Central Asian Orogenic Belt Research Articles

Yanshanian–Himalayan geodynamic transformation of the northwestern Junggar Basin, southwestern Central Asian Orogenic Belt (CAOB), and its significance for petroleum accumulation

The northwestern Junggar Basin in the southwestern Central Asian Orogenic Belt is a typical petroliferous basin. The widely distributed reservoirs in Jurassic–Cretaceous strata indicate that the region records Yanshanian–Himalayan tectonic activity, which affected the accumulation and distribution of petroleum. The mechanism of this effect, however, has not been fully explored. To fill the knowledge gap, we studied the structural geology and geochemistry of the well-exposed Wuerhe bitumen deposit. Our results indicate that deformation and hydrocarbon accumulation in the northwestern Junggar Basin during the Yanshanian–Himalayan geodynamic transformation involved two main stages. During the Yanshanian orogeny, a high-angle extensional fault system formed in Jurassic–Cretaceous strata at intermediate to shallow depths owing to dextral shear deformation in the orogenic belt. This fault system connected at depth with the Permian–Triassic oil–gas system, resulting in oil ascending to form fault-controlled reservoirs (e.g., a veined bitumen deposit). During the Himalayan orogeny, this fault system was deactivated owing to sinistral shear caused by far-field stress related to uplift of the Tibetan Plateau. This and the reservoir densification caused by cementation formed favorable hydrocarbon preservation and accumulation conditions. Therefore, the secondary oil reservoirs that formed during the Yanshanian–Himalayan tectonic transformation and the primary oil reservoirs that formed during Hercynian–Indosinian orogenies form a total and complex petroleum system comprising conventional and unconventional petroleum reservoirs. This might be a common feature of oil–gas accumulation in the Central Asian Orogenic Belt and highlights the potential for petroleum exploration at intermediate–shallow depths.

Triassic Nappe in the Central Part of the Southern Central Asian Orogenic Belt (Ejinaq, NW China): Evidence from Structural Analysis and Geothermochronology

AbstractThe thrust nappe played an important role in the Mesozoic tectonic evolution of the middle part of the Central Asian Orogenic Belt (CAOB). However, the timing, structural style and kinematic processes of the thrust nappe remain controversial, particularly the detail of the thrust nappe in the Guaizihu region (110 km east of Ejinaq). In this study, we investigate new field mapping, seismic sections, geochronology and low‐temperature thermochronometric dating to provide constraints on the history of this thrust nappe in the Chaheilingashun area (northwestern Guaizihu region). The field mapping, seismic sections and structural analysis reveal that the autochthonous system had developed a series of strong fold structures in the upper Permian strata. The allochthonous system mainly contains Devonian monzogranite (U‐Pb age, ranges from 386.7 to 389.0 Ma) and Meso–Neoproterozoic schists (the maximum depositional age, ∼880 Ma), which were thrust upon the upper Permian strata during Middle to Late Triassic. Based on similar rocks, geochronological dating and the Yagan thrust, we suggest that the postulated root zone of this allochthon might have originated from the Huhetaoergai area (40–60 km northwest of the study area). The geochronological results reveal that the lower age limit of this thrust nappe is constrained by the Lower–Middle Triassic syntectonic sediments (tuffaceous sandstone, ∼247 Ma), which is the sedimentary response of the fold structure., The timing of the termination of this thrust nappe is defined by the cooling age (40Ar/39Ar data, 217–211 Ma) of the Devonian monzogranite and Meso–Neoproterozoic schists. Thus, we consider this thrust event in the study area to potentially have occurred in the period from 247 Ma to 211 Ma, which may represent the tectonic response to the closure of the Paleo‐Asian Ocean.

Petrogenesis of the Carboniferous Intrusive Rock in the Xiaobaishitou District of East Tianshan, Northwest China: Magma Evolution and Tectonic Significance

