Petrogenesis And Implications For The Tectonic Nature Of Middle-late Permian Granitic Rocks Within The Eastern Margin Of The Khanka Massif

The Central Asian Orogenic Belt(CAOB)is the largest Phanerozoic accretionary orogens in the world,the eastern segment is a typical area of multi-tectonic systems overprinting and transformation,and has experienced the evolution of the Paleo-Asian Ocean and the Paleo-Pacific Ocean tectonic systems.The Khanka Massif forms the easternmost part of CAOB and represents an ideal location for the late evolution and the closure of the Paleo-Asian Ocean and the onset of the Paleo-Pacific Ocean plate subduction beneath the the Eurasian continent.In this paper,we selected the Permian granitic and volcanic rocks within the eastern margin of the Khanka Massif to explore the petrogenesis and geodynamic significance by the systematic analysis of zircon U-Pb chronology,zircon Hf isotopic composition,whole-rock major-trace geochemistry,and whole-rock Sr-Nd-Pb-Hf isotopic composition,to constrain the tectonic history of the eastern Paleo-Asian Ocean by combining the available regional lithological materials.The Permian magmatism at the eastern margin of the Khanka Massif can be divided into two stages,the middle Permian(264-263 Ma)and the late Permian(?252Ma).The middle Permian igneous rocks include monzogranites and rhyolites,and the late Permian igneous rocks mainly consist of rhyolites.The zircons in the monzogranite show euhedral-subhedral,with fine-scale oscillatory growth zoning visible.This,combined with their high Th/U ratios(0.41-1.45)>0.1,indicate a magmatic genesis.The ²⁰⁶Pb/²³⁸U weighted mean age of the zircon is 264±4 Ma(MSWD=4.5,n=10) suggests the emplacement time of monzogranite.The middle-late Permian rhyolites contain mostly euhedral-subhedral zircons with fine-scale oscillatory growth zoning and high Th/U ratios(0.32-1.46),indicating magmatic genesis.The ²⁰⁶Pb/²³⁸U weighted mean ages of zircons in the rhyolites are 252±4 Ma(MSWD=1.17,n=16)and 263±4 Ma(MSWD=1.01,n=11),respectively,representing the formation time of the rhyolites.The middle Permian monzogranite and the middle-late Permian rhyolite samples in the eastern margin of Khanka Massif are geochemically similar in that they contain high SiO₂ and Al₂O₃ in content and low TiO₂,total Fe₂O₃,and Mg O in content.The monzogranite contains 76.45-76.87 wt.%SiO₂,0.06-0.07 wt.%TiO₂,12.12-12.41 wt.%Al₂O₃,1.53-1.61 wt.%total Fe₂O₃,0.13-0.15 wt.%Mg O,The A/CNK ratios(0.98-1.02)of the monzogranite are indicative of a transitional composition between metaluminous and weakly peraluminous.The monzogranite has flat REE patterns([La/Yb]_N=2.49-2.94)and significantly negative Eu anomalies(Eu/Eu*=0.06-0.08).The monzogranite is enriched in LILEs and depleted in HFSEs(Nb,Ta).The middle-late Permian rhyolites contain 73.01-74.32 wt.%SiO₂,0.22-0.27 wt.%TiO₂,13.40-14.27 wt.%Al₂O₃,2.17-2.68 wt.%total Fe₂O₃,0.48-0.72 wt.%Mg O,The A/CNK ratios(1.08-1.26)which mean they are peraluminous.The rhyolites are featured by enrichment in light REEs(LREEs)and a slight depletion in heavy REEs([La/Yb]_N=5.39-5.65).The Eu/Eu*ratios(0.55-0.65)of the samples reveal negative Eu anomalies.The rhyolites are enriched in large ionic lithophile elements(LILEs;e.g.,Rb,Ba)and deplete in high field strength elements(HFSEs;e.g.,Nb,Ta).Based on the geochemical characteristics,combined with the high differentiation index(DI=94)and some trace element ratios such as Nb/Ta=6.0-6.9(>5)and Zr/Hf=12.9-17.1(<25),suggesting that the middle Permian monzogranite is highly fractionated.Combined with the regional geographical background,we consider that the middle Permian monzogranite formed in an N-S extensional setting associated with the collision of the Jiamusi-Khanka massifs.The middle-late Permian rhyolites have high SiO₂(73.01-74.32 wt.%)in content with low Mg O(0.48-0.72 wt.%)in content and the geochemical features are similar to those of the upper continental crust.These rhyolites have positive ?_Hf(t)values(+2.1 to+13.9).Combining these with their T_DM2of 823-479 and 1036-385 Ma indicates that these rhyolites formed from magmas generated as a result of the partial melting of juvenile crustal material derived from depleted mantle during the early Paleozoic and the Neoproterozoic.Furthermore,The whole-rock?_Nd(t)and?_Hf(t)values of the rhyolites range from-0.54 to-0.16 and+2.73to+3.09,respectively.The middle-late Permian rhyolites with decoupled Nd-Hf isotopic compositions were derived from partial melting of metasedimentary crustal rocks.Their moderate-high zircon saturation temperatures(796?-817?)reveal that they formed in a crustal setting with high heat flow values.Together with the previous identification of the Permian ridge subduction at the western margin of the Khanka Massif,the middle-late Permian rhyolites should be the product of the Paleo-Asian Ocean ridge subduction,which caused the inward migration of the arc-type magmatism.This paper further exclude the possibility of the Paleo-Pacific oceanic plate subduction occurred beneath the eastern margin of the Khanka Massif in Permian.