Carboniferous-Permian Stratigraphy and Sedimentary Environment of Southeastern Inner Mongolia,China:Constraints on Final Closure of the Paleo-Asian Ocean

In this paper we discuss the timing of final closure of the Paleo-Asian Ocean based on the field investigations of the Carboniferous-Permian stratigraphic sequences and sedimentary environments in southeastern Inner Mongolia combined with the geology of its neighboring areas. Studies show that during the Carboniferous-Permian in the eastern segment of the Tianshan-Hinggan Orogenic System, there was a giant ENE-NE-trending littoral-neritic to continental sedimentary basin, starting in the west from Ejinqi eastwards through southeastern Inner Mongolia into Jilin and Heilongjiang. The distribution of the Lower Carboniferous in the vast area is sparse. The Late Carboniferous or Permian volcanic-sedimentary rocks always unconformably overlie the Devonian or older units. The Upper Carboniferous-Middle Permian is dominated by llttoral-neritic deposits and the Upper Permian, by continental deposits. The Late Carboniferous-Permian has no trace of subduction-collision orogeny, implying the basin gradually disappeared by shrinking and shallowing. In addition, it is of interest to note that the Ondor Sum and Hegenshan ophiolitic melanges were formed in the pre-Late Silurian and pre-Late Devonian respectively, and the Solonker ophiolitic melange formed in the pre-Late Carboniferous. All the evidence indicates that the eastern segment of the Paleo-Asian Ocean had closed before the Late Carboniferous, and most likely before the latest Devonian （Famennian）.

Geochronology and geochemistry of Permo-Triassic sandstones in eastern Jilin Province(NE China): Implications for final closure of the Paleo-Asian Ocean

In the eastern part of the Central Asian Orogenic Belt(CAOB) in northeastern(NE) China, scattered outcrops of molasse deposits mark the ending of an orogeny and are crucial for understanding the evolution of the Paleo-Asian Ocean(PAO). However, the timing of tectonic events and the relationships among these strata remain controversial. To better constrain these geologic events, a comprehensive study of the detrital zircon U-Pb geochronology and geochemistry of the sandstones of the Kaishantun(KST) Formation and Kedao(KD) Group in eastern Jilin Province, NE China, was conducted. The KST Formation is traditionally considered a molasse deposit. The sandstones display low CIA, PIA and high ICV values and low Th/U and Rb/Sr ratios, which suggest that the rocks were derived from an immature intermediate-felsic igneous source and experienced a simple sedimentary recycling history with relatively weak chemical weathering. LA-ICP-MS U-Pb dating of detrital zircons from two samples of the KST Formation yields ages of 748-252 Ma, suggesting that the KST Formation was deposited between 254.5 Ma and 252 Ma in Late Permian. The zircons were mainly derived from the continental northern part of the North China Craton(NCC). In contrast, the U-Pb dating of detrital zircons from five samples of the KD Group yields ages of 2611-230 Ma, suggesting that the KD samples were deposited in the Early to Middle Triassic(ca. 248-233 Ma). The detrital zircon ages for the KD samples can be divided into groups with peaks at 2.5 Ga, 1.8 Ga, 800-1000 Ma, 500 Ma and 440-360 Ma, which suggest that the samples were derived from bidirectional provenances in the Jiamusi-Khanka Block and the NCC. These new data,combined with previously published results, suggest that at least three orogenic events occurred in central-eastern Jilin Province during the Early Permian(270-262 Ma), Early Triassic(254-248 Ma) and Middle-Late Triassic(242-227 Ma). The final closure of the PAO occurred during 242-227 Ma in the Middle-Late Triassic along the Changchun-Yanji suture zone. The detrital zircon geochronological data clearly record plate convergence and the scissor-like closure of the PAO in the eastern CAOB.

Zircon U-Pb Ages and Geochemistry of Permo-Carboniferous Mafic Intrusions in the Xilinhot Area, Inner Mongolia: Constraints on the Northward Subduction of the Paleo-Asian Ocean

