South Tianshan Ocean Research Articles

Geochronological and geochemical constraints for a middle Paleozoic continental arc on the northern margin of the Tarim block: Implications for the Paleozoic tectonic evolution of the South Chinese Tianshan

In order to reveal the tectonic evolution of the South Chinese Tianshan orogenic belt, we conducted structural, geochemical, and geochronological studies and identified granitic and volcanic rocks along the northern margin of the Tarim block. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of two samples from granitic plutons yielded crystallization ages from 404.8 ± 2.0 Ma to 388.1 ± 2.2 Ma, and two samples of dacite and rhyolite yielded ages of volcanism at ca. 404 Ma, highlighting a middle Paleozoic igneous event. Geochemical data suggest that these Devonian igneous rocks are metaluminous, high-K calc-alkaline felsic volcanic and plutonic rocks. All the samples display relatively enriched light rare earth element (LREE) contents, weak or no negative Eu anomalies, and relative depletion in Nb, Ta, Ti, and P, but they show enrichment in K, Rb, and Ba. In situ zircon Hf isotopic results show a positive range of e Hf (t). These geochemical and isotopic features suggest that this magmatic event probably occurred in an Andean-type arc. Combined with zircon U-Pb and Hf isotopic studies on the basement rocks, the results indicate multiphase Precambrian (ca. 2600–2300 Ma, 1900–1800 Ma, 1140–830 Ma) continental growth and minor involvement of basement in Devonian arc magmatism. Finally, we integrated the structural, geochemical, and geochronological data into a geodynamic model of the South Chinese Tianshan that emphasizes south-directed subduction of the South Tianshan Ocean along the northern margin of the Tarim block during the early to middle Paleozoic.

Early Paleozoic Tectonic Evolution of the South Tianshan Collisional Belt: Evidence from Geochemistry and Zircon U-Pb Geochronology of the Tie’reke Monzonite Pluton, Northwest China

We report an Early Paleozoic hornblende quartz monzonitic pluton from the Tie’reke region in the central South Tianshan Collisional Belt (STCB). Laser ablation ICP-MS U-Pb zircon dating reveals that the pluton was emplaced during the Late Silurian at ∼ Ma. Our data, together with those from coeval intrusive rocks in the eastern STCB and the eastern Northern Margin of the Tarim Block (NMTB), indicate a major late Early Paleozoic magmatic event in the region. This magmatic event is supported by a detrital zircon U-Pb age population of 462–395 Ma obtained from a Cenozoic sandstone sample from the Kangsu region and an Early Paleozoic metasandstone sample from the Jigen region. Geochemically, the Tie’reke pluton is intermediate in composition, with SiO2 contents ranging from 60.73 to 64.73 wt%, and belongs to the alkali-calcic and shoshonitic series. The pluton displays relative depletion of Nb, Ta, P, and Ti and enrichment of large-ion lithophile elements (Ba, K, and Rb), typical of continental arc–related igneous rocks. Whole-rock Sr-Nd and zircon Hf isotopic data reveal that the magma was derived dominantly from partial melting of the Paleoproterozoic continental crust, with input from juvenile materials from a depleted-mantle wedge. In general, geochronological, geochemical, and isotopic features of the Late Silurian igneous rocks in the present STCB and NMTB, coupled with detrital zircon U-Pb geochronological data from the two sedimentary rocks, suggest that the northern margin of the Paleozoic Tarim Block was an Andean-type active continental margin during Middle Ordovician to Middle Devonian time. Given the coeval magmatism in the Central Tianshan Block, which necessitates a northward subduction of the Paleozoic South Tianshan Ocean, we propose a double-subduction model for the evolution of the Paleozoic South Tianshan Ocean during the Late Ordovician to Middle Devonian period. During the Late Devonian to Middle Carboniferous, the northern margin of the Paleozoic Tarim Block was likely characterized by tectonomagmatic quiescence, whereas the Central Tianshan Block was still extensively affected by arc-type magmatism, furthering the northward subduction of the Paleozoic South Tianshan Ocean.

