Continental Deep Subduction-exhumation Process Of North Qinling Belt:Constraints On Early Paleozoic Tectonic Evolution

High-pressure/ultra high-pressure(HP-UHP)metamorphic rocks within orogenic belts record dynamic earth processes of subduction and exhumation of both oceanic and continental lithospheric material.They mark paleo-subduction zones of convergent plate boundaries,and provide important information on the subduction of oceanic lithosphere,arc activity,back-arc spreading,continental subduction/collision,breakoff and tectonic uplift or exhumation during the extensional collapse of orogen.The processes of subduction and exhumation of HP-UHP metamorphic rocks could be recorded by the P-T-t paths,and it is essential for understanding the orogenic processes.In addition,the anatexis during the exhumation of deely subduction zone and the sedimentary responses to the orogeny exhumation process.The Qinling Orogenic Belt,located between the North China Craton(NCC)and the Yangtze Craton,is considered a composite orogenic belt and an important part of the Central China Orogen.HP-UHP rocks are exposed in the Northern,Southern and central part of Qinling complex,and constitute a HP-UHP metamorphic belt in North Qinling belt.In this dissertation,we carried on intergraded meticulous study of muti-metamorphic ages,petrographic observation，mineral chemistry,and phase equilibrium modeling,established P-T-t paths of all kinds of HP-UHP rocks from the North Qinling belt.And then discussion the relationship between felsic veins from partial melting and the exhumation of deeply subducted continental crust.Further,we employ detrital record from Middle Devonian Liuling group in the South Qinling belt to unravel the uplift of North Qinling orogen.Finally,combined with the previous study,we provide new keys to constraint the tectonic evolution of the North Qinling belt during Early Pelaozoic.The main achievements are as follow:1.Based on detail field investigation and petrographic observation,eclogites and their host rocks from Guanpo、Songshugou、zhaigen and Qinyouhe areas in the North Qinling belt were selected,using phase equilibrium modelling and geothermobarometry calculation methods,combined with zircon/monazite mitistage-ages,we established similar clockwise P-T-t paths and further confirmed they were productions of continental subduction.(1)This is the first report for the relict omphacite inclusions in the garnet from Zhaigen retrograde eclogite.By applying THERMOCALC in the NCFMASHTO system,the peak eclogite-facies P-T condition was constrained loosely constrained on>16.7 Kbar and 679 ℃-765 ℃.Combined sets of mineral inclusions,CL images,trace elements U-Pb LA-ICPMS and SHRIMP dating,and were obtained on different zircon domains from a retrogressed eclogite sample,and these revealed that the timing of the eclogite-facies metamorphism is 497-501 Ma,as recorded by metamorphic zircon cores.The retrogressive zircon rim domains recorded at two age groups of 476-447 and 431-413 Ma probably reflect the timing of middle to low-pressure granulite-facies metamorphism and the late amphibolite-facies retrogressive metamorphism,respectively.A combined CL images,mineral inclusions and REE patterns suggests three stages of metamorphic zircon growth.The eclogite-facies stage occurred at 501～497 Ma,the retrograde growth at 474-447Ma at granulite-facies and at 425 Ma at amphibolite-facies.A clockwise P-T-t path was obtained for Zhaigen retrogressed eclogite.(2)Based on petrographic observation and calculated by THERMOCALC,the peak mineral assemblage and P-T condition of guanpo eclogite are Grt+Omp+Ph+Rt+Ky±Coe and 3.0GPa,725℃,respectively.The peak metamorphic age is 487Ma,which yielded from the domians contained Omp+Grt+Ru inclusions.The host rocks of eclogites record a higher pressure in peak metamorphic stage,their P-T condition is constraint on 3.3～3.6 GPa and 690～760 ℃ by phase equilibrium modeling.The monazite from schist contains low-Si phegite,garnet,rutile,quartz and zircon inclusion,and SIMS U-Pb ages of 480 Ma.This age is similar to the formation age of quartze veins in the eclogite,indicating early exhumation and fuild/melt process was started at 480Ma in the Guanpo area.These two types UHP rocks constitute similar clockwise P-T-t paths.(3)Based on petrograghic observation,mineral chemistry,and phase equilibrium modelling using Perple_X,the predicted peak mineral assemblage of the songshugou retrograde eclogite is Grt+Omp+Ph+Rt+Ky±Coe,and the peak P-T condition is 3.1 GPa and 740～770 ℃,near the quartz-coesite boundary.Then it underwent granulite-facies overprinted and amphibolite-faceis retrogression later.Zircon LA-ICP-MS and SIMS dating revealed the age of eclogite-facies metamorphism is 490 Ma,and recored the metamorphism from granulite-facies to high amphibolite-facies at the age of 470～450 Ma;The UHP felsic gneiss contains Qz+Ru+Ap exsolution in the garnet core,indicating they suffered UHP metamorphism.After peak stage,they fast exhumed to granulite-facies metamorphic condition.Zircon U-Pb dating yielded peak metamorphic ages of 487～495 Ma,monazite U-Pb age of 480 Ma is later than peak ages,indicating granulite-facies age.We established a clockwise P-T-t path for UHP felsic gneiss.