The Central Asian Orogenic Belt(CAOB) resulted from accretion during the Paleozoic subduction of the PaleoAsian Ocean. The Xilinhot area in Inner Mongolia is located in the northern subduction zone of the central-eastern CAOB and outcropped a large number of late Paleozoic mafic intrusions. The characteristics of magma source and tectonic setting of the mafic intrusions and their response to the closure process of the Paleo-Asian Ocean are still controversial. This study presents LA-ICPMS zircon U-Pb ages and geochemical features of mafic intrusions in the Xilinhot area to constrain the northward subduction of the Paleo-Asian Ocean. The mafic intrusions consist of gabbro, hornblende gabbro, and diabase. Their intrusion times can be divided into three stages of 326-321 Ma, 276 Ma and 254 Ma by zircon U-Pb ages. The first two stages of the 326-276 Ma intrusions mostly originated from subduction-modified continental lithospheric mantle sources that underwent a variable degree partial melting(5-30%), recording the subduction of oceanic crust. The third stage of the 254 Ma mafic rocks also show arc-related features. The primary magma compositions calculated by PRIMELT2 modeling on three samples of ~326 Ma and two samples of ~254 Ma show that these mafic samples are characterized by a variable range in SiO2(47.51-51.47 wt%), Al2O3(11.46-15.55 wt%), ΣFeO(8.27-9.61 wt%), MgO(13.01-15.18 wt%) and CaO(9.13-11.67 wt%), consisting with the features between enriched mantle and lower continental crust. The source mantle melting of mafic intrusions occurred under temperatures of 1302-1351°C and pressures of 0.92-1.30 GPa. The magmatic processes occurred near the crust-mantle boundary at about 33-45 km underground. Combined with previous studies, it is concluded that Carboniferous to early Permian(~326-275 Ma) northward subduction of the Paleo-Asian oceanic crust led to the formation of the mafic magmatism in the Baolidao arc zone. The whole region had entered the collision environment at ~254 Ma, but with subduction-related environments locally. The final collision between the North China craton and the South Mongolian microcontinent may have lasted until ca. 230 Ma.

Testing final closure time of the Paleo-Asian Ocean along the Solonker suture by a transition of compressional and extensional setting

The giant Central Asian Orogenic Belt is an extensive accretionary orogen,of which the Solonker suture,as a major regional suture,coincides closely with an early Permian paleobiogeographical boundary.This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane.Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian-Early Triassic,uncertainty remains because of a lack of typical collisionrelated features(e.g.,high-grade regional metamorphism and well-developed fold-thrust structures)and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes(288-275 Ma)and Middle-Late Permian undeformed layered gabbros,strongly peraluminous granites,and I-type granites(265-254 Ma)in the Xinhure area along the northern margin of the North China Craton.The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton.In contrast,the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle-upper crust,respectively,triggered by asthenospheric upwelling.Therefore,a transition from an end-compressional to an extensional environment according to a transition from collision termination to postcollision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma.This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.

Detrital Zircon U-Pb Ages and Geochemistry of the Silurian to Permian Sedimentary Rocks in Central Inner Mongolia, China: Implications for Closure of the Paleo-Asian Ocean

The Solonker suture zone has long been considered to mark the location of the final disappearance of the PaleoAsian Ocean in the eastern Central Asian Orogenic Belt(CAOB). However, the time of final suturing is still controversial with two main different proposals of late Permian to early Triassic, and late Devonian. This study reports integrated wholerock geochemistry and LA-ICP-MS zircon U-Pb ages of sedimentary rocks from the Silurian Xuniwusu Formation, the Devonian Xilingol Complex and the Permian Zhesi Formation in the Hegenshan-Xilinhot-Linxi area in central Inner Mongolia, China. The depositional environment, provenance and tectonic setting of the Silurian-Devonian and the Permian sediments are compared to constrain the tectonic evolution of the Solonker suture zone and its neighboring zones. The protoliths of the silty slates from the Xuniwusu Formation in the Baolidao zone belong to wacke and were derived from felsic igneous rocks with steady-state weathering, poor sorting and compositional immaturity. The protoliths of metasedimentary rocks from the Xilingol Complex were wackes and litharenites and were sourced from predominantly felsic igneous rocks with variable weathering conditions and moderate sorting. The Xuniwusu Formation and Xilingol Complex samples both have two groups of detrital zircon that peak at ca. 0.9-1.0 Ga and ca. 420-440 Ma, with maximum deposition ages of late Silurian and middle Devonian age, respectively. Considering the ca. 484-383 Ma volcanic arc in the Baolidao zone, the Xuxiniwu Formation represents an oceanic trench sediment and is covered by the sedimentary rocks in the Xilingol Complex that represents a continental slope sediment in front of the arc. The middle Permian Zhesi Formation metasandstones were derived from predominantly felsic igneous rocks and are texturally immature with very low degrees of rounding and sorting, indicating short transport and rapid burial. The Zhesi Formation in the Hegenshan zone has a main zircon age peak of 302 Ma and a subordinate peak of 423 Ma and was deposited in a back-arc basin with an early marine transgression during extension and a late marine regression during contraction. The formation also crops out locally in the Baolidao zone with a main zircon age peak of 467 Ma and a minor peak of 359 Ma, and suggests it formed as a marine transgression sedimentary sequence in a restricted extensional basin and followed by a marine regressive event. Two obvious zircon age peaks of 444 Ma and 280 Ma in the Solonker zone and 435 Ma and 274 Ma in Ondor Sum are retrieved from the Zhesi Formation. This suggests as a result of the gradual closure of the Paleo-Asian Ocean a narrow ocean sedimentary environment with marine regressive sedimentary sequences occupied the Solonker and Ondor Sum zones during the middle Permian. A restricted ocean is suggested by the Permian strata in the Bainaimiao zone. Early Paleozoic subduction until ca. 381 Ma and renewed subduction during ca. 310-254 Ma accompanied by the opening and closure of a back-arc basin during ca. 298-269 Ma occurred in the northern accretionary zone. In contrast, the southern accretionary zone documented early Paleozoic subduction until ca. 400 Ma and a renewed subduction during ca. 298-246 Ma. The final closure of the Paleo-Asian ocean therefore lasted at least until the early Triassic and ended with the formation of the Solonker suture zone.

