Study On The Metamorphic Evolution In The Central Sulu UHP Metamorphic Belt

The continental collision orogenic belt,as the most basic constituent unit of the earth’s upper crust and continent,is one of the zones with the strongest tectonomagmatic activity in the history of Earth.The research on the process of continental deep subduction has become the forefront and core subject of developing plate tectonic theory in the 21st century.The Dabie-Sulu high pressure and ultrahigh pressure metamorphic belt is recognized by domestic and foreign geologists as a type locality and the best place to study ultrahigh pressure（UHP）metamorphism and deep dynamics of plate convergence boundary,and has become a natural laboratory for geoscientists.Eclogite,widely exposed in the continental collision orogenic belt,has become the most typical lithology of UHP metamorphic rocks and mantle information carriers.Through the study,it is of great significance to understand the metamorphism in the process of continental subduction and exhumation,the time when continental collision led to the thickening of lithosphere,the formation conditions of UHP metamorphic minerals,and the thermal history of crustal evolution in the continental subduction zone.Predecessors have done a lot of research on eclogite and related rocks in the Sulu high pressure and ultrahigh pressure metamorphic belt,and have made a series of important achievements and breakthroughs.For example,finding UHP metamorphic evidence such as coesite and diamond,determining the age of UHP metamorphism,establishing the tectonic framework of the orogenic belt,and putting forward a variety of related subduction and exhumation models.However,most of these studies are concentrated in the southern and northern parts of the Sulu metamorphic belt and the Yangkou region in the central part,while the studies on Guanshan and Xiaoxinzhuang regions in the central part of Sulu are relatively weak.In addition,previous studies on mineral assemblages and mineral content changes during the metamorphic evolution of UHP metamorphic rock in the Sulu area have been lacking.There are still great disputes on the P-T conditions of UHP metamorphic rock during the peak metamorphic stage and the maximum depth of downward subduction of the Yangtze plate.Taking eclogite and its surrounding rock granite gneiss in the UHP metamorphic zone in the central Sulu as the research object,we conducted systematic petrology,mineral microchemistry and rock geochemistry studies,identified mineral assemblages in different stages,and determined the differences in original rock characteristics and tectonic settings of UHP metamorphic rock in different regions of the central Sulu.Based on phase equilibrium modelling and conventional thermobarometry,we estimated the metamorphic P-T conditions of different metamorphic stages of ultrahigh-pressure metamorphic rock.Combined with zircon U-Pb dating data,the P-T-t path of ultrahigh-pressure metamorphic rock was inversed,revealing the differential geodynamic processes of different plates in the Sulu orogenic belt.The main achievements are as follows:1.Ascertaining the mineral assemblages of different stages of UHP metamorphic rockThe research areas are Yangkou,Xiaoxinzhuang,and Guanshan regions,located in the northern,middle,and southern parts of central Sulu,respectively.Based on the textures and mineralogy of eclogites in the study areas,four stages of mineral paragenesis were identified:Paragenesis-1 is characterized by garnet-_core+omphacite+phengite in which ompacite and phengite are characterized as inclusions in garnet porphyroblast,representing the products of prograde metamorphism.Paragenesis-2 is composed of garnet+omphacite+coesite+phengite,representing peak metamorphism,marked by the appearance of coesite.Paragenesis-3 is composed of garnet+omphacite+phengite+melt,in which melt formed from quartz and feldspar occurs as“clusters”in the phengite and garnet,respectively,representing the initial exhumation stage.Paragenesis-4 is characterized by garnet-_rim+omphacite+amphibole±plagioclase+ilmenite+melt,in which amphibole and ilmenite formed from the breakdown of omphacite and rutile,respectively,corresponding to the stage of retrograde metamorphism.Melt occurred as“veinlets”filling in the fractures of garnet,“strings of beads”and“network vein”along grain boundaries between garnet and omphacite,and melt pseudomorph formed after melt films or a pool of melt in between of garnet