The Xing'an-Inner Mongolia accretionary belt in the southeastern segment of the Central Asian Orogenic Belt (CAOB) was produced by the long-lived subduction and eventual closure of the Paleo-Asian Ocean and by the convergence between the North China Craton and the Mongolian microcontinent. Two ophiolite belts have been recognized: the northern Erenhot-Hegenshan-Xi-Ujimqin ophiolite belt and the southern Solonker-Linxi ophiolite belt. Most basalts in the northern ophiolite belt exhibit characteristics of normal-type to enriched-type mid-ocean ridge basalt affinities with depleted Nd isotopic composition (epsilon(Nd)(t)>+5), comparable to modern Eastern Pacific mid-ocean ridge basalts. Most basaltic rocks in the southern belt show clear geochemical features of suprasubduction zone-type oceanic crust, probably formed in an arc/back-arc environment. The inferred back-arc extension along the Solonker-Linxi belt started at circa 280 Ma. Statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading/subduction, which gave rise to two main epochs of magmatic activity at 500-410 Ma and 360-220 Ma, respectively, with a gap of similar to 50 million years (Myr). The spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction toward the two continental margins in the convergence history, with final collision at similar to 230-220 Ma. In the whole belt, signals of continental collision and Himalayan-style mountain building are lacking. We thus conclude that the Xing'an-Inner Mongolia segment of the CAOB experienced two cycles of seafloor subduction, back-arc extension, and final "Appalachian-type" soft collision.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015TC003948

Ophiolites in the Xing'an-Inner Mongolia accretionary belt of the CAOB: Implications for two cycles of seafloor spreading and accretionary orogenic events

Subduction zones are tectonic expressions of convergent plate margins, where crustal rocks descend into and interact with the overlying mantle wedge. They are the geodynamic system that produces mafic arc volcanics above oceanic subduction zones but high- to ultrahigh-pressure metamorphic rocks in continental subduction zones. While the metamorphic rocks provide petrological records of orogenic processes when descending crustal rocks undergo dehydration and anataxis at forearc to subarc depths beneath the mantle wedge, the arc volcanics provide geochemical records of the mass transfer from the subducting slab to the mantle wedge in this period though the mantle wedge becomes partially melted at a later time. Whereas the mantle wedge overlying the subducting oceanic slab is of asthenospheric origin, that overlying the descending continental slab is of lithospheric origin, being ancient beneath cratons but juvenile beneath marginal arcs. In either case, the mantle wedge base is cooled down during the slab-wedge coupled subduction. Metamorphic dehydration is prominent during subduction of crustal rocks, giving rise to aqueous solutions that are enriched in fluid-mobile incompatible elements. Once the subducting slab is decoupled from the mantle wedge, the slab-mantle interface is heated by lateral incursion of the asthenospheric mantle to allow dehydration melting of rocks in the descending slab surface and the metasomatized mantle wedge base, respectively. Therefore, the tectonic regime of subduction zones changes in both time and space with respect to their structures, inputs, processes and products. Ophiolites record the tectonic conversion from seafloor spreading to oceanic subduction beneath continental margin, whereas ultrahigh-temperature metamorphic events mark the tectonic conversion from compression to extension in orogens.

/pdf/continental-versus-oceanic-subduction-zones-16qv5l92hz.pdf

Continental versus oceanic subduction zones

Ophiolite in the eastern Central Asian Orogenic Belt, NE China

Lithospheric delamination in post-collisional setting: Evidence from intrusive magmatism from the North Qilian orogen to southern margin of the Alxa block, NW China

Introduction to the structures and processes of subduction zones

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

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