柴北缘超高压地体折返过程中地壳深熔的岩石学研究  

Petrological study on anatexis during exhumation of North Qaidam UHP terrane

作　　者：于胜尧[1,2] 张建新 李三忠[1,2] 彭银彪[1] 李云帅 吕沛 姚勇 李卓凡 YU ShengYao;ZHANG JianXin;LI SanZhong;PENG YinBiao;LI YunShuai;LV Pei;YAO Yong;LI ZhuoFan(MOE Key Lab of Submarine Geosciences and Prospecting Techniques,Institute for Advanced Ocean Study,College of Marine Geosciences,Ocean University of China,Qingdao 266100,China;Laboratory for Marine Geology,Pilot National Laboratory for Marine Science and Technology,Qingdao 266037,China;Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Institute of Surface-Earth System Science,Tianjin University,Tianjin 300072,China)

机构地区：[1]中国海洋大学海洋地球科学学院,海洋高等研究院,海底科学与探测技术教育部重点实验室,青岛266100 [2]青岛海洋科学与技术国家实验室,海洋地质过程与环境功能实验室,青岛266037 [3]中国地质科学院地质研究所,北京100037 [4]天津大学表层地球系统科学研究院,天津300072

出　　处：《岩石学报》2019年第10期3130-3140,共11页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41922018、41872050、41572053)资助

摘　　要：宏观、微观岩石学、地球化学和年代学研究表明,柴北缘锡铁山和绿梁山单元富含斜长石的浅色体和富含钾长石的浅色体是超高压地体折返过程中榴辉岩和片麻岩部分熔融的产物。阴极发光图像显示富含斜长石的浅色体中锆石具有明显的核-边双层结构,锆石核部无明显分带特征,并呈现出重稀土平坦和无Eu异常的稀土配分模式,~450Ma的年龄结果与区域上榴辉岩峰期变质时代一致;发光较弱的锆石边部具不明显的环带结构和较低的Th/U比值,~426Ma年龄结果代表了熔体的结晶时代。富含钾长石的浅色体中的锆石U-Pb定年结果记录的~910Ma、~450Ma和~426Ma三组年龄分别代表了片麻岩原岩结晶时代、高压-超高压变质作用时代和熔体结晶时代。富含斜长石的浅色体具有高SiO2、Al2O3、CaO、Na2O、Sr和LREE,而低MgO、FeOT、K2O、Y、Yb和HREE的英云闪长岩-奥长花岗岩的地球化学特征;而富含钾长石的浅色体具有高的SiO2、Al2O3和K2O+Na2O,而较低的CaO、MgO、REE的花岗岩地球化学特征。黝帘石和少量的多硅白云母的脱水分解是触发超高压榴辉岩发生部分熔融形成富含斜长石的浅色体的主要机制;而多硅白云母的脱水分解则是触发超高压片麻岩部分熔融形成富含钾长石浅色体的主要机制。这些浅色体显著的促进了柴北缘超高压地体的快速折返,并对大陆俯冲隧道中的元素迁移和壳-幔作用具有重要的影响。Field,microscopic petrological,geochemical and geochronological observation indicate that the plagioclase-rich and Kfeldspar-rich leucosomes were derived from partial melting of eclogite and gneiss during exhumation of the North Qaidam deeply subducted continental slab. Cathodoluminescence images show that zircons from the plagioclase-rich leucosomes have core-rim texture.The zircon cores exhibit weak zoning and flat heavy rare earth elements( HREE),without obvious Eu anomaly,and the age of~ 450 Ma agrees well with the timing of eclogite-facies metamorphism in this area. The zircon rims show weak zoning and low Th/U ratios,and the age of ~ 426 Ma represents timing of melt crystallization. The K-feldspar-rich leucosomes record three clusters of age including ~ 910 Ma, ~ 450 Ma and ~ 426 Ma,which represent the timing of protolith,HP-UHP metamorphism and anatexis,respectively. The plagioclase-rich leucosomes are characterized by high in SiO2,Al2O3,CaO,Na2O,Sr and light rare earth elements( LREE),but low in MgO,FeOT,K2O,Y,Yb and HREE,similar to tonalite and trondhjemite. In contrast,the K-feldspar-rich leucosomes are characterized by enrichment of SiO2,Al2O3 and K2O + Na2O,but poor in CaO,MgO and REE,similar to granite.Partial melting of UHP eclogite and felsic gneiss during the initial retrogression stage with plagioclase-rich and K-feldspar-rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite. The melt evolution from the leucosomes produced at the early exhumation stage to syn-collisional granitoids produced at the late exhumation might contribute greatly to the exhumation of the North Qaidam UHP metamorphic belt,which will also have important implication on element migratization and crust-mantle interaction in continental subduction channel.

关 键 词：柴北缘 地壳深熔 折返过程 岩石学 

分 类 号：P542[天文地球—构造地质学] P588.3[天文地球—地质学]