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Photomicrographs showing the main mineral assemblages of the HP mafic granulite and associated rocks in the Dunhuang block. (a, b) HP mafic granulite, consisting of Grt, Cpx, Pl, Amp and Rt. Note that the rim of porphyroblastic garnet is partly replaced by the symplectic corona of Pl and Amp; (c) Ky-bearing Grt gneiss, consisting of Pl, Grt, Qz, Rt, Bt and Ky. (d) Grt-Bt schist, consisting of porphyroblastic Grt and matrix Bt and Qz. Mineral abbreviations after Whitney & Evans (2010).
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The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary collages in the world, and records a prolonged sequence of subduction‐accretion and collision processes. The Tarim Craton is located at the southernmost margin of the CAOB. In this study, the discovery of early Palaeozoic high‐pressure (HP) granulites from the Dunhuang block...
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... with diameters of 1-3 mm, with titanite inclusions (Fig. 5a,b). The retrograde minerals occur as symplectites, indicating an amphibolite facies over- printing. The amphibole + plagioclase symplectite forms a worm-like intergrowth corona around the garnet (Figs 4a,b & 5b). Locally, the rim of matrix clinopyroxene is replaced by amphibole (Fig. 4b) or contains oriented plagioclase with amphibole ( Fig. 5a), which is a common texture for clinopyroxene in some typical HP mafic granulites (e.g. Zhao et al., 2001;OÕBrien & Ro¨tzlerRo¨tzler, 2003;Liu et al., ...
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... garnet gneiss displays porphyroblastic texture and is mainly composed of coarse-grained garnet, plagioclase and quartz, with minor kyanite, biotite and rutile (Fig. 4c). Quartz inclusions are abundant in the core domain of large euhedral garnet por- phyroblasts, but are absent in large euhedral garnet rims or small anhedral garnet grains. Rutile occurs either as large crystals in the matrix or as fine exso- lution grains in garnet (Fig. 4c). The garnet-mica schist is strongly foliated and is mainly ...
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... garnet, plagioclase and quartz, with minor kyanite, biotite and rutile (Fig. 4c). Quartz inclusions are abundant in the core domain of large euhedral garnet por- phyroblasts, but are absent in large euhedral garnet rims or small anhedral garnet grains. Rutile occurs either as large crystals in the matrix or as fine exso- lution grains in garnet (Fig. 4c). The garnet-mica schist is strongly foliated and is mainly composed of quartz, biotite and garnet, with minor plagioclase and K-feldspar (Fig. 4d). Coarse quartz and biotite are deformed into bands and garnet occurs as rotational porphyroblast with quartz and biotite inclusions (Fig. 4d). ...
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... garnet por- phyroblasts, but are absent in large euhedral garnet rims or small anhedral garnet grains. Rutile occurs either as large crystals in the matrix or as fine exso- lution grains in garnet (Fig. 4c). The garnet-mica schist is strongly foliated and is mainly composed of quartz, biotite and garnet, with minor plagioclase and K-feldspar (Fig. 4d). Coarse quartz and biotite are deformed into bands and garnet occurs as rotational porphyroblast with quartz and biotite inclusions (Fig. 4d). ...
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... in the matrix or as fine exso- lution grains in garnet (Fig. 4c). The garnet-mica schist is strongly foliated and is mainly composed of quartz, biotite and garnet, with minor plagioclase and K-feldspar (Fig. 4d). Coarse quartz and biotite are deformed into bands and garnet occurs as rotational porphyroblast with quartz and biotite inclusions (Fig. 4d). ...