AbstractThe Xiaobaishitou gabbro‐diorite pluton comprises a medium‐grained gabbro‐diorite suite and a fine‐grained diorite suite, which intrude the Kawabulag Group in the East Tianshan Orogen of the Central Asian Orogenic Belt (CAOB). A combination of mineral chemistry, zircon U‐Pb age, whole‐rock geochemistry, Sr‐Nd isotopes, and in situ zircon Hf isotopes for newly found gabbro‐diorite from the Xiaobaishitou district in the Central Tianshan Terrane (CTT) is presented to investigate the petrogenesis and tectonic or even crustal evolution of the East Tianshan Orogen. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U‐Pb analyses indicate that the gabbro‐diorite was formed at 324.7 ± 2.4 Ma. The isolated clinopyroxene formed under higher P‐T‐ƒO2 melt conditions (10.7–14.6 kbar; 1199–1269°C; high ƒO2) than those for the hornblende, plagioclase, and zircon (557–687°C; moderate ƒO2) in the gabbro‐diorite, which reveals a multilevel, magmatic storage system. The gabbro‐diorite is characterized by fractioned REE patterns, enriched LILEs (e.g., Ba and Pb), negative anomalies of HFSEs (e.g., Nb and Ta), and low La/Yb and Sr/Y ratios, which are typically indicative of crustal contamination and accounted for by subduction‐related fluids. The rock also characterized by typical features of high compatible elements (MgO = 3.14–11.65 wt%, Cr = 1–157 ppm, Ni = 6–830 ppm), high Mg# (47 –74), positive ɛHf(t) values (+5.1 to +10.3) and ɛNd(t) values (+2.3 to +4.4). These features suggest that the Xiaobaishitou gabbro‐diorite was most likely derived from metasomatic mantle and contaminated minor crustal components. Taking into account the spatial and temporal distribution of the Carboniferous magmatic rocks in the CTT, we suggest the formation of the Xiaobaishitou gabbro‐diorite was attributed to southward subduction of the Kangguer Ocean.

Transition from tholeiitic to alkali basalts via interaction between decarbonated eclogite-derived melts and peridotite

Intraplate basalts generally show a geochemical continuum from alkali to tholeiitic basalts. However, the genetic link between these two types of rocks has remained controversial. The Early Jurassic Karamay basalts in the West Junggar terrane, southern Central Asian Orogenic Belt (CAOB), erupted to form a small-volume outcrop in the stable continental intraplate region. The basalts are characterized by aphyric textures without any visible phenocrysts. Thus, they are different from the ubiquitous porphyritic-textured intraplate basalts and have a composition close to that of the mantle-derived primary melt. In contrast to chemically and petrographically well-defined alkali and tholeiitic basalts, the Karamay basalts exhibit transitional compositions spanning from alkali (with normative olivine+nepheline and normative olivine+hypersthene) to tholeiitic (with normative quartz+hypersthene), providing an important case to address the geochemical continuum of intraplate basalts. Similar to the alkali basalts in eastern China, the Karamay basalts have isotopic imprints of sedimentary carbonates, i.e., significantly lighter Mg (δ26Mg = −0.54‰ to −0.34‰) and heavier Zn (δ66Zn = 0.36–0.46‰) isotopes than the normal mantle. However, they display initial ((87Sr/86Sr)t ratios of 0.7047–0.7051, positive εNd(t) values (3.3–4.2) and positive anomalies of Nb-Ta-Ti-Zr-Hf, which are not expected in the case of incorporation of recycled carbonates. This contradiction can be reconciled by considering a decarbonation reaction between carbonates (i.e., dolomite and magnesite) and co-existing eclogite in the subducted oceanic slab at pressure >5 GPa, leaving light Mg and heavy Zn isotope signatures in the stagnant eclogite residue in the deep mantle. Combining the geochemical compositions of our samples with the geologic evidence, and considering the previous results of melt-peridotite reaction experiments, we conclude that the Karamay basalts might have originated from the interaction of silica-rich tholeiitic melt derived from the recycled decarbonated eclogite with fertile peridotite during its ascent. Our study highlights that intraplate alkali basalts, especially silica-rich ones (e.g., with SiO2>45 wt.%), can be transformed from tholeiitic melts through reaction with peridotite mantle, and demonstrates that deeply recycled oceanic crust stagnated in mantle can serve as a main source for alkaline lavas.

Mesozoic Tectonothermal Evolution of the Southern Central Asian Orogenic Belt: Evidence from Apatite Fission-Track Thermochronology in Shalazha Mountain, Inner Mongolia

Mesozoic Tectonothermal Evolution of the Southern Central Asian Orogenic Belt: Evidence from Apatite Fission-Track Thermochronology in Shalazha Mountain, Inner Mongolia

Tectonic evolution of circum-Rodinia subduction: Evidence from Neoproterozoic A-type granitic magmatism in the Central Tianshan Block, northwest China

