Contrasting Metamorphic Evolutions Of Early Paleozoic UHP Metamorphic Rocks In The North Qaidam Mountain: Insight For Exhumation Mechanism Of Deeply Subducted Rocks

Typical continental collision orogeny is characteristized by the outcrop of dominated continual felsic gneiss enclosing eclogites and garnet peridotite.Some medium-low temperature eclogites can preserve the peak eclogitic assemblages and the garnets always preserve the prograde growth zoings.In contrast,some eclogites and their country rocks have been turned to maifc granulite and felsic(pelites)granulites,indicating that they have a prolonged high temperature granulite facies overprinting history during decompression.The two kinds of eclogites may represent different exhumation mechanisms.With the recognition of well preserved eclogites and garnet peridotites,the Yuka-luliangshan region,western north Qaidam ultrahigh pressure metamorphic belt was proviously divided into Yuka-Luofengpo gneiss-eclogite unit and Luliangshan gneiss-garnet peridotite unit.Based on detailed field investigation and geological mapping of key domains,the distribution of different kinds of UHP rocks especially the granulitized eclogites and pelitic granulite were ascertained,and the Yuka-Luliangshan region can be clarified as three units: the Yuka-Luofengpo MLT(medium-low temperature)-UHP metamorphic unit,the Guaijiaoliang-Shuangkoshan arc magmatic-metamorphic rocks unit and the Xiaohongshan-Luliangshan HT(high temperature)-UHP metamorphic unit.As an unusual region with different kinds of UHP rocks,the Yuka-Lulaingshan region is a natural labrotary to invest the differences of exhumation mechanisms of different UHP rocks.Combining with petrographic observation and P-T pseudosections,similar clockwise P-T paths with peak stages at 25-34 kbar,580-633 ?C were obtained for eclogites and metapelites.The box-shaped polyphase mineral inclusions of ep + hb ± ph in garnets of the luofengpo zoisite/clinozoite-rich eclogites as pseudomorphs after former lawsonite.Considering the recognition of low temperature lawsonite(pseudomorph)-bearing eclogite and kyanite-bearing eclogite in the Yuka River,the prograde metamorphic mineral assemblage of the eclogotes in the Yuka-luofengpo unit may all in lawsonite-stable area.Therefore,the recognition of lawsonite(pseudomorph)-bearing eclogite indicates that the subduction and exhumation of the Yuka-Luofengpo unit is fast and in medium-low temperature condition.While the different eclogites have the peak pressure gap of nearly 9 kbar and peak temperature gap of nearly 100 ?C,the peak metamorphic age of eclogites also have a gap of 20-25 Ma,the Yuka-Luofengpo unit may consisit of different metamorphic slices.Petrology,mineral chemistry and phase equilibrium modeling suggest a multi–stage metamorphic history of the granulitized eclogites and pelitic granulites in the Luliangshan terrane.The metamorphic evolution of the granulitized eclogites can be divided into four stages: 1)an eclogite facies metamorphism(P>18.5kbar,T>830°C)characterized by relic omphacite in the matrix and within garnet.;2)a protracted high pressure granulite facies stage(11.3–17.5 kbar and 852–858 °C);3)the later medium pressure granulite facies stage(7.6–7.7 kbar and 878–883 °C);and 4)a retrogressive amphibolite facies stage(P<5 kbar and T<650 °C).The pelitic granulite also record the high pressure granulite facies stage of 13.8 kbar and 810–820 °C,the medium pressure granulite facies stage of 7.2-8.7 kbar and 810-840 °C.The zircons of granulitized eclogites and the pelitic granulites both record the high pressure granulite facies metamorphic age of ~450Ma and the medium pressure granulite facies age of ~430Ma.SHRIMP U-Pb dating of zircon from metapelites in Yuka provides a metamprphic age of 920 Ma,which was interpreted as metamorphic age associated with the Rodinia supercontinent assembly event.LA-ICP-MS U-Th-Pb dating of monazite from metapelites in Yuka yields a nearly peak metamprphic age of ~440Ma for the early Paleozoic continatal collision event.Combining with the Neoproterozoic crystallization age of widespread granitic gneiss(~0.9-1.0Ga)suggest that the North Qaidam Mountains experienced both Grenville-age orogeny and Caledonian stage orogeny,and the high pressure metapelites record a complicated tectono-metamorphic evolution that involved Neoproterozoic and Early Paleozoic orogenic events.The disparate metamorphic evolutions of the UHP rocks from the Yuka-Luofengpo unit and the Xiaohongshan-Luliangshan unit indicate that the two adajacent metamorphic have different exhumation mechanisms.Based on the previous models and the latest geodynamic numerical modeling of the exhumation mechanisms,we infer that the exhumation mechanism of the UHP rocks in the Yuka-Luofengpo unit is along subduction channel,while the exhumation mechanism of the UHP rocks in the Xiaohongshan-Luliangshan unit is through the mantle wedge by diapir(relamination).The gap of peak metamorphic P-T estimation and metamorphic age of the eclogites in the Yuka-Luofengpo unit can be explained by subduction channel model.The deeply subducted rocks were detached from subducted slab at different depths and into subduction channel,and then they were exhumated along the subduction channel.In contrast,the granulitized eclogites and the pelitic granulites of the Xiaohongshan-Luliangshan unit vertically exhumated through the mantle wedge into overriding plate crust by dispar(relamination)prior to uplifting to shallow crust.