Tianshan Suture Research Articles

Paleozoic tectono-thermal history of the amalgamation of the Tarim–North-China and Mongolian collages

In the Mongolian Collage, metamorphic pressure – temperature ( P–T ) and timing reveal a one-stage evolution defined by a duality of late Neoproterozoic–Ordovician subduction-related low T/P metamorphism and supra-subduction high T/P metamorphism recorded in the Mongolia-Manchuria and Baikal-Sayan belts. This was followed by gradual prevalence of supra-subduction high T/P metamorphism towards the late Paleozoic corresponding to the Altai and South Altai cycles. In the Tarim–North-China Collage, metamorphic P–T and timing reveal a two-stage evolution, from dominant intermediate T / P metamorphism possibly resulting from Ordovician–Devonian amalgamation and Andean-type evolution of the collage, to dual low and high T / P metamorphism in the Carboniferous–Permian reflecting subduction-collision processes along the South Tianshan suture in the west and a supra-subduction evolution along the Solonker suture in the east. Altogether, the Paleozoic tectono-metamorphic evolution of the two collages in the Central Asian Orogenic Belt show remarkable differences, with the Mongolian Collage displaying features typical of peripheral accretionary style reflecting recurrent tectonic switches that can be regarded as a single orogenic system, and a two-stage evolution of the Tarim–North-China Collage with features of both peripheral–accretionary and interior-collisional orogenic cycles, but mostly related to recurrent subductions of interior oceans.

Early Cambrian oceanic crust in the Chinese North Tianshan: Evidence of the earliest subduction system within the southwestern Central Asian Orogenic Belt

The long-lived accretion of the southwestern Central Asian Orogenic Belt is directly related to the subduction and demise of multiple Paleozoic oceanic basins. However, the evolution of those different oceans, and their relationships, remain poorly constrained. In this paper, we report on an Early Cambrian ophiolite discovered in the North Tianshan suture, China, which represents the oldest oceanic crustal fragments documented in the North Tianshan belt. Two gabbroic blocks collected from the Gangou ophiolitic mélange yielded consistent zircon U–Pb ages of ca. 521 Ma. These gabbros have similar whole-rock major and trace element and Sr–Nd isotopic compositions to those of gabbroic oceanic crust from mid-ocean ridges. In addition, high-An (∼92) plagioclase and high-Mg# (∼91) clinopyroxene indicate that the gabbros crystallized under hydrous conditions, likely in a subduction-related setting. Our data and those for Early Cambrian subduction-related ophiolites in the Junggar, Western Tianshan, and Beishan orogenic belts record the development of the earliest, large-scale, subduction system within the southwestern Central Asian Orogenic Belt, which was probably associated with assembly of Gondwana. This system may have also connected the Early Cambrian subduction systems between the Western Tianshan and Beishan belts.

Provenance of upper Permian-lowermost Triassic sandstones, Wutonggou low-order cycle, Bogda Mountains, NW China: implications on the unroofing history of the Eastern North Tianshan Suture

This study investigates the provenance of sedimentary rocks in Bogda Mountains, NW China, and reconstructs the lithology and unroofing history of the Eastern North Tianshan Suture. Petrographic point counting data of sandstones and compositions of conglomerates of upper Permian-lowermost Triassic Wutonggou low-order cycle from Zhaobishan, North Tarlong, Taodonggou, and Dalongkou sections in the southern and northern foothills of Bogda Mountains were used to interpret the temporal and spatial variations of lithology of the Eastern North Tianshan Suture, which is the sediment source area. Three compositional trends were identified. A trend of upward-increasing quartz content and granitic pebbles in Zhaobishan section suggests a change from the undissected volcanic arc, accretionary wedge and trench setting to predominantly transitional volcanic arc and subordinate accretionary wedge and trench, in the eastern part of the Eastern North Tianshan Suture. In North Tarlong and Taodonggou sections, however, the lithic content decreases and the contents of quartz and granitic pebbles increase up sections. These trends indicate that the western part of the Eastern North Tianshan Suture changed from an undissected volcanic arc to the transitional volcanic arc, accretionary wedge and trench. No clear trend in the lithic-rich sandstones of the Dalongkou section indicates that sediments were derived from the undissected volcanic arc in the Eastern North Tianshan Suture and local rift shoulders. Compositional variations of studied rocks suggest that the Eastern North Tianshan Suture was an amalgamated complex with great spatial and temporal heterogeneities in lithology and experienced persistent unroofing during late Permian-earliest Triassic. This study reconstructs a key element of the Chinese Tianshan Suture and serves as an example to understand the unroofing processes of ancient sutures.