Neoproterozoic igneous rocks associated with the assembly and configuration of the Rodinia supercontinent are widely distributed in the Central Tianshan Block, southwestern Central Asian Orogenic Belt (CAOB), northwest China. The Central Tianshan Block is an important geological unit in tectonic reconstructions of the CAOB and Rodinia. However, the early Neoproterozoic tectonic framework and evolution of the Central Tianshan Block are poorly understood. Here we present a geochronological, mineral and whole-rock geochemical, and Nd–Hf isotopic study of early Neoproterozoic augen granites from the Bingdaban area. In situ zircon U–Pb dating shows that the augen granites have crystallization ages of 1002–992 Ma. The augen granites have high SiO2 (70.46–76.24 wt%) and K2O (3.36–4.96 wt%) contents, and FeOT/(FeOT + MgO) ratios (0.76–0.84), low Mg# values (29–39), slightly enriched light rare earth element patterns, enriched large-ion lithophile elements, depleted high-field-strength elements (e.g., Nb, Ta, and Ti), and high 10,000 × Ga/Al ratios (2.3–8.3) and Zr + Nb + Ce + Y contents (256–408 ppm). The augen granites have geochemical features typical of A2-type granites, variable εNd(t) values of –1.64 to +6.87 and Nd model ages of 1.78–1.08 Ga, and zircon εHf(t) values of −1.96 to +6.42 with Hf model ages of 1.98–1.48 Ga. These data indicate the granites had a juvenile crustal-dominated source, with subordinate input of ancient crustal components, and formed in a subduction-related extensional setting. The extension might have been related to slab break-off beneath Rodinia. The early Neoproterozoic igneous rocks in the Central Tianshan and adjacent microcontinental blocks in the southwestern CAOB formed at or near an active continental margin, possibly part of the circum-Rodinian subduction zone during or after the assembly of Rodinia. Similar Neoproterozoic magmatism in the Central Tianshan and Yili blocks suggests that they had a comparable crustal evolution and a close tectonic and paleogeographic relationship during the Neoproterozoic. However, significant age and isotopic differences characterize the Central Tianshan–Yili blocks and Tarim Craton, suggesting that the Central Tianshan–Yili blocks were not separated from the Tarim Craton during the early Neoproterozoic.

Petrogenesis and tectonic implications of Devonian granitoids in the central Beishan orogen, NW China: geochemical and Sr-Nd-Hf isotopic constraints

ABSTRACT The Devonian granitoids are widespread in the Beishan area of the southern Central Asian Orogenic Belt, which provides key geological records for revealing the middle Palaeozoic evolution of the Paleo-Asian Ocean. This study presents new zircon U–Pb ages, whole-rock major and trace elements, and Sr-Nd-Hf isotopes of four granitic plutons in central Beishan. The Haergen muscovite monzogranites were emplaced in the Early Devonian (401 Ma) and are considered as S-type based on their peraluminous features and high P2O5 contents. They show high 87Sr/86Sr(i) (0.7097 to 0.7141), low ℇNd(t) (−4.7 to −4.3) and variable ℇHf(t) (0 to 6.9), indicating an ancient crust source. The Balegentai, Haerteerdele, and Miaomiaojing plutons were intruded in the Late Devonian with ages of 381 Ma, 379 Ma, and 361 Ma, respectively. They are I-type granites, as indicated by their metaluminous to weakly peraluminous features and low zircon saturation temperatures. Furthermore, they present depleted 87Sr/86Sr(i) (0.7042 to 0.7079), ℇNd(t) (−1.6 to 2.2) and ℇHf(t) (4.0 to 10.6), suggesting that they were derived from the partial melting of juvenile crustal rocks. The Early Devonian granitoids with an age peak of 410 Ma in central Beishan consist of I-S-A type granites with variable Nd-Hf isotopes, implying extensive interactions between juvenile and/or mantle-derived and ancient crust-derived magma. In contrast, the Middle-Late Devonian granitoids with an age peak of 370 Ma consist of I-A type granites with depleted Nd-Hf isotopes, implying the input of more juvenile materials. We conclude that a collisional event between the Mazongshan and Huaniushan units occurred during the Early Devonian. The Middle-Late Devonian magmatism was formed in an extensional setting, and the Huaniushan unit was also influenced by the northwards subduction of the Liuyuan Ocean.

Origin and tectonic evolution of the Langshan (NW China): Insights from Proterozoic magmatic and sedimentary records

