Southernmost Central Asian Orogenic Belt Research Articles

Permian tectonic switch of the southern Central Asian Orogenic Belt: Constraints from magmatism in the southern Alxa region, NW China

Abstract Late Paleozoic plutons are widely distributed in the Alxa region, southernmost Central Asian Orogenic Belt, and provided an important clue in constraint for the closing time of the Paleo-Asian Ocean (PAO). In this article, we present new zircon U-Pb ages and whole-rock geochemical data from the Permian Huoersen and Zongnaishan plutons in the southern Alxa region. The Huoersen gabbro (ca. 285 Ma) is enriched in large-ion lithophile elements and depleted in high-field strength elements, similar to the features of continental marginal arc. They were most likely generated by partial melting of depleted mantle that was modified by subduction metasomatic fluids. The Zongnaishan granites (ca. 267 Ma) show characteristics of I-type granites and were generated in a syn-collision setting. The Huoersen granites (ca. 259 Ma) are peralkaline and have positive Ɛ Nd(t) (+1.2 to +1.5) values, exhibiting A2-subtype granites affinities. They were formed by melting of lower crust in post-collision extension setting. Based on geodynamic mechanism, a three-stage evolution model is delineated: subduction, syn-collision to post-collisional extension for oceanic branch of the PAO during the Permian. In general, the rock assemblages indicate a tectonic switch from subduction to post-collision extension regimes and the final closure of the PAO.

Geochronology, geochemistry, and Sr–Nd isotopes of Permian dike swarms in the Beishan orogenic belt, southernmost Central Asian Orogenic Belt: Petrogenesis and implications for geodynamic setting

ABSTRACT Permian mafic dikes crop out widely within Beishan area in the southernmost Central Asian Orogenic Belt (CAOB). These mantle-derived products provide important clues for elucidating the genesis and geodynamic setting of Permian magmatism. The mafic dikes within the Beishan area display zircons U–Pb ages of 281 ± 4 Ma. Characterized by low SiO2 (44.88–47.73 wt.%) and high Mg# values (62.2–68.6), the type 1 dikes are relatively depleted in light rare earth elements (LREEs; (La/Yb)n = 0.72–1.43) but enriched in large ion lithophile elements (LILEs). They show high positive εNd(t) values (6.6–8.5) with a range of (87Sr/86Sr) i (0.703477–0.705789) ratios. In contrast, the type 2 dikes have higher SiO2 content (47.47–51.13 wt.%) and lower Mg# (40.9–63.0). They are enriched in LREEs ((La/Yb)n = 1.79–5.02) and LILEs, and have lower εNd(t) values (3.8–7.4) with a variable range of (87Sr/86Sr) i (0.702873–0.706949) ratios. Thus, the type 1 dikes resulted from lower degrees of fractional crystallization and contamination than the type 2 dikes. Considering the LILEs and Pb enrichments, high field strength elements depletions, and high Ba/Th (86–460) ratios, the mantle source was metasomatized by subduction-related fluids. These dikes should be derived from high degrees (>20%) of partial melting of a Sr–Nd isotope-depleted, phlogopite-bearing spinel lherzolite lithospheric mantle according to their high Rb/Sr (0.04–0.24), low Ba/Rb (0.9–3.3), low La/Sm, and constant Sm/Yb ratios in addition to Sr–Nd depletion. Combined with the coeval evidence of regional geology and novel tectonic discriminate diagrams, we suggest that the mafic dikes in Beishan area formed in arc-related setting rather than intracontinental setting during the Permian. The metasomatized lithospheric mantle by subduction-related fluids were heated by upwelling asthenosphere mantle triggered by the slab roll-back, resulting in the formation of the parental magma of the mafic dikes.

Zircon U-Pb-Hf Isotopes, Biotite 40Ar/39Ar Geochronology, and Whole-Rock Geochemistry of the Baogeqi Gabbro in the Northern Alxa, Southernmost Central Asian Orogenic Belt

The final closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic settings in the northern Alxa are very important but controversial tectonic issues. The geochronology and petrogenesis of mafic igneous rocks are superior in clarifying regional tectonic settings. Here, we report on zircon U-Pb-Hf isotopes, biotite 40Ar/39Ar geochronology and whole-rock geochemical data of the hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa. The LA-ICP MS U-Pb analysis of zircon grains from the hornblende gabbro yield a weighted mean age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), manifesting that the Baogeqi gabbro pluton emplacement was during the late Middle Permian (Capitanian). The 40Ar/39Ar dating of biotite grains from the hornblende gabbro yields a plateau age of 231.3 ± 1.6 Ma (2σ, MSWD = 0.55), indicating that the Baogeqi gabbro pluton cooled to below 300 ℃ in the Triassic. The hornblende gabbro samples are calc-alkaline with metaluminous character, and show enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr, and K) but depletion in Nb, Ta, P, Th, and Ti relative to primitive mantle. Combined with the positive zircon εHf(t) values (+4.9–+9.4), we suggest that the magmas formed from the partial melting of depleted mantle were metasomatized by slab-derived fluids. Together with regional geology, these geochemical data suggest that the Baogeqi gabbro pluton was formed in an intracontinental extension setting, further indicating that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region was in a post-collision extensional setting during the Capitanian-Late Permian. In addition, the Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.

