On the Relationship between the Two UHP Metamorphic Belts in the Central Orogenic Belt, China
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In the central erogenic belt of China, at least two UHP metamorphic belts are identified based on detailed structural analysis of the Kanfenggou and Xiangfanggou metamorphic microlithons exposed in the eastern Qinling orogen, together with previous regional structural, petrological and geochronological data at the scale of orogenic domain. The first is the South Altun—North Qaidam—North Qinling UHP metamorphic belt and the second is the Dabie—Sulu UHP and HP metamorphic belts. These two belts are separated by a series of fault-bounded tectonic slices composed of the Qinling Group, Danfeng Group and Liuling or Fuziling Group, and are different in peak age of UHP metamorphism and geodynamic implications for deep subduction and collision of the continent. Regional field and petrological relationships suggest that the Kanfenggou UHP metamorphic microlithon, which contains a large volume of coesite- and microdiamond- bearing eclogite lenses, is compatible with the rocks and structures recognized in the South Altun and North Qaidam UHP metamorphic microlithons exposed in the western part of China, and formed a large UHP metamorphic belt, up to 1000 km long along the erogenic belt. This UHP metamorphic belt represents an intercontinental deep subduction and collision belt between the Yangtze and Sino-Korean cratons, occurring during the Early Palaeozoic (500~400 Ma). On the other hand, the well-constrained Dabie—Sulu UHP and HP metamorphic belts occurred mainly during the Triassic (250~220), and were probably produced by intracontinental deep subduction and collision within the Yangtze craton. The Kanfenggou UHP metamorphic microlithon, situated in the northern Qinling erogenic belt and extending eastwards, does not appear to link with the Dabie—Sulu UHP and HP metamorphic belts, including the northwestern Dabie Mountains and the southern Dabie Mountains, along the orogen. The common occurrence of rutile and representative mineral assemblages of the Xiangfanggou microlithon exposed in the Xixia area, southern Qinling suggest that the Xiangfanggou microlithon underwent high-pressure amphibolite facies metamorphism at pressures close to those of the eclogite facies, and that the Dabie—Sulu UHP and HP metamorphic belts extend from the Tongbai Mountains across the Nanxiang basin to the eastern Qinling Mountains. Consequently, there is no reason to assume the two UHP metamorphic belts as a single giant deep subduction and collision belt in the central orogenic belt between the Yangtze and Sino-Korean cratons. Moreover, any dynamic model for the orogen must account for the development of the UHP metamorphic rocks belonging to two separate tectonic belts of different ages.Keywords:
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There is a giant Central Orogenic Belt (COB) in central China, which extends over 4000 km from West Kunlun on the west, through Altun, Qinlian, Qinling, and Dabie, to Sulu on the east. This paper is to report that micro-grains of diamond were discovered as inclusions in zircons from both eclogite and its gneissic country rocks in North Qinling, middle part of COB. SHRIMP U-Pb dating of zircons from granitic gneiss yielded 507 ?8 Ma for metamorphic rims and 1200~1800 Ma for relic magmatic or old cores, suggesting an Early Paleo-zoic ultrahigh pressure metamorphic event. The discovery indicated that supracrusts in North Qinling have been subducted to depths 120 km, which together with an Early Paleozoic suture reported in the region suggests an ealy collision between North China Plate and Yangtze Plate. Previous study indicates that an over 350 km long coesite-bearing UHP terrane of Early Paleozoic age has been discovered in Altun and Qilian, west part of COB, and an over 1000 km long diamond- and coesite-bearing UHP terrane of both Early Paleozoic and Trias-sic ages was developed in Dabie-Sulu, east part of the COB. This new microdiamond occurrence forms a bridge between two ultra-high pressure metamorphic terranes recognized in the western and eastern parts of China, thereby confirming the existence of the World's largest ultra-high pressure metamorphic belt extending 4000 km. The existence of such a belt, where the ultra-high pressure metamorphic events are dated from west to east as 500 Ma (South Altun) , 495 Ma (North Qaidam) , 507 Ma (North Qinling, this study) , or, 420~400 Ma (Qinling and northern Dabie) and 240-200 Ma (southern Dabie and Sulu) suggests that in general at least two events of large-scale plate subduction and collision, i. e. , Early Paleozoic and Mesozoic, between the North and South China plates have been recorded. Similar evidence of repeated subduction to and exhumation from great depths occurs in Western Europe in the Variscan and Alpine orogenies of Mesozoic and Tertiary ages. These examples of very deep subduction during recurrent continental collision along belts extending thousands of km suggest that UHPM is probably a common rather than an exceptional occurrence during continental colli-sion, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.
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The Sulu ultrahigh-pressure (UHP) belt has been proven to form by deep northward subduction of the Yangtze Block (YB) beneath the North China Block (NCB) in the Triassic. The basement rocks originated from the YB and experienced Triassic UHP metamorphism. In this contribution, we identify for the first time tens of non-UHP metasedimentary blocks enclosed in the regional UHP granitic gneiss in the Weihai terrane in the northeastern Sulu UHP belt. They are mainly composed of paragneiss, marble and calc-silicate rocks, in which UHP-related minerals have not been identified. Detrital zircons from the non-UHP metasedimentary rocks are mostly of magmatic origin and yield U-Pb ages mainly from 2842 to 1792 Ma with two clusters of 2580–2460 Ma and 1900–1800 Ma. This zircon U-Pb age spectrum is typical of the NCB but not the YB, and matches the tectonothermal/magmatic history of the southwestern end of the Jiao-Liao-Ji belt (JLJB), eastern NCB, which lies immediately adjacent to the Weihai terrane. Furthermore, their whole-rock geochemical compositions show great resemblance to those of the Paleoproterozoic metasedimentary rocks within the JLJB. In particular, metamorphic zircons from the non-UHP metasedimentary rocks indicate widespread late Triassic-early Jurassic (213–194 Ma) metamorphic overprinting of the Paleoproterozoic (1850–1820 Ma) metamorphism that characterizes the JLJB. This means that the enclosed non-UHP metasedimentary rocks with NCB signatures in the Weihai terrane were involved in the collision-subduction process during the Triassic. They have not been carried to mantle depths during deep continental subduction. These lines of evidence prompt us to establish a Triassic tectonic erosion model to explain these new discoveries and pre-existing problems along the Sulu UHP orogenic belt.