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... compositions of the zircon analysed by LA-ICP-MS are summarized in Table 4 and shown in Fig. 9b-d. The 207 Pb ⁄ 206 Pb ages of inherited zircon cores range from c. 2.5 to c. 2.0 Ga, and these grains are charac- terized by oscillatory zoning in CL-images, high Th ⁄ U ratios (0.38-1.12), steep HREE patterns and a obvi- ously negative Eu anomaly (Fig. 9d & Table 4). In contrast, the metamorphic zircon domains without zoning in CL images give concordant U-Pb ages with a weighted 206 Pb ⁄ 238 U age of 435 ± 4 Ma (2r, n = 13) (Fig. 9b,c). These domains show low Th ⁄ U ratios (0.004-0.03), absence of Eu anomalies and relatively flat HREE patterns (Fig. 9d & Table ...
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... and a obvi- ously negative Eu anomaly (Fig. 9d & Table 4). In contrast, the metamorphic zircon domains without zoning in CL images give concordant U-Pb ages with a weighted 206 Pb ⁄ 238 U age of 435 ± 4 Ma (2r, n = 13) (Fig. 9b,c). These domains show low Th ⁄ U ratios (0.004-0.03), absence of Eu anomalies and relatively flat HREE patterns (Fig. 9d & Table ...
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... U-Pb dating and trace element analysis of zircon were undertaken using LA-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd. in Wuhan, China. The specifics of the operating conditions and data processing are detailed in Zong et al. [48]. For this study, the laser's spot size and pulse frequency were adjusted to 30 µm and 5 Hz, respectively. ...
The Anshan–Benxi area, situated in the northeast of the North China Craton (NCC), is home to not only the oldest rocks in China (~3.8 Ga) but also a diverse range of granitoids dated between 3.8 and 2.5 Ga. The Lianshanguan batholith, covering an area of approximately 250 km2 with an east–west trend, predominantly consists of syenogranites (K2O > 4 wt. % and K2O/Na2O ratios > 1.3). Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb analyses of the two syenogranites yielded concordant ages of 2541 ± 22 and 2512 ± 13 Ma, respectively. These syenogranites had zircon εHf(t) values ranging from −20 to +4.9 with two-stage Hf model ages (TDM2(Hf)) spanning 3.9–2.7 Ga. Based on petrological, geochemical, and isotopic characteristics, we conclude that the Lianshanguan syenogranites are mainly resulted from the reworking of complicated Eoarchean–Mesoarchean crustal materials, possibly with a small proportion of ~2.7 Ga juvenile crustal materials. When compared with coeval syenogranites from the Northern Liaoning and Western Liaoning–Eastern Hebei areas, ~2.5 Ga syenogranites from the Anshan–Benxi area displayed more complicated TDM2(Hf) ages, hinting at a pronounced late Neoarchean reworking of the Eoarchean to Mesoarchean continental crust (including metasedimentary sources) primarily in the Anshan–Benxi region of the North China Craton. This scenario significantly bolsters the arc–continent collision model.
... The mafic rocks typically occur as lenses/enclaves in a matrix consisting of metapelite, gneiss, and marble, which all together form a ''block-in-matrix" structure or mélange. The metamorphic rocks in the amphibolite, granulite and eclogite facies formed after mafic rocks and yielded similar clockwise P-T-t paths and U-Pb zircon ages of metamorphism at ca. 462-370 Ma (Zong et al., 2012;He et al., 2014;Wang et al., 2016aWang et al., , 2017bWang et al., , 2017cWang et al., , 2018Soldner et al., 2022). Some previous researchers suggest that the Silurian HP granulite (Fig. 1c) formed by collisional processes (Zong et al., 2012;He et al., 2014), although others believe that the Devonian metapelite, amphibolite, mafic granulite, and eclogite units of the Hongliuxia area got metamorphosed in subduction channel at various depths, but later appeared emplaced together in tectonic mélange (Wang et al., 2016a(Wang et al., , 2017b. ...
