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The final subduction and suture of the Paleo-Asia Ocean (PAO) are controversial, and the key to this question is to locate the latest subduction/final suture of the accretionary type orogenic belt. Previous studies mainly focused on the Tianshan, Beishan, Alxa, and Solonker Orogens for this issue. As a Phanerozoic orogenic belt located further south of the Beishan-Alxa Tectonic Belts, the Dunhuang Orogenic Belt (DOB) is becoming a research hotspot for revealing the evolutionary history of the southernmost Central Asia Orogenic Belt (CAOB). The youngest subduction-related arc magmatism is pivotal for constraining the lower limit of subduction of a lost ocean. This study presents integrated investigations of field observation, whole-rock geochemistry, and geochronology, targeting diabase dykes, intermediate-acid magmatic rocks, and garnet-bearing metabasite samples in the middle part of the DOB. LA-ICP-MS U-Pb dating of zircon yielded episodic magmatism of ca. 274–273 Ma for Xiaohongshan granite and Dongbatu syenogranite, ca. 250 Ma for Keziletage syenite, ca. 243–239 Ma for diabase and granitic dykes in the Kalatashitage area and Mogutai syenogranite, and maximum deposition age of ca. 262 Ma for pyroclastic rock in the Kalatashitage area. Most of those Permian-Triassic magmatic rocks show continental arc-related geochemical features during the subduction stage. The negative εHf(t) values and ancient TDM2 model ages of zircon indicate they were mainly derived from the partial melting of ancient crustal materials. The hornblende 40Ar/39Ar ages of ca. 258–250 Ma of the garnet-bearing metabasite samples also reflect a high heat flow setting during this period possibly triggered by arc activities. These Permian-Triassic magmatic and thermal records in the DOB are similar to contemporaneous records in surrounding South Tianshan-Alxa-Beishan orogens. We suggest that the final subduction of the PAO in the DOB possibly at least lasted to the Middle Triassic. [ABSTRACT FROM AUTHOR]

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.

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. [ABSTRACT FROM AUTHOR]

Garnet-bearing mafic granulites and amphibolites from the Hongliuxia area of the southern Dunhuang orogenic belt, northwestern China, commonly occur as lenses or boudinages enclosed within metapelite or marble, which represent the block-in-matrix feature typical of orogenic mélange. Three to four generations of metamorphic mineral assemblages are preserved in these rocks. In the high-pressure amphibolites, prograde mineral assemblages (M1) occur as inclusions (hornblende + plagioclase + quartz ± chlorite ± epidote ± ilmenite) preserved within garnet porphyroblasts, and formed at 550–590 °C and 7.7–9.2 kbar based on geothermobarometry. The metamorphic peak mineral assemblages (M2) are composed of garnet + hornblende + plagioclase + quartz + clinopyroxene, as well as titanite + zircon + rutile + apatite as accessory minerals in the matrix, and are estimated to have formed at 640–720 °C and 14.1–16.0 kbar. The first retrograde assemblages (M3) are characterized by “white-eye socket” symplectites (hornblende + plagioclase + quartz ± biotite ± epidote ± magnetite) rimming garnet porphyroblasts, which formed at the expense of the garnet rims and adjacent matrix minerals during the decompression stage under P – T conditions of 610–630 °C and 5.6–11.8 kbar. The second retrograde assemblages (M4) are intergrowths of actinolite and worm-like quartz produced by the breakdown of the matrix hornblendes, and formed under P – T conditions of ∼490 °C and ∼2.8 kbar. For the high-pressure mafic granulites, the prograde assemblages (M1) are represented by plagioclase + quartz preserved within the garnet porphyroblasts. The metamorphic peak assemblages (M2) are garnet + matrix minerals (clinopyroxene + plagioclase + quartz + hornblende + rutile + zircon) and were estimated to have formed at ∼680 °C and ∼15.4 kbar. The retrograde assemblages (M3) are characterized by fine-grained patches of hornblende + plagioclase + quartz rimming the garnet porphyroblasts, as well as hornblende rimming clinopyroxene in the matrix, and were inferred to have formed at ∼620 °C and ∼4.2 kbar. For the metapelitic gneiss, the metamorphic peak assemblages are the garnet porphyroblasts plus the matrix minerals (biotite + plagioclase + quartz + ilmenite + zircon), which were estimated to have formed at ∼630 °C and ∼8.9 kbar. The mafic granulites and amphibolites record fairly similar clockwise P–T paths that include nearly isothermal decompression processes, which suggest that they experienced subduction and subsequent rapid tectonic exhumation. SIMS and LA-ICP-MS U–Pb dating of zircons and 40Ar/39Ar dating of hornblende suggest that the metamorphism occurred at ∼430–390 Ma. Field occurrences, different protolith ages of the mafic granulites and amphibolites, and the considerable gap in peak P – T conditions between the amphibolite and mafic granulite boudinages and their country rock may suggest a mélange accumulation process during the Paleozoic caused by the Silurian–Devonian orogeny, which is possibly associated with the closure of the Liuyuan ocean, a branch of the Paleo-Asian ocean near the southern Central Asian Orogenic Belt. [ABSTRACT FROM AUTHOR]