Ediacaran to Paleozoic magmatism in West Junggar Orogenic Belt, NW China, and implications for evolution of Central Asian Orogenic Belt

The formation of the Central Asian Orogenic Belt is thought to be related to the evolution of the Paleo-Asian Ocean surrounded by the Siberian, European, and Tarim-North China cratons. The West Junggar Orogenic Belt (WJOB) in the southern Central Asian Orogenic Belt is part of the Kazakhstan-Junggar terrane between the Altay Orogenic Belt to the north and the Tianshan Orogenic Belt to the south. It is characterized by the development of the latest Ediacaran to Paleozoic intra-oceanic island arcs and accretionary complexes, including the latest Ediacaran to Ordovician Tangbale-Mayile arc-accretionary system in the south and Ordovician to Early Carboniferous Boshchekul-Chingiz arc and Zharma-Saur arc in the north, and a remnant ocean between them. Our work reveals that the Shinaizha, Kekesayi, and Barleik dioritic plutons within the Tangbale-Mayile arc are typical of arc-related magmatism and were formed between 572 and 505 Ma, which indicate the Ediacaran to Middle Cambrian subduction of the southern Paleo-Asian Ocean. However, these arc plutons were strongly deformed and surrounded by younger rocks, implying that the magmatic arc might be dismembered and involved into the Ordovician to Devonian accretionary complexes. The Boshchekul-Chingiz arc was recorded from Late Ordovician and its arc magmatism peaked in Late Silurian to Early Devonian. The arc moved northward due to the retreat of the south-facing Irtysh-Zaysan subduction zone, which resulted in the formation of contemporaneous A-type or peralkaline magmatic rocks or bimodal volcanics in response to the within-arc extension. The northward migration of the subduction zone led to the formation of the Devonian to Early Carboniferous Zharma-Saur arc. The earliest Late Carboniferous I-type granitoids occurred sporadically in WJOB, followed by pervasive occurrences of Late Carboniferous to Early Permian A- and I-type granitoids in WJOB and adjacent areas. The Late Carboniferous granitoids, combined with the Late Carboniferous stitching plutons within the Irtysh-Zaysan suture zone to the north and the North Tianshan suture zone to the south of WJOB, suggest the Kazakhstan-Junggar terrane collided with Altay and Tianshan orogenic belts in the latest Early Carboniferous (ca. 320 Ma).

Geochronology and Geochemistry of Paleozoic to Mesozoic Granitoids in Western Inner Mongolia, China: Implications for the Tectonic Evolution of the Southern Central Asian Orogenic Belt

AbstractSituated between the South Tianshan suture zone to the west and the Solonker suture zone to the east, the Yagan and Zhusileng-Hangwula arcs (YZHAs) in western Inner Mongolia in China occupy...

ZIRCON U-PB AND HF ISOTOPIC STUDY OF THE KAWABULAKE OPHIOLITE EASTERN TIANSHAN: IMPLICATION FOR THE TECTONIC EVOLUTION OF CAOB

The Eastern Tianshan belt, located in the southern CAOB, played an important role in the crustal evolution, particularly because it links the Southern Tianshan suture to the west with the Inner Mongolia Solonker suture to the east. However, some critical issues, such as the exact position and formation age of the final suture zone of the Paleo-Asian ocean are still obscure or in controversy. Thus, here we have performed detailed studies of the Kwabulake ophiolit zone, a key part of the southern suture of the CAOB. New LA- ICPMS zircon U–Pb ages, Hf isotopic values, and whole-rock geochemical data have been presented to: (1) constrain the age of the Kawabulake ophiolite, (2) understand the petrogenesis of the granodiorites and their tectonic setting, and (3) reveal their implications for geodynamics of the Eastern Tianshan belt.