The Paleozoic evolution of Central Tianshan: Geochemical and geochronological evidence

A suite of Paleozoic granitoids in Central Tianshan was studied for both geochemistry and geochronology in an effort to constrain their origin and tectonic setting. We combined LA-ICP-MS dating of zircon, standard geochemical analyses and Hf-isotopic studies of zircon to develop our tectonic model. Based on our analysis, the granitoids formed in three distinctive stages: ~450–400Ma, ~370–350Ma and ca. 340Ma. The first stage (450–400Ma) granitoids exhibit metaluminous, magnesian, high-K to shoshonitic characteristics of I-type granitoids (arc-setting), that are enriched in LREE relative to HREE with high (La/Yb)CN values, show negative Eu anomaly and are depleted in Nb, Ta and Ti. This phase of granitoid emplacement was most likely related to the southward subduction of the Paleo-Tianshan Ocean beneath the Tarim block and the subsequent Central Tianshan arc. In contrast, the second stage granitoids (370–350Ma) are distinctly different and are classified as calc-alkaline or shoshonitic plutons with a weak positive Eu anomaly. Within the second stage granitoids, it appears that the earlier (~365Ma) granitoids fit within the A-type field whereas the younger (~352Ma) granitoids plot within the post-collisional potassic field. These granitoids formed during collisions between Central Tianshan and the Tuha terrane that occurred along the northern margin of Central Tianshan. Lastly, the ca. 340Ma granitoids are typical of volcanic arc granitoids again that probably formed during the northward subduction of the South Tianshan Ocean beneath the Central Tianshan landmass or the subsequent southward subduction of the residual Paleo-Tianshan Ocean.The Hf isotopic data of zircons from all the studied granitoids were pooled and yielded three prominent Hf TDMC model age populations: ca. 2400Ma, ca. 1400Ma and ca. 1100Ma. The Hf-data shows a significant input of juvenile crust in addition to crustal recycling. We interpret these three phases of juvenile crustal addition to phases of global growth of continental crust (~2400Ma), the addition of juvenile crust during the breakup of the Columbia supercontinent (~1400Ma) and the assembly of Rodinia (~1100Ma).

Late Permian to Triassic intraplate orogeny of the southern Tianshan and adjacent regions, NW China

The South Tianshan Orogen and adjacent regions of Central Asia are located in the southwestern part of the Central Asian Orogenic Belt. The formation of South Tianshan Orogen was a diachronous, scissors-like process, which took place during the Palaeozoic, and its western segment was accepted as a site of the final collision between the Tarim Craton and the North Asian continent, which occurred in the late Palaeozoic. However, the post-collisional tectonic evolution of the South Tianshan Orogen and adjacent regions remains debatable.Based on previous studies and recent geochronogical data, we suggest that the final collision between the Tarim Craton and the North Asian continent occurred during the late Carboniferous. Therefore, the Permian was a period of intracontinental environment in the southern Tianshan and adjacent regions. We propose that an earlier, small-scale intraplate orogenic stage occurred in late Permian to Triassic time, which was the first intraplate process in the South Tianshan Orogen and adjacent regions. The later large-scale and well-known Neogene to Quaternary intraplate orogeny was induced by the collision between the India subcontinent and the Eurasian plate. The paper presents a new evolutionary model for the South Tianshan Orogen and adjacent regions, which includes seven stages: (I) late Ordovician–early Silurian opening of the South Tianshan Ocean; (II) middle Silurian–middle Devonian subduction of the South Tianshan Ocean beneath an active margin of the North Asian continent; (III) late Devonian–late Carboniferous closure of the South Tianshan Ocean and collision between the Kazakhstan-Yili and Tarim continental blocks; (IV) early Permian post-collisional magmatism and rifting; (V) late Permian–Triassic the first intraplate orogeny; (VI) Jurassic–Palaeogene tectonic stagnation and (VII) Neocene–Quaternary intraplate orogeny.