The Langshan connects the North China Craton (NCC) with the Alxa Block and occupies a key position to unraveling their Precambrian tectonic affinity. The tectonic origin and evolutionary history of the Precambrian basement of the Langshan is debated. To clarify these issues, we report zircon U-Pb ages and geochemical data for magmatic and metasedimentary rocks in the northern Langshan. The new results combined with previous data indicated that the northern Langshan experienced multistage magmatism at ∼2.53–2.49, ∼2.02, and ∼1.92–1.97 Ga, similar to those in the NCC. Late Paleoproterozoic (∼1.64–1.71 Ga) gneissic alkalic feldspar granite and monzogranite show A-type granite geochemical signatures. Detrital zircon U-Pb-Hf isotopic data indicated that the Baoyintu Group was deposited in the Mesoproterozoic and mainly sourced from the basement rocks and coeval rift-related granites in the northern Langshan and NCC. The A-type gneissic granites and Baoyintu Group were resulted from the breakup of the Columbia supercontinent. In addition, a K-feldspar granite with A-type granite geochemical characteristic was dated at 855 ± 5 Ma. A schist displays detrital zircon age peaks at ∼1577, ∼1443, ∼1348, and ∼1174 Ma with predominantly positive εHf(t) values, and the Alxa Block might provide significant detritus for it. A continental fragment was probably detached from the Alxa Block due to regional back-arc extension along the northern margin of the Rodinia supercontinent during the early Neoproterozoic, which was finally incorporated into an early Paleozoic accretionary complex in the southern Central Asian Orogenic Belt. The available data indicate that the Langshan can be subdivided into the northeastern Langshan and southwestern Langshan, with the former belonging to the NCC and the latter as a part of the Alxa Block. We advocate that the Alxa Block was an independent microcontinent with a different tectono-thermal evolution from the NCC at least during the Neoproterozoic.

Metamorphic and chronological constraints on the early Paleozoic tectono‐thermal evolution of the Olkhon Terrane, southern Siberia

AbstractTerranes accreted to the southeastern margin of the Siberian Craton record an important early Paleozoic tectono‐thermal event (known as the Baikal orogenic cycle) in the evolution of the Central Asian Orogenic Belt (CAOB). However, the precise metamorphic conditions and relative timing of this event and its linkage to the wider CAOB remain far poorly constrained. The best exposed of these terranes is the Olkhon Terrane on the western bank of Lake Baikal. Here, late Neoproterozoic through early Paleozoic island arc and back‐arc assemblages were metamorphosed to form a thin granulite facies belt cropping out adjacent to the Siberian Craton and lower temperature/pressure paragneiss and migmatite towards the southeast. Phase equilibria modelling suggests that the granulite facies belt preserved moderate pressure (c. 0.80 GPa) and high temperature (up to 900°C) conditions while the paragneiss and migmatites in the southeast have peak metamorphic conditions around 700–770°C at 0.60–0.80 GPa. New geochronological data (zircon U–Pb in granulite and monazite U–Pb in paragneiss/migmatite) in combination with phase equilibria modelling and petro‐structural analysis suggest that the tectono‐metamorphic evolution of the Olkhon Terrane was controlled by a long‐lasting (535–450 Ma) and pervasive thermal anomaly. Discrete maxima in the zircon and monazite U–Pb ages at c. 535, 500, and 450 Ma are linked to different stages of a semi‐continuous high‐temperature metamorphic evolution. Based on existing geological data of the region, a generalized geodynamic model for the Baikal orogenic cycle involving switching between compressional and extensional regimes during the early Paleozoic accretion of ‘exotic’ CAOB‐derived material to the southern margin of Siberia is proposed. The tectono‐metamorphic evolution of the Olkhon Terrane may represent a world‐class example of polyphase shortening of a long‐lived hot intra‐continental arc–back‐arc system during its collision with cratonic blocks.

Late Paleozoic tectonics of Southern Central Asian orogenic belt: Evidence from magmatic rocks in the northern Alxa, Northwest China

Late Paleozoic magmatic rock outcrops are common in the Northern Alxa, Southern Central Asian Orogenic Belt (CAOB), which is a key area for understanding tectonic processes and defining the final closure time of the Paleo-Asian Ocean (PAO). We present zircon U‒Pb chronology and whole-rock geochemistry data for late Paleozoic magmatic rocks from the Yagan area of northern Alxa. This paper reveals two periods of magmatism: syenogranite (374.8 Ma) and bimodal intrusive rocks, which consist of gabbro (261.4 Ma), diabase (262.9 Ma) and biotite monzogranite (263.4 Ma). The syenogranite is high-K calc-alkaline and shows enrichments in Th, Zr, Hf and LREEs; depletions in Sr, Nb, Ta, and Ti; and low Mg# values (6.9–13.2); the syenogranite was derived from partial melting of the crust and has volcanic arc characteristics. The gabbro and diabase have similar geochemical characteristics, such as enrichments in Pb, Rb, Sr, Zr, and Hf and depletions in Nb, Ta, and Ti, with positive εHf(t) values (+0.9–+2.7 and +2.6–+3.6, respectively), indicating that they originated from partial melting of depleted mantle and experienced crustal contamination during magma emplacement. The biotite monzogranite shows depletions in Nb, Ta, and Ti and εNd(t) values of -2.6 to −2.4 and resulted from partial melting of the lower crust caused by asthenospheric underplating. The bimodal intrusive rocks formed in a postcollision extensional setting. Combined with previous data, we conclude that northern Alxa was an active continental margin during the late Devonian and that the final closure of the Yagan branch ocean of the PAO occurred prior to the middle Permian.