... The metamorphic rocks in the amphibolite, granulite and eclogite facies formed after mafic rocks and yielded similar clockwise P-T-t paths and U-Pb zircon ages of metamorphism at ca. 462-370 Ma (Zong et al., 2012;He et al., 2014;Wang et al., 2016aWang et al., , 2017bWang et al., , 2017cWang et al., , 2018Soldner et al., 2022). Some previous researchers suggest that the Silurian HP granulite (Fig. 1c) formed by collisional processes (Zong et al., 2012;He et al., 2014), although others believe that the Devonian metapelite, amphibolite, mafic granulite, and eclogite units of the Hongliuxia area got metamorphosed in subduction channel at various depths, but later appeared emplaced together in tectonic mélange (Wang et al., 2016a(Wang et al., , 2017b. The emplacement of the metamorphic rocks into the tectonic mélange was possibly linked to the processes of subduction at an active convergent margin of the Liuyuan Ocean, a southern branch of the PAO (Wang et al., 2016a(Wang et al., , 2017c. ...
... The Dunhuang block is suggested to belong to the stable Precambrian metamorphic basement of the Dunhuang-Alxa craton (Lu et al. 2008;He et al. 2013;Zhang et al. 2013;. Recent studies on high-grade metamorphic rocks such as high-pressure mafic granulites, amphibolites, and eclogites exposed in the Dunhuang area indicate that the Dunhuang Block is a part of the Central Asian Orogenic Belt (Zong et al. 2012;Zhao et al. 2016;Pham et al. 2018;Zhao and Sun 2018). The eastern margin is the Central-South Qilian block, and the Hongliugou-Lapeiquan ophiolite mélange belt lies to the west. ...
Corundum is rarely found in situ within alkali syenites. A corundum-bearing syenite was found in the Yushishan rare metal deposits of the eastern section of the Altyn Tagh fault in the northeastern Tibetan Plateau, but the characteristics and formation of corundum remain unknown. We describe a corundum-bearing syenite dike emplaced in biotite plagioclase gneiss that suffered overprinted deformation with characteristics of mylonitization. The corundum crystals have variable grain sizes, and the largest ones are megacrystic with growth zoning. The corundum crystals contain a variety of mineral inclusions that are divided into primary and secondary. The primary mineral inclusions within the corundum include variable contents of Fe-Ti oxide needles, ilmenite, zircon, monazite-(Ce), potassium feldspar, pyrochlore, columbite-(Fe), magnetite, samarskite-(Y), and pyrite that indicate corundum crystallized in peraluminous Zr-rich and Si-poor alkali rock with variable TiO2 contents. Secondary mineral inclusions include Zn-rich hercynite, ilmenite, magnetite, annite, fluorapatite, and intergrowths of ilmenite with columbite-(Fe) and goethite that reveal late-stage influx of Zn-, Ti-, Fe-, and F-bearing fluids into corundum that caused metasomatism and element migration and mineral precipitation. The trace element analysis of corundum shows high-Fe and -Ga contents and low-Mg and -Cr contents that are consistent with the characteristics of corundum of magmatic origin. The trace element characteristics and the oxygen isotopes (6.2–8.2‰) results indicate that corundum crystallized in melts with the involvement of Al-rich and Si-poor crustal material.
... The metamorphic rocks in the Dunhuang area are known as the Dunhuang Complex. Recent studies demonstrate that it is composed of metamorphosed igneous and sedimentary rocks of different ages, juxtaposed together during a Paleozoic tectonothermal event (Meng et al., 2011;Zhang et al., 2012aZhang et al., , 2013Zong et al., 2012Zong et al., , 2013Zhao et al., 2013Zhao et al., , 2015aZhao et al., , 2015bZhao et al., , 2019Wang et al., 2017Wang et al., , 2021Soldner et al., 2022). The oldest rock identified so far in this area is a ~3.05 Ga granodioritic gneiss, which has variable but predominantly negative zircon εHf values (average − 3.6 ± 4.8, 2SD, except for two analyses with positive εHf values), suggesting an origin from Paleo -Eoarchean crustal materials (Zhao et al., 2015a). ...