Geochemistry and geochronology of Paleozoic intrusions in the Nalati (Narati) area in western Tianshan, Xinjiang, China: Implications for Paleozoic tectonic evolution

The Nalati (Narati) area in the Chinese western Tianshan is characterized by abundant Paleozoic intrusions, including granites, diorites and gabbros. They are not only indicators of the interaction between crust and mantle, but also useful clues for tracing the tectonic history of the Tianshan Orogen. Most Early Paleozoic granitoids (biotite monzonitic granites and muscovite granites) of this study are from the Yili Block. The biotite monzonitic granites (mixed-source-derived I-type granites) have a zircon U–Pb age of 497.0±5.9Ma, indicating the time of the subduction of the Terskey Oceanic crust in the Late Cambrian. The 427.2±5.7Ma Zircon U–Pb age of the S-type muscovite granites let us interpret that these granites may have been formed during the crust thickening process after the collision between the Yili Block and the Nalati Block. In western Tianshan the Late Paleozoic biotite granites, muscovite granites, quartz diorites, biotite monzonitic granites, granodiorites and alkali granites have respectively a LA-ICPMS zircon U–Pb age of 371.8±6Ma, 357.2±7.5Ma, 313.9±2.5Ma and 296.9±2.4Ma. The biotite granites display I-type geochemical features and are considered to have been derived from a lower continental crust source. The muscovite granites have a pronounced S-type affinity and are considered to have been formed by the partial melting of thickened continental crust after the collision between the Central Tianshan Belt and the South Tianshan Belt. The quartz-diorites are adakite-like and have an I-type affinity, which are considered to have been formed by partial melting of a delaminated lower crust in a post-orogenic extension setting. The granodiorites also show some typical geochemical features of adakite. Their formation is considered to be related to the lower crustal delamination and the ascending of asthenosphere after the collision event. The alkali granites show an A-type granite affinity. They may have been formed in a within-plate tectonic setting and possibly related to rifting as a consequence of extension.The gabbros can be divided into two groups based on their geochemical characteristics and ages. The older gabbros (415–429Ma) with lower TiO2 contents are formed in an arc-related setting. The younger ones (340–318Ma) with higher TiO2 content are formed in an intracontinental setting and influenced by crustal contamination.Magmatic activities recorded by Paleozoic intrusions can constrain the tectonic history of the Tianshan region from subduction of oceanic crust to intraplate extensional collapse and to rifting, and can also corroborate that the final closure of the South Tianshan Ocean occurred in the Late Devonian–Early Carboniferous time.

Paleozoic multiple accretionary and collisional tectonics of the Chinese Tianshan orogenic collage

Subduction-related accretion in the Junggar–Balkash and South Tianshan Oceans (Paleo-Asian Ocean), mainly in the Paleozoic, gave rise to the present 2400km-long Tianshan orogenic collage that extends from the Aral Sea eastwards through Uzbekistan, Tajikistan, Kyrgyzstan, to Xinjiang in China. This paper provides an up-to-date along-strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution.The northern part of the orogenic collage developed by consumption of the Junggar–Balkash Ocean together with Paleozoic island arcs (Northern Ili, Issyk Kul, and Chatkal) located in the west, which may have amalgamated into a composite arc in the Paleozoic in the west and by addition of another two, roughly parallel, arcs (Dananhu and Central Tianshan) in the east. The western composite arc and the eastern Dananhu and Central Tianshan arcs formed a late Paleozoic archipelago with multiple subduction zones. The southern part of the orogenic collage developed by the consumption of the South Tianshan Ocean which gave rise to a continuous accretionary complex (Kokshaal–Kumishi), which separated the Central Tianshan in the east and other Paleozoic arcs in the west from cratons (Tarim and Karakum) to the south. Cross-border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early Paleozoic to Permian as recorded by successive southward juxtaposition of ophiolites, slices of ophiolitic mélanges, cherts, island arcs, olistostromes, blueschists, and turbidites, which are mainly Paleozoic in age, with the youngest main phase being Late Carboniferous–Permian. The initial docking of the southerly Tarim and Karakum cratons to this complicated late Paleozoic archipelago and accretionary complexes occurred in the Late Carboniferous–Early Permian in the eastern part of the Tianshan and in the Late Permian in the western part, which might have terminated collisional deformation on this suture zone. The final stages of closure of the Junggar–Balkash Ocean resembled the small ocean basin scenario of the Mediterranean Sea in the Cenozoic. In summary, the history of the Altaids is characterized by complicated multiple accretionary and collisional tectonics.

The Paleozoic northern margin of the Tarim Craton: Passive or active?