Final-stage Southward Subduction of the Eastern Paleo-Asian Ocean:Evidence from the Middle Permian Mafic Intrusions in the Northern Margin of the North China Craton

The northern margin of the North China Craton(NCC)contains widespread Permian magmatic rocks,but the origin of these rocks remains controversial.This uncertainty hampers us from better understanding of tectonic framework and evolution of the eastern Paleo-Asian Ocean,particularly with respect to its final-stage subduction and closure time.To address these questions,this study presents petrological,zircon U-Pb geochronological,whole-rock geochemical and in situ zircon Hf isotopic data for these Permian mafic intrusions in the northern margin of the NCC.Precise zircon U-Pb dating results indicate that these mafic intrusions were emplaced in the Middle Permian(ca.260 Ma).Geochemically,the studied mafic intrusions have high MgO and transition metals element contents,with high Mg^(#) values,indicating a mantle origin.These mafic intrusions are characterized by enrichments in large ion lithophile elements(LILEs;e.g.,Rb,Ba,and K)and light rare earth elements(LREEs),and depletions in high field strength elements(HFSEs;e.g.,Nb,Ta,and Ti)and heavy rare earth elements(HREEs),indicating that they were formed in a subduction-related setting.These geochemical features,together with zircon ε_(Hf)(t)values(-1.1 to+11.2),indicate that their parental magmas were derived from partial melting of heterogeneous mantle wedge metasomatized by subduction-related fluids,with the contributions of slab sediments.The studied mafic intrusions also show wide range of major and trace elements contents,and variable Mg^(#) values,Eu and Sr anomalies,suggesting that their parental magmas had undergone variable degrees of fractional crystallization.Together with the E-W trending Permian continental arc along the northern margin of the NCC,we confirm that the generation of the Middle Permian mafic intrusions was related to southward subduction of the Paleo-Asian oceanic lithosphere beneath the NCC and the Paleo-Asian Ocean had not closed prior to the Middle Permian.

Final Closure Time of the Paleo-Asian Ocean: Implication from the Provenance Transformation from the Yangjiagou Formation to Lujiatun Formation in the Jiutai Area, NE China

The Jiutai area is tectonically situated at the eastern segment of the Central Asian Orogenic Belt(CAOB) and is close to the North China Craton(NCC) to the south, serving as an ideal place to investigations of the closure of the PaleoAsian Ocean(PAO). Sandstone samples collected from the Yangjiagou Formation and the Lujiatun Formation in this area have been studied in detail in terms of petrology, geochronology and geochemistry. The maximum depositional time of the Yangjiagou and Lujiatun formations has been constrained to early Middle Triassic(ca. 245 Ma) and middle Late Triassic(ca. 219 Ma), respectively. The Yangjiagou Formation, with a major provenance of dissected island arcs, is dominantly composed of Phanerozoic sediments from Northeastern China(NE China) massifs. The Lujiatun Formation, with major sediments from active continental margins, has a relatively larger proportion of Precambrian sediments, in which the ~1.85 Ga and ~2.5 Ga sediments are typical of the crystalline basements of the NCC and NE China massifs, which were uplifted and eroded during the closure of the PAO. Besides, both formations show the enrichment in LREEs and the depletion in HREEs, the common Eu negative anomalies, and trace element contents similar to that of the upper continental crust. Based on the provenance analysis of these two formations, the final closure time of the PAO in this area is constrained as from the early Middle Triassic(ca. 245 Ma) to the middle Late Triassic(ca. 219 Ma).

