Geochronology And Geochemistry Of The Ophiolites And Island-arc-type Igneous Rocks In The Western Qinling Orogen And The Eastern Kunlun Orogen

The east-west striking "Central Orogenic System(COS)" lain in the central China is considered to be the collisional zone between the Northern China blocks (represented by the North China Block and the Tarim Block)and the Southern China blocks(represented by the South China Block and the Qiangtang Block).The formation of the "COS" was closely related to the evolution of the Proto-Tethys and the Paleo-Tethys.In the eastern part of the "COS",there's the uniform Qinling-Dabie orogen which resulted from the collision between the North China Block and the South China Block,while to the west of the Western Qinling orogen,the "COS" has two major branches:i.e.the Qilian orogen in the north and the Kunlun orogen in the south,between which a lot of micro-continents are distributed.Trying to find out the joining relationship between the Qinling-Dabie orogen and the Qilian-Kunlun orogenic belts is a key project for understanding the tectonic evolution of China continental blocks.The Western Qinling orogen and the Eastern Kunlun orogen are the key areas to link up the Qinling-Dabie orogen and the Qilian-Kunlun orogens. Lots of ophiolites and island-arc-type igneous rocks,which are related to the evolution of the Proto-Tethys and the Paleo-Tethys,develop along those major suture zones in the area of "Qinling-Qilian-Qaidam-Kunlun" of the "COS".Accurate geochronological and geochemical studies for ophiolites and island-arc-type igneous rocks in these areas will provide crucial constraints not only on the evolution of the Western Qinling and the Eastern Kunlun orogens but also on the joining relationship between the Qinling-Dabie orogen in the east and those orogenic belts in the west,as well as on the evolution of the Tethys in the Central- Western China.Previous studies on such rocks and remained questions are summarized as following:â‘ Two main suture zones have been identified in the Eastern Qinling orogen: i.e.the "Shang-Dan" suture and the "Mian-Lüe" suture.An early Paleozoic arc-back arc basin system along the southern margin of the North China Block has been recognized in the "Shang-Dan Ocean"(belonged to the Proto-Tethyan domain),while the opening of the "Mian-Lüe Ocean"(belonged to the Paleo-Tethyan domain)split the South Qinling micro-continent from the Gondwana during the Late Paleozoic. How the "Shang-Dan" suture extends westward is a key question in the tectonic research on the Qinling orogen.Based on stratigraphic correlation,the TianshuiWushan suture in the western Qinling has been suggested to be the westward extension of the Shang-Dan suture.However,accurate geochemical and geochronological data for ophiolites and island-arc-type igneous rocks in the Western Qinling orogen have not been reported yet,which are critical to test such a view point. Furthermore,some important ages concerning the evolution history of the arc-back arc basin system in the "Shang-Dan Ocean" have not been well constrained,such as the age for the initial formation of the "Shang-Dan" oceanic crust,the age for the opening of the Ealangping back-arc basin and the times for the closure of the "Shang-Dan Ocean".â‘¡To the west of the Western Qinling orogen,there are numerous microcontinentals in the area of "Qilian-Qaidam-Kunlun",which are geochemically similar to the Gondwana and separated from each other by several Early Paleozoic sutures. Therefore,the tectonic framework of this area is generally considered to be the Proto-Tethyan archipelagic ocean during the Late Neoproterozoic to the Early Paleozoic.According to previous geochronological data,the southern margin of such an archipelagic ocean was located in the central east Kunlun tectonic belt,while the southern east Kunlun tectonic belt,to the south,is considered to belong to the Paleo-Tethyan domain because only Late Paleozoic ages for the ophiolites in this belt have been reported.However,the recent discovery of the Early Paleozoic ophiolite in the Buqingshan area,located in the western section of the southern east Kunlun tectonic belt,suggests that either there is undiscovered Early Paleozoic ophiolite in the eastern section of this belt,i.e.A'nyemaqen area,or the evolution history of the Buqingshan area differed from the A'nyemaqen area.To clarify this question may help us to determine the location of the southern margin of the Proto-Tethyan archipelagic ocean,which needs further geochronological investigations on the ophiolites in the southern east Kunlun tectonic belt.