When and how continental crust formed and evolved to its current state is a fundamental question regarding crust – mantle evolution and geodynamic regimes on the early Earth. Earth's Archean continental crust is dominated by sodic granitoids comprising tonalite – trondhjemite – granodiorite (TTG), whereas diverse high-K granitoids appear relatively late in the geological record. Therefore, the formation and secular evolution of Archean TTGs and high-K granitoids is key to many issues regarding early Earth geodynamics. The Tarim Craton, NW China, is an ancient continental block containing some of the oldest rocks (∼3.7 Ga TTGs) on Earth, but its Archean evolution is poorly known due to limited outcrop. Here we review the Archean geological record of the Tarim Craton and present the new findings of ∼3.2–3.0 Ga TTG, ∼2.8 Ga high-K granite and ∼2.35 Ga A-type granitoids (mostly syenogranite and syenite) in the southwestern Tarim. The ∼3.2–3.0 Ga TTG gneisses have homogenous and highly radiogenic zircon Hf isotopic compositions (weighted mean εHf = +3.5 to +4.4), indicating crustal growth from a long-term depleted mantle. In contrast, the ∼2.8 Ga high-K granite and ∼2.35 Ga A-type granitoids have lower εHf values (weighted mean + 2.0 and − 4.3 to −7.8, respectively) that plot on the evolution trend of the Mesoarchean juvenile crust, indicating repeated crustal reworking and progressive crustal differentiation. Thermodynamic – geochemical modelling demonstrates that water-fluxed melting is an efficient way to produce large volumes of TTG melts, and that melting pressure plays a primary role in determining the trace element systematics of diverse TTGs. Water-fluxed melting at crustal (10–12 kbar) and mantle (18–20 kbar) depths explains not only the compositions of the ∼3.2–3.0 and ∼3.7 Ga TTGs in the Tarim Craton, respectively, but also the global average compositions of median- to low-pressure and high-pressure TTGs. Partial melting of the Mesoarchean gneisses at shallow depth (≤5 kbar) reproduces the trace element patterns of the ∼2.35 Ga A-type granitoids in SW Tarim, but cannot explain the low SiO2 of the syenites, which probably originated from partial melting of an enriched mantle resulting from recycling of Archean continental crust during continental rifting. This process also contributed to the diversification of granitoids and maturation of continental crust. Based on these results, we propose a new model for the formation and evolution of Archean continental crust. In our model, most Archean TTGs were produced by water-fluxed melting of thick proto-arcs built on thick Archean oceanic crust, whereas rare high-pressure TTGs were formed by water-fluxed melting of subducted arcs during intermittent subduction and arc accretion. Our model links the origin of Earth's continental crust to an early hydrosphere and subduction – accretion tectonics, and explains the decline of TTGs and rise of high-K granitoids by secular cooling of the mantle, which resulted in thinner oceanic crust and arcs, facilitating arc subduction, rather than arc accretion, and enhancing the rate of crustal reworking relative to crustal growth.
... Importantly, the occurrence of~465 Ma high-pressure (HP) eclogites in the northern of the Liuyuan basaltic belt (Saktura et al., 2017;Liu et al., 2011;Qu et al., 2011) implies an Early Paleozoic subduction and collision event in the southern Beishan Orogen. Moreover, the discovery of~440 -430 Ma HP granulites in the Dunhuang Block indicates that the Dunhuang continental margin has been subducted and collided with the southern Beishan Orogenic belt in the Early Paleozoic (He et al., 2014b;Zong et al., 2012). During the Early Paleozoic, therefore, the terranes in the southern Beishan Orogen have been progressively accreted together in the northern margin of the Dunhuang block and the Paleo-Asian Ocean has been closed in this orogen. ...