Geochronology, Geochemistry and Sr-Nd-Hf Isotopes of Early–Middle Triassic Adakitic Plutons in Central-eastern Jilin Province, NE China: Constraints on the Non-synchronous Closure of Paleo-Asian Ocean

The Changchun–Yanji suture zone(CYSZ) in NE China is considered as the suture between the North China Craton(NCC) and Central Asian Orogenic Belt(CAOB). The geochronology, geochemistry and Sr-NdHf isotopes of Early–Middle Triassic adakitic plutions from the CYSZ, are presented in this paper to discuss their petrogenesis and tectonic setting, as well as to constrain the timing and style of the Paleo-Asian Ocean's final closure. In Early Triassic, the Dayushan pluton(ca. 250 Ma) from western CYSZ has negative ε_(Nd)(t) values, bidirectional provenances(NCC and CAOB) of ε_(Hf)(t), which are formed in a collision tectonic setting. In contrast, in eastern CYSZ, the early Triassic samples in Liangshan(ca. 242 Ma) were high Mg~# values, positive ε_(Nd)(t), single provenances(CAOB) of ε_(Hf)(t) resulting from a subduction setting. In the Middle Triassic, the Atype granites in western CYSZ are found in previous studies representing a post-collisional extensional environment, whereas syn-collisional Lianyanfeng granites(ca. 237 Ma) in eastern CYSZ with low ISr and large scale ε_(Nd)(t) and ε_(Hf)(t) values from bidirectional provenances(NCC and CAOB), represent a collisional setting. The Paleo-Asian Ocean's occurred in a scissor-like fashion along the CYSZ during the Triassic period.

New Evidence of Detrital Zircon Ages for the Final Closure Time of the Paleo-Asian Ocean in the Eastern Central Asian Orogenic Belt(NE China)

Objective The NE China is located in the eastern segment of the Central Asian Orogenic Belt（CAOB）,which is a large accretionary orogen between the Siberian Craton and the North China Craton（NCC）.Many researches have discussed about the evolution of the Paleo-Asian Ocean（PAO）in the eastern CAOB.However,

Molecular Biomarker Characteristics of the Linxi Formation Source Rocks in the Middle-Western Region of Inner Mongolia:New evidence for late-stage tectonic evolution of the Paleo-Asian Ocean

Objective Two important geological issues have long been controversial in the Xing-Meng area of North China. The first concerns the final closure of Paleo-Asian Ocean in Xing-Meng area, and the other concerns the folding and lifting of the Xing-Meng Trough. The focus of thses issues is the Late Permian sedimentary environment, which is generally considered to be either an exclusively continental environment or from the closed inland sea environment in the Early to Middle stage to continental lacustrine environment in the late stage. In recent years, we have successively discovered abundant typical marine fossils （e.g., bryozoans and calcareous algae） in the Upper Permian thick limestone layer from Linxi County and Ar Horqin Banner in eastern region of Inner Mongolia and Jiutain County in Jilin Province. These significant findings have attracted the attention from fellow academics.

Petrology, Geochronology and Geochemistry of the Xar Moron River Ophiolite: Implications for the Tectonic Evolution of the Paleo-Asian Ocean

As the largest accretionary orogen, the crustal tectonic framework and evolution of the Central Asian Orogenic Belt(CAOB) have always been one of the hot topics among geologists(Seng?r et al., 1993, 1996;Jahn et al., 2000 a;Badarch et al., 2002;Windley et al., 2007;Li et al., 2009). The formation of the main part of the crust in the CAOB involved continuous lateral accretion of island arcs and accretionary complexes along the margins of the Siberian, Sino-Korean and Tarim paleocontinents and the final collision between these continental margins because of the subduction of the Paleo-Asian Ocean plate since Mesoproterozoic. The ophiolites, which represent the fragments of ancient oceanic lithosphere, are the direct evidence for the study of the evolution of orogenic belts. Based on field geological survey, the mantle peridotite(serpentinite), gabbro, basalt and radiolarian bedded chert, which were deemed as the "ophiolite trinity", were identified as isolated blocks in the matrix of pelitic siltstone and silty mudstone in the Kedanshan, Xingshuwa and Jiujingzi areas along the Xar Monron River in southeast Inner Mongolia of China. Besides, there were plenty of other exotic blocks, such as limestone and sandstone, in the matrix. Both of the matrix and blocks underwent strong foliated deformation. All of these rocks above constitute a tectonic mélange. Zircon U-Pb dating for the gabbro blocks in the Xingshuwa and Jiujingzi ophiolites reveals that they were formed in early Permian(275-280 Ma). The ages of the gabbros, together with the middle Permian radiolaria fossils in the chert reported by Wang and Fan(1997), indicate that the oceanic basin was not closed in early-middle Permian. The geochemical compositions of the basaltic blocks distributed in different locations in the Xingshuwa tectonic mélange display different genetic types of normal mid-ocean ridge basalt(N-MORB), enriched mid-ocean ridge basalts(E-MORB), oceanic island basalt(OIB), island arc basalt and continental marginal arc basalt, which indicates what they represented is a complex oceanic basin. Combining with the studies on regional magmatism, strata and structure data, it is suggested that the Xar Moron River Ophiolite belt represented the final suture zone of the Paleo-Asian Ocean in the southeast Inner Mongolia, and the ocean did not close before late Permian.