Late Carboniferous high-pressure metamorphism of the Kassan Metamorphic Complex (Kyrgyz Tianshan) and assembly of the SW Central Asian Orogenic Belt

High-pressure (HP) metamorphism of the Kassan Metamorphic Complex (KMC) in the western Kyrgyz Tianshan has been related to either late Ordovician or late Carboniferous-Permian subduction processes. We report Sm-Nd ages for retrogressed eclogite samples and 40Ar/39Ar cooling ages for enclosing garnet-muscovite samples from the KMC as new age constraints on HP metamorphism and rock exhumation. These data will be used for an upgraded paleogeographic model for late Paleozoic crustal consolidation in the southwestern Central Asian Orogenic Belt. The retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace-element patterns consistent with protoliths derived from garnet-bearing mantle sources at rifting plate margins. Geothermobarometric data for a retrogressed eclogite sample indicate peak-metamorphic conditions of 540 ± 30 °C at 1.6 ± 0.1 GPa. Samples from different lithotectonic units of the KMC provide coherent Sm-Nd garnet-whole rock ages of 317 ± 4 Ma and 316 ± 3 Ma (2σ). The prograde major-element zoning in the mm-sized garnets in combination with the moderate peak-metamorphic temperature, support our interpretation of the Sm-Nd garnet ages as unambiguous evidence for late Carboniferous HP metamorphism. The Sm-Nd garnet growth ages overlap within-error with the 40Ar/39Ar mica cooling ages of 314 ± 2 Ma and 313 ± 2 Ma (2σ) indicating rapid uplift of the subduction complex after peak metamorphism. The ca. 317-313 Ma HP-exhumation event of the KMC is contemporaneous with those of the Atbashi and Akeyazi (ca. 500 km east in NW China) HP complexes and implies similar collision histories at the South Tianshan Suture to the east and west of the Talas-Fergana Fault (TFF). The exhumation of the KMC and Atbashi HP complexes overlaps with the initiation of the TFF (Rolland et al., 2013) suggesting incipient separation of the Chatkal and Atbashi complexes during rock exhumation and early plate collision.

Late Paleozoic subduction and collision processes during the amalgamation of the Central Asian Orogenic Belt along the South Tianshan suture zone

The provenance of late Paleozoic siliciclastic sedimentary strata on the northern margin of the Tarim Craton and the South Tianshan Orogenic Belt provides important insights into subduction and collision processes during the formation of the southern Central Asian Orogenic Belt. Detrital zircons from Carboniferous and Permian sedimentary rocks in the South Tianshan belt show two predominant age populations of 500–400 and 305–270Ma, and three subordinate clusters around ~2.5Ga, 2.0–1.7Ga, and 1.2–0.6Ma. Such age patterns are similar to major magmatic episodes in the Tarim Craton but are distinct from those in the Central Tianshan–Yili Block, implying that the Carboniferous–Permian strata in the South Tianshan belt were deposited on the northern margin of the Tarim Craton. These data, in combination with Carboniferous passive margin deposition along the South Tianshan and northern Tarim regions and intense arc magmatism in the Central Tianshan area, support the northward subduction of the South Tianshan oceanic crust. The abrupt decrease of zircon εHf(t) values at ~310Ma indicates the tectonic transformation from oceanic subduction to continental collision. However, syn- and post-collisional sediments in the South Tianshan and northern Tarim regions did not contain detritus from the Central Tianshan–Yili Block, as evidenced by the absence/paucity of 380–310Ma detrital zircons in the late Paleozoic strata and by the stratigraphic record in the regions. Insignificant surface erosion and uplifting during the collision and (ultra-)high-pressure rock exhumation require divergence in the frontal wedge. We propose that the switch from a convergent to a divergent regime was triggered by the arrival of the Tarim mantle plume in the latest Carboniferous, which possibly had profound effects on regional sedimentation and exhumation of (ultra-)high-pressure rocks in the orogenic belt.

