Magmatism And Tectonic Evolution In The Xinjiang Junggar And Adjacent Regions Since The Late Paleozoic

The Central Asian Orogenic Belt(CAOB),which is one of the world's largest Phanerozoic accretionary orogens,not only records long-lived evolution history of the Paleo-Asian Ocean(PAO)but also represents an ideal natural laboratory to probe the Phanerozoic crustal growth.The Junggar and adjacent regions in northern Xinjiang occupies a critical southwestern segment of the CAOB and is mainly characterized by multistage subduction and collision of accretionary complexes and magmatic arcs.Its unique magmatic orogeny-mineralize is a frontier topic in international geosciences,which is important for dissecting the type of continental orogeny and exploring the crustal growth.The late Paleozoic was the peak of magmatism and mineralization,and it was also a critical period for the evolution and tectonic transition of the PAO in the area.Although many geologists have concentrated on its formation and evolution during the late Paleozoic,some key scientific issues are still controversial and remain to be solved,such as the history of subduction,accretion and closure of the PAO,the growth mechanism of Phanerozoic continental crust and whether there are Precambrian microcontinents in the region.In addition,studies on Mesozoic-Cenozoic magmatism in the Junggar and adjacent regions are rare,which hampers our understanding of the mantle evolution and crust-mantle interaction during this period.Based on detailed field geology,mineral chemistry,geochronology and geochemical studies of the magmatic rocks exposed and obtained by drilling in the Junggar and adjacent regions,we hope to provide new insights into not only the magmatism and tectonic evolution of the studied area since the late Paleozoic,but also the Phanerozoic crustal growth in CAOB.The preliminary conclusions have been obtained as follows:(1)The newly discovered diabase dykes from the Chinese Altai have zircon U-Pb ages of?386 Ma.The dykes have low Si O₂(44.5-48.2 wt%)and K₂O(0.17-0.33 wt%)contents,placing them within the low-K tholeiitic series.With both N-MORB-and arc-like geochemical characteristics as well as highly depleted Sr-Nd isotopic compositions(⁸⁷Sr/⁸⁶Sr_i=0.7039-0.7042,?_Nd(t)=+8.3 to+8.6),the diabase dykes were probably derived by partial melting of a N-MORB-like asthenospheric mantle source metasomatized by subduction-related fluids under spinel-facies conditions.Melting temperatures and pressures for the primary magmas were estimated at 1371-1394?and2.2-2.4 GPa,respectively.We propose that the Altai Devonian diabase dykes formed in a back-arc extensional environment,in response to the roll-back subduction of the Paleo-Asian oceanic slab.Combined with the coeval arc magmatism in the study area,we envisage that an active arc-back-arc basin system developed in the Chinese Altai during the Devonian.(2)We have identified four periods of Carboniferous magmatism in the East Junggar.These igneous rocks have compositions ranging from basic to acidic,and are mainly subalkaline,calc-alkaline and metaluminous.They are all characterized by arc-like trace-element patterns and depleted Sr-Nd-Pb-Hf isotopic compositions.Among them,the early Carboniferous(340-330 Ma)volcanic rocks were derived from partial melting of a lithospheric mantle wedge fluxed by slab-derived fluids under spinel-facies conditions.The late Carboniferous(320 Ma)mafic dikes were formed by partial melting of a spinel-bearing lithospheric mantle metasomatized by minor subduction-related fluids.The late Carboniferous(305 Ma)felsic volcanic rocks originated from melting of a juvenile basaltic lower crust.We suggest that the early Carboniferous volcanic rocks erupted in an island-arc setting related to the northward subduction of Kalamaili oceanic crust(a branch of the PAO),whereas the late Carboniferous magmatism formed in a post-collisional extensional regime in response to slab break-off or lithospheric delamination.Combined with regional geological information,we propose that a rapid tectonic transition from oceanic subduction to post-collisional extension may have occurred in East Junggar during the Carboniferous,marking the final closure of the Kalamaili Ocean,which most likely took place ca.330-320 Ma.This transition recorded the final amalgamation and terminal accretion of the southwestern CAOB.(3)Jurassic basalts from the Karamay area,West Junggar erupted at?190 Ma.They show a columnar joint structure and porphyritic texture with phenocryst minerals of olivine(Fo_63-79),clinopyroxene(Wo_39-46En_38-47Fs_12-22)and plagioclase(An_25-54).The basalts have relatively depleted Sr-Nd isotopic compositions(⁸⁷Sr/⁸⁶Sr_i=0.7048-0.7049,?_Nd(t)=+2.95 to+3.02)and are characterized by OIB-like geochemical features.They underwent insignificant crustal contamination and were probably generated by low degrees(?5-10%)of partial melting of an asthenospheric mantle under garnet-facies conditions.We suggest that the Jurassic basalts formed in a continental intraplate setting,possibly related to asthenospheric upwelling triggered by small-scale removal of thickened lithospheric root.(4)A large number of Archean zircon xenocrysts were found in the drilling-sampled Carboniferous volcanic rocks beneath the Junggar Basin.The oldest zircon population with U-Pb ages of 3.8Ga shows chondritic?_Hf(t)of-0.7 to+0.7,and Hf model ages of 3900-3952Ma(T_DM)and 3973-4062Ma(T_crust).Three slightly younger populations at 3.7Ga,3.62Ga and 3.45Ga have?_Hf(t)of-5.7 to+2.4 as well as consistent Eoarchean-Hadean Hf model ages.The youngest xenocrysts yield U-Pb ages of 2.55Ga with?_Hf(t)values of+3.7 to+8.0.Crustal mineral inclusions and the trace-element compositions of all these zircon xenocrysts show that they probably crystallized from granitic magmas.Our results imply the presence of unexposed Archean crust beneath the Junggar Basin.This understanding emphasizes the important contribution of ancient materials in the formation of young accretionary orogens and is of great importance for reassessing the continental growth of the CAOB.By comparing the Hf isotopic signatures of ancient zircons(>3.4Ga)worldwide,we infer that there may have been a great change in early crustal evolution at the Hadean-Archean transition(?3.9Ga),possibly linked to the Late Heavy Bombardment(LHB).(5)Based on our results and previous published geological data,we suggest that the tectonic evolution of the Junggar and adjacent regions since the late Paleozoic is as follows:(1)The Devonian is the subduction-accretion stage of the PAO that is characterized by the lateral accretion of island arc magma,accompanied by the episodic formation of back-arc basin;(2)The Carboniferous is the tectonic transition stage,and is the key stage of ocean-continent transition.Island arc magmatism was developed in the early stage,while post-collisional magmatism was dominant in the late stage;(3)The Permian was a stage of post-collisional extension,which was dominated by post-collisional magmatism,possibly influenced by the Tarim mantle plume;(4)The Mesozoic-Cenozoic entered into a stable intracontinental evolution stage,and intraplate basaltic magmatism developed locally.In addition,we emphasize the role of ancient microcontinents in the subduction accretion processes of the PAO.These microcontinents may have been collaged into the juvenile crust during the long-lived evolution of the PAO,highlighting the potential contribution of ancient continental materials in the formation of young accretionary orogens.The tectonic evolution of the Junggar and adjacent regions since the late Paleozoic has also recorded important Phanerozoic continental crust growth.During the subduction stage of the PAO,lateral accretion of island arc magma was the main mechanism of crustal growth,accompanied by episodic roll-back processes in the formation of back-arc basin and Precambrian materials reworking.After the closure of the PAO,the growth mechanism is dominated by vertical growth,which is characterized by post-collisional vertical underplating of mantle-derived magma and mantle plume activity.