Formation of South Tianshan suture and its geological significance

South Tianshan–Solonker suture,is the largest and southernmost suture within the Central Asian orogenic belt(CAOB).It records the ultimate collision between Tarim–North China cratons and Siberia craton,and is commonly interpreted as marking the eventual closure of Paleo-Asian Ocean.South Tianshan suture belongs to the western segment of the suture zone,and its evolutionary features are important for defining the formation age of the South Tianshan–Solonker suture.In this paper,the authors review the geochronological,geochemical,petrographic,and paleontological evidence within South Tianshan suture to delineate its formation era and closure characteristics,and thus further revealing the ultimate evolutionary pattern of the western segment of Paleo-Asian Ocean.This suture records strong plate collision before Late Carboniferous,forming a series of high-pressure metamorphic rocks,characterized by the presence of blue schist,eclogite and mica schist.In Permian,the whole area was under a relatively stable post-orogenic setting,with the formation of bimodal volcanic rocks,post-collisional granites,and terrestrial molasses.Sedimentary facies gradually changed from marine to either lacustrine or fluvial during this period.An Early Permian granite dike crosscuts the HP metamorphic belt,and the HP metamorphic rocks also underwent retrograde metamorphism at this time,indicating the formation of the South Tianshan suture was earlier than Permian.Hence,the western section of Paleo-Asian Ocean closed during Late Carboniferous,and Tarim Craton moved northward to collide with Kazakhstan–Yili Block,leading to the formation of the South Tianshan suture.

中亚造山带东段深地震反射剖面大炮揭露下地壳与Moho结构--数据处理与初步解释

深地震反射大炮数据能够准确地获得下地壳和Moho的精细结构及其横向变化信息,揭露岩石圈尺度的构造样式与深部过程。中亚造山带东段位于古亚洲洋、蒙古—鄂霍茨克洋和古太平洋三大构造域的叠合区域,其岩石圈结构记录了大洋,特别是古亚洲洋消亡方式和大陆增生的深部过程。本文选用横过中亚造山带东段(奈曼旗—东乌珠穆沁旗,长约400 km)深地震反射剖面中的24个大炮数据和2个中炮数据,通过数据处理获得了近垂直反射的大炮单次剖面,揭露出中亚造山带东段下地壳及Moho的精细结构,刻画出古亚洲洋消亡极性与中亚造山带增生造山的深部过程:西拉木伦缝合带与贺根山缝合带构成古亚洲洋消亡的双缝合带,西拉木伦缝合带下方古亚洲洋板块以向南消亡为主,贺根山缝合带下方古亚洲洋板块以向北消亡为主,后者规模大于前者。在两个缝合带之间下地壳呈现出几个大规模的块状弧状反射体,推测是大洋中的残余微地块,在古亚洲洋消亡过程中拼接在一起,成为中亚造山带增生造山的一部分,并遭受了碰撞挤压和后造山伸展作用。Moho位于双程走时12 s附近(厚度约36 km),近于水平展布,沿整条剖面起伏不大。平缓的Moho成因与造山后的地壳伸展作用相关。

Early Paleozoic subduction of the Paleo-Asian Ocean: Geochronological and geochemical evidence from the Dashizhai basalts, Inner Mongolia

Zircon U-Pb results of basalt from the Dashizhai Town in Inner Mongolia, NE China, shows that the basaltic lava was erupted at 439±3 Ma, much older than the "Permian basalts" as previously thought. These rocks show arc-type trace element patterns (i.e., Nb-Ta depletion and light REE and large ion lithophile element enrichment) and unradiogenic Sr and highly radiogenic Nd and Hf isotope compositions. They can be subdivided into two petrogenetic groups: Group 1 basalts have relatively high TiO2, MgO and compatible elements and low Sr and Th, characterized by mid-oceanic ridge basalt (MORB)-type Sr-Nd-Hf isotope compositions (87Sr/86Sr(i)=0.7028―0.7032, εNd(t)=+9.8-+11.2, εHf(t)=+16.1- +18.4). Group 2 has lower TiO2, MgO and compatible elements and higher Sr and Th, and relatively evolved Sr-Nd-Hf isotope compositions (87Sr/86Sr(i)=0.7037-0.7038, εNd(t)=+5.7-+7.3, εHf(t)=+12.6- +13.0). Both groups were interpreted as melts derived from a metasomatized mantle wedge formed during the subduction of Paleo-Asian Ocean. The mantle source for Group 1 was probably a highly isotopically depleted oceanic mantle modified by predominant slab fluids; whereas subducted sediments had an important contribution to the melting source for Group 2. The petrogenesis of the Dashizhai basalts provides clear evidence for early Paleozoic subduction of the Paleo-Asian Ocean, and the highly radiogenic Nd and Hf compositions in these rocks suggest that these lavas and their possible intrusive counterparts were one of the important components for Phanerozoic crustal growth. Our and previous studies on the "Dashizhai Formation" volcanic rocks yield an unrealistic eruption range of 440-270 Ma for different rock types, we thus advise to disassemble the previously defined "Dashizhai Formation" into multiple lithologic units and to reinterpret the spatial and temporal distributions of different volcano-sedimentary associations.

