Metamorphic evolution and geochronology of garnet amphibolite, the Dongbatu tectonic-metamorphic mélange, eastern middle Paleozoic Dunhuang Orogenic Belt, northwest China

Mafic granulite and amphibolite in the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt, northwest China, southernmost Central Asian orogenic belt, occur as lenses within the matrix of metapelite and marble, exhibiting typical block-in-matrix fabrics of tectonic mélange. Three stages of metamorphic mineral assemblages were identified in these lenses. Clockwise metamorphic pressure-temperature (P-T) paths were obtained through geothermobarometry and thermodynamic pseudosection modeling, passing from 656 °C and 10.9 kbar through 830 °C and 16.5 kbar to 657 °C and 4.9 kbar for the mafic granulite, and from 564–645 °C and 3.2–9.6 kbar through 634–727 °C and 6.1–14.2 kbar to 615–664 °C and 3.2–4.2 kbar for the amphibolites, respectively. Metamorphic peak P-T conditions in the metapelitic country rocks were estimated to be 635–675 °C and 6.0–6.9 kbar. The metamorphic peak of the mafic granulite approaches the high P-T facies series, indicative of a subduction zone. Secondary-ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of metamorphic zircons suggests that the metamorphic event occurred between ca. 420 and 372 Ma. These data further certify that the subduction of the continental margin and subsequent uplift of the Dunhuang orogenic belt represent a long-lived tectono-metamorphic event in the Paleozoic.

Garnet amphibolites can provide valuable insights into geological processes of orogenic belts, but their metamorphic evolution is still poorly constrained. Garnet amphibolites from the Wutai–Hengshan area of the North China Craton mainly consist of garnet, hornblende, plagioclase, quartz, rutile and ilmenite, with or without titanite and epidote. Four samples selected in a south–north profile were studied by the pseudosection approach in order to elucidate the characteristics of their metamorphic evolution, and to better reveal the northwards prograde change in P–T conditions as established previously. For the sample from the lower Wutai Subgroup, garnet exhibits obvious two‐substage growth zoning characteristic of pyrope (Xpy) increasing but grossular (Xgr) decreasing outwards in the core, and both Xpy and Xgr increasing outwards in the rim. Phase modelling using thermocalc suggests that the garnet cores were formed by chlorite breakdown over 7–9 kbar at 530–600 °C, and rims grew from hornblende and epidote breakdown over 9.5–11.5 kbar at 600–670 °C. The isopleths of the minimum An in plagioclase and maximum Xpy in garnet were used to constrain the peak P–T conditions of ~11.5 kbar/670 °C. The modelled peak assemblage garnet + hornblende + epidote+ plagioclase + rutile + quartz matches well the observed one. Plagioclase–hornblende coronae around garnet indicate post‐peak decompression and fluid ingress. For the samples from the south Hengshan Complex, the garnet zoning weaken gradually, reflecting modifications during decompression of the rocks. Using the same approach, the rocks are inferred to have suprasolidus peak conditions, increasing northwards from 11.5 kbar/745 °C, 12.5 kbar/780 °C to 13 kbar/800 °C. Their modelled peak assemblages involve diopside, garnet, hornblende, plagioclase, rutile and quartz, yet diopside is not observed petrographically. The post‐peak decompression is characterized by diopside + garnet + quartz + melt = hornblende + plagioclase, causing the diopside consumption and garnet compositions to be largely modified. Thus, the pesudosection approach is expected to provide better pressure results than conventional thermobarometry, because the later approach cannot be applied with confidence to rocks with multi‐generation assemblages. U–Pb dating of zircon in the Wutai sample records a protolith age of c. 2.50 Ga, and a metamorphic age of c. 1.95 Ga, while zircon in the Hengshan samples records metamorphic ages of c. 1.92 Ga. The c. 1.95 Ga is interpreted to represent the pre‐peak or peak metamorphic stages, and the ages of c. 1.92 Ga are assigned to represent the cooling stages. All rocks in the Wutai–Hengshan area share similar clockwise P–T morphologies. They may represent metamorphic products at different crustal depths in one orogenic event, which included a main thickening stage at c. 1.95 Ga followed by a prolonged uplift and cooling after 1.92 Ga.