Continuous denudation and pediplanation of the Chinese Western Tianshan orogen during Triassic to Middle Jurassic: Integrated evidence from detrital zircon age and heavy mineral chemical data

The Mesozoic tectonic framework of the Chinese Tianshan orogen is ambiguous. However, thick Mesozoic sediments both inside and in front of the orogen can provide clues to solve this problem. LA-ICPMS U–Pb dating on detrital zircons and electron microprobe analysis on detrital heavy minerals are carried out on the Triassic to Middle Jurassic sediments along the Kuqa River section in the northern Tarim basin and in two intermountain basins (Bayanbulak basin and Gongnaisi valley), to reveal the change in provenance through time and space. Zircons from the Kuqa River section are mainly of magmatic origin and their ages have a wide range from 2800 to 234Ma, 70.5% of which are Palaeozoic. This age signature indicates a main provenance from the Tianshan orogen. Detrital heavy minerals (garnet and tourmaline) from the Kuqa River section suggest a complex provenance from granitic and metamorphic rocks of the South Tianshan belt, the South Tianshan suture and the Yili-Central Tianshan block. Detrital zircon U–Pb ages of the Early-Middle Jurassic sandstones from the Gongnaisi valley and Bayanbulak basin are predominantly Palaeozoic, without Precambrian, indicating a simple source from the Yili-Central Tianshan block. The different features of detrital zircon ages and heavy mineral chemical compositions in individual sample reflect provenance changes due to continuous denudation and pediplanation of the Chinese West Tianshan orogen in an extensional setting during Triassic to Middle Jurassic: (1) during Early to Middle Triassic, the South Tianshan belt towered to the north of Tarim basin, acting as a watershed and being the main source area for northern Tarim basin; (2) since Late Triassic to Early Jurassic, material from southern Yili-Central Tianshan block was largely transported to Tarim basin because of the continuous denudation of the South Tianshan belt; (3) in Middle Jurassic, the Chinese Western Tianshan orogen reached a peneplain state, resulting in the direct deposition in intermountain basins and the decrease of sediment supplying from Yili-Central Tianshan block to Tarim.

Paleozoic ophiolitic mélanges from the South Tianshan Orogen, NW China: Geological, geochemical and geochronological implications for the geodynamic setting

Abstract Two ophiolitic melange belts, the South Central Tianshan Ophiolite Belt (SCTOB) and the South Tianshan Ophiolite Belt (STOB), extend almost parallel to the Chinese South Tianshan Orogen — the southernmost part of the Central Asian Orogenic Belt (CAOB). The SCTOB occurs interlayered in the South Central Tianshan Suture Zone, the STOB as exotic blocks in Palaeozoic sedimentary strata. However, their tectonic settings and ages, which are crucial for understanding the formation of the CAOB, are still controversially discussed. In order to better understand these geological problems, geological, geochemical and geochronological investigations were conducted on the Guluogou (SCTOB) as well as on the Serikeyayilake and Aertengkesi ophiolitic melanges (STOB). The ophiolitic suites are composed of basalts, gabbros and mantle peridotites. Laser ablation ICP-MS (LA-ICP-MS) U–Pb zircon ages of 334 ± 5 Ma and 332 ± 7 Ma were obtained for the Guluogou gabbros, while SHRIMP U–Pb zircon ages of 423 ± 10 Ma and 423 ± 4 Ma were determined for the Serikeyayilake and Aertengkesi gabbros. The mineral composition of the mantle peridotites and the geochemical characteristics of the basalts suggest a mid-ocean ridge (MOR) type affinity for the Guluogou ophiolite and a SSZ type affinity for the Serikeyayilake and Aertengkesi ophiolites. The present data, combined with previously published results, indicate that the STOB may have been derived from a forearc rifting setting that existed during the early Silurian to the early Carboniferous (ca. 439–356 Ma), whereas the SCTOB may represent fragments of a normal wide ocean that lasted to the early Carboniferous (ca. 332 Ma).

