Timing,Petrogenesis And Metamorphic Evolution Of Corundum-Bearing Gneisses And Related Rocks In The Dabie Orogen

The continental collision orogenic belts have often undergone a long and complicated metamorphic evolutional history.The ultrahigh-pressure(UHP)metamorphic rocks have undergone a series of processes of deep subduction to the depth of the mantle and subsequent exhumation to the earth surface.The P-T-t paths of these UHP rocks record the whole dynamic earth processes from subduction to exhumation of the crustal material.Therefore,the study of P-T-t paths of these metamorphic rocks is the essential for understanding the orogenic processes.On the other hand,anatexis(i.e.partial melting)plays an important role during the formation and evolution of continental collision orogens,and is of great significance for understanding the exhumation mechanism of deep subducted continental crust,post-collisional metamorphic superpsition and the development of plate tectonics.It is one of the most popular area in the earth sciences fields.In addition,the research on the anatectic effect of orogenic belts is mainly focused on the silica-saturated rock system,However,in some case,the rock system is silica-unsaturated with the absence of quartz such as Al-rich Si-poor metapelite and mafic rock(syenite).At present,the studies on the partial melting effect in the silica-undersaturated system are scarce.The relevant reactions,melting behavior and the formation mechanism of the peritectic minerals need to be further studied.The Dabie orogen was formed by the subducting of the South China Block beneath the North China Block in Triassic.It is an important part of the Central China orogen and consists of five fault-bounded lithotectonic units with various metamorphic grades and evolutional histories.Among them,the North Dabie complex zone(NDZ)is one of the three eclogite-bearing ultrahigh-pressure(UHP)units of the Dabie orogen,and underwent a complex evolution including deep subduction,ultrahigh pressure metamorphism,multistage exhumation and post-collisional collapse as well as multiple episodes of anatexis.Among them,the corundum-bearing gneisses are a rare petrological group in the Mozitan area of the NDZ,providing an excellent opportunity to study the anatexis under the silica-unsaturated system.Therefore,in this PhD’s thesis,the corundum-bearing gneisses and related rocks(eclogites and granitic gneisses)in the NDZ have been systematically studied by petrology,petrography,zircon U-Pb geochronology,element and isotope geochemistry.The obtained results summary the different formation mechanisms of corundum,and study the formation mechanism and temperature and pressure conditions of anatectic corundum,finally proposed the the geological background of the formation of anatectic corundum.Combined with the study of other rocks related with the corundum-bearing gneisses,such as eclogite,providing new constraints on the ages and mechanisms of metamorphism and anatexis in NDZ,especially bringing the new insights on mechanisms and ages of anatexis in the silica-unsaturated system.The results also provide new constraints on subduction-exhumation tectonic evolution of the collision orogen.Based on the petrographic observations,the corundum-bearing gneisses from the Mozitan area of the NDZ consist of corundum,biotite,K-feldspar and plagioclase,and show clear macro-and micro-structural evidence of anatexis by dehydration melting of muscovite in the absence of quartz.Mineral textures and chemical data integrated with phase equilibria modeling,indicate that coarse-grained corundum porphyroblasts in leucosome domains are peritectic phase,reflecting dehydration melting of muscovite through the reaction:Muscovite=Corundum+K-feldspar+Melt.Aggregates of fine-grained,oriented,small corundum grains intergrown with alkali feldspar in the mesosome domains are,instead,formed by the dehydration melting of muscovite with aluminosilicate,through the reaction:Muscovite+Al-silicate=Corundum+K-feldspar+Melt.Moreover,the petrogenetic grid shows that the second reaction,that is,the formation of aggregates of fine-grained,oriented,small corundum grains intergrown with alkali feldspar in the mesosome domains is earlier than the formation of coarse-grained corundum in leucosome domains.This is also consistent with the petrographic observation that all corundum-bearing gneisses contain the small corundum aggregates in the mesosome domains,while the coarse-grained corundum in leucosome domains only appeared in some samples,indicating that the coarse-grained corundum porphyroblasts