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Coesite from the Dabie Shan Eclogite, Central China
Abstract
A coesite inclusion in garnet from an amphibolitized eclogite is described from the Dabie Shan region in central China. The eclogite with coesite occurs as a nodule (boudin?) in a several metres thick marble band in a regional eclogite facies terrain dominated by leucocratic garnet-bearing gneisses and eclogite bands. Probable quartz pseudomorphs after coesite have also been discovered as inclusions in garnet and omphacite from other localities in the Dabie Shan region, suggesting that a crustal slice of over 1000 km2 in area has been subducted to depths of over 90 km. -Authors
... Eclogite, a typical metamorphic rock exposed in the ocean-continent/continent-continent subduction zone, is of great significance for investigating the formation and evolution of orogenic belts (Wei et al. 2009;Li et al. 2018Faryad et al. 2019;Miladinova et al. 2022;Pan et al. 2023). Since the discovery of ultrahigh-pressure minerals, such as coesite and diamond, in the eclogites and its country rocks of the Dabie-Sulu orogenic belt in eastern China (Okay et al. 1989;Wang 1989;Xu et al. 1992), this orogenic belt has become a classic area for studying high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks (Chopin 2003;Zheng et al. 2003;Liou et al. 2004;Ernst et al. 2007;Stern et al. 2018). ...
... Since the 1980s, the understanding of subduction depth of crustal materials has been continually refined based on the discovery of ultra-high pressure minerals and special exsolution structures in eclogites from the Sulu and Dabie orogenic belts. The initial discovery by Okay et al. (1989) found coesite within eclogites in the Dabie orogenic belt, suggesting a minimum subduction depth of 90 km. Further evidence came from the presence of fine-grained diamonds as inclusions in garnets of eclogites in the eastern Dabie orogenic belt, indicating pressures greater than 4.0 GPa and temperatures higher than 900°C and implying a formation depth of over 120 km ). ...
... Influenced by the Tanlu fault zone (a left-lateral strikeslip fault), the Sulu orogen has been displaced northward by about 500 km in relation to the Dabie orogen ( Figure 1A; Okay, 1993;Zhu et al., 2005). The presence of ultrahigh-pressure (UHP) metamorphic minerals such as coesite (Okay et al., 1989;Wang et al., 1989; and diamond (Xu et al., 1992;2003;Xu et al., 1998;Liu et al., 2007) in the metamorphic rocks of this orogenic belt indicate that crustal rocks have subducted to mantle depths of >100 km and subsequently exhumated to the crustal level. The UHP metamorphic rocks in the Dabie orogen are dominated by orthogneiss with subordinate eclogite, granulite, amphibolite, migmatite and marble (Zheng et al., 2003). ...
Post-collisional alkaline intrusive rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Hf-Pb isotopes, as well as zircon U-Pb ages and Hf-O isotopes. The results provide geochemical constraints on the nature of their mantle sources and thus insight into crust-mantle interaction in the continental collision zone. The alkaline intrusive rocks are composed of syenite and nepheline syenite. Syn-magmatic zircon U-Pb dating by LA-ICP-MS for them yielded Early Cretaceous ages of 131.3 ± 1.4 Ma to 122.6 ± 0.6 Ma, coeval with the post-collisional magmatism in the Dabie orogen. One relict zircon with U-Pb age of 211 Ma is consistent with the timing of metamorphism for the ultrahigh-pressure (UHP) metamorphic rocks in this orogen. They have arc-like trace element distribution patterns, such as enrichment in LILE (large ion lithophile element) and LREE (light rare earth element) but depletion in HFSE (high field strength element), and enriched whole-rock Sr-Nd-Hf isotope compositions with high (⁸⁷Sr/⁸⁶Sr)i ratios of 0.7077–0.7131 but negative εNd(t) values of −16.0 to −9.4 and εHf(t) values of −17.5 to −12.7. Their syn-magmatic zircons have three groups of Hf-O isotope compositions comparable to those of UHP metamorphic rocks in Central-South Dabie and North Dabie, which represent the upper and middle continental crust of the subducted South China Block, respectively. In this regard, slab–mantle interaction is evident during the Triassic continental collision. We suggest that the melts derived from the subducted South China Block reacted with the lithospheric mantle wedge of the North China Block, resulting in phlogopite-bearing metasomatites, whose partial melting would generate the post-collisional alkaline intrusive rocks during the Early Cretaceous.
... The Dabie Shan in central China was created by the collision of the South China block and the North China block around 240 Ma (Li et al., 1999;Zheng, 2008). The area is renowned for its vast collection of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks, including eclogite, gneiss, and marble (e.g., Okay et al., 1989;Xu et al., 1992). UHP marble, a significantly carbon-rich rock type, is frequently found in coexistence with UHP eclogite in the Dabie Shan region (e.g., Liu et al., 2006Liu et al., , 2015. ...
Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.
