Protolith Recovery And Metamorphic PTt Path Of Parametamorphic Rock Series In The Precambrian Basement Of The Easten Central Qilian

The Qilian orogenic belt is located in the northeast margin of the Qinghai-Tibet Plateau.As an integral part of the northern boundary of the Proto-Tethys Ocean,it is the junction zone of the North China plate,the Yangtze plate and the Tarim plate.From north to south,the Qilian orogenic belt and its adjacent areas are divided into the Alxa block,the North Qilian accretionary belt,the Central Qilian block,the Quanji block,the South Qilian accretionary belt,the north Qaidam ultra-high pressure metamorphic belt and the Qaidam block.The Qilian orogenic belt is a natural laboratory for exploring the northern boundary of the Proto-Tethys Ocean and recorded the opening-closing process of the Proto-Tethys Ocean.As a Precambrian basement in the Proto-Tethys domain,the formation age,material source,tectonic affinity,metamorphic evolution and tectonic thermal events of the continental crust of the Central Qilian block are of great significance to understand the assembly and breakup process of the Rodinia supercontinent and the opening-closing process of the Proto-Tethys Ocean.Taking the parametamorphic rocks from the Maxianshan Group and the Huangyuan Group of the Central Qilian block as the research object,studies of petrography,mineral chemistry,whole rock geochemistry and isotopic geochronology were carried out to discuss the formation age,tectonic setting and characteristics of source area of their protoliths.The results revealed the tectonic affinity of the Central Qilian block,and constrained the assembly of the Rodinia supercontinent.The metamorphic ages and metamorphic PTts path of the basement rocks were also conducted to constrain the closure process of the Proto-Tethys Ocean.The main conclusions obtained in this study are as follows:(1)The micaschists from the Huangyuan Group have detrital zircon ages of2895-928 Ma that peaking at 1.80-1.40 Ga with two-stage Hf model ages of 3423-1407 Ma.They show strongly increasing zircon εHf(t)values of-8.1 to +12.1 from 1.6 Ga to1.4 Ga,suggesting that voluminous juvenile materials were added to the crust during this period.Detrital zircon ages from the felsic gneisses are dominantly 960-913 Ma that peak at 927 Ma with two stage Hf model ages of 1977-1694 Ma and εHf(t)values of-0.1 to-10.7,suggesting that reworking of the crust was dominant during this period.The detrital zircon ages from metasedimentary rocks of the Maxianshan Group are between 876 and 2465 Ma that peaking at 1062-876 Ma with εHf(t)values from-11.3 to+17.7 and two-stage model ages from 1672 Ma to 3380 Ma.The protoliths of the micaschists from the Huangyuan Group are dominantly shales and minor wackes with maximum depositional ages from ca.1317 to 928 Ma.The protoliths of the felsic gneisses are mostly wackes with a maximum depositional age of ca.927 Ma.The protoliths of the metasedimentary rocks from the Maxianshan Group are dominantly shales and minor wackes with maximum depositional ages from ca.1160 to 910 Ma.The variable maximum depositional ages of the metasedimentary rocks indicate that their protoliths constituted a turbidite series with a long history of deposition starting at ca.1317 Ma in an oceanic island arc-related basin that developed through a transitional continental arc-related basin into an active continental marginal basin at ca.910 Ma.(2)It is inferred that the 1795-1321 Ma detritus from the Huangyuan Group and Maxianshan Group was sourced from juvenile arc crust at the margin of the Indian craton or the Western Australian craton.The source rocks for the 1317-913 Ma detritus were arc magmatic rocks formed during assembly of Rodinia.A sequence of initial intra-oceanic subduction(ca.1317-967 Ma)and continuous oceanic crust-continent subduction with formation of a mature continental arc(ca.967-896 Ma)at the margin of Rodinia during the formation of the Central Qilian block is suggested.(3)Based on petrographic observation,mineral chemistry and phase equilibrium modeling,three clockwise metamorphic stages of prograde,peak and retrograde recorded in the garnet zone,the staurolite zone and the sillimanite zone of the Huangyuan Group are identified.The sample from the garnet zone recorded a prograde stage of 4.5-5.0 kbar and 520-530°C,a peak P stage of 9.8-10.2 kbar and 560-570°C,a thermal relaxation stage of 8.0-8.5 kbar and 580-590°C and a retrograde stage of 6.8-7.0kbar and 560-580°C.A sample from the staurolite zone recorded a prograde stage of 5.0kbar and 510℃,a peak stage of 5.2-5.9 kbar and 570-600℃ and a retrograde stage of~5.0 kbar and ~540℃.Two samples from the sillimanite zone recorded a peak stage of5.0-8.5 kbar and 660-790°C and a retrograde stage of 4.0-6.0 kbar and 540-650℃.The metamorphic ages of 466±2 Ma and 435-428 Ma were obtained by zircon U-Pb dating and muscovite40Ar-39 Ar dating,respectively.We suggest that the 466 ± 2 Ma represents the peak metamorphic timing of the Huangyuan Group,and the 435-428 Ma represents the cooling history during retrograde metamorphism.(4)The HT/LP metamorphic belt(508-459 Ma)recorded by the Huangyuan Group at the northern margin of the Central Qilian block and the HP/LT metamorphic belt(490-462 Ma)recorded by the eclogite and blue schist in the Qingshuigou-baijingsi-bianmagou area in the south of the North Qilian constitute a quasi contemporaneous paired metamorphic belts,which resulted from the southward subduction of the North Qilian oceanic crust.The southward subduction of the North Qilian oceanic crust should be earlier than 508 Ma and lasted until at least 459 Ma.The North Qilian oceanic crust began to detach and exhume at 460 Ma,resulting in the extension and cooling of the overlying crust of the Huangyuan Group at 435-428 Ma.