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.

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）.

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,

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.

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 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.

Tectonic Framework of Late Paleozoic Intrusions in Xingxingxia： Implications for Final Closure of South Tianshan Ocean in East Tianshan

This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during（304±3）–（278±3） Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during（268±5）–（259.9±2.6） Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities（i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian） and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of（268±5）–（259.9±2.6） Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around（268±5）–（260±3） Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.

From arc-continent collision to ocean closure: Lower Cretaceous Shamuluo Formation in the western segment of the Bangong-Nujiang suture zone, central Tibet

One of most hotly debated topics concerning the Late Mesozoic evolution of Tethyan and the Tibetan Plateau is the timing of the closure of the Meso-Tethys ocean,which is represented by the Bangong-Nujiang suture zone.The Upper Jurassic-Lower Cretaceous Shamuluo Formation,which unconformably overlies the older Mugagangri Group accretionary complex,provides important information on the closure of the Meso-Tethys Ocean.This paper precisely confines the depositional age of the Shamuluo Formation in the western segment of the Bangong-Nujiang suture zone,extending it from the Late Jurassic to the Albian.Combined with the results of previous studies,we suggest that the Shamuluo Formation in the Awengco-Baerqiong region mainly contains a bathyal Berriasian-Hauterivian subunit and a shallow-marine Albian subunit.Provenance analysis indicates that the Berriasian-Hauterivian subunit was mainly derived from the Jurassic southern Qiangtang magmatic arc,while the Albian subunit was derived from the coeval volcanic rocks and the Upper Carboniferous-Upper Permian strata in the southern Qiangtang terrane.Thus,the two subunits of the Shamuluo Formation have significant distinct sedimentary facies and provenances,indicating that they were deposited in different tectonic settings.Based on the regional geological data,we suggest that the bathyal Berriasian-Hauterivian subunit and the shallow-marine Albian subunit of the Shamuluo Formation should be interpreted as a record of the oceanic arc-continent collision and the Lhasa-Qiangtang soft-collision,respectively.Thus,the closure time of the Meso-Tethys Ocean is at least limited to the Albian.

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.

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.

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.

Optical closure of parameterized bio-optical relationships

An optical closure study on bio-optical relationships was carried out using radiative transfer model matrix operator method developed by Freie Universitt Berlin.As a case study,the optical closure of bio-optical relationships empirically parameterized with in situ data for the East China Sea was examined.Remote-sensing refl ectance(R rs)was computed from the inherent optical properties predicted by these biooptical relationships and compared with published in situ data.It was found that the simulated R rs was overestimated for turbid water.To achieve optical closure,bio-optical relationships for absorption and scattering coeffi cients for suspended particulate matter were adjusted.Furthermore,the results show that the Fournier and Forand phase functions obtained from the adjusted relationships perform better than the Petzold phase function.Therefore,before bio-optical relationships are used for a local sea area,the optical closure should be examined.