Whether the northern Tarim Craton was an active or passive continental margin during the Paleozoic is vital to our understanding of the subduction polarity of paleo-oceanic plates in the Tianshan Orogen and the accretion history of the southwestern Central Asian Orogenic Belt. We identified undeformed granitoids intruding into the Paleoproterozoic metamorphic rocks in the Korla area at the northern Tarim Craton. Zircon LA–ICP-MS U–Pb dating of three samples from a porphyritic granodiorite pluton yields three indistinguishable crystallization ages: 420.6±2.3Ma (MSWD=1.02, n=27), 421.7±2.8Ma (MSWD=0.75, n=17) and 419.8±3.3Ma (MSWD=1.10, n=13), highlighting a late Early Paleozoic magmatic event. In-situ zircon Hf isotope data of the youngest zircon population (ca. 420Ma) vary in large ranges (up to 11 epsilon Hf units for single sample), with the lowest εHf (t) value (−14.2) approaching the evolution line of the Early Paleoproterozoic crust in the northern Tarim Craton, suggesting magma mixing between the old crust-derived magma and the relatively juvenile materials probably derived from depleted mantle. Geochemical data suggest that these granitoids are metaluminous, high K calc-alkaline granodiorite. They show a moderately differentiated REE pattern with insignificant Eu anomalies and are relatively depleted in Nb, Ta, Ti, P, but enriched in K and Ba, resembling Andean arc granite. In various discrimination diagrams, all samples consistently plot into the subduction-related areas. These geochemical and isotopic features suggest that this magmatic event probably occurred in an Andean-type continental arc. Therefore, if the northern Tarim Craton was a passive continental margin in the Early Paleozoic, it must have changed into an active margin at least since ca. 420Ma, probably as early as the Ordovician–Silurian boundary, necessitating a southward subduction of the South-Tianshan Ocean. Considering the coeval arc magmatism on the Central Tianshan Block, a divergent double subduction model for the South-Tianshan Ocean is tentatively proposed.

Subduction-induced mantle heterogeneity beneath Eastern Tianshan and Beishan: Insights from Nd–Sr–Hf–O isotopic mapping of Late Paleozoic mafic–ultramafic complexes

Extensive late Paleozoic mafic–ultramafic complexes in the Eastern Tianshan and Beishan regions in the southern Central Asian Orogenic Belt (CAOB) represent an ideal opportunity to assess the characteristics of the mantle in this area. Whole-rock Nd–Sr and zircon Hf–O isotopic data from the mafic–ultramafic complexes of the Jueluotage Belt, Middle Tianshan Massif and Beishan Rift were compiled for isotopic mapping. εNd(t) and (87Sr/86Sr)i values range from +5.5 to +9.5 and 0.702 to 0.706 respectively in the Jueluotage Belt, and from −2 to +11 and 0.703 to 0.710 respectively in the Middle Tianshan and Beishan. Zircon εHf (t) and δ18O values are in the range of +6.0–+17.0 and 4.14‰–8.00‰ respectively in the Jueluotage Belt, +1–+8 and 5.17‰–6.30‰ respectively in the Middle Tianshan, and show large variations of −1–+17 and 4‰–12‰ respectively in the Beishan. Spatially, the (87Sr/86Sr)i ratios of mafic–ultramafic complexes increase from the Jueluotage Belt and Beishan to the Middle Tianshan, while the εNd(t) values show an opposite trend and reflect a mixing character between depleted mantle and EM-II reservoirs. Zircon εHf (t) and δ18O values exhibit an apparent southward decreasing and increasing trends respectively, from the Jueluotage Belt, through Middle Tianshan, to Beishan Rift. Their corresponding Hf model ages increase continuously from north to south. These observations demonstrate that the mantle sources beneath the Eastern Tianshan and Beishan are compositionally heterogeneous, which presumably result from significant and variable degrees of subduction-related modification. We therefore suggest that the late Paleozoic subcontinental lithospheric mantle beneath the Jueluotage Belt was modified by slab-derived melts and fluids during the subduction of the Junggar Ocean, whereas the mantle beneath the Beishan Rift was influenced mainly by slab-derived fluids during the subduction of the South Tianshan Ocean. The unique similarities of the Middle Tianshan mafic–ultramafic complexes reflect a two-stage modification of their mantle source, namely the sequential subduction of the early South Tianshan Ocean, followed by subduction of the Junggar Ocean. Thus, the subduction-induced mantle heterogeneity beneath the CAOB is probably regional in scope.