Geology of the Liuyuan Complex, NW China: A Permian back-arc basin ophiolite at the southern edge of the Central Asian orogenic belt

Located in the southern margin of the Central Asian orogenic belt, the stratigraphy and tectonic setting of the mafic 290−280 Ma Liuyuan Complex have been controversial for decades, with workers arguing for a forearc ophiolite or a continental rift setting. Here, we present the results of a detailed field study, where the Liuyuan Complex was subdivided into troctolite, melatroctolite, layered gabbro, varitextured olivine gabbro, hornblende gabbro, plagiogranite, sheeted dike, and mafic tectonite, in addition to previously identified and studied basalt and chert. All contacts between the igneous facies are intrusive, with gabbroic rocks separated from the overlying basalt by a well-developed and laterally continuous sheeted dike complex. Based on their geochemical affinities, two groups of basalt were identified: group I (low-TiO2) and group II (high-TiO2). A modeled liquid line of descent, assuming perfect mineral fractionation, with a liquidus temperature of 1212 °C, pressure of 1 kbar, fO2 at the quartz-fayalite-magnetite (QFM) buffer, and initial melt H2O of 0.5 wt%, provides an excellent fit to group I lavas, with group II basalts interpreted as having formed from a distinct arc source. The stratigraphy, extended trace-element patterns, and tectonic fingerprinting of the lavas suggest the Liuyuan Complex formed as an ophiolite in a fast-spreading back-arc basin, a setting inconsistent with previously proposed tectonic models for the southern Central Asian orogenic belt. We expand on a tectonic model that proposes the Liuyuan Complex formed as a back-arc to the recently identified Ganquan arc. The back-arc basin was then consumed in a north-dipping subduction zone beneath the active margin of composite Siberia. The magmatic center of this arc migrated southward, likely caused by slab roll-back, with the Liuyuan Complex becoming the basement of an arc. Exhumation of the Liuyuan Complex took place by 267 Ma, as constrained by the age of a subaerial dacite that unconformably overlies the basalts of the Liuyuan Complex.

Evidence of Grenville and Valhalla Tectonic Events at the Western Margin of the Siberian Craton from Rocks of the Garevka Complex (North Yenisei Ridge)

The tectonic evolution of the Siberian cratonic margins offers important clues for global paleogeographic reconstructions, particularly with regard to the complex geological history of Central Asia. The Yenisey Ridge fold-and-thrust belt at the western margin of the Siberian Craton forms part of the Central Asian Orogenic Belt (CAOB) and is a key to understand the Precambrian tectonic evolution of the Siberian Craton and crustal growth in the CAOB. Understanding the tectonic evolution of the Yenisei Ridge is crucial for solving the debate related to the role of the Siberian Craton within the Rodinia supercontinent assemblies and breakup with the opening of the Paleoasian ocean. Here we report new data on the petrogenesis, tectonic settings, thermodynamic conditions, metamorphism and protolith ages for compositionally contrasting rocks of the Garevka metamorphic complex, obtained from the results of mineralogical-petrological, geochemical, and isotope-geochronological studies. Possible models and geodynamic settings for their formation are discussed. Based on the results of U-Pb dating of zircons, two new pulses of Neoproterozoic endogenous activity at the western margin of the Siberian craton were established, associated with Grenville (930–900 Ma) and (880–845 Ma) post-Grenville Valhalla (880–845 Ma) accretion-and-collision processes. These episodes of regional crustal evolution are correlated with the synchronous successions and similar style of rocks along the Arctic margin of Rodinia and supports the spatial proximity of Siberia and North Atlantic cratons (Laurentia and Baltica), which is consistent with the proposed Neoproterozoic paleogeographic reconstructions of the Rodinia configuration.

Discovery of a >1,000 km Cambrian Eclogite‐Bearing High‐Pressure Metamorphic Belt in the Central Asian Orogenic Belt: Implications for the Final Closure of the Pan‐Rodinian Ocean