in leucosome domains were formed late and required a higher temperature.The P-T pseudosections modeling in the constrains peak pressure-temperature conditions at 900-950℃ and 9-14 kbar.This P-T condition consists with the ultrahigh-temperature granulite facies recorded by the eclogites during exhumation in the NDZ.The SHRIMP U-Pb dating results of zircons from the corundum-bearing gneisses and related rocks(eclogite,gneiss)suggest these rocks all have experienced the Triassic deep subduction and UHP metamorphism.Studies on the zircons of corundum-bearing gneisses indicate that they have undergone multistage of metamorphism during the Triassic subduction and exhumation process.According to the different age records in the metamorphic zircons and the studies of eclogites and other rocks in this area,the evolutional processes for the corundum-bearing gneisses from the NDZ can be divided into:prograde metamorphism(234～249 Ma),UHP metamorphic peak(221～225 Ma),quartz eclogite-facies metamorphism(213～217 Ma),granulite-facies metamorphic metamorphism(203～209 Ma),amphibolite-facies metamorphism(167～193 Ma)and Cretaceous thermal metamorphism(135～157 Ma).Apart from that,multiphase crystal inclusions(MCIs)of albite+K-feldspar+muscovite+biotite in zircon mantle domains record high-T anatexis with late Triassic ages(203～209 Ma)which represent the retrograde metamorphism stage of high-temperature granulite facies(900～950℃,9-14 kbar)during initial exhumation.It also reveals that anatectic corundum are formed in the early stage of exhumation and the genetic mechanism is that muscovite decompression dehydration melting in the silica-unsaturated system,producing the peritectic minerals,such as corundum and K-feldspar.The whole-rock geochemistry of the corundum-bearing gneisses indicates that their protoliths were of sedimentary origin and belong principally to clay rocks and the notably low-Si and high-Al bulk compositions were inherited from the protolith,which is inferred to have been high-Al sediments derived from weathering of a dominantly felsic rock.The results are consistent with the mineral assemblage of corundum-bearing gneisses.In combination with geochronological and isotopic results,the sourced rocks of the aluminous gneisses in this study are products of rifting magmatism during the Neoproterozoic extension of the Yangtze Craton in response to the breakup of the super continental Rodinia.Therefore,the formation mechanism of the corundum-bearing gneisses is as follows:large-scale magmatism occurred on the northern margin of the Yangtze block at Neoproterozoic,forming felsic magmatic rocks with the Neoproterozoic age.After a series of weathering,the chemical composition of the whole rock gradually became rich in aluminum and poor in silicon.Subsequently,these Al-rich and Si-poor sedimentary rocks have participated in the deep continental subduction process in the Triassic,and experienced multistage metamorphic evolution processes during the subduction and exhumation process.In the early stage of exhumation(Late Triassic,203～209 Ma),muscovite decompression dehydration melting at high temperature and finally,corundum-bearing gneisses was formed.According to the petrographic observations,geothermobarometry of minerals and phase equilibrium modeling,the estimated peak P-T condition of eclogites in NDZ are 2.5 GPa and 770～929℃ and the Ab and Rt exsolutions in the omphacite core indicate these eclogites suffered the UHP metamorphism and belong to UHP eclogite facies metamorphic stage.Then eclogites underwent granulite facies overprinted(0.73～1.68 GPa,886～937℃)and amphibolite-facies retrogression(0.6～0.9GPa,739～781℃)later.Zircon SHRIMP dating revealed the age of eclogite facies metameophism is 220 Ma,and recorded the granulite facies metamorphic of 208 Ma.These eclogites all belong to high temperature(>750℃).Among them,the eclogites of Gongjialin area have higher peak temperature(850～910℃)than Robazhai eclogites.According to the detailed field investigation and petrographic observation,the corundum-bearing gneisses and related rocks such as eclogites in NDZ were selected,using phase equilibrium modelling and geothermobarometry of minerals,combined with zircon SHRIMP dating,we established the clockwise P-T-t path which reveals these rocks all underwent the same multistage high temperature metamorphism from UHP eclogite facies to granulite facies overprinting.Furthermore,different from the CDZ and SDZ,the NDZ underwent a multistage high-temperature metamorphism showing a rapid exhumation and slow cooling during the early stage of uplift.