Paleozoic Geological Evolution Of The Chinese Altai Orogen In The Fuyun-qinghe Region:Constraints On The Accretionary Orogenic Processes Of The Paleo-asian Ocean

The Altai Orogen,as a crucial part of the Central Asian Orogenic Belt(CAOB),and recording geological history of subduction-accretion of the Paleozoic Asian Ocean(PAO),is regarded as a key area to understand accretionary orogenesis.This Ph D study selected the Fuyun-Qinghe region as the key research area,which is located at the eastern Altai orogen,focusing on the Habahe group and the Paleozoic magmatic rocks.I carried out a systematic study including field geology,petrology,petrography,peochemistry,Zircon U-Pb age dating and Hf isotope composition analyses in order to constrain the subudction polarity,arc-basin system,slab rollback and PAO closure processes,which provided important insight into understanding Paleozoic tectonic evolution of Chinese Altai Orogen.Detailed achievements are listed as the following:(1)As the most widely exposed meta-sedimentary rocks in the Chinese Altai,the Habahe Group was previously considered as representing a component of the Precambrian micro-continent,forming at a passive continental margin or a magmatic arc.However,our new zircon U-Pb data for the detrital zircons from the Habahe Group in the Qinghe region indicate that the strata were deposited in the period from the late Silurian to early Devonian(427~405Ma)and experienced regional metamorphism in the middle Devonian(383±9Ma),as constrained by the ages of the youngest detrital zircons(427±13Ma),the age of 405±3Ma obtained from the tonalite that intrudes the Habahe Group,as well as the metamorphic age of383±9Ma.Additionally,the majority of zircons from the Habahe Group yield positive ?Hf(t)values(+0.22~+15.02).These data,along with geochemical features and maturity of the sedimentary rocks,indicate an immature provenance for the Habahe Group,with the source materials mainly derived from a juvenile crust with some old continental components.The juvenile crust is considered as having formed in a subduction zone,developing in the Chinese Altai during early-middle Paleozoic time.New geochemical data presented in this study suggest that the Chinese Altai was a transitional arc during early Paleozoic time,similar to the Japan-type arc with involvements of old continental components.Therefore,it is proposed that the Chinese Altai was likely rifted from the south margin of the Siberia Craton,probably related to the break-up of the Rodinia Supercontinent,and then experienced accretion and reworking associated with the subduction of the Paleo-Asian Ocean during early Paleozoictime.(2)The Devonian is a main period of subduction accretion of the PAO,generating a major of magmatic event in the Altai orogen.The Fuyun-Qinghe area is here considered as an ideal place to study the subduction polarity and style of crustal growth,given that the numerous magmatic rocks are exposed.Systematic investigations were also carried out on the gabbro,diorite,granitoid and rhyolite in the study area.Geochronological data indicate that these igneous rocks were emplaced at 415~382Ma extending along a SE-ward younging direction.The geochemical data argue for a magmatic arc setting for these rocks.Moreover,the chemical data display a spatial variation,with northwestward increasing in the contents of K2 O,large ion lithophile elements(LILEs)and rare earth elements(REEs)and decreasing in Ni,Co,Sr concentration,reflecting a northeastward subduction polarity.The positive zircon?Hf(t)values also display a southeastward increasing pattern,ranging from +1.56~+6.98 to+11.21~+15.89.However,the model ages show a southeastward younging,varying from1.3-0.9Ga to 0.66~0.43 Ga.The data pattern,together with the migration of the Paleozoic magmatic rocks toward the southeast,suggesting a slab rollback event happening in the Devonian.(3)Controversy has long surrounded the closure time of the Paleo-Asian Ocean.The Permian granitoids are widely distributed in the southern Chinese Altai and show typical alkaline or A-type affinities.The Qinghe pluton on the northside and within Erqis fault is composed mainly of granite with minor diorite.New geochronological data reveal the crystallization age ranging from 286 to 269 Ma for these intrusive rocks.The Qinghe granite is characterized by high Si O2,alkali(K2O+Na2O=5.8%~8.72%)and low Ca O(1.2%~3.76%),belonging to high kalium calc-alkaline series.They are also depleted in Ba,Sr,P,Ti,Nb and Ta,with negative Eu anomalies(?Eu=0.46~0.78),comparable to A2-type granitoids that were emplaced at the end of the collision orogeny.In addition,they display positive ?Hf(t)values(ranging from +4.04 to +11.78)and model ages(TDM2)of 880~694Ma,923~633Ma,875~555Ma and 1030~635Ma for these intrusions,suggesting that they derived from partial melting of Neoproterozoic mantle material or juvenile crust.Given that the contemporary enriched alkali(A-type)granite along the southeast margin of the Altai orogenic belt,this study suggests that the Qinghe pluton was generated by partial remelting of Neoproterozoic juvenile lower crust,probably caused by the underplating of mantle-derived magma with subsequent crystal fractionation.Therefore,the collision orogeny of the Altai orogenic belt and closure of Paleo-Asian Ocean in the Chinese Altai orogen must have occurred in the early Permian,followed by a post-collisional extensional event.(4)The Paleozoic tectonic evolution of the Chinese Altai Orogen is closely related to the subduction process of the PAO.According to the formation age,source nature,sedimentary features and depositional environment of the Habahe group,associated with the time-spatial distribution regular of early Paleozoic,a trench-arc-basin tectonic system was proposed in this Ph D study.In the Devonian,widespread igneous rocks were generated with southeastward younging direction,due to a subduction slab rollback event.In the early Permian,the Erqis ocean closed,leading to the collision between the Altai and Junggar blocks,followed by a post-collisional extension,resulting in massive A-type granites or alkaline rocks along the Erqis fault.Hence,the Paleozoic tectonic evolution of the Altai orogen in Fuyun-Qinghe region recorded entire evolution history of the Chinese Altai resulting from the subduction accretion and the closure of the PAO.