â‘¢It is a hot subject in geological society that whether the distinct isotopic features of MORB from the Indian Ocean has been inherited from the Tethys.It has been shown that "the Indian MORB-type isotopic signature" has been possessed by ophiolites form the Tethyan(including Proto-Tethys,Paleo-Tethys and Neo-Tethys) domain in the Western China.Thus,a hypothesis has been advocated by some scholars that the distinct Indian Ocean mantle domain may be largely "inherited" Tethyan mantle.However,contrasting with present-day MORB,ubiquitous enrichments of Pb exist in MORB-type basalts and gabbros from the Tethyan ophiolite suites.Therefore,the conclusion mentioned above,which is based on the Pb isotopes of the ophiolite samples with positive Pb anomaly,is still doubtful. â‘£Following the closure of the Paleo-Tethys and the Neo-Tethys,several continental collisions had taken place during Mesozoic and Cenozoic in Eastern China and Western China,respectively.The continent of Eastern China resulted from the collision between the North China Block and the South China Block in Triassic, while the continent of Western China was the product of the successive collisions between the blocks in Tibet plateau from the north to the south,such as Middle Qilian, South Qilian,Qaidam,North Kunlun and South Kunlun,which had been combined with the North China Block during the Paleozoic,and the Qiangtang Block,the Lhasa Block and the Indian Plate,which successively collided each other in the Masozoic and the Cenozoic.It is interested to know that what tectonic responses to those continental collision events developed in Eastern China and Western China, respectively,had happened in the junction part of the "COS",i.e.the area of Western Qinling and "Qilian-Qaidam-Kunlun".In order to answer the above four scientific questions,geochemical and geochronological studies on the ophiolites and island-arc-type igneous rocks,as well as some Mesozoic tectono-magmatic events in the Western Qinling orogen and the Eastern Kunlun orogen have been carried out.The following new findings and knowledge are achieved in this study.1 Geochronological framework of the Early Paleozoic "Tianshui-Wushan" ocean and its arc-back arc basin system in the Western Qinling orogenThe tectonic setting and ages of the ophiolite and igneous rocks developed along the Tianshui-Wushan suture and its adjacent area in the Western Qinling orogen have been determined based on comprehensive geochemical analyses and SHRIMP U-Pb dating of zircons as well as ⁴⁰Ar/³⁹Ar dating of hornblende.Thus,the geochronological framework of the Early Paleozoic "Tianshui-Wushan" ocean and its arc-back arc basin system in the Western Qinling orogen has been established,which supports the viewpoint that "the Tianshui-Wushan suture could be the westward extension of the Shang-Dan suture".The Guanzizhen ophiolite complex occurred in the central section of the Tianshui-Wushan tectonic belt.Two types of protolith have been distinguished for the samples from the Guanzizhen ophiolite complex,i.e.the paleo-oceanic crust and the paleo-island arc igneous rocks:samples related to the paleo-oceanic crust are characterized by LREE depletion or slight LREE enrichment((La/Yb)_N＜1.4)with no Nb negative anomalies in spider-diagram((La/Nb)_N＜1.5)and depleted Nd isotopes (ε_Nd(t)=2.1～4.0).Such geochemical features are similar to those of the MORB. Moreover,"oceanic plagiogranite" formed at the middle ocean ridge has been identified for the first time in the Guanzizhen area.In contrast,one gabbro sample related to the paleo-island arc is characterized by high LREE enrichment ((La/Yb)_N=17.1)with remarkable Nb negative anomalies in spider-diagram ((La/Nb)_N=3.74)and enriched Nd isotopes(ε_Nd(t)=-6.9).Zircon SHRIMP U-Pb dating yielded the ages of 534±9Ma and 517±8 Ma for meta-gabbro and meta-plagiogranite related to the paleo-oceanic crust,respectively,and an age of 489±10 Ma for meta-gabbro related to the paleo-island arc.The ⁴⁰Ar/³⁹Ar plateau age of hornblende from the meta- plagiogranite is 394.9±5.2 Ma,indicating the time of amphibolite-facies metamorphism.The Wushan ophiolite complex occurred in the western section of the Tianshui-Wushan tectonic belt.The samples from the Hualingou gabbro and the Yuanyangzhen gabbro-diorite are characterized by LREE enrichment((La/Yb)_N=7.1 and 10.8,respectively)and remarkable Nb,Ta and Ti negative anomalies in spiderdiagram, indicating they were formed in the island arc setting.The zircon SHRIMP U-Pb ages are 440±5 Ma and 456±3 Ma for the Hualingou gabbro and the Yuanyangzhen gabbro-diorite,respectively.Their lowε_Nd(t)values(-3.8～-6.9) suggest that the