The Permian basalts in the Central Asian Orogenic Belt (CAOB) are crucial for constraining the closure of the Paleo-Asian Ocean. However, the origin of these basalts is still under discussion. Here, we present comprehensive bulk-rock geochemical, Sr−Nd−Pb−Hf isotopic, and zircon U−Pb−Lu−Hf isotopic data of the Liuyuan basalts and coexisting gabbros, which are located in the Beishan Orogen in the southern CAOB, to constrain their emplacement setting and tectonic implications. Our new gabbro ages of ca. 288–294 Ma are interpreted to represent the formation time of the Liuyuan basaltic belt. The Liuyuan basalts show MORB-like rare earth element (REE) patterns and bulk-rock εHr(t) and εNd(t) values of 11.0–15.4 and 4.6–9.2, respectively, suggesting an origination mainly from a depleted mantle source. However, positive Pb anomalies, Nb−Ta depletions, and high Th/Yb ratios as well as evolved Sr−Nd−Pb−Hf isotopic compositions of some samples indicate variable continental crustal contribution. According to the covariation of Pb anomalies (Pb* =2×PbN/(CeN+PrN)) with Sr−Nd−Pb−Hf isotopic compositions, we speculate that parent magma of the Liuyuan basalt was contaminated by continental crustal materials during the eruption rather than having been generated from an enriched mantle source. As revealed by mixing modelling, the Liuyuan basaltic magmas would require a minor (<10%) upper continental crustal assimilation to explain the enriched trace elemental and radiogenic Sr−Nd−Pb−Hf isotopic signatures. Consequently, the Liuyuan basaltic belt is believed to have been generated in a continental extensional environment instead of an oceanic setting and does not constitute a Permian ophiolitic suture zone as previously suggested, since the Paleo-Asian Ocean was already closed in the southern Beishan Orogen in the Early Permian.
... The tectonic affinity of the Dunhuang Block has always been a controversial topic. Some researchers favour the idea that it was part of the Tarim Craton (Meng et al. 2011;He et al. 2013) or the western North China Craton (Zhang et al. 2012Zhao 2017). Recently, with the development of high-precision analysis and testing methods, voluminous Paleozoic rocks have been identified in the Dunhuang Block that are regarded as the products of orogenic processes related to the subduction and closure of the PAO. ...
... 3.06-2.5 Ga) and metamorphosed supracrustal rocks, as well as minor Paleoproterozoic maficfelsic intrusive rocks (Zhang et al. 2012Zong et al. 2013;Zhao 2017;Gan et al. 2020a). The metamorphosed supracrustal rocks (termed the Dunhuang Group) consist of paragneisses, quartzites, amphibolites, schists, marbles and metasedimentary rocks (BGMRG 1989;Lu et al. 2008). ...
... The Paleoproterozoic high-pressure mafic granulites and amphibolites were metamorphosed at c. 1.86-1.82 Ga and recorded clockwise P-T-t paths in the Shuixiakou area, possibly related to the assembly of the supercontinent Columbia (Zhang et al. 2012;Zong et al. 2013). ...
Late Ordovician–Early Silurian intermediate igneous rocks have been detected in the Dunhuang Block, NW China. These igneous rocks help to better constrain the tectonic evolution of the Dunhuang Block and even the southern Central Asian Orogenic Belt during the early Paleozoic. In this contribution, zircon U–Pb dating, whole-rock geochemistry, and zircon Hf isotope analyses are conducted on the early Paleozoic gneissic diorites from the Sanweishan and northern Huangshuihou areas in the Dunhuang Block. Zircon U–Pb geochronology demonstrated that the emplacement ages of the gneissic dioritic rocks were ca. 443–440 Ma. The dioritic rocks show varying SiO 2 (48.1–63.1 wt.%) and MgO (1.87–3.52 wt.%) contents with high Mg # (46–52) values, and negative Eu anomalies (Eu/Eu* = 0.62–0.94). Zircons in the gneissic diorites from the northern Huangshuigou and Sanweishan areas yield variable ε Hf (t) values of −4.4 to +10.4 and –8.6 to –6.2, respectively, coupled with low Rb/Sr (0.04–0.34), Rb/Ba (0.06–0.31), and Al 2 O 3 /(MgO + FeO T ) (0.72–1.47) ratios, indicating that the they were predominately generated by disequilibrium melting of the juvenile crust and mixed with ancient crustal and minor mantle melts. In conjunction with previously published data, we propose that the early Paleozoic gneissic dioritic rocks were formed in a continental margin arc setting related to the southward subduction of the Paleo-Asian Ocean. Additionally, our findings point out that the Dunhuang Block was generally modified and reactivated by the Paleozoic orogenic events related to the evolution of the Paleo-Asian Ocean, resulting in extensive Paleozoic magmatism–metamorphism activities in the whole Dunhuang area.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.6174601
... In addition, some Paleozoic metamorphic rocks have been documented in the Dunhuang Block, including granulite-facies meta-mafic rocks (ca. 440-400 Ma, Zong et al., 2012;He et al., 2014) and eclogite (ca. 428-391 Ma, Wang et al., 2017). ...
