The generation and evolution of Archean continental crust in the Dunhuang block, northeastern Tarim craton, northwestern China

Multistage anatexis during tectonic evolution from oceanic subduction to continental collision: A review of the North Qaidam UHP Belt, NW China

Continental orogens on Earth can be classified into accretionary orogen and collisional orogen. Magmatism in orogens occurs in every periods of an orogenic cycle, from oceanic subduction, continental collision to orogenic collapse. Continental collision requires the existence of prior oceanic subduction zone. It is generally assumed that the prerequisite of continental deep subduction is oceanic subduction and its drag force to the connecting passive-margin continental lithosphere during continental collision. Continental subduction and collision lead to the thickening and uplift of crust, but the formation time of the related magmatism in orogens depends on the heating mechanism of lithosphere. The accretionary orogens, on the other hand, have no strong continental collision, deep subduction, no large scale of crustal thrusting, thickening and uplift, and no UHP eclogite-facies metamorphic rocks related to continental deep subduction. Even though arc crust could be significantly thickened during oceanic subduction, it is still doubtful that syn- or post-collisional magmatism would be generated. In collisional orogens, due to continental deep subduction and significant crustal thickening, the UHP metamorphosed oceanic and continental crusts will experience decompression melting during exhumation, generating syn-collisional magmatism. During the orogen unrooting and collapse, post-collisional magmatism develops in response to lithosphere extension and upwelling of asthenospheric mantle, marking the end of an orogenic cycle. Therefore, magmatism in orogens can occur during the continental deep subduction, exhumation and uplift after detachment of subducted oceanic crust from continental crust, and extensional collapse. The time span from continental collision to collapse and erosion of orogens (the end of orogenic cycle) is 50-85 Myr. Collisional orogens are the key sites for understanding continental deep subduction, exhumation, uplift and orogenic collapse. Magmatism in collisional orogens plays important roles in continental reworking and net growth.

Magmatism during continental collision, subduction, exhumation and mountain collapse in collisional orogenic belts and continental net growth: A perspective

Abstract The Dabie–Sulu orogenic belt in east-central China contains one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world. The UHP eclogites are associated with gneiss, peridotite and marble. But all these rocks underwent in situ UHP metamorphism during the continental collision in the Triassic. Although fluid action is not significant during cold subduction, it becomes prominent during hot exhumation of UHP slices. Whereas the composition of UHP metamorphic rocks is primarily inherited from their protoliths, they were locally modified by partial melting to varying extents. The partial melting of UHP rocks is highly heterogeneous along the collisional orogen, and achieves a maximum during rifting orogeny at the post-collisional stage. This paper outlines the petrographical features and geochemical compositions of UHP metamorphic rocks, and presents an overview of collisional modification and post-collisional reworking of the subducted continental crust. Further discussions are devoted to continental subduction tectonics, as well as to subduction style, subduction polarity and exhumation mechanism. The results also provide insights into the initiation of continental rifting in a Wilson cycle. Consequently, studies of the Dabie–Sulu orogenic belt have contributed greatly to our understanding of tectonic processes, fluid regime and chemical geodynamics in continental subduction zones.

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Ultrahigh-pressure metamorphic rocks in the Dabie-Sulu orogenic belt: compositional inheritance and metamorphic modification

Timing and mechanisms of multiple episodes of migmatization in the Korla Complex, northern Tarim Craton, NW China: Constraints from zircon U–Pb–Lu–Hf isotopes and implications for crustal growth

Thin layers and lenses of granitic leucosome are widely distributed within amphibolites, paragneisses and orthogneisses of the Sulu UHP terrane. They are parallel to, or cross-cut, foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre- to decimetre-scale segregations. Variously migmatized rocks extend at least 350 km from SW Sulu (Maobei) to NE Sulu (Weihai), in a band at least 50 km wide. A combined study of mineral inclusions, cathoduluminescence (CL) images, U–Pb LA-ICP-MS dates, and in-situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these UHP rocks. Most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited (igneous or metamorphic) cores or as new, euhedral crystals. The overgrowths and new crystals commonly show perfectly euhedral shapes, have pronounced oscillatory zoning and contain felsic mineral inclusions, such as Kfs + Pl + Qtz ± Ilm ± monazite (Mon). In contrast, the inherited igneous or metamorphic cores are rounded or irregular, contain low-P or UHP mineral inclusions and show clear dissolution textures. These data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the UHP rocks. The REE patterns of the anatectic zircon show steep slopes from the HREE to LREE with strongly to moderately negative Eu anomalies (Eu/Eu* = 0.31–0.72) and pronounced positive Ce anomalies (Ce/Ce* = 6.8–26.5). Abundant U–Pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the Sulu UHP terrane. Anatectic zircon from 12 granitic leucosomes within amphibolites, paragneisses, and orthogneisses from Sulu UHP slices II and III yields consistent mean U–Pb ages of 219.0 ± 1.2 to 218.3 ± 1.6 Ma, 218.8 ± 2.0 to 217.3 ± 1.7 Ma and 218.2 ± 1.4 to 215.0 ± 1.5 Ma, respectively. In contrast, anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from Sulu UHP slice III records younger mean U–Pb ages of 151.9 ± 1.3 to 151.1 ± 1.8 Ma and 155.9 ± 1.8 to 153.7 ± 1.7 Ma, respectively. These data imply that the Sulu UHP terrane experienced two Mesozoic partial melting events. The first partial melting event (219–215 Ma) was probably associated with a Late Triassic granulite facies stage of ‘hot’ exhumation, whereas the second (156–151 Ma) is interpreted as the result of Middle-Late Jurassic extension and thinning of the previously thickened crust of the Sulu UHP terrane. Both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks.

P. H. Liu

Papers

Multiple partial melting events in the Sulu UHP terrane: zircon U–Pb dating of granitic leucosomes within amphibolite and gneiss