Guanzizhen Paleozoic island arc and the Wushan Paleozoic island arc were epicontinental arcs developed on the southern margin of the North China Block.To the north of the Qinling Group developed on the northern side of the Tianshui-Wushan Fault,acidic and basic volcanic rocks occured in the area of Qingshui-Hongtubao.Their SiO₂ contents range from 47.69%～50.80%and 68.83%～70.14%,displaying characteristics of bimodal volcanic rocks.The geochemical and Sr-Nd isotopic features of the all basalts either from the "Chenjiahe Group" bimodal volcanic rocks at the Qingshui area or from the Hongtubao area are similar.Moreover, relatively higher radiogenic Pb isotopic compositions,especially higher ²⁰⁷Pb/²⁰⁴Pb ratios,suggest that the formation of these basalts were related to the subduction zone. Zircon SHRIMP U-Pb ages of 447±8 Ma and 448±8 Ma for two dacite samples from Xincheng at the Qingshui area are consistent with the previously published age of 443.4±1.7 Ma for the basalt at the Hongtubao area.Thus,it's suggested that the "Chenjiahe Group" volcanic rocks were formed at the initial stage of a back-arc extension instead of the island arc setting suggested by previous studies. According to these new results,a model for the tectonic evolution of the "Tianshui-Wushan Ocean" in the Western Qinling orogen during the Early Paleozoic is proposed.The "Tianshui-Wushan Ocean" formed in the Early-Middle Cambrian (534±9 Ma～517±8Ma)followed by the initial subduction during the Late Cambrian to the Early Ordovician.Two subduction-related events took place at the Wushan area during the Late Ordovician(456±3Ma)to the Early Silurian(440±5 Ma)together with the initial extension of the back-arc basin in Qingshui and Hongtubao area at 448±8 Ma.Metamorphism resulted from the arc-continent collision happened during the Early-Middle Devonian(394.9±5.2Ma).The evolution history of the "Tianshui-Wushan" ocean and its arc-back arc basin system established in this study is well comparable to that developed along the "Shang-Dan" tectonic belt.Therefore,the "Shang-Dan Ocean" and the "Tianshui- Wushan Ocean" constitute,on the southern margin of the North China Block,the Early Paleozoic "Qinling Ocean",whose evolution history may reflect that of a marginal sea in the Proto-Tethys.2 The Late Sinian to the Early Ordovician oceanic basin existed along the southern east Kunlun tectonic beltThe fragments of paleo-oceanic crust with zircon SHRIMP U-Pb ages of the Late Sinian to the Early Ordovician are firstly observed in the Huashixia-Maqin section along the southern east Kunlun tectonic belt.The Kuhai gabbro,with the Late Sinian age(555±9Ma)is characterized by slight LREE enrichment((La/Yb)_N=5.15),high Ni abundance(102 ppm)and slight depletion of Nb and Ta,which are similar to the features of the OIB,while the Majixueshan gabbro with the Early Cambrian age(535±10 Ma)is characterized by LREE depletion((La/Yb)_N=0.61),high Ni abundance (160 ppm)with no Nb or Ta negative anomalies,which are typical features of the MORB.On the other hand,the Dur'ngoi diorite with age of 493±6 Ma is characterized by LREE enrichment((La/Yb)_N=52.64),low Ni abundance(18 ppm) and remarkable depletion of Nb and Ta,which are typical features of the IAB.So its age indicates the subduction time of the paleo-oceanic crust.The above results together with the Early Paleozoic ophiolite in the Buqingshan area reported previously document the existence of the Late Sinian to the Early Ordovician ocean in the southern east Kunlun tectonic belt,indicating that the southern east Kunlun tectonic belt has had an evolution history differing from the "Mian-Lüe" tectonic belt during the Late Neoproterozoic to the Early Paleozoic.The above dating results of the ophiolites occured along the southern east Kunlun tectonic belt are comparable to those reported ages for ophiolites and island-arc volcanic rocks from the Northern Qilian belt to the central east Kunlun tectonic belt. Thus,it is concluded that the southern margin of this Proto-Tethyan archpelagic ocean could not be limited in the central east Kunlun tectonic belt as suggested previously, but in the southern east Kunlun tectonic belt.3 The introduction of Pb during metamorphism of ophiolites and its effect on the Pb isotopic compositions—implication for origin of the isotopic signature of the Indian Ocean MORBPb isotopic compositions of the Tethyan ophiolites have been studied to test whether the distinct isotopic signatures of the Indian Ocean are inherited from the Tethys Ocean.However,we first point out that the positive Pb anomaly existed in MOR-type ophiolites may result in invalidation