... The Sulu terrane is regarded as the eastern extension of the Dabie terrane which was displaced by the Tanlu fault (TLF) (Fig. 1a). Coesite and diamond have been identified in the terrane (Okay et al., 1989;Xu et al., 1992). The Dabie orogen is subdivided into five major lithotectonic units from north to south, including (i) the Beihuaiyang zone (BZ); (ii) the North Dabie high-T/UHP zone (NDZ); (iii) the Central Dabie mid-T/UHP zone (CDZ); (iv) the South Dabie low-T eclogite zone (SDZ) and (v) the Susong metamorphic zone (SZ) (Liu et al., 2007c;Liu et al., 2011a). ...
In the Dabie orogen, central China, the ultrahigh pressure metamorphism (UHPM) was best recorded from meta-mafic and ultramafic rocks occurring as lenses within voluminous amphibolite-facies metafelsic rocks. However, whether or not these metafelsic rocks experienced ultra-deep subduction (>5 GPa) has been rarely quantified because of the rare preservation of peak diagnostic assemblages. In this study, we reconstruct the P-T-t paths for paragneisses and metagranites from the Dabie region to better
elucidate their metamorphic evolution based on detailed petrological, mineralogical and Raman
micro-spectrometer analysis of zircon inclusion combined with conventional geothermobarometry,
phase equilibrium modelling and U-Pb dating. At least three stages of metamorphic evolution are identified for these lithologies. The UHPM is represented by the assemblages of coesite + garnet Ia/Ib + jadeite + phengite + K-feldspar + aragonite for paragneiss and diamond + aragonite + magnesite + phengite + rutile + garnet for metagranite, respectively. The paragneiss experienced peak conditions of 4.5–6.0 GPa/690–720°C, followed by a ecompression heating to 3.5–4.6 GPa/750–850 °C. Also, the diamond + aragonite + magnesite inclusions in zircon from the metagranite provide the first direct mineralogical evidences for ultra-deep subduction. Furthermore, the minimum peak pressure was defined at 5.4 GPa based
on pseudosection for metagranite. As a result, this study provides the first mineralogical and modelling evidences of ultra-deep subduction for the metafelsic rocks in the Dabie orogen. The dating results suggest that the UHPM, anatexis and amphibolite-facies overprint occurred at 236 ± 3 Ma, 226 ± 1 Ma and 195 ± 4 Ma, respectively. The estimated average exhumation rate is 5.8–7.2 km/Ma from peak UHP to amphibolite-facies stages. Moreover, a comparison of different collisional orogens suggests that the preservation of UHP minerals in metafelsic rocks strongly depends on exhumation rate.
... High-grade metamorphic rocks such as granulite and eclogite are mainly exposed in the North, Middle, and South Dabie UHP zones. Since the discovery of the UHP index minerals such as coesite and diamond (Okay et al., 1989;Xu et al., 1992), the Dabie orogen has long been a focused area to study continental subduction. The metamorphic units exposing eclogite and granulite have been extensively studied. ...
The China Central Orogenic System (CCOS), extending in an east-west direction in the middle part of China, is composed of the Early Paleozoic Altyn-North Qilian-North Qaidam-East Kunlun-North Qinling-North Tongbai orogens in the west and the Late Paleozoic to Early Mesozoic South Tongbai-Hong’an-Dabie-Sulu orogens in the east. They were produced by oceanic subduction and continental subduction/collision during the closure of the Proto-Tethys and the Paleo-Tethys oceans, respectively. Different types of metamorphic rocks with various ages are extensively exposed in these orogens, and they were produced at different geothermal gradients in different stages during the tectonic evolution of convergent continental margins, making them ideal targets to reconstruct the spatiotemporal evolution of the Eastern Tethys tectonic domain. In this article, an integrated study of metamorphic temperature (T)-pressure (P)-time (t) records is presented for metamorphic rocks along the CCOS, aiming to ascertain the change of metamorphic T/P ratios in both time and space, and then shed light on the tectonic evolution of the East Tethys tectonic domain in association with the thermal state change of convergent continental margins. The results indicate that despite the difference in metamorphic ages, metamorphic rocks in different orogens show a common trend with clockwise P-T-t paths. With respect to plate convergence for subduction and collision, regional metamorphism is categorized into three stages: (1) an early convergent stage, corresponding to low T/P Alpine-type blueschist- to eclogite-facies high-P to ultrahigh-P metamorphism; (2) a later convergent stage, corresponding to the medium T/P Barrovian-type medium-P amphibolite to high-P granulite-facies metamorphism; and (3) a post-convergent stage, corresponding to the high T/P Buchan-type low-P amphibolite to MP granulite-facies metamorphism. Nonetheless, a few metamorphic rocks only record a two-sage metamorphic evolution, with an early Barrovian-type high-P granulite-facies metamorphism and a late Buchan-type low-P granulite-facies metamorphic overprinting. In modern convergent plate margins, Alpine-type metamorphism mainly occurs in the stages of oceanic subduction and continental collision, Barrovian-type metamorphism takes pace in both stages of crustal thickening during continental hard collision and slab exhumation when continental subduction zones have evolved from compressional to extensional regimes, and Buchan-type metamorphism occurs in intracontinental rifting