Neoproterozoic igneous rocks are widely distributed in the Dunhuang-Alxa Block, northwest China, and record geodynamic processes caused by the assembly and break-up of Rodinia. In this study, we present new petrological, geochemical, and zircon U-Pb-Hf isotope data for the Langshan gabbros in the northeastern Alxa Block. Langshan gabbros (827−819 Ma) have depleted whole-rock Nd (+3.2 to +4.9) and zircon Hf (+4 to +21) isotopic compositions and weak enrichments in large-ion lithophile elements. These rocks were probably derived by partial melting of a depleted mantle wedge metasomatized by fluids released from a subducted slab in a back-arc basin environment. Geochemical and zircon Hf isotope data for early Neoproterozoic igneous rocks are consistent with the presence of a long-lived (ca. 930−810 Ma) retreating subduction zone along the northern margin of the Dunhuang-Alxa Block, indicating it was located at the northern margin of Rodinia. Some mantle plume-related magmatism, including A1-type silicic volcanic rocks in the Langshan Group and Jinchuan ultramafic-mafic intrusions, occurred in the interior of the Dunhuang-Alxa Block, which was similar to early Neoproterozoic plume-related magmatism (ca. 850−820 Ma) in the interior of northern Rodinia. The temporal link between subduction and extension suggests that this retreating subduction zone along the northern margin of Rodinia, in combination with a mantle plume beneath its interior, resulted in lithospheric extension and break-up of Rodinia during ca. 850−810 Ma.
... By contrast, high-grade metamorphic rocks from the southernmost Tarim-North-China collage consist of Late Palaeozoic UHP eclogite lenses enclosed in volcano-sedimentary units resembling incomplete Alpine-type sequences (Gao et al., 1995) or Early Palaeozoic eclogite units formed in a collisional setting (Saktura et al., 2017;Soldner et al., 2020). The Dunhuang block, located east of Tarim, forms a part of the Tarim-North China collage (e.g., Xiao et al., 2017;Zong et al., 2012) and represents a specific tectonic domain, where HP granulite, amphibolite and rare eclogite lenses are enclosed in metapelite matrix (e.g., He et al., 2014;H.Y.C. Wang, Chen, et al., 2017). ...
... Wang et al., 2016a;Zhao et al., 2017;Zhu et al., 2018). Eclogite, HP granulite and amphibolite yield zircon U-Pb metamorphic ages of 440-425, 410-400 and 370 Ma in the northern belt (Meng et al., 2011;Zhao et al., 2016), 444, 431 and 420-372 Ma in the central belt (He et al., 2014; H.Y.C. Wang, Zhang, Lu, et al., 2018;Zhao et al., 2019;Zong et al., 2012) and 438-355 Ma in the southern belt (H.Y.C. H.Y.C. Wang, Wang, et al., 2017;H.Y.C. ...
