地幔橄榄岩,且第二期橄榄石高Mg的特征表明其经历过俯冲变质。锆石U Pb定年显示原岩时代约为520 Ma,变质时代为440~445 Ma。对野马滩和沙柳河2条蛇绿岩剖面的深入研究有助于理解大洋深俯冲向大陆深俯冲和碰撞的转换过程。The North Qaidam ultra-high pressure(UHP)metamorphic belt is a typical early Paleozoic continental orogenic belt of subduction and collision.Two meta-ophiolite cross-sections in Dulan area is among the oceanic relicts before the continental subduction.Taking two meta-ophiolite cross-sections from Yematan and Shaliuhe as examples,the authors described the research progress in this area from aspects of petrology,geochemistry and geochronology,and discussed their geological significance as well.The Yematan meta-ophiolite cross-section consists of eclogite exposed in gneiss,garnet pyroxenite and serpentinized peridotite.Eclogite witnessed UHP metamorphism in k-cymrite and exsolution muscovite in omphacite.The peak metamorphism conditions are P=2.9—3.2 GPa,T=631—687℃.Protoliths of the eclogites resemble N-MORB and E-MROB geochemically,and zircon U Pb age of metamorphic geochronology is of 445—457 Ma.The Shaliuhe meta-ophiolite cross-section consists of eclogites,harzburgites and rodingite.Coesite is preserved in eclogite with the peak metamorphic conditions of P=2.7—3.4 GPa and T=610—725℃.Geochemical information of eclogite indicates that the protoliths include cumulate gabbro and E-MORB while the harzburgite is abyssal peridotite and the high Mg#in the second-generation olivine indicates that it underwent a subduction metamorphism.Zircon U Pb of the protolith age is at about 520 Ma and the metamorphic age is at 440~445 Ma.Detailed studies of these two meta-ophiolite cross-sections are of great help to understand the transition process from the oceanic subduction to the continental subduction and collision.

关 键 词：蛇绿岩 榴辉岩 都兰 野马滩 沙柳河 柴北缘超高变质带 

分 类 号：P588.3[天文地球—岩石学]