Provenance and tectonic settings of Permian turbidites from the Beishan Mountains, NW China: Implications for the Late Paleozoic accretionary tectonics of the southern Altaids

The Beishan orogenic belt, which connects the Tianshan suture on the west and Solonker suture on the east, contains key evidence for the termination time of the southern Altaids. Critical for evaluating different controversial tectonic models are Permian marine volcaniclastic arenites in the Liuyuan and Heishankou areas, which are dominated by greywackes and lenticular pebbly litharenites that contain grading, groove marks, and erosional bases, which provide evidence of turbidity action. Sandstones from the Liuyuan section are dominated by angular basaltic, andesitic, and feldspar fragments, but sandstones from the Heishankou section mainly consist of andesitic and felsic volcanic fragments. These relations suggest derivation from two different sources. Major element compositions suggest that the source rocks of the Heishankou litharenites were more SiO2-rich than those at Liuyuan. Sandstones at Heishankou are characterized by lower Ni–Co–Cr–V and slightly higher Th and La contents than those at Liuyuan. This indicates that the litharenites in the Liuyuan and Heishankou areas were derived from intermediate-mafic and intermediate-felsic source rocks, respectively. Tectonic setting discrimination plots suggest that the Liuyuan sandstones were deposited as detritus from an oceanic island arc, but the Heishankou sediments from an Andean continental margin. Our petrological and geochemical data from these two types of Permian turbidites suggest that an arc-continent collision took place in the Early Permian, and this is consistent with the Permian termination of the southern Altaids.

Palaeozoic tectonic evolution of the Tianshan belt, NW China

The Chinese Tianshan belt is a major part of the southern Central Asian Orogenic Belt, extending westward to Kyrgyzstan and Kazakhstan. Its Paleozoic tectonic evolution, crucial for understanding the amalgamation of Central Asia, comprises two stages of subduction-collision. The first collisional stage built the Eo-Tianshan Mountains, before a Visean unconformity, in which all structures are verging north. It implied a southward subduction of the Central Tianshan Ocean beneath the Tarim ac-tive margin, that induced the Ordovician-Early Devonian Central Tianshan arc, to the south of which the South Tianshan back-arc basin opened. During the Late Devonian, the closure of this ocean led to a collision between Central Tianshan arc and the Kazakhstan-Yili-North Tianshan Block, and subsequently closure of the South Tianhan back-arc basin, producing two su-ture zones, namely the Central Tianshan and South Tianshan suture zones where ophiolitic melanges and HP metamorphic rocks were emplaced northward. The second stage included the Late Devonian-Carboniferous southward subduction of North Tianshan Ocean beneath the Eo-Tianshan active margin, underlined by the Yili-North Tianshan arc, leading to the collision between the Kazakhstan-Yili-NTS plate and an inferred Junggar Block at Late Carboniferous-Early Permian time. The North Tianshan Suture Zone underlines likely the last oceanic closure of Central Asia Orogenic Belt; all the oceanic domains were consumed before the Middle Permian. The amalgamated units were affected by a Permian major wrenching, dextral in the Tianshan. The correlation with the Kazakh and Kyrgyz Tianshan is clarified. The Kyrgyz South Tianshan is equivalent to the whole part of Chinese Tianshan (CTS and STS) located to the south of Narat Fault and Main Tianshan Shear Zone; the so-called Middle Tianshan thins out toward the east. The South Tianshan Suture of Kyrgyzstan correlates with the Central Tianshan Suture of Chinese Tianshan. The evolution of this southern domain remains similar from east (Gangou area) to west until the Talas-Ferghana Fault, which reflects the convergence history between the Kazakhstan and Tarim blocks.

Paleozoic tectonics of the southern Chinese Tianshan: Insights from structural, chronological and geochemical studies of the Heiyingshan ophiolitic mélange (NW China)