Geosphere, April 2018, v. 14, p. 883-906, doi:10.1130/GES01514.1, Supplemental Item A. Analytical methods.

High-pressure and ultrahigh-pressure minerals tend to be preserved in mafic and ultramafic metamorphic rocks, such as eclogites and garnet amphibolites, rather than felsic rocks. Generally, the garnet amphibolites preserve particular porphyroblastic and corona textures that provide important information of geological processes. Therefore, identification of garnet amphibolite might hint that subduction or collision processes were likely to have occurred.The Yili Block is one microcontinent in southwest of Central Asian Orogenic Belt, with Precambrain basement rocks exposed in the northern and southern margin. The Middle to Late Ordovician arc-type magmatic rocks were identified in the northern margin of the Yili Block with a subduction-related calc-alkaline affinity infer that the southward subduction of the Junggar Ocran beneath the Yili Block, but the record of coeval metamorphism is rarely reported. The Toksai garnet amphibolites idientified from the Wenquan Group in the northern margin of Yili Block records a clockwise P-T-t path. Its near isothermal depressive retrogressive metamorphism was typical characteristic of the Western Alps P-T path, recording the process of subduction and collision. The protolith belongs to tholeiite, with high TiO2 and low K2O+Na2O contents (3.10~3.89 wt.%, 0.76~2.01 wt.% respectively), enrichment of large ionic lithophile elements and depletion of high field strength elements, and enrichment of rare earth elements, showing the geochemical characteristics of tholeiite in intra-continental rift setting (Th/Ta=1.70~2.76, Ta/Hf=0.23~0.37). The geochemical characteristics reveal that the magmatic rocks derived from an OIB-like mantle source. The garnet amphibolites also has low contents of MgO (4.82~6.40 wt.%), Cr (70.8~224 ppm), Ni (9.68~65.7 ppm) and low values of Mg# (34.0~41.3), Nb/U (14.3~36.3), Nb/Ta (9.70~16.2), indicating that their protolith are not primitive magma, were formed by separate crystallization of different mineral phases with a small amount of crustal contamination. The zircon U-Pb dating results suggest that the garnet amphibolites protolith was formed in the middle to late Neoproterozoic, and the metamorphic age is end of Late Ordovician (450~440 Ma). The zircon and monazite from surrounding rocks also record the coeval tectonic thermal event. Consequently, it is inferred that the protolith of the garnet amphibolites may have formed in an intraplate rifting setting as a result of the breakup of Rodinia, and indicating that the Yili Block maybe a continental fragment separated from the Tarim Block during the middle to late Neoproterozoic. In the Middle to Late Ordovician, the Wenquan Group as a part of Aktau-Wenquan contineantal domain was involved in the continental–arc collision and continuing accretion in north of the Yili/Kazakhstan Block with the southward subduction of the Junggar–Balkhash oceanic lithosphere, and experience high amphibolite facies metamorphism in the end of Ordovician.

Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

Metamorphic evolution of (ultra)-high-pressure subduction-related transient crust in the South Tianshan Orogen (Central Asian Orogenic Belt): Geodynamic implications