(4)Integrate previous studies on retrograde eclogite in the Qinyouhe area and our new discovery of the relict phengite in these rocks,we proposed that Qinyouhe garnet amphibolite is the retrogressed eclogite,the predicted peak mineral assemblage is Grt+Omp+Phe+Qtz.Zircon SIMS and LA-ICPMS dating yielded the protolith age is 783～818 Ma,eclogite-facies metamorphic age is 490 Ma,and the high amphibolite-facies to amphibolite-facies metamorphic is 460～416Ma.(5)The mineral assemblages,P-T conditions,and zircon/monazite U-Pb data together define clockwise P-T-t paths for the North Qinling eclogite/retrograde eclogite and their country rcoks,suggesting that the Neoproterozoic protolith of these HP-UHP rocks was subjected to continental subduction and suffered eclogite-facies metamorphism in the early Paleozoic(496-492 Ma)before undergoing retrograde metamorphism during an initial stage of fastly exhumation to middle-upper crust level at 480-470 Ma and subsequent slow exhumation to shallow upper crust at 450～420 Ma in a single cycle from subduction to continuous multi-stage exhumation.(6)Integrate new studies on Palaeozoic ophiolites and geological studies,to evaluate the formation mechanism of North Qinling belt HP-UHP rocks.HP-UHP rocks occur as lenses or layers surrounded within by gneissic rocks in the northern,central,and southern Qinling Complex of the North Qinling belt,considered to have formed through deep continental deep subduction.The protoliths of the HP-UHP metamorphic rocks show formation ages(ca.800 Ma),geochemical characteristics,and Pb-Nd isotopic compositions similar to the Neoproterozoic igneous rocks from the South Qinling belt.Continental materials of the South Qinling belt were dragged down to mantle depths by the Shangdan oceanic crust and was subjected to HP-UHP metamorphism at 492-496 Ma.2.By means of the studies of the felsic veins and granites from the Qinling Complex in the North Qinling belt,the geochemical features and formation age of the veins and granites are determined,and the relationship with between veins and exhumation process of deeply subducted continental crust.It can be devided into three major phase of anatecyic veins:470～485 Ma mainly occured as net and bedding veins in Zhaigen and Qinyouhe area,455 Ma outcropped as granitic vein in the retrograde eclogites,and 420～400 Ma are pegmatite veins and felsic granites from the Xixia area.Stage one(485～485 Ma):they are characterd with high SiO2 and K2O contents and Low TiO2 and MgO contents and defined a peraluminous series.The LREE content is obvious low to the gneiss,which is similar to granitic melts originate from partial melting of continental crust minerals.These results indicated that the felsic or pelitic rocks in the continental crust experienced incipient melting during the early exhumation and the anatectic melts originate from dehydration melting due to muscovite breakdown.Stage two(455 Ma):the felsic vein have high SiO2,K2O and Na2O content,high Sr/Y,La/YbN ratios,and low MgO contents,are similar to high-Si adakites.The high positive εHf(t)values(3.4～10.4)is consist with the eclogite in the area(eHf(t)=+6.0～+13.1),indicating the the anatectic melts derived from partial melting of eclogite lenses or blocks during exhumation,and while contaminated by the host gneissic rocks slightly.Stage three(420～400 Ma):pegmatite veins have high SiO2,Na2O and low K2O content,belong to tonalite and trondhjemite trend;while garnet-bearing felsic granites have high SiO2,Na2O and low Mg2O content content.They are the products from partial melting in different depth source areas during the uplift of the thickened crust after collision.3.Integrated with cathodoluminescence(CL)imaging and trace element analysis,U-Pb age,and Lu-Hf isotopes of detrital zircons,provenance of the metaclastic rocks from Middle Devonian Liuling group areas are well constrained,and provide insights into the uplift process of North Qinling belt.The Liuling Group is exposed in the Northern part of the South Qinling orogenic belt.LA-ICP-MS U-Pb analysis of detrital zircons from the meta-sandstones in this Group yields ages ranging between 400 Ma to 3200 Ma,with three prominent age clusters at 500-400 Ma,850-700 Ma and 1000-900 Ma.A few older zircon populations with U-Pb ages of 1750-1450 Ma,2000 Ma and 2600-2400 Ma are also present.Age data integrated with cathodoluminescence,trace element data and εHf(t)values of zircon grains show that the Liuling sediments have a complex source.Source rocks mainly include Early Neoproterozoic and Early Paleozoic granitoids,together with minor ultra-high pressure/high pressure(HP-UHP)metamorphic rocks,and paragneiss in the North Qinling belt,and Middle-Late Neoproterozoic magmatic rocks in the South Qinling belt.The dominant population of detrital zircon grains with ages between 500 Ma and 400 Ma show the characteristics of both magmatic and metamorphic zircons.They show three age clusters at 497 Ma,451 Ma,and ca.420 Ma and show marked correlation with the three stages of Palaeozoic magmatism,as well as with the peak and retrograde HP-UHP metamorphic stages in the North Qinling belt.This correlation demonstrates that these Early Palaeozoic granitoids and HP-UHP metamorphic rocks in the North Qinling belt were already exhumed to the surface,underwent erosion prior to Middle Devonian time and were then deposited in an extensional basin.That means the Early Paleozoic orogeny of North Qinling belt has ended.