The Dunhuang region is located in the east of the Tarim Craton and has long been considered as part of a Precambrian terrane of the Tarim or North China Craton. Aiming to further understand the tectonic evolution of the Dunhuang region and its relations with the adjacent tectonic units, this study is to investigate the metamafic rocks in the Sanweishan area, Dunhuang region, and in turn to summarize the tectono-thermal events and re-examine the tectonic characteristics of the region. The metamafic rocks in the Sanweishan area include three types of amphibolites. They are garnet–clinopyroxene amphibolites, garnet-free amphibolites and garnet–mica amphibolites. The whole-rock major and trace element compositions suggest that their protoliths probably are E-MORB type tholeiitic basalts with some crustal addition. The LA-ICP-MS zircon U–Pb data of the garnet–clinopyroxene amphibolite and the garnet-free amphibolite yielded lower intercept ages of 410 ± 11 Ma and 403 ± 8 Ma, respectively, while the garnet–mica amphibolite gave three metamorphic ages of 441 ± 3 Ma, 408 ± 12 Ma and 370 ± 2 Ma. Three suites of metamorphic conditions have been estimated at 1.14–1.55 Gpa and 458–548 °C, 0.67–0.82 Gpa and 590–600 °C, and 0.47–0.66 Gpa and 557–586 °C for garnet–clinopyroxene amphibolite, using different mineral thermobarometers, which illustrate a collisional orogeny-related clockwise P–T path. Integrated with the study on the high-pressure granulite in the Mogutai area, it can be inferred that the metamafic rocks in the Sanweishan area obviously experienced amphibolite–facies metamorphism at ca. 410–400 Ma, and they could also underwent high-grade metamorphism at ca. 440–430 Ma. Therefore, we suggest that the E–W-trending collisional orogeny of ca. 440–400 Ma could sweep cross the Dunhuang region. The Dunhuang region records multiple tectono-thermal events occurring at ca. 3.1–2.5 Ga, ca. 2.3–1.6 Ga, ca. 440–400 Ma and ca. 370–310 Ma. However, compared with the limited exposure of the Precambrian complex, the Early Silurian–Late Carboniferous (ca. 440–310 Ma) orogeny-related metamorphic and magmatic rocks are widely distributed in the Dunhuang region, making up nearly 70% of the total exposure. Thus, it is proposed that the geological bodies in the Dunhuang region were mainly produced by a Paleozoic orogeny and represent a Paleozoic orogen, while the limited Precambrian complex probably is a microcontinental fragment or part of the margin of the Tarim or North China Cratons that was involved in this Paleozoic orogeny during the Early Silurian–Late Carboniferous. We infer that the Dunhuang orogenic belt is part of the Central Asian Orogenic Belt (CAOB), and thus the CAOB probably extends beyond the Beishan orogenic collage and southward into the Dunhuang region. [ABSTRACT FROM AUTHOR]

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried. [ABSTRACT FROM AUTHOR]

• The central Dunhuang Orogenic belt (DOB) can be split into two contrasting terranes. • The granitoid rocks were emplaced in two phases: ca. 284–275 Ma and ca. 255–238 Ma. • The two phases of granitoids are arc-related and post-collisional rocks, respectively. The Dunhuang orogenic belt (DOB), located in the southern Central Asian Orogenic Belt (CAOB), exposes Precambrian and Paleozoic intrusive-metamorphic rocks. To better understand its evolution, we carried out detailed petrological and geochronological investigations on the major lithologies of the central DOB. The central DOB can be divided into two terranes, the north and the south, with contrasting geological history. The north is characterized by Precambrian crystalline rocks (ca. 3.1–1.6 Ga) overprinted by Devonian tectono-thermal events. The Precambrian rocks show evidence for major juvenile crust growth with additions of older crustal materials at 2.7–2.5 Ga, a major crustal reworking event at 2.0–1.9 Ga, and juvenile additions with minor crustal reworking at 1.8–1.7 Ga. In contrast, the south is characterized by early Devonian (ca. 406 Ma) TTG-gneiss and amphibolite gneiss, and some Late Paleozoic–early Mesozoic granitoid rocks (ca. 284–238 Ma). The Late Paleozoic–early Mesozoic granitoid rocks from the south were emplaced in two phases: early Permian (ca. 284–275 Ma) and late Permian–middle Triassic (ca. 255–238 Ma). They belong to the high-K calc-alkaline I-type series which were generated from mixing juvenile materials with Mesoproterozoic continental crust. The early Permian and the late Permian–middle Triassic granitoid rocks are considered as arc/subduction-related and post-collisional rocks, respectively. These suggest that the final assembly of the DOB with the Beishan or final assembly within the DOB happened in the middle Permian (ca. 275–255 Ma). The DOB formed by accretion/collision of multiple terranes/microcontinents, similar to other areas in the CAOB. [ABSTRACT FROM AUTHOR]

High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly represented by the symplectic minerals surrounding the embayed garnet and the retrograded hornblende rimming matrix-type clinopyroxene. Metamorphic pressure and temperature (P-T) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by thermobarometry are all clockwise, passing from 640 to 720 °C/6.2–12.6 kbar (M1) through 840–920 °C/14.6–16.2 kbar (M2) to 750–815 °C/5.5–7.9 kbar (M3) for high-pressure mafic granulite, from ~650 °C/5.7 kbar (M1) through ~750 °C/9.2 kbar (M2) to ~780 °C/8.1 kbar (M3) for amphibolite, and from ~615 °C/7.9 kbar (M1) through 730–820 °C/8.6–11.7 kbar (M2) to 675–740 °C/5.4–8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore, pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits prograde metamorphism in a P-T range of 510–800 °C/8.5–13 kbar and demonstrates peak metamorphic P-T conditions of ~850 °C/16 kbar, which are consistent with the thermobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U Pb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430–420 Ma (M2) and ca. 400–390 Ma (M3), respectively. Metamorphism was followed by the intrusion of granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subduction-collision complexes in this orogenic belt. • Three stages of metamorphic mineral assemblages have been found in the Kalatashitage area. • These rocks record clock-wise P-T paths. • The peak and retrograde events occurred at ca. 430–420 Ma and ca. 400–390 Ma, respectively. • Diverse subduction and exhumation were confirmed in NW Dunhuang Orogenic Belt, NW China. [ABSTRACT FROM AUTHOR]