... Wang, Zhang, Chen, et al., 2018;Q.W. L. Zhang et al., 2020). Host kyanite-bearing gneiss and micaschist in the central belt yield U-Pb metamorphic ages of 435-429 Ma (He et al., 2014;Zong et al., 2012) and 372 Ma (H.Y.C. Wang, Zhang, Lu, et al., 2018). ...
High‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure‐temperature‐time‐deformation (P−T−t−D) paths. In the current study, P−T−t−D paths of early Paleozoic HP granulite‐facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by: 1) Relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, 2) a N‐S trending sub‐vertical fabric (S2) preserved in low‐strain domains and 3) upright folds (F3) associated with an ubiquitous steep E‐W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8−11 kbar and ~850–900°C during crustal melting (M2) followed by partial re‐equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re‐equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P−T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure‐temperature (P−T) evolution M3 suggests formation of a mélange‐like association during the late stages of orogeny. The high‐pressure event D1‐M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre‐orogenic crust. The high‐temperature (HT) re‐equilibration D2‐M2 is interpreted as a result of Mid‐Devonian shortening of the previously thickened crust, possibly due to “Andean‐type” underthrusting. The D3‐M3 event reflects Late Devonian supra‐subduction shortening and continuous erosion of the sub‐crustal lithosphere. This tectono‐metamorphic sequence of events is explained by polyphased Andean‐type deformation of a “Cascadia‐type” active margin, which corresponds to a supra‐subduction tectonic switching paradigm.
... Y. C. and Silurian-Devonian HP mafic granulites (H. Y. C. Zong et al., 2012) demonstrates that the Dunhuang Complex is a tectonic mélange that contains diverse blocks that were subducted to variable depths and P-T conditions (H. Y. C. , 2017H. ...
... Y. C. Wang et al., 2018), as documented by many studies of the HP metamorphism (He et al., 2014;H. Y. C. Wang et al., 2018;Zong et al., 2012). Metabasic rocks of different metamorphic grades form lenses, centimeters to meters in size, entrained in a metasedimentary matrix H. Y. C. Wang, Chen, et al., 2016, 2017 (Figures 2 and 3; Figure S1 in Supporting Information S1); the overall structure is a typical "block-in-matrix" mélange (Silver & Beutner, 1980;M. ...
... The presence of predominant north-dipping shear zones, duplexes, and imbricate thrusts (Feng et al., 2018) also supports northward subduction polarity. The relationships between the Middle Ordovician HP granulites in NW Dunhuang (this study), the Early Silurian HP granulites (He et al., 2014;Zong et al., 2012) in central Dunhuang, and the Early Devonian eclogite (H. Y. C. in southern Dunhuang suggest a southward younging and a deepening polarity of exhumed HP rocks. ...
Subduction zones provide important constraints on reconstruction of tectonic configurations and convergent geodynamics. The timing and evolution of the subduction zones at the interaction between the Paleo‐Asian and Tethyan Oceans remain ambiguous, casting significant uncertainty on the reconstructions of proto‐Asia and subduction dynamics. Here, we report new petrologic and geochronologic data of a high‐grade metamorphic complex in the Dunhuang area, NW China, together with thermodynamic modeling, geothermobarometry, and second ion mass spectrometry U‐Pb chronology to reveal the complex metamorphic structure and history of the Paleozoic subduction‐exhumation channel at the intersection of the Paleo‐Asian and Tethyan domains. The subduction zone contains a diverse collection of eclogite, high‐pressure (HP) granulites, amphibolites, and metasedimentary rocks with a broad spectrum of P‐T‐t paths, which were buried and exhumed at different depths at ∼463–411 Ma. Our first dating of the oldest (∼463 Ma) HP granulite extends the orogenic period earlier to the Middle Ordovician. The uneven change in subduction gradients from ∼18°C/km to ∼10°C/km illustrates the thermal evolution of the subduction zone from infancy to maturity. A dramatic drop of the gradient at ∼420 Ma suggests a potential short‐term switch of the subduction dynamics from “hot” to “cool,” possibly due to plate geometry reorganization in response to slab roll‐back. Our data demonstrate that the Dunhuang Complex was chaotically mixed and juxtaposed at different levels in the subduction‐exhumation channel. The southward younging and increase in depth of the HP metamorphism indicate that the Dunhuang Complex was formed by north‐dipping subduction of the Proto‐Tethyan Ocean from the Early Paleozoic.