and omphacite.The formation and evolution of mineral assemblages are of great significance to the formation and differentiation of continental crust.2.Revealing the differences of protolith characteristics and tectonic setting of UHP metamorphic rock（eclogite and granite gneiss）in different regions of central SuluThe Guanshan and Xiaoxinzhuang eclogite protoliths exhibit geochemical characteristics of the high Fe tholeiitic basalt series.The Yangkou eclogite protoliths are partly characterized by geochemical characteristics of the high Fe calcium alkaline basalt series,partly andesitic rocks,and the Yangkou granitic gneiss protoliths are granite.In the central Sulu UHP metamorphic zone,the protoliths and tectonic settings of eclogite granitic gneiss are different with the changing trend of IAT→ICA→CAB from south to north,indicating that the protoliths and tectonic settings of different plates in the evolution of the orogenic belt are different.3.Reversing the complete P-T-t path of UHP metamorphic rock in different regions of central Sulu,and revealing the differential exhumations of different plates in the Sulu ultrahigh-pressure metamorphic beltEclogites from the central Sulu ultrahigh-pressure metamorphic belt consist of garnet,omphacite,phengite,amphibole,epidote,quartz/coesite,rutile,ilmenite,apatite and kyanite.Granitic gneisses consist of garnet,epidote,phengite,biotite,plagioclase and quartz.Yangkou eclogites and granitic gneisses samples records three stages of metamorphism:（I）subduction stage and（Ⅱ）exhumation stage.The P-T conditions during subduction stage are recorded in garnet cores,with P-T range of 24-26 kbar and 595-600℃.Based on zircon U-Pb dating,the prograde metamorphism was restricted to～240 Ma,and UHP metamorphism occurred at～224 Ma.The P-T conditions in the exhumation stage are preserved in granitic gneiss.with P-T range of 22-23 kbar and 795-820℃.Late exhumation stage is recorded in garnet rim,with P-T range of 12-15kbar and 765-795℃,and the age of the retrograde metamorphic stage was～205 Ma.The retrograde P-T path is nearly parallel to the prograde P-T path.Such a hairpin style of P-T path marks rapid exhumation of the deeply subducted crustal slice through slice thrust along a subduction channel from mantle to crustal depths at the terminal stage of continental collision.Xiaoxinzhuang eclogites samples record three stages of metamorphism:（Ⅰ）prograde stage,the P-T conditions are constrained at 23-24kbar/645-655℃and 24-26 kbar/625-630℃,and the age of prograde metamorphism was～241 Ma.The P_max,35-36 kbar at 756-775℃,have been calculated from a combination of the garnet-clinopyroxene Fe-Mg exchange thermometer and garnet-phengite-clinopyroxene-kyanite-quartz?coesite thermobarometers.And UHP metamorphism occurred at～227 Ma.（Ⅱ）post-P_max decompression and heating to the T_max stage,the P-T conditions are 21-23 kbar and 870-925℃.（Ⅲ）retrograde stage,the P-T conditions are 13-16 kbar and 815-850℃,and the retrograde metamorphism was restricted to～213 Ma.Guanshan eclogite samples records two stages of metamorphism:（Ⅰ）prograde associated with quick subduction,the P-T conditions are 19-22kbar/610-640℃,22-23 kbar/600-610℃and 25-26 kbar/605-615℃,and the prograde metamorphism was restricted to～245 Ma.The peak P-T conditions are 33kbar and 685℃,and UHP metamorphism occurred at～228 Ma.（Ⅱ）retrograde associated with quick exhumation,the P-T conditions are 14-15 kbar/805-820℃and 12 kbar/780-790℃,and the retrograde metamorphism was restricted to～214 Ma.4.Revealing the geodynamic process of the Yangtze Plate subducting and exhumating towards the North China PlateThe prograde to peak stage proceeded at low geothermal gradients 7.0-9.5℃/km to 6.2-6.5℃/km for HP（245-240 Ma）to UHP eclogite facies（228-224 Ma）,representing a depth from 65-85 km to more than 115 km,and the subduction rates is 3km/Ma.The exhumation stage proceeded at thermal gradients 10.5-12.5℃/km for HP eclogite facies（224-214 Ma）,corresponding to depths from 115 km to 70-75 km,and rapid exhumation rates is≥4 km/Ma.Exhumation has resulted in the elevation of thermal gradients and the change of metamorphism from HP eclogite facies to HP granulite facies.The late exhumation stage proceeded at moderate thermal gradients 15.5-19℃/km for amphibole eclogite facies（214-205 Ma）,corresponding to depths 40-50 km,and slow exhumation rates is≤3 km/Ma.The data suggest that ultrahigh-pressure metamorphic rocks in central Sulu subducted to a depth of more than 115 km.