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Metamorphic core complex
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Metamorphic core complex
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The general geology of ultrahigh pressure(UHP) metamorphic ophiolitic belt in Zermatt-Saas, western Alps, especially the petrologic characteristics and metamorphic evolution, is introduced in this paper. Based on the comparative study of geology, petrology and geochronology between two typical UHP ophiolitic belts in the Zermatt- Saas zone, western and the Chinese Tianshan belt, the general characteristics of UHP ophiolitic belt such as lower geothermal gradient and preservation of pillow lava structure of oceanic basalt are discussed in this paper.
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Two processes are suggested to explain how UHP rocks are exhumed from mantle depths. One is removal of the overburden either by erosion or by extension, whereas the other involves the uplifting of the UHP rocks through the overburden. Application of either of these mechanisms to the Dabie Mountains, however, is fraught with difficulty. When combined with previously published data, new studies on metamorphic P-T paths, regional structures, and deep upper-mantle architecture revealed by seismic tomography lend support to a multi-stage exhumation process that operated in the Dabie Mountains. The first stage (230 to 200 Ma) is characterized by ductile deformation, produced during eclogite-facies recrystallization under a geothermal gradient as low as 10°C/km, implying a synsubduction exhumation. Some of the UHP rocks evidently were exhumed to a depth of ~60 km, as indicated by petrological study of the Shuanghe eclogite. The second stage (200 to 170 Ma) attended ductile deformation and amphibolite-facies retrograde metamorphism. Subduction of the Yangtze block was halted by slab breakoff at a depth of ∼200 km. The resultant geothermal gradient recovered to ∼20″C/km. Slab breakoff permitted buoyancy-driven ascent of the UHP low-density melange to shallow crustal levels in a diapir structure. When the UHP portion of the mountain root rose, the shallow portion was heated to a temperature higher than that of the peak metamorphic pressure. The third stage (170 to 120 Ma) is characterized by extension and thermal uplift, as well as erosion. Sedimentary basins and volcanic rocks developed on both sides of the Dabie Mountains. Gab-bro-pyroxenite intruded the hanging wall of the UHP terrane, and granite, as well as migmatite, developed in that stage. Exhumation mechanisms might include corner flow for the first stage, buoyancy-driven squeezing-up for the second stage, and crustal extension, as well as erosion, for the third. Rupture and loss of the subducted lithospheric plate generated the gravity instability that resulted in exhumation of the subducted UHP section.
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Table S1 Chemical compositions (wt%) of garnet in zircons from Danfeng amphibolite a)
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The Dabie orogen underwent deep continental subduction, rapid exhumation, and the huge amount of erosion during the Mesozoic. Its tectonic evolution, especially how its evolution was recorded by sedimentary basins at the flanks of the Dabie orogen is one of the most important issues of the world’s attention. These years, newly studies of basin sedimentology, combined with structural geology, have shown a fundamental progress. The overall distribution of different basin types in the orogen indicates that shortening and thrusting at the margins of the orogen from the Late Triassic to the Early Cretaceous controlled the foreland basins, and extension, doming and rifting were initiated in the core of the orogen from the Jurassic to the Early Cretaceous and were expanded to the whole orogen after the Late Cretaceous. Therefore, The Dabie orogen records gradual transition from overall shortening and thrusting to dominantly extension and rift basin formation expanded from its core to its margins, although these shortening and extension overlapped in time from the Jurassic through Early Cretaceous at crustal levels. The unroofing ages of the ultra-high pressure (UHP) metamorphic rocks in the Dabie orogen change from Early Jurassic to Late Jurassic westward. The depth of exhumation increases eastwards. The sediment sources for the Hefei basin are mostly composed of the deeply exhumed, axial Dabie metamorphic complex, and the sediment sources for the Middle Yangtze basin are mostly from cover strata in the southern orogen and related strata with subjacent (i.e. subsequently overthrusted) Mianlue suture belt. Geodynamic analysis represents that continental collision between the North China Block and the South China Block along the Shangdan and Mianlue sutures, subsequently northwestward progradation of the Jiangnan fold and thrust belt, and the underthrusting of the North China Block along the Northern Boundary Fault of Qinling Range led to crustal thickening, gravitational spreading and balanced rebound of the resultant thick crustal welt, and multi-episodic exhumation of the HP/UHP metamorphic rocks. The future studies by the methods of tracing the Dabie orogeny through deposition in the marginal basins should focus on eastward extension of the Mianlue suture, thrust and overlap of the Dabie HP/UHP metamorphic block on different lithotectonic zones and basins along the northern South China Block, the structural framework of the source area of the basins in the syn-depositional stage, the basin lateral extension, huge amount of erosion and sediment transportation from the Dabie blanket and basement rocks, and recovery of subducted and re- moved structural units within the Dabie orogen, etc.
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