Neoproterozoic To Early Paleozoic Tectonic Evolution In The South Qilian Orogen

The South Qilian Orogen(SQO)is an important component of the Qilian Orogen,which located in northeastern of Tibetan Plateau and sandwiched between the Oulongbuluk and the Qilian block,also can considered as a key part to understanding the tectonic evolution of the Qilian Orogen and its tectonic relationship with the adjacent orogens.However,the SQO remain under-researched.In this thesis,we focused on the Neoproterozoic—early Paleozoic medium-mafic,intermediate-acidic intrusive rocks,and Balonggonggaer Formation(BF)within the South Qilian Orogen.A series of petrology,lithofacies,geochemistry,zircon U-Pb-Hf isotopes analysis has been performed after our detailed geological mapping.Combined with the regional geological studies,we initially established the Neoproterozoic-early Paleozoic chronology of the tectonic evolution in the SQO.Finally,the breakup of Rodinia,formation of ocean basin,oceanic subduction and continent-continent collision in the South Qilian during Neoproterozoic to early Paleozoic were revealed.Several vital achievements of this study are as follows.(1)Xialanuoer gabbros,as a product of the breakup of Rodinia,formed in a continental rift setting(738±11Ma)in middle Neoproterozoic.The magma source was mainly derived from the partial melting of the paleo-Proterozoic enriched lithospheric mantle.While the quartz diorites formed in an active continental margin arc setting(758.9±5.5Ma)during middle Neoproterozoic,which might be the geologic record of plate subduction in the south Qilian during the breakup of Rodinia.The magma source of quartz diorites was mainly originated from the Neoarchean to early Paleoproterozoic partial melting of the ancient crust.In addition,the Hf isotope characteristics of minor inherited zircons from quartz diorites indicated that a strong crustal growth event occured in South Qilian during early Paleoproterozoic.These results showed that the South Qilian experienced continental breakup event after the early Neoproterozoic consolidation of the basement,responding to the breakup of Rodinia.(2)The late Ordovician intermediate-acid intrusive rocks in the central SQO are mainly composed of biotite granite and biotite diorite(454~446Ma),which belonged to the I and I-S granite,and formed in an island arc evironment under the plate subduction.The magma source were mainly originated from the partial melting of the ancient crust,and may be accompanied by the addition of a few mantle-derived materials.It is suggested that a magmatic intrusive event dominated by multi-source magma mixing within the South Qilian during late Ordovician.Combined with previous studies,the early Paleozoic magmatic intrusions in South Qilian mainly concentrated in the late Ordovician(~453Ma)and early Silurian(~438Ma),corresponding to the oceanic crust subduction and continent-continent collision,respectively.(3)The BF possibly formed during a continental rift to the oceanic basin development stage,and was involved in the Caledonian orogeny in the ~444Ma.It was also subjected to the intense deformation and metamorphism.The fold,ductile shear zone and penetrating foliation of BF were all caused by Caledonian orogeny,while the fold of foliation and veins should be caused by the late tectonic superposition.The sediments of BF were mainly derived from the Paleoproterozoic basement rocks and Neoproterozoic magmatic rocks of the Qilian and Oulongbuluk blocks,whereas the small amount of Archean zircons were possibly derived from the Tarim Craton.The Hf isotopic characteristics of detrital zircons indicated that provenance occured 3 episodic crustal growth events: early Neoproterozoic(986~882Ma),late Paleoproterozoic-Mesoproterozoic(1970~1242Ma)and Neoarchean-early Paleoproterozoic(2892~2332Ma),and accompanied with the ancient crustal material recycling.Combined the age distribution of Neoproterozoic magmatic events,the zircon Hf isotope characteristics of regional Precambrian detrital zircons,indicating that the Qilian block and Yangtze Craton have closely tectonic affinities.In addition,the Neoproterozoic inherited zircons(762~804Ma)in late Ordovician granite can be found with great amount in BF.These zircons had similar Hf isotope characteristics,suggesting that majority magma of granites were originated from BF.(4)An ocean(South Qilian Ocean)may be existed between Oulongbuluk and Qilian Block during the Neoproterozoic-early Paleozoic,which was based on the the breakup of Rodinia.Before the Caledonian orogeny,the North Qaidam ocean,South Qilian ocean,North Qilian ocean,Qaidam block,Oulongbuluk block,Qilian block and Alxa block were constituted the palaeotectonic pattern of "four blocks and three oceans" in Qilian area.(5)The distribution characteristics,the mineralization age and tectonic setting of early Paleozoic intrusive rocks in South Qilian indicated that the northward subduction of South Qilian ocean occurred at the end of the middle Ordovician.Large-scale oceanic crust subduction occurred in late Ordovician.In the early Silurian,the Oulongbuluk block collided with the Qilian block.Subsequently,the orogen suffered a strong uplift and denudation,which resulted in the widespread deletion of Silurian strata in South Qilian.(6)Based on the qualitative and timing of key geological events,and combined with previous studies on the Neoproterozoic-early Paleozoic tectonic evolution in the North Qaidam Orogen,we analyzed the tectonic evolutionary history in the South Qilian region during Neoproterozoic to early Paleozoic:In middle Neoproterozoic,the north Qaidam and the South Qilian area were cracked,dispersed and developed to the ocean,the Olombrouch massif in the northern margin of Qaidam basin formed the aulacogen with Quanji Group,South Qilian formed the continental rift with Xialanuoer gabbros,respectively.The subduction of North Qaidam ocean occurred at 537~460Ma.The close of this ocean occurred at 460~450Ma,while the subduction of South Qilian ocean ocurred at the end of middle Ordovician,and the continent-continent collision occurred in the early Silurian(~438Ma).The BF might be the product of the extensional stage of South Qilian,representing the initial sedimentary formation during early breakup of ocean basin in the South Qilian.The North Qaidam Orogen and the SQO were subjected to uplift and denudation during the Silurian period,resulted in the missing of Silurian strata.In the early Devonian,the North Qaidam entered into the post-orogenic extensional stage,accompanied by intense magmatic activity(422~400Ma),and formed the Maoniushan Formation,indicating the end of the Caledonian orogeny in the North Qaidam.However,post-orogenic granites(416.7±4.3Ma),lamprophyres,and a small number of molasse formation indicate that the a major tectonic transition from compression to extension in SQO probably commenced in the Early Devonian.