松辽盆地西缘突泉地区晚侏罗世过铝质流纹岩和英云闪长玢岩的发现:从蒙古-鄂霍茨克洋闭合到陆陆碰撞的地质记录

蒙古-鄂霍茨克洋南向俯冲、大洋闭合和陆-陆碰撞是东北亚地区晚中生代的重要区域构造事件,它与本区的岩浆活动、变质作用、成盆和造山作用都密切相关。准确界定洋壳俯冲、大洋闭合和陆-陆碰撞这三个相互关联地质过程的时空范围是客观认识区域构造演化的前提。然而,这项研究通常难以进行,因为很难找到与之相关的合适的地质记录。作者在松辽盆地西缘突泉地区发现从大洋闭合到陆-陆碰撞相关的过铝质流纹岩和英云闪长玢岩,锆石LA-ICP-MS U-Pb定年结果显示其结晶年龄分别为156±1Ma和155±1Ma,系晚侏罗世岩浆事件产物。流纹岩和英云闪长玢岩为钙碱性过铝质岩石,具有较高的铝饱和指数A/CNK(1.32~2.13),较低的MgO+FeO T含量(0.96%~3.37%)和FeO T/MgO比值(2.84~5.02),岩矿鉴定表明它们含绢云母等原生和次生高铝矿物,在CIPW标准矿物计算中出现刚玉分子(3.77%~9.65%),结合流纹岩和英云闪长玢岩在花岗岩的S-I-M-A型分类方案相关地球化学图解投影结果,综合表明它们具有过铝质S型花岗岩特征。流纹岩和英云闪长玢岩具有较低的Rb/Sr(0.35~0.55)、Rb/Ba(0.08~0.26)和Al_(2)O_(3)/TiO_(2)(38.41~61.36)比值,表明其原始岩浆源于杂砂岩部分熔融。锆石饱和温度计算表明这两类岩石的岩浆形成温度在837~876℃之间,低于A型花岗岩岩浆形成温度(900℃)。两类岩石富集Rb、Ba、K等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素,具有较低的Y(5.29×10^(-6)~19.75×10^(-6))、Nb(7.44×10^(-6)~8.50×10^(-6))、Sr(60.6×10^(-6)~154.9×10^(-6))和Yb(0.53×10^(-6)~2.40×10^(-6))含量,具有弧岩浆属性。在Y-Nb图解中流纹岩和英云闪长玢岩投点在火山弧和同碰撞型花岗岩区域内;在Rb/10-Hf-Ta×3图解中投点在火山弧和碰撞型花岗岩区域内;在R1-R2构造判别图解中样品投点在同碰撞和造山期区域内;在Sr-Yb图解中样品投点在地壳加厚区域内。结合全球典型俯冲岛弧和同碰撞酸性岩地球化学特征和同时期本区域岩浆岩成因,表明流纹岩和英云闪长玢岩形成于洋壳俯冲和陆-陆碰撞转换背景,暗示其形成与蒙古-鄂霍茨克茨洋闭合和陆-陆碰撞作用有关。流纹岩(La/Yb)N值为6.62~8.77,指示源区起源深度为40~46km,英云闪长玢岩(La/Yb)N值为7.93~13.39,指示源区起源深度为44~55km,表明在156±1Ma~155±1Ma地壳处于持续加厚过程。结合区域地质资料,构建了蒙古-鄂霍茨克洋从俯冲闭合到陆-陆碰撞的构造-岩浆演化模型。本文认为蒙古-鄂霍茨克构造体系的影响范围到达了松辽盆地西缘突泉地区,156±1Ma~155±1Ma研究区处于大洋闭合到陆-陆碰撞转换阶段。

冀北梁头岩体年代学和地球化学特征及对古亚洲洋闭合时限的启示

华北板块北缘古亚洲洋闭合时间一直存在较大分歧,冀北张北梁头二长花岗岩体位于华北克拉通北缘中段,毗邻古亚洲洋构造域东南缘,对揭示古亚洲洋闭合时限具有重要启示。笔者等以梁头岩体为研究对象,开展了岩石学、地球化学、锆石U-Pb年代学和Hf同位素研究,探讨了其形成时代、岩石成因和构造环境。锆石LA-ICP-MS U-Pb测年表明,梁头岩体的侵位年龄为262.5±2.6 Ma,形成时代为晚二叠世。岩石学和岩石地球化学研究表明,梁头岩体具有较高的SiO_(2)(70.27%~73.89%),Na_(2)O(4.2%~4.55%),K_(2)O(3.87%~4.64%),Sr(417×10^(-6)~827×10^(-6))含量,较低的Yb(0.286×10^(-6)~0.518×10^(-6)),Y(2.74×10^(-6)~6.98×10^(-6))含量。岩石富集Sr等大离子亲石元素和LREE,亏损Ti、P、Nb、Ta等高场强元素和HREE,轻重稀土分馏明显(LaN/YbN=15.6~67.3),具有弱的正Eu异常。该岩石显示高锶低钇中酸性岩(Adakite,埃达克质岩)的地球化学特征,形成于陆缘弧环境,为陆弧岩浆岩。锆石的ε_(Hf)(t)值为-17.50~-11.12,模式年龄tDM2为1989~2390 Ma。结合华北克拉通北缘晚石炭世—早二叠世陆弧岩浆岩的时空分布与演化特征,梁头陆缘弧二长花岗岩体的存在表明古亚洲洋在晚二叠世(262.5±2.6 Ma)可能仍处于向华北克拉通北缘的俯冲消减过程中,古亚洲洋闭合时间可能应在晚二叠世末。

Evolution and final closure of the Mongol-Okhotsk Ocean

Mongol-Okhotsk Orogenic Belt is the last main orogen that constructs modern tectonic framework of northeastern Asia. It has recorded the long-term evolution of the Mongol-Okhotsk Ocean(MOO) from its Early Paleozoic initial opening,through the Late Paleozoic-Early Mesozoic subduction, to its Mesozoic final closure, leading to the amalgamation of the Siberian Craton(SIB) and North China-Amuria Block(NCC-AMB). Opening of the MOO can be traced to the early stage of the Early Paleozoic. Northward subduction of the Mongol-Okhotsk oceanic slab beneath the southern margin of the SIB initiated in the Silurian, whereas the southward subduction beneath the northern margin of the NCC-AMB started in the Late Devonian. The bidirectional subduction of the Mongol-Okhotsk oceanic slab resulted in pulse arc magmatism, with three main peaks in the earliest Carboniferous, Late Permian, and Late Triassic-Early Jurassic. In the Late Triassic, the collision between the AMB and Western Mongolian Blocks led to the bending of the Western Mongolian Blocks, which caused the initial closure of the MOO in its western segment. Due to the clockwise rotation of the SIB and counterclockwise rotation of the NCC-AMB, the MOO showed a scissor-like closure from west to east. The final closure of the MOO occurred in the Middle-Late Jurassic, which also resulted in the formation of the Mongol Orocline. Since then, the amalgamation of blocks in northeastern Asia has finished, and the northeastern Asian continent went into the intraplate evolutional stage.