In the southern Chinese Tianshan, the southernmost part of the Central Asian Orogenic Belt (CAOB), widespread ophiolitic mélanges form distinct tectonic units that are crucial for understanding the formation of the CAOB. However, the timing of tectonic events and the subduction polarity are still in controversy. In order to better understand these geological problems, a comprehensive study was conducted on the Heiyingshan ophiolitic mélange in the SW Chinese Tianshan. Detailed structural analysis reveals that the ophiolitic mélange is tectonically underlain by sheared and weakly metamorphosed pre-Middle Devonian rocks, and unconformably overlain by non-metamorphic and undeformed lower Carboniferous (Serpukhovian) to Permian strata. The igneous assemblage of the mélange comprises OIB-like alkali basalt and andesite, N-MORB-like tholeiitic basalt, sheeted diabase dikes, cumulate gabbro and peridotite. Mafic rocks display supra-subduction signatures, and some bear evidence of contamination with the continental crust, suggesting a continental marginal (back-arc) basin setting. Zircons of a gabbro were dated at 392 ± 5 Ma by the U–Pb LA-ICP-MS method. Famennian–Visean radiolarian microfossils were found in the siliceous matrix of the ophiolitic mélange. Mylonitic phyllite which displays northward-directed kinematic evidence yielded muscovite 40Ar/ 39Ar plateau ages of 359 ± 2 Ma and 356 ± 2 Ma. These new data, combined with previously published results, suggest that the mafic protoliths originally formed in a back-arc basin in the Chinese southern Tianshan during the late Silurian to Middle Devonian and were subsequently incorporated into the ophiolitic mélange and thrust northward during the Late Devonian to early Carboniferous. Opening of the back-arc basin was probably induced by south-dipping subduction of the Paleo-Tianshan Ocean in the early Paleozoic, and the Central Tianshan block was rifted away from the Tarim block. Closure of the back-arc basin in the early Carboniferous formed the South Tianshan Suture Zone and re-amalgamated the two blocks.

Mineral ages and P-T conditions of Late Paleozoic high-pressure eclogite and provenance of melange sediments from Atbashi in the south Tianshan orogen of Kyrgyzstan

Ages derived from various isotope systems in high-pressure (HP) rocks of the western Tianshan orogen of NW China have been interpreted as evidence for late Carboniferous and/or Triassic collision of the accretionary margin of the Central Asian Orogenic Belt (CAOB) with the Tarim Craton. In order to elucidate this controversy, we present new P-T data as well as Sm-Nd and 40Ar/39Ar cooling ages for an eclogite sample from Atbashi in the accretionary melange of the South Tianshan suture in Kyrgyzstan, some 500 km along strike to the west of the controversial locality in the upper Akeyazhi River Valley in NW China. A clockwise P-T path for the eclogite with peak pressures of 18 to 24 kbar at 520 to 600 °C is consistent with near-isothermal decompression and exhumation in a subduction zone before collision of the CAOB with the Tarim Craton. Geochemical data and an initial eNd value of ∼ +9 suggest an N-MORB protolith for the eclogite. The high-pressure mineral assemblage of the eclogite yielded a statistically robust Sm-Nd isochron age of 319 ± 4 Ma (2σ, 5 data points, MSWD = 0.4) for equilibration and closure of the Sm-Nd system during HP metamorphism. 40Ar/39Ar dating of phengite from the same sample yielded a cooling age of 316 ± 3 Ma (2σ) implying rapid exhumation. Docking of the Tarim Craton with the southern margin of the Middle Tianshan-North Tianshan blocks in Kyrgyzstan during the late Carboniferous is supported by widespread emplacement of A-type granitoids of early Permian age that suggest a setting of consolidated crust. An unmetamorphosed and little deformed molasse-type conglomerate of latest Carboniferous age, overlying the HP rocks, indicates that HP metamorphism, exhumation, and exposure of the HP melange occurred from 320 to ∼300 Ma. The detrital zircon age spectrum of a metagraywacke sample from the accretionary melange suggests sources in the Tarim Craton and/or from the Middle and North Tianshan that possibly comprise rifted blocks from Tarim.