Structures and chronology of the Yabrai shear zone in the Alxa, NW China: Constraints on the late Paleozoic shear system in central segment of the Central Asian Orogenic Belt

Large-scale strike-slip faults often develop in the collisional and accretionary orogenic belts around the world. The Central Asian Orogenic Belt (CAOB) contains numerous strike-slip faults and provides opportunities to decipher the relative movements among different components. Here, we report new data from the ENE-trending Yabrai dextral ductile shear zone in the Alxa region (NW China) of the southernmost CAOB. We performed field mapping, structural analysis, and muscovite 40Ar/39Ar dating to constrain the geometries, kinematics and timing of the shear zone. The Yabrai shear zone crops out for more than 150 km, and the kinematic indicators from the outcrops, microstructures, and quartz c-axis fabrics suggest a consistent dextral shear motion. The microstructures and EBSD results indicate intermediate-to low-temperature deformation (∼300–530 °C). Muscovite 40Ar/39Ar dating from mylonitic samples yield ages of 254–251 Ma, which imply that the shear zone was active in the Late Permian. The displacement of Yabrai shear zone is estimated to be ca. 30 km. An integration of spatiotemporal patterns of shear zones and regional aeromagnetic data suggests that a giant “S–C”-type shear system occurred in the Alxa region in the Late Permian. Based on this work and previous studies, a crustal-scale strike-slip system of late Paleozoic is developed between the CAOB and the southern cratons. It formed in a transcurrent regime after the closure of the Paleo-Asian Ocean, which was a response to the deformation of the whole CAOB in the end of late Paleozoic.

The Carboniferous-Permian tectonic setting for the southernmost Central Asian Orogenic Belt: Constraint from magmatic and sedimentary records in the Alxa area, NW China

The northern Alxa orogenic belt, a junction between the western and eastern Central Asian Orogenic Belt (CAOB), is not well understood, especially in its Carboniferous to Permian tectonic history. In this paper, an integrated study incorporating the geochronology, geochemistry, and isotopes of magmatic rocks and provenance analysis of clastic rocks is conducted to address the above issue. The zircon UPb age data, both in this study and in previous studies, indicates that the Permian magmatic activities in the Alxa Block were intensive and peaked at ca. 276 Ma. The magmatic zircons yield negative εHf(t) values with a wide range, indicating their dominant derivation from the crustal reworking of the Alxa Block with the addition of juvenile materials. The Permian high-K to shoshonitic, I-type granitoids combined with the A-type granitoids/rhyolites were most likely formed in extensional settings. The late Carboniferous sandstones collected along the Badain Jaran fault yield a high proportion of early Paleozoic zircons peaked at ca. 429 Ma with both positive to negative Hf(t) values and a low proportion of Precambrian zircons with positive to negative Hf(t) values. Thus, their provenance were likely the Precambrian to Paleozoic rocks in the Alxa Block and in the terranes in the CAOB. The zircon age spectra with no indicators of Carboniferous zircons approximating their depositional age supports an extensional setting. The Permian A-type granites and volcanic rocks in the Alxa area have some geochemical affinity with the Permian alkaline granitoids in the eastern Tianshan–Beishan and central-north Inner Mongolia regions. These findings support the extension of the Gobi–Tianshan belt in the Alxa area.

Permian oceanic slab subduction in the southern Beishan: Reply to comment by Liu et al. on “Permian oceanic slab subduction in the southernmost Central Asian Orogenic Belt: Evidence from adakite and high-Mg diorite in the southern Beishan”

Permian oceanic slab subduction in the southern Beishan: Reply to comment by Liu et al. on “Permian oceanic slab subduction in the southernmost Central Asian Orogenic Belt: Evidence from adakite and high-Mg diorite in the southern Beishan”