辽北开原地区晚古生代—早中生代混杂岩的厘定及其地质意义

辽宁省开原地区地处华北克拉通北缘,域内沿清河断裂出露一套中浅变质岩系,前人先后将其称为“辽河群”、“清河镇群”、“开原岩群”、“清河构造混杂岩”,但对其物质组成和形成时代尚未形成统一认识。通过系统总结不同岩石锆石U-Pb测年结果和地球化学数据,该套变质岩系是由新太古代、古元古代、晚二叠世-中三叠世不同时代地质体组成,其中早三叠世的海相沉积盖层和大面积的晚二叠世-中三叠世活动大陆边缘岛弧岩块指示古亚洲洋的闭合过程,碰撞背景下形成的变质英安岩(244 Ma)将古亚洲洋的最终闭合时间限定为中三叠世。综合对比区域地质资料,古亚洲洋在开原地区的俯冲闭合作用导致形成这套变质程度不同以新太古代华北板块基底岩石、晚二叠世-中三叠世火山岩为“岩块”,变粒岩、浅粒岩等变质碎屑岩为“基质”的混杂岩系。

黑龙江塔溪地区中侏罗世侵入岩地球化学特征、成因及构造环境

为丰富对兴蒙造山带东端中生代构造背景的认识,选取贺根山—嫩江—黑河构造带内塔溪地区的石英二长岩、石英闪长岩及石英正长岩开展岩相学、年代学和地球化学研究。结果表明,石英闪长岩的LA-ICP-MS锆石U-Pb定年结果为(167.2±1.3)Ma,形成于中侏罗世。根据微量元素地球化学特征可将塔溪地区侵入岩划分为高Sr型和低Sr型两类,早期侵入的石英闪长岩为高Sr型,晚期侵入的石英二长岩和石英正长岩为低Sr型。高Sr型与低Sr型侵入体在主量、微量和稀土元素特征上均存在明显差异,具有类似I型和A型花岗岩的过渡特征,属复合型侵入岩体。该侵入岩体的形成与中生代强烈的陆内伸展造山和蒙古—鄂霍次克洋与法拉隆板块的双向俯冲闭合过程有关,是地壳在挤压增厚向伸展减薄转变的背景下,软流圈地幔上隆及玄武质岩浆底侵于下地壳并混染地壳物质部分熔融的产物。塔溪地区侵入岩体的存在,表明中侏罗世期间存在一次重要的地壳挤压增厚向伸展减薄转换事件,同时其形成时代也限定了蒙古—鄂霍次克洋东段的闭合时间应为中侏罗世。

Petrology, geochemistry and Ar-Ar geochronology of eclogites in Jinshajiang orogenic belt, Gonjo area, eastern Tibet and restriction on Paleo-Tethyan evolution

As one of the important Paleo-Tethys suture zones in eastern Tibet,the Jinshajiang orogenic belt is of great significance to study the tectonic evolution of the main suture zone of Paleo-Tethys.In this paper,eclogites developed in the Jinshajiang suture zone in Gonjo area,eastern Tibet,are selected as specific research objects,and petrological,geochemical and Ar-Ar geochronological analyses are carried out.The major element data of the whole rock reveals that the eclogite samples have the characteristics of picritic basalt-basalt and belong to the oceanic low potassium tholeiites.The results of rare earth elements and trace elements of the samples show that the protoliths of eclogites have characteristics similar to oceanic island basalt(OIB)or normal mid ocean ridge basalt(N-MORB).Muscovite(phengite)from two eclogite samples yield the Ar-Ar plateau ages of 247±2 Ma and 248±2 Ma respectively,representing the peak metamorphic age of eclogite facies and the timing of complete closure of the Jinshajiang Paleo-Tethys Ocean.Muscovite and biotite selected from the hosting rocks of eclogite yield the Ar-Ar plateau ages are 238±2 Ma and 225±2 Ma respectively,reflecting the exhumation age of eclogites and their hosting rocks.Combined with the zircon U-Pb dating data(244 Ma)of eclogites obtained in previous work,it can be concluded that the Jinshajiang Paleo-Tethys ocean was completely closed and arc-continent collision was initiated at about 248-244 Ma(T21).Subsequently,due to the large-scale arc(continent)-collision orogeney between Deqin-Weixi continental margin arc and Zhongza block(T31-T32),the eclogites were rapidly uplifted to the shallow crust.