Permian Tectonic Evolution in Southwestern Khanka Massif:Evidence from Zircon U-Pb Chronology,Hf isotope and Geochemistry of Gabbro and Diorite

Laser ablation-inductively coupled plasma mass spectrometry （LA-ICP-MS） zircon U-Pb dating and geochemical data for the Permian gabbros and diorites in the Hunchun area are presented to constrain the regional tectonic evolution in the study area. Zircons from gabbro and diorite are euhedral-subhedral in shape and display fine-scale oscillatory zoning as well as high Th/U ratios （0.26-1.22）, implying their magmatic origin. The dating results indicate that the gabbro and diorite formed in the Early Permian （282-2 Ma） and in the Late Permian （255-3 Ma）, respectively. In addition, the captured zircons with the weighted mean age of 279-4 Ma are also found in the diorite, consistent with the formation age of the gabbro within uncertainty. The gabbros belong chemically to low-K tholeiitic series, and are characterized by low rare earth element （REE） abundances, fiat REE pattern, weak positive Eu anomalies （JEu）, and depletion in high field strength elements （HFSEs, Nb, Ta, and Ti）, similar to the high-aluminum basalts from island arc setting. Initial Hf isotopic ratios of zircons from the gabbro range from ＋7.63 to ＋14.6, suggesting that its primary magma could be mainly derived from partial melting of a depleted lithospheric mantle. The diorites belong to middle K calc-alkaline series. Compared with the gabbros, the diorites have higher REE abundance, weak negative Eu anomalies, and more depletion in HFSEs （Nb, Ta, and Ti）, similar chemically to the volcanic rocks from an active continental margin setting. Initial Hf isotopic ratios and Hf two-stage model ages of zircons from the diorite range from ＋11.22 to ＋14.17 and from 424 to 692 Ma, respectively, suggesting that its primary magma could be mainly derived from partial melting of the Early Paleozoic and/or Neoproterozoic accretted lower crust. Taken together, it is suggested that geochemical variations from the Early Permian gabbros to the Late Permian diorites reveal that the subduction of the Paleo-Asian oceanic plate beneath the Khanka Massif and collision between the arc and continent （Khanka Massif） happened in the late stage of the Late Paleozoic.

Construction of the Continental Asia in Phanerozoic:A Review

This is a review of the formation and tectonic evolution of the continental Asia in Phanerozoic.The continental Asia has formed on the bases of some pre-Cambrian cratons,such as the Siberia,India,Arabia,North China,Tarim,South China,and Indochina,through multi-stage plate convergence and collisional collages in Phanerozoic.The north-central Asia had experienced the expansion and subduction of the Paleo-Asian Ocean(PAO)in the early Paleozoic and the closure of the PAO in the late Paleozoic and early Mesozoic,forming the PAO regime and Central Asian orogenic belt(CAOB).In the core of the CAOB,the Mongol-Okhotsk Ocean(MOO)opened with limited expansion in the Early Permian and finally closed in the Late Jurassic–Early Cretaceous.The south-central Asia had experienced mainly multi-stage oceanic opening,subduction and collision evolution in the Tethys Ocean,forming the Tethys regime and Himalaya-Tibetan orogenic belt.In eastern Asia,the plate subduction and continental margin orogeny on western margin of the Pacific Ocean,forms the West Pacific regime and West Pacific orogenic belt.The PAO,Tethys,and West Pacific regimes,together with Precambrian cratons among or surrounding them,made up the major tectonic and dynamic systems of the continental Asia in Phanerozoic.Major tectonic events,such as the Early Paleozoic Qilian,Uralian,and Dunhuang orogeneses,the late Paleozoic East Junggar,Tianshan and West Junggar orogeneses,the Middle to Late Permian Ailaoshan orogeny and NorthSouth Lhasa collision,the early Mesozoic Indochina-South China and North-South China collisions,the late Mesozoic Mongolia-Okhotsk orogeny,Lhasa-Qiangtang collision,and intra-continental Yanshanian orogeny,and the Cenozoic IndoAsian,Arab-Asian,and West Pacific margin collisions,constrained the formation and evolution of the continental Asia.The complex dynamic systems have left large number of deformation features,such as large-scale strike-slip faults,thrustfold systems and extensional detachments on the continental Asia.Based on past tectonics,a future supercontinent,the Ameurasia,is prospected for the development of the Asia in ca.250 Myr.