Origins of the meta‐mafic rocks in the southern Dunhuang Block (NW China): Implication for tectonic framework of the southernmost Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogenic collages in the world with prominent juvenile crust addition, where the southernmost margin of the CAOB located is one hot topic in the international studies. The Dunhuang Block located to the south of the Beishan Orogen is a key position for outlining the tectonic framework of the south margin of the CAOB. However, at present, the tectonic attribution of the Dunhuang Block is still controversial. In this paper, we focus on the origins and protoliths of the meta‐mafic rocks from the Hongliuxia complex in the southern Dunhuang Block for determining the tectonic attribution of the Dunhuang Block. In the field, the meta‐mafic rocks are mainly exposed as tectonic blocks within the metasedimentary rocks displaying the “block‐in‐matrix” fabrics or as tectonic slices juxtaposed by faults. Geochemically, the meta‐mafic rocks could be divided into three groups based on the rare earth elements (REE) and trace elements: the group 1 samples are depleted in the LREE relative to HREE and show flat HREE patterns, similar to the mid‐oceanic ridge basalt; the group 2 samples show flat REE patterns without obvious differentiation between the LREE and HREE, like the oceanic plateau basalt; the group 3 samples are enriched in LREE relative to LREE, and have negative Nb, Ta, and Ti anomalies, similar to the island arc basalt. Conclusively, the protoliths of the meta‐mafic rocks dominantly belong to the oceanic plate components. They were metamorphosed during Early Silurian to Middle Devonian in different depths of the subduction zone documented in previous studies. So, we suggest the Hongliuxia complex is an exhumed Palaeozoic subduction–accretion complex (SAC) formed during the oceanic plate subduction. This result confirms the Palaeozoic subduction event occurred in the Dunhuang region and indicates that the Dunhuang Block is a Palaeozoic Orogen rather than a part of Precambrian continent block. Considering the regional geology context, we speculate the Dunhuang Orogen belongs to the southernmost margin of the CAOB.

Missing adakitic granite and syn-subduction mafic dikes within Permian volcanic belts of the southern margin of the CAOB? Comment on “Permian oceanic slab subduction in the southernmost Central Asian Orogenic Belt: Evidence from adakite and high-Mg diorite in the southern Beishan”

The petrogenesis of adakite and adakitic rocks and their tectonic settings are debated. Adakites may be derived by many processes which include: partial melting of subducted oceanic slab or lower crustal material; fractional crystallization of basaltic magma; magma mixing processes associated with arc-related and non-arc tectonic settings. It may be inaccurate to assume that adakitic rocks are solely derived from subducted oceanic slabs. The development of accretionary orogens, as it relates to adakite formation, is a topic of significant interest. Geochemical data coupled with field observations are vitally important to address the interplay between tectonics and magma genesis. In this comment, we discuss the petrogenetic processes and possible tectonic setting of the “adakite” in the southern Beishan region of the Central Asian Orogenic Belt (CAOB), to provide insight on the use of geochemical data to discriminate the tectonic setting of their formation. Based on our field observations, we disagree with the model presented by Zheng et al. (2020) as it pertains to the existence of an “ophiolitic mélange” and the evidence of a Permian arc system but rather, advocate that the adakitic rocks at Beishan are generated at an extensional setting.

Recognition of a Permian to Triassic foreland basin in the central Beishan Orogen, NW China: Provenance variations and their constraints on latest Palaeozoic orogeny

The Beishan Orogen, located in the southernmost Central Asian Orogenic Belt, is a key area for research on accretionary orogens. The formation of the Beishan Orogen is closely related to the evolution of the Paleo-Asian Ocean, but the closure time of a significant branch of the Paleo-Asian Ocean in the Beishan area, i.e., the Hongliuhe–Niujuanzi–Xichangjing (HNX) Ocean, remains controversial. This study investigates the sedimentary sequences of Baihu Basin adjacent to the HNX suture zone and presents new detrital zircon UPb geochronology and whole-rock geochemistry data to reveal depositional ages, provenance variations, paleogeography changes and attributes of Baihu Basin. Detrital zircon ages indicate that deposition began in the Permian and lasted until the Triassic. Two coarse sandstones in Member 1 and one siltstone in Member 2 contain abundant detrital zircons older than 700 Ma, in addition to the typical age peak that characterises the Gongpoquan Arc at ca. 450 Ma, while two sandstones in Member 3 contain rare detrital zircons older than 700 Ma. The whole-rock geochemistry shows that the lower part of the Hongyanjing Formation possesses numerous old components and higher chemical index of alteration values. The sediment in the lower part of the Hongyanjing Formation was derived from both the Gongpoquan Arc, mainly comprising Palaeozoic arc-related magmatic rocks, and the Shuangyingshan Arc, including large quantities of Precambrian meta-sedimentary rocks. The upper part originated solely from the juvenile Gongpoquan Arc. During the collision, the Shuangyingshan Arc played a role similar to a craton. Craton-derived source rocks and fining-upward sequences in the lower part indicate uplift of the Shuangyingshan Arc, which may represent the development of a forebulge and foredeep. In the upper part, the source rocks were derived from the Gongpoquan Arc and accretionary wedges, with coarsening-upward sequences reflecting the springback of the Shuangyingshan Arc and disappearance of the forebulge. Based on the sedimentary sequences, provenance variations, and paleogeographic changes, we suggest that Baihu Basin is a foreland basin associated with the collision between the Gongpoquan Arc and Shuangyingshan Arc, marking the final closure of the HNX Ocean from the Late Permian to Early Triassic. We investigate the closure of the HNX Ocean and reconstruct the evolution of the central Beishan Orogen in the youngest Palaeozoic.