AbstractThe immense Central Asian Orogenic Belt (CAOB) records the transition from the Pan‐Rodinian Mirovoi Ocean to the Paleo‐Asian Ocean, but the final closure of the Mirovoi Ocean remains unexplored. Here we document two new eclogite occurrences, located between CAOB microcontinents and the Ediacaran–Cambrian island arc in Mongolia. Pseudosection modeling and geothermobarometry constrain peak conditions of ∼560°C and ∼2.2 GPa for the Urgamal eclogite and ∼715°C and ∼2.0 GPa for the Tsengel eclogite. Zircon U‐Pb dating results suggest the Urgamal eclogite‐facies metamorphism occurred at ∼522 Ma, and the granite gneiss enclosing the eclogite yielded ∼811 Ma crystallization age and ∼519 Ma metamorphic age. Detrital zircon age spectra of the paragneisses associated with the eclogite suggest a probable provenance from the adjacent Zavkhan microcontinent and the Neoproterozoic arc superimposed on the microcontinent. The Tsengel eclogite has a protolith age of ∼853 Ma and an eclogite‐facies metamorphic age of ∼522 Ma, and the associated migmatite gneiss record a ∼520 Ma migmatization event. The rock associations and geochemical affinities indicate that their protoliths represent mafic rocks formed in a continental rift setting and the Early Neoproterozoic subduction–accretion complex, respectively. Their similar tectonic position and metamorphic age suggest that these rocks are part of a single >1,000 km long Cambrian high‐pressure metamorphic belt. The rocks record closure of the Mirovoi Ocean within the CAOB and constrain this crucial event to ∼520 Ma along the western margin of the microcontinents in Mongolia.

Melting modeling of mixed peridotitic and mafic lithologies at shallow depths of the continental metasomatized lithospheric mantle: Implementation for the Early Cretaceous volcanic rocks of Eastern Mongolia

The Eastern Mongolia volcanic area formed in the Late Mesozoic–Early Cenozoic within Central Asian Orogenic Belt. The main volcanic events of the area occurred in the Early Cretaceous when alkaline basaltic lavas erupted and formed the so-called cover volcanic complex. Geochemical and isotope features of the cover volcanic complex allowed researchers to identify the following mantle rocks as their source: metasomatized peridotites, eclogites, and pyroxenites. Thermodynamic modeling in alphaMELTS program was performed to determine whether the simultaneous melting of these rocks with subsequent processes of crystallization differentiation could lead to the formation of the studied rocks. The modeling results show that the melting of the most enriched with incompatible trace elements peridotites, eclogites, and pyroxenites cannot produce the rocks of the cover volcanic complex. At the same time, the mixing of peridotite- and eclogite-derived melts corresponds most closely to the mechanism of rock formation. However, Ti, K, P, Rb, and Sr enrichment of the studied rocks also requires participation in magma generation processes of mantle metasomatic veins enriched with rutile, apatite, phlogopite, and amphibole.

A Preliminary study on deep geodynamic process of Xar Moron tectonic belt in eastern Central Asia Orogenic Belt

板块汇聚边缘的陆壳厚度变化与构造和岩浆过程的动态相互作用有着错综复杂的联系,也是对深部地球动力学背景的直接响应。西拉木伦构造带是中亚造山带东部重要的汇聚板块边界,查明其浅部构造变形及深部动力学过程对于理解中亚造山带构造演化具有重要意义。本文通过野外地质工作查明晚二叠世-早三叠世西拉木伦构造带的上地壳发育一系列北东向、轴面向南东倾的宽缓褶皱以及向南东逆冲的断层,变形样式属于薄皮构造,显示出由北西向南东挤压的单向构造应力背景,平衡剖面恢复显示此时期构造变形造成地表~30%的缩短以及~4km的浅部地壳增厚。利用林西地区火成岩全岩La/Yb比值和锆石Eu/Eu^*参数构建的年龄-地壳厚度曲线揭示,二叠纪早-中期地壳厚度从49km连续减薄到33km,反映此时期整体处于伸展环境。二叠纪晚期至三叠纪初期,地壳厚度增加了~15km,峰值厚度达~48km,这个迅速的地壳增厚过程可能是岩浆作用导致的地壳垂向增生和构造作用产生的造山带物质堆叠综合作用的结果。本文根据构造带同汇聚期岩浆岩面积和地壳厚度估算造山作用形成了~11%的新生陆壳。同时,两个时期的深部壳幔相互作用方式也有不同,二叠纪早期西拉木伦构造带火成岩锆石的<i>ε</i>_Hf(<i>t</i>)值相对较高(6.1~19.9;均值10.1),<i>δ</i>¹⁸O值较分散(5.1‰~8.3‰),指示岩浆在形成过程中有幔源物质的加入,示踪了林西地区深部与软流圈上涌有关的伸展过程;而晚二叠世至三叠纪初期花岗岩锆石<i>ε</i>_Hf(<i>t</i>) 值相对下降(-1.1~17.2;均值9.3),<i>δ</i>¹⁸O值仍高于幔源值(＞5.9‰),揭示同源地幔岩浆的持续重融改造过程。综合沉积环境、地壳厚度变化、岩浆岩同位素变化、地壳增生量及地表单向构造应力背景等特征,本文提出西拉木伦构造带可能经历了地幔俯冲;而早-中二叠世的软流圈上涌和晚二叠世-早三叠世的下地壳及岩石圈地幔密度增大,可能是发生地幔俯冲的深部地球动力学原因。