Zircon U-Pb-Hf isotopic and geochemical characteristics of the Xierzi biotite monzogranite pluton,Linxi,Inner Mongolia and its tectonic implications

The opening, subduction and final closure of the Paleo-Asian Ocean led to the formation of the Central Asian Orogenic Belt. Controversy has long surrounded the timing of final closure of the Paleo-Asian Ocean. Here we present zircon U-Pb ages and petrological, geochemical and in situ Hf isotope data for the Xierzi biotite monzogranite pluton, Linxi, SE Inner Mongolia. U-Pb dating of zircon by LA-ICP-MS yields a middle Permian emplacement age(268.7 ± 2.3 Ma) for the Xierzi pluton that is dominated by biotite monzogranites with high SiO_2(71.2-72.8 wt.%),alkali(Na_2 O + K_2 O =8.05-8.44 wt.%), Al_2 O_3(14.4-15.2 wt.%) and Fe_2 O_3~T relative to low MgO contents, yielding Fe_2 O_3~T/MgO ratios of 2.87-3.44, and plotting within the high-K calc-alkaline field on a SiO_2 vs. K_2 O diagram. The aluminum saturation indexes(A/CNK) of the biotite monzogranites range from 1.06 to 1.19, corresponding to weakly to strongly peraluminous. They are enriched in rare earth elements(REE), high field strength elements(HFSEs; Zr,Hf). and large ion lithophile elements(LILEs; Rb, U, Th). The LREEs are enriched relative to the HREEs,with a distinct negative Eu anomaly in a chondrite-normalized REE diagram. Geochemically, the Xierzi biotite monzogranite is classified as an aluminous A-type granite, with all samples plotting within the A2-type granite field on a Y/Nb vs. Rb/Nb diagram. Zircon ε_(Hf)(t) values and two-stage modal ages of the zircons within the pluton range from +4.80 to +13.65 and from 983 to 418 Ma, respectively, indicating that the primary magma was generated through partial melting of felsic rocks from juvenile crust.Consequently, these results demonstrate that the Xierzi pluton formed under the post-orogenic extensional setting after arc-continent collision in the middle Permian.

Geology, geochronology and geochemistry of large Duobaoshan Cu-Mo-Au orefield in NE China: Magma genesis and regional tectonic implications

Duobaoshan is the largest porphyry-related Cu-Mo-Au orefield in northeastern(NE)Asia,and hosts a number of large-medium porphyry Cu(PCDs),epithermal Au and Fe-Cu skarn deposits.Formation ages of these deposits,from the oldest(Ordovician)to youngest(Jurassic),have spanned across over 300 Ma.No similar orefields of such size and geological complexity are found in NE Asia,which reflects its metallogenic uniqueness in forming and preserving porphyry-related deposits.In this study,we explore the actual number and timing of magmatic/mineralization phases,their respective magma genesis,fertility,and regional tectonic connection,together with the preservation of PCDs.We present new data on the magmatic/mineralization ages(LA-ICP-MS zircon U-Pb,pyrite and molybdenite Re-Os dating),whole-rock geochemistry,and zircon trace element compositions on four representative deposits in the Duobaoshan orefield,i.e.,Duobaoshan PCD,Tongshan PCD,Sankuanggou Fe-Cu skarn,and Zhengguang epithermal Au deposits,and compiled published ones from these and other mineral occurrences in the orefield.In terms of geochronology,we have newly summarized seven magmatic phases in the orefield:(1)Middle-Late Cambrian(506-491 Ma),(2)Early and Middle Ordovician(485-471 Ma and~462 Ma),(3)Late Ordovician(450-447 Ma),(4)Early Carboniferous and Late-Carboniferous to Early Permian(351-345 and 323-291 Ma),(5)Middle-Late Triassic(244-223 Ma),(6)Early-Middle and Late Jurassic(178-168 Ma and~150 Ma),and(7)Early Cretaceous(~112 Ma).Three of these seven major magmatic phases were coeval with ore formation,including(1)Early Ordovician(485-473 Ma)porphyry-type Cu-Mo-(Au),(2)Early-Middle Triassic(246-229 Ma)porphyry-related epithermal Au-(Cu-Mo),and(3)Early Jurassic(177-173 Ma)Fe-Cu skarn mineralization.Some deposits in the orefield,notably Tongshan and Zhengguang,were likely formed by more than one mineralization events.In terms of geochemistry,ore-causative granitoids in the orefield exhibit adakite-like or adakite-normal arc transitional signatures,but those forming the porphyry-/epithermal-type Cu-Mo-Au mineralization are largely confined to the former.The varying but high Sr/Y,Sm/Yb and La/Yb ratios suggest that the ore-forming magmas were mainly crustal sourced and formed at different depths(clinopyroxene-/amphibole-/garnet-stability fields).The adakite-like suites may have formed by partial melting of the thickened lower crust at 35-40 km(for the Early Ordovician arc)and>40 km(for the Middle-Late Triassic arc)depths.The Early Jurassic Fe-Cu skarn orecausative granitoids show an adakitic-normal arc transitional geochemical affinity.These granitoids were likely formed by partial melting of the juvenile lower crust(35-40 km depth),and subsequently modified by assimilation and fractional crystallization(AFC)processes.In light of the geological,geochronological and geochemical information,we proposed the following tectonometallogenic model for the Duobaoshan orefield.The Ordovician Duobaoshan may have been in a continental arc setting during the subduction of the Paleo-Asian Ocean,and formed the porphyry-related deposits at Duobaoshan,Tongshan and Zhengguang.Subduction may have ceased in the latest Ordovician,and the regional tectonics passed into long subsidence and extension till the latest Carboniferous.This extensional tectonic regime and the Silurian terrestrial-shallow marine sedimentation had likely buried and preserved the Ordovician Duobaoshan magmatic-hydrothermal system.The south-dipping Mongol-Okhotsk Ocean subduction from north of the orefield had generated the Middle-Late Triassic continental arc magmatism and the associated Tongshan PCD and Zhengguang epithermal Au mineralization(which superimposed on the Ordovician PCD system).The Middle Jurassic closure of Mongol-Okhotsk Ocean in the northwestern Amuria block(Erguna terrane),and the accompanying Siberia-Amuria collision,may have placed the Paleo-Pacific subduction system in NE China(including the orefield)under compression,and formed the granodiorite-tonalite and Fe-Cu skarn deposits at Sankuanggou and Xiaoduobaoshan.From the Middle Jurassic,the consecutive accretion of Paleo-Pacific arc terranes(e.g.,Sikhote-Alin and Nadanhada)onto the NE Asian continental margin may have gradually distant the Duobaoshan orefield from the subduction front,and consequently arc-type magmatism and the related mineralization faded.The minor Late Jurassic and Cretaceous unmineralized magmatism in the orefield may have triggered mainly by the far-field extension led by the post-collisional(Siberia-Amuria)gravitational collapse and/or Paleo-Pacific backarc-basin opening.