Duobagou Permian–Triassic granites from the Dunhuang orogenic belt, NW China: implications for the tectonic evolution of the southernmost Central Asian Orogenic Belt

AbstractThe Duobagou Permian–Triassic granites of the Dunhuang orogenic belt are of great importance in understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt. LA-ICP-MS U–Pb zircon ages indicate that Permian–Triassic granitic intrusions from the Duobagou area formed at 276–274 Ma and 246 ± 1 Ma. These granites have high SiO2, Na2O and K2O, but low Al2O3, CaO and MgO contents and belong mainly to the high-K calc-alkaline I-type granite series. Based on whole-rock geochemistry and Sr–Nd and zircon Hf isotopes, the Duobagou Permian–Triassic granites were dominantly derived from the partial melting of lower continental crust formed during late Palaeoproterozoic to Mesoproterozoic times in a post-collisional extensional setting. Permian granites with zircon ϵHf(t) values of −5.4 to +3.1 and Hf model ages of TDM2 = 1.14–1.70 Ga indicate the involvement of a mantle component in their petrogenesis. Triassic granites with higher zircon ϵHf(t) values (+0.5 to +3.8) and TDM2 = 1.08–1.31 Ga suggest more juvenile sources caused by a greater contribution of mantle-derived melts, indicating a significant crustal growth. Regional extension from lithospheric delamination and heating from asthenospheric upwelling were proposed to have triggered the partial melting of lower crust, resulting in the generation of the Permian–Triassic magmatism. This may have been the mechanism for the significant crustal growth during Permian and Triassic times in the southernmost Central Asian Orogenic Belt.

Middle Cambrian granites in the Dunhuang Block (NW China) mark the early subduction of the southernmost Paleo-Asian Ocean

The formation and evolution of the Central Asian Orogenic Belt (CAOB) were closely associated with the opening and closure of the so-called “Paleo-Asian Ocean” (PAO). The subduction of the southern PAO is expected to have resulted in widespread Paleozoic arc magmatism in the southernmost CAOB. However, the precise location and timing of such subduction remain uncertain. The Dunhuang Block (DHB) in the southernmost CAOB is thought to have recorded such subduction. In this contribution, we report the results of the zircon U–Pb ages, Hf isotopes, and whole-rock geochemical data for the Lianghu, Dapokouzi, and Xiaowan plutons in the northeastern DHB, NW China. Zircon U–Pb dating reveals that these plutons formed at ca. 510 Ma (the Middle Cambrian) and may represent the oldest arc-related magmatism in the DHB. Geochemically, the Dapokouzi pluton is characterized by high SiO2 (70.7–71.2 wt%), Na2O (5.11–5.26 wt%), Al2O3 (15.5–15.6 wt%), and Sr (730–733 ppm) contents, high Sr/Y (380–398) ratios, moderate Mg# (49–50) values, low MgO (0.61 wt%), K2O (2.00–2.01 wt%) and Y (1.84–1.93 ppm) contents, and weakly positive Eu (1.39–1.44) anomalies, indicating geochemical affinities of adakitic rocks. They have positive εHf(t) values (+11.2 to +13.9) close to the depleted mantle line and young two-stage model ages (TDM2 = 571–742 Ma). All the geochemical characteristics show that the Dapokouzi pluton was derived from the remelting of juvenile crust with addition of minor metasomatized mantle-derived materials and a residue phase dominated by garnet-bearing amphibolite. Similarly, the Lianghu and Xiaowan plutons have high SiO2 (70.2–74.4 wt%) and Al2O3 (12.8–14.3 wt%), low MgO (0.61–1.01 wt%) with Mg# values of 35–43 and K2O (0.43–1.55 wt%) contents. Compared to the Dapokouzi pluton, they exhibit low Sr (121–155 ppm) and Y (15.3–23.1 ppm) contents, low Sr/Y (5.25–9.54) ratios and Cr-Ni-Co concentrations, and pronouncedly positive εHf(t) values (+11.0 to +14.7), indicating that the magmas originated from the remelting of juvenile lower crust followed by significant fractional crystallization at relatively shallow crustal depths. Data from this and previous studies convincingly suggest the DHB was located in a thickened continental arc setting during the early Paleozoic, probably related to the early subduction of the PAO lithosphere. Crustal thickening, as indicated by the geochemical signature of the Dapokouzi pluton, was most likely associated with the underplating of mantle-derived mafic magmas during the slab subduction. This study therefore highlights that the subduction of the southern PAO might have initiated prior to 510 Ma, possibly dating back to the early Cambrian.