A synthesis of Carboniferous stratigraphy in Xinjiang, Northwest China with emphasis on regional and global correlations

The Carboniferous is the most widely distributed system in the pre-Mesozoic strata of Xinjiang, Northwest China, and they are exposed in structurally independent units within various tectonic settings across the southwest Central Asian Orogenic Belt and Tarim Craton in Northwest China. According to the lithofacies associations and biotic assemblages, the Carboniferous successions of Xinjiang can be divided into 18 stratal regions, and each of these stratal regions may be further separated into several subregions in terms of lithology, biota, and geochronologic data. The depositional succession of each region is described on the basis of the key section. The marine fossils such as foraminifers (fusulinids), brachiopods, corals, and conodonts, as well as nonmarine fossils such as plants and pollens are reviewed in this study. Combined with the geochronologic data from the volcanic rocks and ash layers, a refined scheme of the Carboniferous successions in different regions across Xinjiang, Northwest China is obtained. The detailed and newly established stratigraphic framework of the Carboniferous strata could be very helpful for the regional and global stratigraphic correlations. It can also offer a new and reliable chronostratigraphic foundation for the comprehensive understanding of the tectonic evolution and petroleum exploration in Central Asia.

A new model for the segmentation, propagation and linkage of the Tan-Lu fault zone, East Asia

• The Tan-Lu Fault formed by coalescence of two kinematically independent segments. • The northern and southern Tan-Lu faults exhibit contrasting displacement profiles. • The northern and southern Tan-Lu faults propagated in opposite directions. • Southward indentation of the Siberian craton initiated the northern Tan-Lu Fault. • The northern and southern Tan-Lu faults linked in the early Late Jurassic. The Tan-Lu fault zone (TLF) is a major strike-slip fault with a long and complex history in East Asia, whose evolution provides a new perspective on the formation of large-scale faults (>1000 km long). Fault displacement analysis, geological mapping and U-Pb LA-ICP-MS dating have been performed to understand the evolution of the TLF. Along-strike displacement variation reveals that the TLF consists of two kinematically independent segments, the northern and southern TLF, with opposite long-term propagation directions. Structural and geochronological studies in the eastern Yanshan belt, located around the linkage area of the southern and northern TLF, indicates that NNE-trending sinistral strike-slip faults initiated at 167-164 Ma and were reactivated at 124-121 Ma. Structural analysis suggests that these early NNE-trending strike-slip faults transferred sinistral motion along the northern TLF into southward thrusting along the Yanshan belt, representing the Middle Jurassic southern termination of the northern TLF. Our studies suggest that the through-going TLF formed when the younger southward-propagating northern TLF merged with the older northward-propagating southern TLF in the Late Jurassic. A new model is thus proposed for the Mesozoic evolution of the TLF. The initiation and southward propagation of the northern TLF is interpreted to have resulted from the southward indentation of the Siberian craton into the amalgamated Central Asian Orogenic Belt and North China block. The divergent mega-splays of the northern TLF likely resulted from westward-younging formation during the clockwise rotation of northeast Asia. Coalescence of two genetically unrelated faults could be an alternative mode for large-scale fault formation.

Tectono-magmatic evolution from accretion to collision in the southern margin of the Central Asian Orogenic Belt

造山带演化及增生到碰撞的转变是板块构造与大陆动力学研究中的前沿科学问题。中亚造山带被认为是古亚洲洋长期俯冲-增生演化形成的显生宙最大的增生造山带,以发育巨量的面状展布的俯冲-增生相关的弧岩浆岩为特征。并且,由于中亚增生型造山带在潘吉亚最后聚合过程中发生弧弧(陆)碰撞,因此缺乏大规模且跨构造单元的碰撞相关的构造和变质等物质标志。显然,能否识别出大洋闭合期间碰撞作用的岩浆标志成为确定增生造山带增生过程终止的关键之一。本文系统研究确定:中亚造山带东南缘二叠纪到三叠纪钙碱性-碱钙性岩浆在空间分布上显示出由北西向南东迁移演化的特征;在岩浆性质上具有从二叠纪新生地壳来源的弧岩浆向早-中三叠世碰撞挤压背景下古老陆壳组分逐渐增多的高Sr/Y岩浆以及晚三叠世后造山伸展相关的A型花岗岩演化的特征。这些特征提供了俯冲-增生向碰撞造山演变的关键岩浆岩证据。结合区域资料,厘定出增生造山带最后碰撞相关的标志性岩浆为沿缝合带呈零星线性展布的增厚下地壳源区的高Sr/Y花岗岩类,构建了中亚造山带南缘从双向俯冲-增生到增生楔-增生楔碰撞及后造山伸展的三阶段构造-岩浆演化模型。系统对比研究,揭示出增生-碰撞相关的岩浆记录沿横向展布在中亚造山带南缘甘肃北山到吉林中部一带,表明碰撞挤压相关的岩浆作用在中亚造山带南缘具有一定的普适性。中亚造山带南缘从增生到碰撞的岩浆演化记录的厘定,证实显生宙最大的巨型增生造山带演化末期经历了碰撞造山作用,对进一步深入探索增生造山演化末期碰撞相关的标志性岩浆具有重要意义。

