Differential Exhumation And Partial Melting Of HP-UHP Metamorphic Rocks In South Altyn Tagh,NW China

The Altyn Tagh is located at the northeast margin of the Tibet Plateau,between with the Tarim basin to the north and the Qaidam basin to the south.During the past decades,various HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh,distributed in Jianggalesayi,Yinggelisayi,Danshuiquan and Munabulake areas.Among of them,the metamorphic rocks in Jianggalesayi and Yinggelisayi areas have been confirmed to be underwent UHP metamorphism.However,credible mineral evidence such coesites is still lack to further confirm the UHP metamorphism of south Altyn Tagh.Meanwhile due to the peak metamorphic condition and retrograde metamorphic age of those HP metamorphic rocks in the Danshuiquan area have not been well constrained yet,resulted in the metamorphic evolution relationship of these HP-UHP metamorphic rocks from different areas of the South Altyn Tagh is ambiguity.Furthermore,previous studies reported that peak temperature of the HP-UHP metamorphic rocks in Danshuiquan-Yinggelisayi areas can reach 900~1000 °C,however,information about whether they experienced partial melting in such high temperature is very limited.In present study,an integrated study including petrography,metamorphic modeling,geochemistry and geochronology was conducted on the HP-UHP metamorphic rocks from Keqike Jianggalesayi and Danshuiquan areas and the anatectic veins from Danshuiquan and Yinggelisayi areas,the main achievements are as follows:1.The identification of coesite from Keqike Jianggalesayi eclogite for the first time provides convicing evidence for that the South Altyn Tagh indeed experienced UHP metamorphism.(1)Coarse coesite grains inclusions in omphacite from Keqike Jianggalesayi eclogite have been identified by identical raman waves of 512 cm-1,which defined the peak mineral assemblage of Grt+Omp+Coe+Ru in the coesite stability field.The discovery of coesite provides robust evidence for South Altyn Tagh undergoing UHP metamorphism.(2)The P-T condition of eclogite-facies stage was estimated at 728~880°C,>2.8GPa.Then it underwent granulite-and amphibolite-facies two stages of retrogression.The inclusions in different domains and zircon U-Pb ages indicated peak eclogite-facies age is ~500 Ma and the granulite-facies age is ~450Ma.A clockwise P-T-t path was obtained.2.Retrograde eclogite was discovered in Danshuiquan area,and the peak P-T condition of all previous HP granulites in this area can reach eclogite-facies metamorphism.(1)In Danshuiquan area,mineral assemblage of Grt+Omp+Q+Ru was identified as inclusions in zircons formed at peak stage(~500Ma domain)in the retrograde eclogite xenolith of granitic gneiss(granitic HP granulite),and zircons in this stage with flat HREE patterns and no Eu anomalies,which indicate that the basic xenolith is retrograde eclogite.This discovery proves that the peak stage of metamorphic rocks in this area reach eclogite-facies.The metamorphic conditions of graniulite-facies and amphibolite-facies are estimated as P=1.22-1.31 GPa,T=845-900°C and P=0.9~1.1Gpa,T=660~700°C respectively according to the geothermobarometer,and consequently,a clockwise P-T-t path for the retrograde eclogite is established.(2)In granitic gneiss(granitic HP granulite)with high Ca component,fine preserved garnet with prograde growth zoning is discovered,and a intergrated P-T path is established according to the THERMOCALC 3.40,which prescribe a eclogite-facies peak condition of P=2.2~2.5GPa,T=950~1000°C.(3)In the kynite-garnet bearing pelitic gneiss(pelitic HP granulite),the garnet is fine preserved with prograde growth zoning,according to the component and mineral inclusions in the garnet and the mineral assemblage,a intergrated P-T path is established combined with the THERMOCALC 3.40 and traditional geothermobarometer,which prescribe a peak metamorphic condition of P=0.78~0.96 Gpa,T=723~800°C,prove that the peak stage of the politic gneiss reach eclogite-facies.3.We ascertain the granulite-facies retrograde age of the metamorphic rocks in Danshuiquan area as ~483-488 Ma,which is ~30Ma earlier than the granulite-facies retrograde age in Jianggalesyi area,and indicate that the HP-HUP metamorphic rocks in different area of the South Alty Tagh with the character of differential exhumation.4.Combined with the chronology,geochemistry and zircon Hf isotope analysis,we confirmed the age of the felsic veins and granitic dike in Danshuiquan-Yinggelisayi area,and the characteristic and the source of the veins and dike are well defined.(1)The age of the felsic vein in retrograde eclogite,felsic gneiss and garnet pyroxenite in Danshuiquan area are estimated as 484~488Ma,which is in accordance with the graniulite-facies retrograde age in this area.Combined the geochemistry and zircon Hf isotopic result,melt in granitic gneiss is believed derived from the own partial melt of gneiss,while melt in retrograde eclogite suggested a contaminated source from both the retrograde eclogite and the granitic gneiss,and melt in garnet pyroxenite show a contaminated source from both the garnet pyroxenite and the pelitic gneiss.(2)Age of the felsic vein and granitic dike in granitic gneiss of Yinggelisayi area are 485 Ma and 486 Ma respectively.The geochemical and Hf isotopic result indicate that the vein is formed by the partial melt of the granitic gneiss,and is contaminated by the melt of the basic UHP rocks during melt migration.The granitic dike show similar geochemical and Hf isotopic characters with the granitic gneiss in Danshuiquan-Yinggelisayi area,indicate the dike is formed by the partial melt of the gneiss.5.Combined with the research results of the HP–UHP metamorphic rocks in South Altyn Tagh,the differential exhumation of metamorphic rocks might be accounted for the detachment and exhumation of rocks in different layers with different thermal conditions in subduction channels,and HP-UHP rocks near the mantle wedge or mantle hot spot will be heated and triggered partial melt,thus lead to the rapid exhumation.This might be a probable causation which resulted in the high metamorphic temperature and rapid exhumation of rocks in Danshuiquan-Yinggelisayi area.