A mid-Palaeozoic ocean–continent transition in the Mazongshan subduction–accretion complex, Beishan, NW China: new structural, chemical and age data constrain the petrogenesis and tectonic evolution

AbstractAccretionary orogens contain key evidence for the conversion of oceanic to continental crust. The late tectonic history and closure time of the Palaeo-Asian Ocean are recorded in the Mazongshan subduction–accretion complex in the southern Beishan margin of the Central Asian Orogenic Belt. We present new data on the structure, petrology, geochemistry and zircon U–Pb isotope ages of the Mazongshan subduction–accretion complex, which is a tectonic mélange with a block-in-matrix structure. The blocks are of serpentinized peridotite, basalt, gabbro, basaltic andesite, chert and seamount sediments within a matrix that is mainly composed of fore-arc-trench turbidites. U–Pb zircon ages of two gabbros are 454.6 ± 2.5 Ma and 434.1 ± 3.6 Ma, an andesite has a U–Pb zircon age of 451.3 ± 3.5 Ma and a tuffaceous slate has the youngest U–Pb zircon age of 353.6 ± 5.1 Ma. These new isotopic ages, combined with published data on ophiolitic mélanges from central Beishan, indicate that the subduction–accretion of Beishan in the southernmost Central Asian Orogenic Belt lasted until Late Ordovician – Early Carboniferous time. Structure and age data demonstrate that the younging direction of accretion was southwards and that the subduction zone dipped continuously to the north. Accordingly, these results record the conversion of oceanic to continental crust in the southern Beishan accretionary collage.

Provenance of the Permo–Carboniferous sediments in the northern Alxa and its tectonic implications for the southernmost Central Asian Orogenic Belt

As the southernmost segment of the Central Asian Orogenic Belt (CAOB), the northern Alxa orogenic belt (NAOB) connects the southeastern and southwestern segments of the CAOB. The NAOB amalgamated with the closure of the Paleo-Asian Ocean; however, the closure time of the Paleo-Asian Ocean is still on great debate. In this study, we reported new detrital zircon U–Pb geochronology and Hf–O isotopes for the Permo–Carboniferous sediments in the northern Alxa to constrain the provenance and its tectonic implications. The Permo–Carbonifereous Amushan Formation is composed of volcanic-carbonite-clastic rocks and was deposited in a shallow marine environment. Based on the zircon U–Pb geochronology, the Amushan Formation was deposited in the late Carboniferous to early Permian, but some outcrops of volcanic and clastic rocks in the Quaganqulu area were likely formed in the middle to late Permian. The integrated zircon age spectrum for the clastic rocks shows a wide range from late Archean to Paleoproterozoic, Mesoproterozoic (with a peak age at 1458 ​Ma), early Neoproterozoic (with peak ages of 988 ​Ma and 929 ​Ma), early Paleozoic (with a peak age at 447 ​Ma) and late Paleozoic (with a peak age at 294 ​Ma). Combined with the zircon Hf–O isotopes, the provenance was considered to be the Alxa Block, the Shalazhashan terrane and the Zhusileng–Hangwula block (and the southern Beishan orogenic belt). The multiple source regions to the south and north of the Paleo-Asian Ocean indicate the closure of this ocean before the late Carboniferous. The absence or small proportion of depositional age-approximated zircons in most samples makes their age spectra similar to extensional basins. Combined with the intra-plate volcanism, the deposits were considered to be formed in extensional settings. Accordingly, after the closure of the Paleo-Asian Ocean, the NAOB stepped into an extensional stage.

Palaeozoic diorites from the south‐western Dunhuang terrane, NW China: Constraints on tectonic evolution of southernmost CAOB

The petrology, geochemistry, zircon U–Pb geochronology, and Hf isotopes of Duobagou diorite and Karatashtage diorite were studied to establish the evolution of Palaeozoic orogeny in the Dunhuang area in Western China. The weighted mean 206Pb/238U age of 448 ± 2 Ma in zircon grains from Duobagou diorite indicated an origin in the late Ordovician Period; age‐corrected εHf(t) values of −0.98 to +4.46 correspond to a two‐stage zircon Hf model age (TDM2) of 1,145 Ma to 1,477 Ma. Karatashtage diorite has a weighted mean 206Pb/238U age of 436 ± 3 Ma, indicating an origin in the early Silurian Period; age‐corrected εHf(t) values of −1.13 to +7.96 correspond to a two‐stage zircon Hf model age (TDM2) of 718 Ma to 1,301 Ma. These findings indicate that Duobagou and Karatashtage diorites were derived from ancient crust. Duobagou diorite has high levels of Al2O3 (15.76%~17.64%), MgO (2.18%~5.44%), Mg# (40~51), Na2O (2.78%~4.19%), Sr (466~1,160 ppm), and Ba (523~892 ppm), low levels of K2O (1.29%~3.13%) and TiO2 (0.64%~1.0%), variable levels of Cr (73.4~253 ppm) and Ni (42.2~98.8 ppm), and depleted HREEs. Karatashtage diorite is characterized by high levels of Al2O3 (16.33%~17.59%), MgO (4.15%~7.10%), Mg# (50~59), Na2O (3.39%~4.41%), Cr (73.4~253 ppm), Ni (42.2~98.8 ppm), Sr (466~1,160 ppm), and Ba (523~892 ppm), low levels of K2O (1.29%~3.13%) and TiO2 (0.64%~1.0%), and depleted HREEs. The Duobagou pluton and the Karatashtage pluton display arc signatures, suggesting that they were formed by the partial melting of a subducted slab under garnet–amphibolite facies in an active continental margin, and an interaction between melt and mantle materials during emplacement. Subduction of the oceanic plate in the Dunhuang terrane occurred during the Early Silurian Period (435–430 Ma), which is close to that of the Beishan orogenesis but later than the formation of the Hongliugou–Lapeiquan ophiolitic belt. We suggest that the Dunhuang terrane underwent an orogenic process related to the Palaeo‐Asian Ocean during the Early Palaeozoic Era, and that it belongs to the Central Asian Orogenic Belt (CAOB).