LateTriassic A‐type granite boulders in Lower Cretaceous conglomerate of theHida belt, Japan: Their origin and bearing on theYamatotectonic line inFar East Asia

AbstractTo identify the origin of the Hida belt in central Japan, geochemistry and U–Pb age of zircons were analyzed for the extra‐large granitoid boulders in the Lower Cretaceous fluvial conglomerate of the Jinzu Group. This study clarified that the boulders of granitoids have geochemistry of typical A‐type granite, as characterized by high Nb + Y and high Ta + Yb values. U–Pb ages of igneous zircons from three individual granite boulders are concentrated at ca. 220 Ma (Late Triassic). As to Late Triassic A‐type granites, there is no corresponding body previously recognized within Japan, whereas identical A‐type granites occur in the eastern Songliao block on the immediate west of the Jiamusi block in NE China. The large size of boulders and the fluvial facies of the hosting conglomerate indicate their origin in the Hida belt per se, suggesting the cryptic and/or past occurrence of A‐type granite, rather than in NE China. Together with the eastern Songliao block (China), Laoelin‐Grodekov (LG) belt (Primorye, Russia), and Yamato Ridge (Japan Sea), the Hida belt forms a unique domain on the immediate west of GSC in Far East Asia, which is characterized commonly by the co‐occurrence of Permo‐Triassic and Jurassic granitoids, and probably with Late Triassic A‐type granite. These confirm that Hida belt represents an allochthonous unit tectonically emplaced onto the rest of Japan, which is composed of the Phanerozoic subduction‐related orogenic belt (Nipponides) developed along the Pacific side of Greater South China (GSC) since the Cambrian. For emphasizing a major geotectonic boundary of the Mesozoic granitoid provinces in Far East Asia, we propose the Yamato tectonic line between the easternmost Central Asian orogenic belt and GSC/Nipponides, which is traced for up to 3000 km from the Russia/China/North Korea border to SW Japan.

Late Jurassic intraplate extension in southern Central Asian Orogenic Belt: Evidence from Beidashan mafic rocks in Alxa

阿拉善地块北缘晚中生代(晚侏罗世-早白垩世)时期发育有广阔的伸展盆地体系,其形成可能与陆内伸展作用有关。本文首次在阿拉善地块西北缘北大山陶来地区识别出一套晚侏罗世镁铁质-超镁铁质岩,对其进行系统的岩石学、年代学和地球化学研究,从而为该地区陆内伸展提供岩浆作用方面的证据。LA-ICP-MS锆石U-Pb定年结果表明,北大山陶来地区角闪辉长岩形成时代为155.7±4.6Ma (MSWD=1.5),即晚侏罗世。角闪辉长岩显示出高铝基性岩的地化特征,如低SiO₂(48.0%~52.5%),高Al₂O₃(15.3%~18.2%)、MgO (8.78%~10.6%)和CaO (12.8%~16.7%)。此外,它们还显示出高的Mg^#(72~82)、Cr (295×10^-6~960×10^-6)和Ni (97.4×10^-6~186×10^-6),N-MORB型和平坦的HREE配分形式、Eu正异常,较高的Nb/La比值,这些都表明其可能为软流圈地幔减压熔融产生的原始岩浆与壳幔过渡带中含斜长石堆晶岩相互作用后分离结晶的产物。富铝和富钙源区以及岩浆中高的水含量抑制斜长石早期成核与生长共同控制了高铝角闪辉长岩的形成。辉长岩表现出N-MORB型全岩地球化学特征和板内成因岩浆锆石特征(高Nb/Hf及低Hf/Th、Th/Nb),均揭示了阿拉善西北缘陶来地区晚侏罗世镁铁质-超镁铁质岩体是板内岩石圈伸展作用的产物。在古亚洲洋最终碰撞闭合后伸展基础上,蒙古-鄂霍茨克洋闭合与古太平洋俯冲后撤叠加作用引发了岩石圈伸展,并沿中亚造山带南缘多个古生代缝合带产出了194~124Ma多期镁铁质-超镁铁质岩。