Differential uplift of the Chinese Tianshan since the Cretaceous: constraints from sedimentary petrography and apatite fission-track dating

The Chinese Tianshan is one of the important international natural laboratories for studying continental geodynamics, but its uplift time, kinematics and mechanism of formation are key unresolved questions. In order to constrain the timing, kinematics and mechanism of uplift of the Chinese Tianshan, we applied sedimentary petrography and apatite fission-track dating to Cretaceous-Tertiary series from the northern Tarim Basin and the Tianshan Mountains. We collected and analyzed 79 sandstone samples and 75 detrital heavy mineral samples from the Kezilenuer-Kuqa profile (northern Tarim Basin) with a well-defined magnetostratigraphy. Our data indicate that detrital mineral maturity abruptly decreased at the disconformity between the Upper and Lower Cretaceous, and decreased again after 15(−12) Ma. The sediments in the northern Tarim Basin changed their provenance at 124 Ma, 26(−24) Ma and 15(−12) Ma, respectively. In addition, we collected and analyzed 36 primary apatite samples from the South, Central and North Tianshan. Our results show that the Chinese Tianshan underwent three phases of differential uplift. The first phase of uplift started at the southern Central Tianshan during the Early Cretaceous and propagated southward. The second phase of uplift started at the northern Central Tianshan during the Late Cretaceous and propagated also southward. The third phase of uplift started at the northern Central Tianshan during the Eocene and propagated both northward and southward. These differential uplifts have caused development of disconformity and drops in mineral maturity of detrital sediments in the northern Tarim Basin. Such a differential and heterogeneous uplift process might have been trigged by collisions of different microcontinents (i.e., Lhasa, Kohistan-Dras and India) at the southern margin of Asia. These collisions reactivated the South Tianshan Fault and then the North Tianshan Suture, and uplifted the Chinese Tianshan step by step.

Late Paleozoic tectonic evolution of the northern West Chinese Tianshan Belt

The northern West Chinese Tianshan is divided into three subunits: Carboniferous turbidite, ophiolitic mélange and Yili magmatic arc. Stratigraphical and petrological studies suggest that the turbidite and ophiolitic mélange form a subduction complex. The ophiolitic mélange that forms the North Tianshan suture was a result of intra-oceanic tectonism and subsequent redeposition and deformation during the subduction of the North Tianshan oceanic basin. The Yili arc-type granitoids are constained by single zircon U-Pb radiochronology between 361 and 309 Ma. The first-hand kinematic results on the deformed turbidite suggest that this suture zone was reworked by a Permian ductile dextral strike-slip fault. An evolutionary model of the study area allows three events to be distinguished: 1) Late Devonian to Carboniferous subduction of the oceanic basin below the Yili Block producing Yili magmatic rocks and subduction complex, 2) Late Carboniferous complete closure of this basin, 3) Permian right-lateral strike-slip faulting generating pull-apart basins and alkaline magmatism. A prominent reactivation during the Indo-Eurasia collision provoked the northward thrusting of the Paleozoic units upon the Cenozoic sediments of the Junggar Basin, consequently, hiding the bulk of this Late Paleozoic suture.

P–T evolution of glaucophane–omphacite bearing HP–LT rocks in the western Tianshan Orogen, NW China:new evidence for ‘Alpine‐type’ tectonics

AbstractThe late Palaeozoic western Tianshan high‐pressure /low‐temperature belt extends for about 200 km along the south‐central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P–T conditions of mafic garnet omphacite and garnet–omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high‐pressure rocks. Maximum pressure conditions are represented by the assemblage garnet–omphacite–paragonite–phengite–glaucophane–quartz–rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 °C. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca‐amphibole and chlorite. The post‐eclogite facies metamorphic conditions between 9 and 14 kbar at 480–570 °C suggest an almost isothermal decompression from eclogite to epidote–amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post‐eclogite facies conditions are evidence for a clockwise P–T path. Analysis of phase diagrams constrains the P–T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high‐pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to ‘Alpine‐type’ tectonics. This contradicts previous models which favour ‘Franciscan‐type’ tectonics for the southern Tianshan high‐pressure rocks.

P–T path of high‐pressure/low‐temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high‐pressure/low‐temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta‐sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km2 in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro‐glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre‐peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre‐peak lawsonite/epidote‐blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote‐blueschist to greenschist facies conditions. The P–T path indicates that the high‐pressure rocks formed in a B‐type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili‐central Tianshan plates.