The eastern Paleo-Tethyan Jinshajiang–Ailaoshan–Song Ma orogenic belt in the Southeast Tibetan Plateau represents the tectonic boundary between the Indochina and South China blocks. Regional Cenozoic lithospheric strike-slip movements caused by the India–Asia collision have modified many of the pristine geological records associated with the Indosinian orogeny. The lack of reliable petrological evidences, particularly of high-pressure (HP) metamorphism, has hindered the tracing of the evolutionary history of the Paleo-Tethyan orogenic belt. We report the mineralogy, geochemistry, geochronology and thermodynamic modelling of eclogite lenses/blocks and their host garnet–phengite schists from the Song Ma Suture Zone of Northwest Vietnam and reveal their protolith origins and metamorphic evolution. The eclogites are geochemically similar to mid-ocean ridge basalt, showing weak depletions in high field-strength elements, positive to slightly negative whole-rock ɛNd(t) ratios (−1.69 to +5.15) and slightly high 87Sr/86Sr(t) ratios (0.70466–0.70834). The igneous protolith of the eclogites formed in the late Silurian (425.4 ± 3.6 Ma; igneous zircon dating, 638–459 Ma TDM dating), which makes them the oldest known mid-ocean ridge mafic rocks in an ancient Paleo-Tethyan Ocean. Various eclogites preserve similar peak mineral assemblages (garnet + omphacite + phengite + rutile + quartz ± epidote/clinozoisite ± kyanite ± winchite) under indistinguishable peak conditions. Their prograde P–T trajectories show a uniform pattern of progressive heating and weak compression along a geothermal gradient of 8°C–10°C/km, indicating a warm subduction setting at a low angle that contrasts with the cold oceanic subduction recorded in the adjacent Paleo-Tethyan suture zones. Combined with detrital U–Pb ages from previous studies, our geochronological data indicate that the host schists originated from continental crust within the South China Block and underwent a pervasive Triassic HP metamorphism related to the Indosinian orogeny. The eclogites and host schists yielded HP metamorphic ages of 239–234 Ma and retrogressive amphibolite-facies metamorphic ages of 231–229 Ma, suggesting rapid cooling (14.0°C–21.6°C/Myr) and exhumation (6.4 km/Myr) from the mantle to the crust. The Song Ma eclogites and their host schists formed when the final ocean closure transitioned to the initial Indochina–South China collision. The Song Ma eclogites and their host schists provide essential information on the opening and closure of the eastern Paleo-Tethys and the subsequent amalgamation of Southeast Asian continental fragments.

<p>A 1:50000 regional survey, covering an area of about 2000 km<sup>2</sup>, was carried out in the Shangrimuce area of Qilian Mountain in Northwest China. The results show that during Caledonian, the northern margin of the Central Qilian block experienced collision with mature island arcs and subsequently northward expansion. In the Shangrimuce study area, five geological units have been identified; they are, form south to north, back-arc basin, early Ordovician island arc, inter arc basin, middle Late Ordovician island arc, and fore-arc and oceanic lithosphere amalgamation zone. </p><p>(1) back-arc basin. In the Yangyuchi- Shule River- Cuorigang- Wawusi area, there may be a back-arc spreading basin, and there should be spreading basins in this area. It is speculated that there was a northward reverse subduction in the late Ordovician, accompanied by a syenite body, a broad spectrum dyke swarms and an accretionary wedge zone in the whole area.</p><p>(2) early Ordovician island arc. In the Shangrimuce-Dander area, the Proterozoic basement granitic gneiss, the early Ordovician island arc block and the high-pressure geological body all occur in the form of thrust horses, forming a double metamorphic belt, which reveals the existence of ocean subduction to south in the early Ordovician. </p><p>(3) inter arc basin. On both banks of Tuolai River to the east of Yanglong Township, there are early Middle Ordovician inter-arc basins with oceanic crust. </p><p>(4) middle Late Ordovician island arc. To the north of Tuolai River, there is a middle Late Ordovician island arc belt. Both sides of the island arc zone experienced strong ductile shear deformation, which recorded a complex arc-continent collision. </p><p>(5) fore-arc and oceanic lithosphere amalgamation zone (Fig.1). The Yushigou area has developed a fore-arc and oceanic lithospheric amalgamation zone, with weakly deformed fore-arc flysch basin, strongly deformed siliceous rocks, pillow Basalt, diabase, gabbro, peridotite and other rock assemblages.</p><p>Combined with the characteristics of arc-continent collision zone in the Western Pacific, there are two stages of shear zone series (Fig.2). One is ductile shear zones formed by the South dipping gneissic belt, revealing the existence of oceanic subduction accretion wedge and emplacement of high-pressure rocks. Another superimposed one is north dipping. This indicates that the arc-continent collision caused by back-arc reverse subduction, which ultimately controls the overall geometric and kinematic characteristics of the shear zones in the region.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.8219836ca50067454890161/sdaolpUECMynit/12UGE&app=m&a=0&c=40b3389c641f2d0ca723e1527c32927e&ct=x&pn=gepj.elif&d=1" alt=""></p><p>Figure 1 United sections showing a Caledonian trench-arc system in the Qilian Mountain, NW China.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.8def566da50066084890161/sdaolpUECMynit/12UGE&app=m&a=0&c=e82258ecc235c4e618abd6c035b58232&ct=x&pn=gepj.elif&d=1" alt=""></p><p>Figure 2 Structural analysis at Hongyahuo, indicating two stages of deformation.</p><p>The research has been supported by projects from the Ministry of Land and Resources (No.201211024-04; 1212011121188) and the 2020 undergraduate class construction project from China University of Geosciences (Beijing) (No. HHSKE202003).</p><p> </p>