stage after the plate convergence. Therefore, the tectonic evolution of convergent continental margins can be reconstructed by combining metamorphic T/P ratios with their corresponding metamorphic facies series and metamorphic timing of metamorphic rocks. Based on the reported metamorphic rocks of different types and ages along the CCOS, it appears that the continental subduction/collision occurred at 500–490 Ma in the Altyn-North Qinling-North Tongbai orogens but 450–430 Ma in the North Qaidam-East Kunlun orogens, and the intracontinental rifting occurred at 460–450 Ma in the Altyn-North Qinling-North Tongbai orogens but 410–400 Ma in the North Qaidam-East Kunlun orogens, respectively, in the western Proto-Tethys domain. For the eastern Paleo-Tethys domain, in contrast, the continental subduction/collision occurred at 250–220 Ma and post-collisional intracontinental rifting occurred at 140–120 Ma. Furthermore, metamorphic evolution from low T/P ratios in the subduction/collision stage to high T/P ratios in the intracontinental rifting stage needs 40–60 Myr in the Proto-Tethys domain but about 110 Myr in the Paleo-Tethys domain. For the two different orogenic domains, therefore, the convergent continental margins underwent a common tectonic evolution from warm collision/cold subduction to hot rifting, which starts from continental subduction/collision characterized by the formation of medium-P amphibolite to high-P granulite facies series or high-P to ultrahigh-P eclogite facies series in compressional regimes, through exhumation of the deeply subducted crustal rocks, and terminates with intracontinental rifting featured by high-T to ultrahigh-T granulite facies series in extensional regimes.
Zircon is a common mineral in nature that survives varied pressure and temperature conditions in the subduction process. It has excellent ability to reveal progressive metamorphic history. Hence it is useful in reconstruct the subduction tectonics in the collisional orogenic belts. In the Tso Morari Gneiss of Indus Suture Zone, Himalaya, eclogite boudins have registered imprint of subduction related ultrahigh-pressure (UHP) metamorphism, this imprint is however missing in the host gneisses. To search the missing link, zircons of the gneisses are studied. The zircons overgrowth and the numerous mineral inclusions indicating, metamorphic responses of the gneisses. The Raman spectra of minerals show, cores of the zircon consist of apatite and quartz, and in the surrounding overgrowth preserves quartz-coesite, c-polymorphs, and other metamorphic minerals. The distribution pattern of these minerals in the zircons is consistent with the Th/U ratios ranging 0.30 to 0.01 recognizes inner magmatic and outer metamorphic domains. The U-Pb ages from inner magmatic, at c. 500 Ma, and from outer metamorphic growth at c.45-42 Ma, suggests the former is the protolith age and later is metamorphic ages of the gneisses. The tectonic interpretation reveals, the subduction of Indian felsic crust to UHP depth (>100km) at c. 45 Ma.
Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change
The timing of the initiation of the present‐day tectonic architecture and drainage systems in eastern China remains debated. This study presents a comprehensive provenance study of the Early Jurassic peripheral basins surrounding the Dabie orogen including framework petrography, heavy‐mineral analysis, single‐grain chronology and chemistry. Clasts of high‐grade schist, muscovite grains, rare gneissic fragments, abundant metamorphic garnet and phengite (Si > 3.3 pfu), combined with a main 216–256 Ma rutile U–Pb population found in these Early Jurassic sandstones, indicate a source from the Triassic (U)HP belt in the Dabie orogen. Sedimentary lithics and ultra‐stable heavy‐mineral assemblages indicate an additional source of recycled sedimentary rocks. Combined with the continuous shift of the youngest detrital rutile age population toward younger ages toward the north that mimics the pattern of metamorphic bedrock ages in the Dabie orogen, we infer that the present surface tectonic architecture and paleodrainage patterns of the Dabie orogen were established in the Early Jurassic. Thus, the Early Jurassic exhumation of the Dabie orogen marked the development of the watershed between Northern and Southern China, namely the Huai River and several principal tributary systems of the middle‐lower Yangtze River.
The continental crust is strongly depleted in copper compared with its building blocks-primary arc magmas-and this depletion is intrinsically associated with continental crust formation. However, the process by which Cu removal occurs remains enigmatic. Here we show, using Cu isotopes, that subduction-zone processes and mantle melting produce limited fractionation of Cu isotopes in arc magmas, and, instead, the heterogeneous Cu isotopic compositions of lower crustal rocks, which negatively correlate with Cu contents, suggest segregation or accumulation of isotopically light sulfides during intracrustal differentiation of arc magmas. This is supported by the extremely light Cu isotopic compositions of lower crustal mafic cumulates and heavy Cu isotopic compositions of differentiated magmas in thick continental arcs. Intracrustal differentiation of mantle-derived magmas and subsequent foundering of sulfide-rich mafic cumulates preferentially removes isotopically light Cu, leaving a Cu-depleted and isotopically heavy continental crust.
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