of Pb isotopes in tracing the sources of ophiolites.To determine the possible reasons causing the positive Pb anomaly in MOR-type ophiolites,the differences in Pb concentrations obtained using different pretreatment methods(acid leach vs.unleach)and different analytical methods(by ICP-MS vs.by isotopic dilution)are investigated.It shows that there are three major causes of the positive Pb anomaly observed in ophiolites,i.e.the high Pb blank during ICP-MS analyses,the adsorption of Pb during the surface processes,and the external Pb introduced by metamorphic fluids.The negative correlation between Nb/Pb(indicating the quantity of fluid flux) andδPb(indicating the degree of positive Pb anomaly)suggest that the excess Pb in MOR-type ophiolites may be related to the activity of metamorphic fluids during or after the tectonic emplacement of ophiolites.The Pb isotopic compositions of ophiolite samples are results of mixing between metamorphic fluids and unaltered MORB rocks.Therefore,in this study,only those samples with the lowest value ofδPb in each ophiolite should be selected to constrain the Pb isotopic compositions of their mantle sources.Following this way,our new data obtained from the Proto-, Paleo- and Neo-Tethyan ophiolites in Western China suggest that most of the Indian MORB-type isotopic signatures are basically inherited from the Tethyan mantle, consistent with previous conclusion.However,the Indian Ocean mantle trends to have lowerε_Ndvalues and higher ²⁰⁶Pb/²⁰⁴Pb ratios than the Paleo-Tethyan mantle,which could be a result of recycling of crustal materials during the closing of the Paleo-Tethys.4 The tectonic responses in the Western Qinling and the "Qiliang-QaidamKunlun" orogens to the continental collision events in Easten China and Western China during the Late Paleozoic to Mesozoic—Geochronological recordsOblique collision between two continents may lead to large-scale strike-slip faults parallel to active continental margin.The ⁴⁰Ar/³⁹Ar plateau of biotite from the mylonite dacite at Xincheng in the Qingshui area gives an age of 355.2±2.6 Ma, indicating the time of strike-slip movement for the Chenjiahe Fault.Zircon SHRIMP U-Pb age of 951±13Ma for the granitic mylonite from the Lijiahe profile in the north of Wushan area indicates that the granitic protolith was formed during Neoproterozoic, while the ⁴⁰Ar/³⁹Ar plateau age of 226.8±2.2 Ma given by white mica from the same sample indicates the mylonitization time of the gneiss,suggesting a strike-slip movement for the Tianshui-Wushan Fault occurred at the Triassic.These two ages are just corresponding to the closure times of the "Shang-Dan Ocean" and the "Mian-Lüe Ocean",respectively,which suggest that these strike-slip movements on continental margin resulted from each oblique collision and suturing between the North China Block,North Qinling and South Qinling micro-continents.Thus,the age of 355Ma indicates the oblique collision between the South Qinling Block and the North Qinling Block,while the age of 227Ma indicates the oblique collision between the Yangtze Block and the South Qinling Block.To those suture zones without any ultrahigh-pressure metamorphic rocks,the consistency observed in this study between the formation ages for the strike-slip movement on continental margin and the times of continental matching provides us a valid means to constrain the times of continental collision.Because the Tianshui-Wushan Fault,on the northern margin of the South Qinling micro-continent,took place right-lateral strike-slip movement during the Middle-Late Triassic,while the "Mian-Lüe" Fault,on the southern margin of the South Qinling micro-continent,took place left-lateral strike-slip movement in the same time.It's suggested that the South Qinling micro-continent had undergone "Westward Extrusion" during the Triassic collision between the North China Block and the South China Block.The westward compression given by such a westward extrusion may cause an extension in the "Qilian-Qaidam-Kunlun"area,which had been assembled during the Early Paleozoic.The tentative dating result of 233±9 Ma for the gabbro from the Zongwulong ophiolite indicates an extensional event happened during the Triassic on the northern margin of the Qaidam Block.Such an extensional event could be the tectonic responses to the "Westward Extrusion" of the South Qinling micro-continent.Furthermore,the ⁴⁰Ar/³⁹Ar plateau age 87.2±0.9 Ma for the Madang alkaline basalt is considered as the tectonic responses in the Western Qinling area to the collision between the Lhasa Block and the Qiangtang Block in Tibet during the Late Cretaceous.