Paleozoic tectonothermal event in Mt. Dongbatu, Dunhuang terrane, southernmost Central Asian Orogenic Belt (CAOB): Implications for petrogenesis and geological evolution

In this contribution, systematic petrographical, whole-rock geochemical, geochronological and zircon Lu-Hf isotopic investigations were conducted on Paleozoic (ca. 410–400 Ma) metamorphic-magmatic rocks from Mt. Dongbatu in the central Dunhuang terrane. The metamorphic rocks in the Shangkouzi area are of meta-calcsilicate rocks, and granitoid rocks occurring within these rocks are geochemically of trondhjemite. Whereas, metamorphic rocks in the Gangou area are metabasaltic rocks. LA-ICP MS zircon U-Pb dating results reveal that these metamorphic-magmatic rocks recorded ca. 1.6 Ga magmatic event and metamorphic events of ca. 410–400 Ma and ca. 370 Ma. Field occurrence, whole-rock geochemistry, mineral composition and zircon Hf isotopes suggest that the 1.6 Ga mantle-derived basaltic magma, together with iron-rich and Ca-rich components and mica-quartz schist of Paleoproterozoic supracrustal rocks probably contributed to protolith of meta-calcsilicate rocks in the Shangkouzi area, and granitoid rocks within meta-calcsilicate rocks were possibly generated by partial melting of calcsilicate rocks.Combined with previously published data, it is suggested that the protoliths of Paleozoic high-grade metamafic rocks in the Dunhuang terrane are predominantly composed at least of basaltic rocks produced by magmatic events of 2.5–1.8 Ga, ca. 1.8 Ga and ca. 1.6 Ga. The Paleozoic granitoid rocks in the Dunhuang terrane might have been derived from different magma sources as partial melting of subducted plate, or thickened lower continental crust, or pre-existing basaltic rocks produced by 1.6 Ga and 1.8 Ga magmatic activities, and calcsilicate rocks genetically associated basaltic magmatism at ca. 1.6 Ga. Considering regional tectonic evolution, it can be concluded that the Dunhuang terrane was probably in a post-collisional extension setting at ca. 1.6 Ga, producing certain amount of basaltic and calcsilicate rocks. From the beginning of Silurian (ca. 440 Ma), the Dunhuang terrane was involved in the orogenic processes related to the closure of the southern Paleo-Asian Ocean, becoming the active continental margin of the southernmost CAOB.

Paleozoic subduction of the southern Dunhuang Orogenic Belt, northwest China: metamorphism and geochronology of the Shuixiakou area

Amphibolites in the Shuixiakou area of the southern Dunhuang Orogenic Belt, southernmost Central Asian Orogenic Belt (CAOB), occur as lenses within hornblende-biotite-plagioclase gneiss or pelitic schist, exhibiting block-in-matrix feature of tectonic mélange. Three generations of metamorphic mineral assemblages (M1, M2, and M3) have been recognized in the garnet-bearing amphibolite lenses. The metamorphic prograde assemblage (M1) is documented with inclusion trails (hornblende + plagioclase + quartz) within garnet porphyroblasts, and are estimated to be formed under 610–690 °C and 6.5–10.2 kbar. The metamorphic peak assemblage (M2) consists of garnet + hornblende + clinopyroxene + plagioclase + quartz in the matrix and records metamorphic peak P-T conditions of 720–750 °C and 13.4–14.7 kbar. The retrograde assemblage (M3) is represented by the symplectic assemblage (hornblende + plagioclase + quartz ± biotite ± magnetite) rimming the garnet porphyroblast, formed in the decompression stage under P-T conditions of 630–730 °C and 3.8–7.2 kbar. The derived metamorphic P-T paths show similar tight clockwise loops including nearly isothermal decompression processes, typical of orogenic metamorphism. SIMS dating of metamorphic zircons from the amphibolites confirm that the high-pressure metamorphism (M2) occurred at ca. 438–398 Ma.