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt(CAOB)in NE China is a key area for investigating continental growth.However,the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood.NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts.The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities,respectively.In contrast,the Xing’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes.These blocks and terranes were separated by the Xinlin-Xiguitu,Heilongjiang,Nenjiang,and Solonker oceans from north to south,and these oceans closed during the Cambrian(ca.500 Ma),Late Silurian(ca.420 Ma),early Late Carboniferous(ca.320 Ma),and Late Permian to Middle Triassic(260-240 Ma),respectively,forming the Xinlin-Xiguitu,Mudanjiang-Yilan,Hegenshan-Heihe,Solonker-Linxi,and Changchun-Yanji suture zones.Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean(PAO),namely,the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans.The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west.The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south.The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner.A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO,which led to“soft collision”of tectonic units on each side,forming huge accretionary orogenic belts in central Asia.

Episodic Paleozoic Adakitic Magmatism in the Eastern Tianshan,Southern Altaids:Implications for Petrogenesis and Geodynamics

The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth,as well as the important evidence to determine the location of the major oceanic basin and evolution history of lost oceanic basins,which are longstanding conundrums for understanding the tectonics of the Paleo-Asian Ocean(PAO).As the processor of the Altaids,the PAO was a long-lived ocean,and its multiple subductions led to substantial continental growth in the Central Asian continent.Here we report our newly discovered Paleozoic adakitic rocks(granite and dacite)developed in Jijitaizi in the Eastern Tianshan of the southern Altaids to address the above issues.The Jijitaizi granite and dacite contain high concentrations of Sr(203-343 ppm)and low concentrations of Y(3.00-6.36 ppm)and Yb(0.23-0.62 ppm),indicative of the adakitic affinity.Zircon U-Pb dating results show that the adakitic rocks were formed in three episodes at 372.26±1.51 Ma,356.02±1.11 Ma and 334.51±0.68 Ma from the Late Devonian to Carboniferous.The rocks are depleted in Nb,P and Ti,and possess variable Mg#values(28.51-57.22),high Na_(2)O/K_(2)O ratios(1.39-4.94),as well as relatively depleted Hf isotopic compositions,all of which point to a subduction origin from slab melting.Our new finding with the date from the Jijitaizi complex(ca.321 Ma)suggests continuing subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan from the Late Devonian to Carboniferous.Our work provides a solid line of key evidence demonstrating that continuous oceanic subduction of the major PAO evolved into its mature stage following subducting beneath the Central Tianshan from ca.372 to 321 Ma.