... However, the formation of ca. 462 Ma magmatic belts (e.g., Wang et al., 2016b;Zhao et al., 2017;Zhu et al., 2020) and the recent discovery of a middle Cambrian arc (Gan et al., 2020) suggest that its suprasubduction history could have already commenced in the early Paleozoic. It is likely that this magmatic evolution could also be preserved in protoliths of the HP metabasaltic rocks and surrounding gneisses. ...
... Throughout the Cascadia-type back-arc region, high mantle temperatures originate from vigorous, thermally driven convection with no significant crustal extension (Currie and Hyndman, 2006). Finally, the hot and rheologically weak lithosphere acted as a precursor to remarkable HP crustal reworking of this margin during the Silurian-Devonian orogeny (e.g., Zong et al., 2012;He et al., 2014). This multistage evolution is finally discussed in the framework of recent full-plate tectonic models for Central Asian blocks (Domeier, 2018;Merdith et al., 2021) in order to assess the evolving position of the Dunhuang block with respect to various oceanic systems existing between the northern Gondwana margin, Siberia, and the Panthalassa Ocean in the early Paleozoic. ...
... Paleozoic magmatic rocks form as much as one third of the exposed crust in the Dunhuang block (Fig. 2). Early Paleozoic subductionrelated magmatic rocks include 511-402 Ma I-type granitoids (Wang et al., 2016a(Wang et al., , 2016bZhao et al., 2017;Zhu et al., 2020;Gan et al., 2020;Shi et al., 2020) and ca. 436 Ma islandarc volcanic rocks (Wang et al., 2017c), whereas minor granitoids formed during melting of the thickened crust at ca. 440 Ma and 412-409 Ma (Zhang et al., 2009;Zhao et al., 2019). ...
Origins of early Paleozoic metabasites (granulites and amphibolites) and their host metasedimentary rocks in the Dunhuang block, NW China, are addressed by new geochronological and geochemical data. The metabasites show back-arc basalt−like geochemical features, marked by high Zr/Nb ratios and Zr-Hf troughs, but they can be classified into two groups based on their dissimilar protolith ages and distinct Nd signatures. Most group I metabasaltic rocks were emplaced before 455 Ma and possess high Ba/Nb ratios (11.46−224), low (Nb/La)PM (0.10−0.71), and negative whole-rock εNd(t) values (−12.7 to −2.7), whereas group II rocks have protolith ages around 445 Ma, low Ba/Nb ratios (0.70−22.93), low (Nb/La)PM (0.78−1.51), and less evolved whole-rock Nd isotopic features (εNd[t]: −2.0 to +2.7). It is proposed that group I metabasites originated from an enriched lithospheric mantle, while group II metabasites were derived from the depleted asthenospheric mantle. The metasedimentary rocks received detritus mainly from the neighboring Cambrian magmatic arc, and they are compositionally similar to active-margin sediments. Metamorphic zircon U-Pb ages ranging 462−422 Ma from the investigated rocks together with prominent magmatism further suggest high-grade metamorphism prevailing during the Late Ordovician−early Silurian. Based on these data, a Cascadia-type evolution is proposed involving an Ordovician−early Silurian suprasubduction stretching of the Cambrian active continental margin, which culminated with mantle upwelling. Recent paleogeographic reconstructions support the evolution and assembly of interior Proto−Tethys-Ran oceanic and continental plates, including the Dunhuang block, between 510 and 440 Ma, followed by Panthalassan subduction beneath the Tarim−North China continental assemblage at 440−430 Ma.