Nature and provenance of the Beishan Complex, southernmost Central Asian Orogenic Belt

The ages and origins of metasedimentary rocks, which were previously mapped as Precambrian, are critical in rebuilding the orogenic process and better understanding the Phanerozoic continental growth in the Central Asian Orogenic Belt (CAOB). The Beishan Complex was widely distributed in the southern Beishan Orogenic Collage, southernmost CAOB, and their ages and tectonic affinities are still in controversy. The Beishan Complex was previously proposed as fragments drifted from the Tarim Craton, Neoproterozoic Block or Phanerozoic accretionary complex. In this study, we employ detrital zircon age spectra to constrain ages and provenances of metasedimentary sequences of the Beishan Complex in the Chuanshanxun area. The metasedimentary rocks here are dominated by zircons with Paleoproterozoic–Mesoproterozoic age (~1160–2070 Ma), and yield two peak ages at 1454 and 1760 Ma. One sample yielded a middle Permian peak age (269 Ma), which suggests that the metasedimentary sequences were deposited in the late Paleozoic. The granitoid and dioritic dykes, intruding into the metasedimentary sequences, exhibit zircon U–Pb ages of 268 and 261 Ma, respectively, which constrain the minimum deposit age of the metasedimentary sequences. Zircon U–Pb ages of amphibolite (274 and 216 Ma) indicate that they might be affected by multi-stage metamorphic events. The Beishan Complex was not a fragment drifted from the Tarim Block or Dunhuang Block, and none of cratons or blocks surrounding Beishan Orogenic Collage was the sole material source of the Beishan Complex due to obviously different age spectra. Instead, 1.4 Ga marginal accretionary zones of the Columbia supercontinent might have existed in the southern CAOB, and may provide the main source materials for the sedimentary sequences in the Beishan Complex.

Phanerozoic granitoids in the central and eastern parts of Central Asia and their tectonic significance

The widespread occurrence of voluminous granitoids is a typical characteristic of the Central Asian Orogenic Belt (CAOB) in the central and eastern parts of Central Asia. The granitoids occupy a total area more than 8 million km2, with three major area peaks at Cambro-Ordovician, Carboniferous-Permian, and Jurassic. Six zircon age peaks at ca. 500, 430–370, 330–300, 290–260, 210–170 and 145–110Ma.The early Paleozoic granitoids (540–440Ma, with age peak at ca. 500Ma) mainly occur in the northwestern CAOB, and are of calc-alkaline I-type. They probably formed in a subduction setting (500–480Ma) within the Paleo-Asian Ocean (PAO) and subsequent microcontinental-collision (ca. 460Ma). The middle Paleozoic granitoids (430–370Ma) predominately occur in the NW and southern CAOB, and are characterized by a high-K calc-alkaline affinity, suggesting an accretionary and/or a collisional setting. The late Paleozoic (350–260Ma) granitoids are widespread in the CAOB and formed in variable tectonic settings within an intra-continental regime or active continental margins in the PAO and in the evolving Mongol-Okhotsk Ocean (MOO). Carboniferous granitoids (350–300Ma) in the northern and northwestern CAOB exhibit A-type, alkaline features and formed in a post-accretionary settings, whereas granitoids in the southern CAOB show I-type, calc-alkaline features and were generated in an arc setting. Permian granitoids (290–270Ma) are predominantly of I-A, A-type with alkaline signatures, associated with coeval mafic magmatism and suggesting a post-collisional or intraplate setting, contemporaneous with a Large Igneous Province (LIP) in the Tarim Craton. Some 270–250Ma calc-alkaline granitoids also occur in the southernmost CAOB. Combined with 250–245Ma high Sr/Y granitoids, these rocks reflect final closure of the PAO during 270–250Ma.Early Mesozoic (mainly Triassic) granitoids occur mainly in the eastern CAOB or eastern Central Asia. The early-phase granitoids (250–230Ma) are of I- and S-types, whereas the mid- and late-phase granitoids (ca. 230–190Ma) are of A-type and transitional I-A type or highly fractionated I-type. All these granitoids occur in variable settings (intraplate, post-orogenic, and continental arc) related to three tectonics regimes, namely the Paleo-Asian Ocean (PAO), the Mongol-Okhotsk Ocean (MOO), and the Paleo-Pacific Ocean (PPO). Late Mesozoic granitoids mainly occur in eastern Central Asia and were emplaced during 6 major magmatic events at 190–180, 180–165, 165–145, 145–135, 135–100 and 100–60Ma. The 190–165Ma granitoids are of I-type, calc-alkaline/high-K calc-alkaline, and evolved along active margins related to the Mongol-Okhotsk and Paleo-Pacific ocean domains. The 145–120Ma granitoids are highly fractionated I-, transitional I-A or, A-types, with high-K calc-alkaline/shoshonitic signatures. The magmatic evolution from the Late Jurassic to Early Cretaceous coincided with a tectonic transition from contractional, thickened crust to extensional thinning. The 120–65Ma granitoid magmatism displays a southeastward younging trend, suggesting a roll-back of the Paleo-Pacific oceanic plate.The granitoids in the CAOB of eastern Central Asia show variable whole-rock Nd and Hf-in-zircon isotopic compositions, and most have juvenile signatures. In some regions (e.g., Altai-Junggar and Great Xing’an), the volume of juvenile and slightly juvenile crust (<1.0Ga) is much higher than that of other orogens (such as the Qinling in Central China. The proportion of juvenile vs. old crust may be a marker to identify a typical accretionary orogen from a collisional orogen.