Tectonic evolution of the South Tianshan orogen and adjacent regions, NW China: geochemical and age constraints of granitoid rocks

Geochemical and geochronological evidence was obtained from granitoids of the South Tianshan orogen and adjacent regions, which consist of three individual tectonic domains, the Kazakhstan–Yili plate, the Central Tianshan Terrane and the Tarim plate from north to south. The Central Tianshan Terrane is structurally bounded by the Early Paleozoic ‘Nikolaev Line–North Nalati Fault’ and Late Paleozoic ‘Atbashy–Inyl’chek–South Nalati–Qawabulak Fault’ zones against the Kazakhstan–Yili and Tarim plates, respectively. The meta-aluminous to weakly peraluminous granitic rocks, which are exposed along the Kekesu River and the Bikai River across the Central Tianshan Terrane, have a tholeiitic, calc-alkaline or high-potassium calc-alkaline composition (I-type). Geochemical trace element characteristics and the Y versus Rb–Nb or Y versus Nb discrimination diagrams favor a continental arc setting for these granitoid rocks. SHRIMP U–Pb and LA-ICP-MS U–Pb zircon age data indicate that the magmatism started at about 480 Ma, continued from 460 to 330 Ma and ended at about 275 Ma. The earlier magmatism (>470 Ma) is considered to be the result of a simultaneous southward and northward subduction of the Terskey Ocean beneath the northern margin of the Tarim plate and the Kazakhstan–Yili plate, respectively. The later magmatism (460–330 Ma) is related to the northward subduction of the South Tianshan Ocean beneath the southern margin of the Kazakhstan–Yili–Central Tianshan plate. The dataset presented here in conjunction with previously published data support a Late Paleozoic tectonic evolution of the South Tianshan orogen, not a Triassic one, as recently suggested by SHRIMP U–Pb zircon dating for eclogites.

Ultrahigh-Pressure Metamorphic Belts in China: Major Progress in the Past Several Years

The Triassic Dabie-Sulu collision zone along with recently identified ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic terranes of the western Tianshan, Altyn Tagh, northern Qaidam, and northern Qinling belts forms a huge chain of orogens from northwestern to east-central China. The western Tianshan HP/UHP terrane formed by Pacific-type subduction of a Paleozoic South Tianshan ocean, and was exhumed to higher crustal levels during collision of the Tarim and Yili-central Tianshan plates. HP/UHP recrystallization took place either at ~345 Ma or 232-215 Ma. The Altyn Tagh, northern Qaidam, and northern Qinling UHP belts are Early Paleozoic collisional orogens. Coesite-bearing eclogites, garnet peridotites, and the country rock gneisses in the Dabie-Sulu UHP terrane were subjected to in situ UHP metamorphism based on the presence of abundant coesite inclusions in zircons and identical metamorphic ages in both eclogite and gneiss. Some garnet peridotites are crust-hosted whereas others were mantle derived. The later are depleted mantle fragments that experienced mantle metasomatism and subduction-zone UHP metamorphism. P-T estimates for eclogites and garnet peridotites combined with investigations of mineral exsolution textures and high-P polymorphs indicate that recovered depths of continental subduction in China reached 200 km or more.

Paleozoic accretionary and collisional tectonics of the eastern Tianshan (China): Implications for the continental growth of central Asia

This paper deals with the various tectonic units in the Chinese Eastern Tianshan orogenic collage in the Central Asian Orogenic Belt, and discusses the Paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the Tarim and southern Angaran continental margins. The Late Ordovician-Silurian to Early Devonian Eastern Tianshan archipelago was characterized by (a) the Harlik-Dananhu subduction system with a S-dipping polarity in the north; (b) a southerly N-dipping subduction system beneath the Central Tianshan arc in the middle; and (c) the South Tianshan ocean against Tarim in the south. During the Devonian to Early Carboniferous, N-dipping subduction led to the Harlik-Dananhu arc and the Kanggurtag forearc basin/accretionary complex. In the Early to Mid-Carboniferous, the magmatic front associated with the N-dipping subduction beneath the Dananhu-Harlik arc migrated southwards, forming the Yamansu arc constructed upon the Kanggurtag accretionary forearc. By the Late Carboniferous the Dananhu-Harlik arc was attached northwards to the Angaran margin, resulting in lateral enlargement of the Angaran continent. In the latest Carboniferous to Early Permian a multiple soft collision left wide suture zones in the south that include the ophiolite-strewn Aqikkuduk-Shaquanzi and Kumishi accretion-collision complexes, which were stitched by Early Permian post-collisional plutons. By re-defining and re-interpreting the various tectonic terranes, this paper presents a new, improved model for the Paleozoic evolution of this part of Central Asia.

