Tectonic Erosion Related To Continental Subduction:Insights From The Formation And Reworking Of The Precambrian Basement In Eastern North Qaidam Mountains

Tectonic(subduction)erosion has been commonly recognized in oceanic subduction zones,but evidence of tectonic erosion related to continental subduction is rarely reported.In this contribution,based the studies on the Precambrian basement in Oulongbuluke block and Dulan Ultrahigh-Pressure(UHP)Metamorphic unit in eastern North Qaidam Mountains,we sugesst a tectonic erosion process during the Early Paleozoic continental subduction.To the north of the North Qaidam high-ultrahigh pressure(UHP)metamorphic belt,the microcontinent Oulongbuluke Block(OB)is characterized by Precambrian metamorphic basement.Based on the new and existing research data,Oulongbuluke Block consists mainly of granitic gneisses,metamorphic supracrustal rocks and small amount of metabasite(mafic granulite),with peak metamorphic P-T conditions of 6.5-8.8kbar,718-771?,reflecting a medium-low pressure granulite facies condition.The zircon U-Pb datings show that the OB underwent the Early Paleoproterozoic(2.4-2.3Ga)magmatism and Late Paleoproterozoic(1.93-1.8Ga)high grade metamorphism and anataxis.Zircon Hf isotope data yield Hf isotope two stage model ages ranging mainly from 2.88 to 2.58Ga,suggesting that they are derived from the reworking of the latest-Mesoarchean to Neoarchean rocks.New data show that the north Wulan area is not the metamorphic basement of OB,as previously considered,whereas an Early Proterozoic arc metamorphic-magmatic complex belt that superimposed on Meso-Neoproterozoic magmatic-metamorphic rock series.The arc metamorphic rocks underwent a peak granulite facies metamorphism with P-T conditions of 0.4-0.5 GPa and 700-800?,being characteristic of low pressure-high temperature metamorphism(LP/HT).Zircon and monazite U-Pb data illustrate that their metamorphic age are within 500-460Ma.Arc magmatic rocks associated with the LP/HT metamorphic rocks include mafic-ultramafic intrusions,diorite and granite with magmatic ages of 510-450Ma.Thus,to the north of the North Qaidam UHP metamorphic belt,an Early Proterozoic arc metamorphic-magmatic complex belt is recognized,suggesting that an earlier continental arc formed by the north-dipping oceanic subduction prior to the continental deep subduction(450-420Ma).The UHP rocks in the North Qaidam UHP metamorphic belt mainly consist of Meso-Neoproterozoic orthogneiss,paragneiss and the enclosing eclogite.The Meso-Neoproterozoic gneisses are also considered to be dominant for crystallization basement of the Qaidam block.In this contribution,we recognize Paleoproterozoic gneiss and assocaited eclogites in Dulan UHP metamorphic units,eastern North Qaidam Mountains.These Paleoproterozoic gneiss with eclogite lenses show tectonic contact with the Meso-Neoproterozoic gneisses,separated by a ductile shear zone.Zircon U-Pb dating data suggest that the Paleoproterozoic gneisses recorded the protolith crystallization ages of 2.4-2.3Ga,the metamorphic age of 1.9-1.8Ga and an early Paleozoic metamorphic age in outer rims of zircons.Zircon U-Pb dating data from eclogite lenses enclosed in gneisses also gave the metamorphic ages of?1.9-1.8Ga and ca.447Ma,the latter is interpreted to reflect the timing of eclogite facies metamorphism.Hf isotopic data of the Paleoproterozoic gneisses gave two stage model age within 2.88-2.58Ga.These data indicate that the Paleoproterozoic gneiss and the enclosing eclogites in the Dulan UHP metamorphic unit share consistent characteristics with the metamorphic basement of OB(upper plate),and obviously different from the Meso-Neoproterozoic gneisses from the Qaidam block(lower plate).Based on the data above,we propose an Early Paleozoic subduction-tectonic erosion evolution model of the North Qaidam Mountains:The North Qaidam HP-UHP metamorphic belt was formed by the north-dipping subduction of the Qaidam block,and it underwent the oceanic subduction prior to the continental subduction:1)At ca.510Ma or more earlier,the oceanic crust between Qaidam block and Oulongbuluke block started to subduct towards the north under the Oulongbuluke block,and producted the corresponding arc magmatism and metamorphism;2)At ca.450Ma or a little earlier,the ocean closed,and subsequently continental crust begin to subduct due to the drag of the subducted oceanic crust,and the arc magmatism and metamorphism ended;3)During 450-420Ma,when continental crust underwent deep subduction towards the north under the Oulongbuluke block,slices of Palaeoproterozoic metamorphic basement from the upper plate(Oulongbuluke block)were tectonically buried to more than 80 km depth in the subduction channel,and experienced UHP metamorphism together with the subducted Qaidam block prior to their exhumation.