A synthesis of Carboniferous stratigraphy in Xinjiang, Northwest China with emphasis on regional and global correlations

Metamorphic evolution of two types of garnet amphibolite from the Qingyuan terrane, North China Craton: Insights from phase equilibria modelling and zircon dating

The well-preserved seamount buildups are documented from the northwestern Qinling (秦岭) orogenic belts, Northwest China. The study sections are located in the Ganjia (甘加) area of the Xiahe (夏河) County, Gansu (甘肃) Province. The dark basalt and overlying massive reef carbonate characterize the Xiahe seamount buildup. Basalts are dominated by the olivine type of rocks and bear distinct porphyritic textures, and fumarole and amygdaloidal structures. The basalts are dominated by SiO2 (up to 48.49 wt.%–52.29 wt.%) followed by (Na2O+ K2O) (3.80 wt.%–4.96 wt.%) and TiO2 (2.04 wt.%–2.52 wt.%). They are featured by considerably high content of Ti. The tholeiite-series rocks dominate the basalts, while calc-alkali-series rocks are also present. The REE of the basalts shows the LREE-enrichment type with distinct positive Eu abnormal. The trace elements of the basalts are characterized by the lack of P and high content of Ti. These geochemical signals suggest that the Xiahe basalts were formed in an ocean-island setting. The LA ICP-MS zircon U-Pb age of the basalts is 267.6±5 Ma, which is reinforced by the presnce of the fusulinid Neoschwagerina Zone of the Wordian (Middle Permian) in the limestone interbeds of the basalts. Integration of petrological and geochemical studies of seamount basalts and lateral correlation of seamount buildups reveals that the Qinling-Qilian-Kunlun orogenic belts were probably the archipelagic oceans during the Permian.

Anatomy of zircon growth in high pressure granulites: SIMS U–Pb geochronology and Lu–Hf isotopes from the Jiaobei Terrane, eastern North China Craton

As one of the major components of the Himalayan–Tibetan Orogeny, the Lhasa terrane plays a key role in understanding the origin and evolution of this giant orogenic belt and the opening and closure of the Tethys oceans. The eclogite-bearing Sumdo Complex in the central Lhasa terrane was recognized as the main suture of the Palaeo-Tethys Ocean between the north and south Lhasa sub-terranes. Despite the eclogite having been studied for a long time, no attempts have been applied to studying the country rocks, causing confusion in understanding the relationship between the eclogite and the adjacent schist. Petrological investigations and phase equilibrium calculations on the garnet-bearing mica schist of the Sumdo Complex have been performed to constrain its metamorphic evolution. The P–T conditions for three metamorphic stages are constrained as P1 (480–500°C, 2.6–2.7 GPa), P2 (580–600°C, 1.3–1.4 GPa) and R (530°C, 0.9 GPa), which represent the prograde, temperature peak and retrograde stages. Two possible P–T paths were constructed, which experienced isothermal decompression (PT1) or heating with a decompression process (PT2), corresponding to the growth and extinction of garnet porphyroblasts in the matrix. The LA-MC-ICP-MS U–Pb dating method yielded a metamorphic age of 229.7±3.5 Ma, which was interpreted as the age of amphibolite-facies metamorphism at c. 600°C, 1.2–1.4 GPa during the closure of the Palaeo-Tethys Ocean, resulting in the aggregation of the north and south Lhasa sub-terranes. The close relationship between the eclogite and garnet-bearing mica schist, and their similar P–T–t paths indicate an in situ tectonic evolution rather than tectonic juxtaposition during exhumation.