Paleozoic tectonic evolution of the Tianshan Orogen, northwestern China

The Chinese Tianshan orogenic belt which extends west-east for about 1500 km is one of the most important Paleozoic orogenic belts in central Asia. It includes parts of the Tarim, Yili-Central Tianshan and Junggar plates, which are microcontinental blocks with Precambrian basements and Meso-Cenozoic depositional covers. The Tarim plate is separated from the Yili-Central Tianshan plate by the Southern Central Tianshan suture (Early Carboniferous). Ophiolitic melanges and blueschists are widespread in this suture and are believed to be produced by the collision between a passive continental margin on the north side of the Tarim plate and an active continental margin on the south side of the Yili-Central Tianshan plate. The collision occurred at the end of Early Carboniferous. The Yili-Central Tianshan and the Junggar plates are separated by the Northern Central Tianshan suture, also considered as an Early Carboniferous collisional zone, along which the Middle Ordovician ophiolitic melanges, blueschists and Late Devonian-Early Carboniferous ophiolites are exposed. In early Paleozoic times, the subduction of the paleo-Junggar ocean took place in an active margin on the north side of the Yili-Central Tianshan plate along the Northern Central Tianshan suture. At the end of Early Carboniferous, the north side of the Yili-Central Tianshan plate collided with the North Tianshan Arc along this suture. The paleo-oceanic crust in the Tianshan and adjacent region was entirely consumed at the end of Early Carboniferous. Meanwhile, the Tarim, Yili-Central Tianshan and Junggar plates were accreted to Eurasia. Epicontinental sediments were then deposited throughout the Tianshan region during the Middle-Late Carboniferous times. In the Early Permian, Tianshan began to uplift with accompanying volcanic eruption and granitic intrusion. The uplift was accelerated in the Late Permian and continental molasse was extensively deposited.

The mineralogy, petrology, metamorphic PTDt trajectory and exhumation mechanism of blueschists, south Tianshan, northwestern China

The south Tianshan blueschist belt extends for about 200 km along the south Tianshan suture from Changawuzhi to the upper Kekesu river. The high-pressure—low-temperature (high- P/low- T) metamorphic rocks contain sodic amphibole, garnet, phengite, albite, quartz, epidote, chlorite and stilpnomelane. These blueschists occur as small blocks, lenses, laminae and bedded slabs or sheets in greenschist facies country rocks. A few dismembered ophiolitic nappe slices and marble lenses are interlayered in the high- P/ T metamorphic belt. Metabasite blueschists are mainly E-type MORB, less commonly N-type. The protoliths of the meta-sedimentary rocks are graywackes, which contain volcaniclastic material of E-type MORBs, and probably some continental basement materials, derived from the Tarim plate. These protoliths may represent subduction complexes or meta-melanges, which formed within an accretionary wedge on the southern side of the Yili-Central Tianshan plate (YCTP) at the end of the early Paleozoic. The mineral assemblages include sodic amphibole (mainly crossite) + garnet + phengite + epidote + chlorite + albite, estimated to have crystallized at 450°C and 9–10 kbar. These conditions correspond to the epidote-blueschist facies and are the peak metamorphic conditions. The prograde metamorphism is represented by lawsonite-blueschist facies and the retrograde is greenschist facies. The metamorphic PTDt trajectory of the blueschists runs clockwise and reveals that the metamorphism revolved from laumontite facies (?), lawsonite-blueschist facies (?), epidote-blueschist facies (408 Ma?) to greenschist facies (345 Ma), during the creation and exhumation of the blueschists in the south Tianshan suture. We have recognized six stages of deformation in the blueschists. The earlier three stages were concomitant with metamorphism, the later three postdated the metamorphism. On the basis of the deformation characteristics of the blueschists and coherent rocks, the regional geology and the metamorphic PTDt path we discuss the exhumation mechanism of the blueschists. The post-collisional ductile thrust within the suture probably exhumed the blueschists and coherent rocks, allowing them to escape retrograde overprinting in the greenschist facies. The influence of erosion on the exhumation of blueschists could not be considered directly and quantitatively because of the absence of Devonian strata and the few exposures of Early Carboniferous clastic rocks, all without detritus of blueschists.