Formation of HP–LT rocks and their tectonic implications in the western Tianshan Orogen, NW China: geochemical and age constraints

The Western Tianshan high-pressure/low-temperature metamorphic belt in northwestern China formed within a Paleozoic accretionary wedge on the south side of the Yili–Central Tianshan plate. It comprises various lithologies, including mafic metavolcanic rocks, metavolcaniclastic rocks, metagreywacke, marble and serpentinite. The metavolcanics comprise eclogite, omphacitite, epidosite and blueschist. The trace and rare-earth element characteristics of these rocks indicate MORB to OIB affinities suggesting a seamount-like setting for the protoliths of these rocks. The ε Nd value of the eclogite (+8.9) indicates that the protolith was derived from a long-term depleted mantle, i.e. an N-MORB source, supporting the trace and rare-earth element characteristics of these samples. Sm–Nd isochron dating of 343±44 Ma (Omp–Gln–Grt–WR) and an age of 346±3 Ma (Grt–Gln) were determined using an eclogite boudin within blueschist. A well-defined 40Ar/ 39Ar plateau age of 344±1 Ma was obtained for crossite from an omphacite–phengite-bearing blueschist and is concordant with the Sm–Nd isochron age for the eclogite, suggesting that neither argon loss nor excess argon affected the crossite. A well-defined 40Ar/ 39Ar plateau age of 331±2 Ma was determined for phengite from the same sample. The high-pressure metamorphic rocks were formed by the B-type subduction of a Paleozoic ‘South Tianshan Ocean’ about 344 Ma ago. They were, however, exhumed to higher crustal levels during the collision of the Tarim and Yili–Central Tianshan plates about 331 Ma ago, similar to an ‘Alpine-type’ tectonic regime.

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.

Vol. 27, nos. 1–8 author index

The Chinese Tianshan occupies the southernmost part of the Central Asian Orogenic Belt (CAOB). High-grade metamorphic rocks are widely distributed in its central uplift zones, composing the crustal basement of the Central Tianshan Block of the Chinese Tianshan. However, the origin of the crustal basement and whether the high-grade metamorphism was associated with the Paleozoic orogeny are still unresolved. Here, we present precise LA-ICP-(MC)-MS zircon U–Pb dating and Hf isotopic data for three meta-sedimentary rocks and one orthogneiss from the Xingxingxia Complex, which represents the Precambrian basement of the Central Tianshan Block. Zircon U–Pb dating results show that the protolith age of the orthogneiss is ca. 880 Ma and that the meta-sedimentary rocks consist dominantly of 0.8–1.0 Ga and 1.3–2.0 Ga materials with minor early Paleoproterozoic components. In combination with the zircon Hf isotopic data, it is suggested that the crustal basement of the Central Tianshan Block was formed in the early Mesoproterozoic, which is evidently different from the Archean basement formation of the Tarim Craton. We suggest that the Central Tianshan Block may have formed through a magmatic arc accretionary orogen along the continental margin of Baltica during the Mesoproterozoic. Moreover, both the meta-sedimentary rocks and the orthogneiss of the Xingxingxia Complex simultaneously suffered a metamorphic overprint at ca. 380 Ma. Zircon REE and Hf isotopic data indicate that the metamorphic zircon rims were formed by new zircon overgrowths in partial melts. Geothermobarometry and average P–T calculations using THERMOCALC on the Grt–Bt gneiss yielded ca. 720–730 °C and 4–6 kbar, suggesting amphibolite- to granulite-facies conditions. The Late Devonian metamorphism of the Xingxingxia Complex is possibly related to orogenic low-pressure/high-temperature metamorphism in the middle or upper crust during the closure of the eastern segment of South Tianshan Ocean. Our new data, together with the occurrences of Ordovician–Silurian eclogites and HP mafic granulites along the northeastern margin of the Tarim Craton, indicate that progressive continental collisions occurred in the southern CAOB during the Paleozoic.