The Dabie orogen has been primarily studied through metaigneous rocks, whereas metasedimentary rocks remain poorly constrained in metamorphic evolution and protolith ages. This study presents petrography, mineral chemistry, phase equilibrium modelling, and geochronology of two representative metasedimentary rocks. Jadeite quartzite records four metamorphic stages (M1, M2, M3, and M4) with pressure and temperature (P-T) of 3.25–3.33 GPa/595–647 °C; 2.51–2.73 GPa/493–543 °C; 1.0–1.1 GPa/629–691 °C; and <0.8 GPa/317 ± 43 °C, respectively. However, the epidote-biotite-plagioclase gneiss records only retrograde metamorphism. Zircon and rutile U-Pb dating yields two age clusters: 234–225 Ma for ultrahigh-pressure metamorphism (UHPM) and 204–203 Ma for retrogression. Detrital zircon age spectra, with a youngest population at ~440 Ma, indicate Paleozoic deposition. Integrated with protolith ages of the Susong Complex and Foziling Group, these metasedimentary rocks of the Dabie orogen are interpreted as Paleozoic sediments deposited along the northern passive margin of the Yangtze Plate. Compared with P-T-t paths of UHPM rocks, the studied units record a complex exhumation path marked by initial decompression with cooling, subsequent decompressional heating, and late-stage decompressional cooling, highlighting the critical role of metasedimentary rocks in resolving the tectonic architecture of the Dabie orogen.

The Eastern Kunlun Orogenic Belt (EKOB) in Northwest China, is an ideal area for understanding the tectonic evolution of the Palaeo-Tethys Ocean. The EKOB is marked by widespread Palaeozoic to Early Mesozoic magmatism. However, the Late Permian–Early Mesozoic tectonic evolution of the Palaeo-Tethys Ocean tectonic regime is poorly understood. Here, we report new zircon LA-ICP-MS U–Pb dating and geochemical analyses of Late Permian–Early Triassic intrusive rocks in the Xiangyanggouxi and Dageledong areas in the EKOB, Northwest China, to discuss their petrogenesis and geodynamic setting and reconstruct the tectonic evolution of Palaeo-Tethys oceanic tectonic regime. Zircon U-Pb dating results indicate that these granitoids crystallized during ca. 256–248 Ma, i.e. the Late Permian to Early Triassic. These granitoids are mainly comprised of granodiorites and monzonitic granites that possess high concentrations of SiO2 (66.81–75.98 wt.%), K2O+Na2O (6.21–7.92 wt.%), and low contents of MgO, CaO, Cr, Ni, and Mg# (<0.44). They are classified as medium- to high-K calc-alkaline and metaluminous I-type granitoids. These granitoids are rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs) with arc affinity. In addition, combined with the εHf(t) values of −2.4 to +2.4 with two-stage model ages ranging from 1409 Ma to 1027 Ma, and the εNd(t) values from −3.73 to −2.95, it is suggested that the primary magma of these granitoids was derived from the partial melting of the Late Meso–Proterozoic mafic lower crust. These findings, combined with spatio-temporal distributions of regional magmatism, reveal that the Late Permian and Early Triassic granitoids formed in a continental magmatic arc setting related to the northward subduction of the Palaeo-Tethys Ocean plate beneath the EKOB-Qaidam terrane.