Tectonic mélange records the Silurian–Devonian subduction-metamorphic process of the southern Dunhuang terrane, southernmost Central Asian Orogenic Belt

The Hongliuxia tectonic melange of the southern Dunhuang terrane, northwestern China, southernmost Central Asian Orogenic Belt (CAOB), consists of eclogite, mafic granulite, and amphibolite as puddingstones within a matrix of metapelitic gneiss and marble; these rocks are interpreted to be part of an ancient subduction zone setting. Secondary ion mass spectrometry U-Pb dating of metamorphic zircons obtained from the puddingstones and matrix metapelite suggests that the metamorphism occurred at ca. 428–391 Ma. The metamorphic rocks all record similar clockwise metamorphic pressure-temperature-time ( P - T - t ) paths of the western Alpine type. However, remarkable differences between metamorphic peak P - T conditions ranging from 830 °C and 24.2 kbar for the eclogite puddingstone to 700 °C and 10.2 kbar for the metapelite matrix were found in the melange rocks. This indicates the mixing of rocks from significantly different depths to create a tectonic melange in a subduction channel, possibly juxtaposed during the uplift stage. These data suggest that the southernmost CAOB underwent subduction and subsequent exhumation caused by subduction of the Paleozoic Hongliuxia ocean during the middle Silurian to middle Devonian.

Paleozoic intrusive rocks from the Dunhuang tectonic belt, NW China: Constraints on the tectonic evolution of the southernmost Central Asian Orogenic Belt

The Dunhuang tectonic belt (DTB) is of great importance for understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt (CAOB). In this study, the temporal-spatial distribution, petrogenesis and tectonic setting of the Paleozoic representative intrusive rocks from the DTB were systematically investigated to discuss crustal evolution history and tectonic regime of the DTB during Paleozoic.Our results reveal that the Paleozoic magmatism within the DTB can be broadly divided into two distinct episodes of early Paleozoic and late Paleozoic. The early Paleozoic intrusive rocks, represented by a suite metaluminous-slight peraluminous and medium- to high-K calc-alkaline I-type granitoids crystallized at Silurian (ca. 430–410Ma), are predominantly distributed along the northern part of the DTB. They were probably produced with mineral assemblage of eclogite or garnet+amphibole+rutile in the residue, and were derived from magma mixing source of depleted mantle materials with various proportions of Archean-Mesoproterozoic continental crust. The late Paleozoic intrusive rocks can be further subdivided into two stages of late Devonian stage (ca. 370–360Ma) and middle Carboniferous stage (ca. 335–315Ma). The former stage is predominated by metaluminous to slight peraluminous and low-K tholeiite to high-K calc-alkaline I-type granitic rocks distributed in the central part of the DTB. They were also generated with mineral assemblage of amphibolite- to eclogite-facies in the residue, and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust. The later stage is represented by adakite and alkali-rich granite exposed in the southern part of the DTB. The alkali-rich granites studied in this paper were possibly produced with mineral assemblage of granulite-facies in the residue and were generated by partial melting of thickened lower continental crust. Zircon Hf isotopes and field distribution of those Paleozoic intrusive rocks reveal that both the Silurian and the late Devonian magmatic activities predominantly represent crustal growth processes in the DTB, accompanied by different degrees of reworking of pre-existing continental crust. However, the middle Carboniferous (ca. 335–315Ma) magmatic activity reflects a crustal reworking process. The Silurian and late Devonian intrusive rocks were most likely formed in the arc-related subduction zones, whereas, the middle Carboniferous intrusive rocks were possibly formed in a transitional tectonic setting from compression to extension, representing the final stage of Paleozoic orogeny in the DTB.These Paleozoic magmatic rocks further suggest that the DTB has reactivated from a stable block to an orogen and undergone two episodes (the early Paleozoic and the late Paleozoic) of orogeny during Paleozoic. It represents a Paleozoic accretionary orogen of the southernmost margin of the CAOB between the Tarim Craton and North China Craton, and tectonically extends northward to the Beishan orogen and westward to the eastern South Tianshan Belt.