The key to defining the termination of accretion in an accretionary orogen is to recognize the initial magmatic processes that are generated at the time of ocean closure. We present new age, geochemical, and isotopic data for magmatic rocks related to terminal collision along the Solonker‐Xar Moron suture zone in the southern Central Asian Orogenic Belt (CAOB) that record such processes following closure of the Paleo‐Asian Ocean. These magmatic rocks were emplaced in the Early‐Middle Triassic (251–245 Ma) and show high Sr/Y signatures. Their low MgO, Cr, and Ni contents and variable whole‐rock εNd(t) values (+5.8 to −5.3), together with the range in zircon εHf(t) (+15.6 to −9.8) and δ18O values (5.1 to 7.9‰), indicate an origin from partial melting of juvenile lower crustal rocks with some old components, including supracrustal recycling under garnet amphibolite facies conditions. Our data, along with available geological and geophysical evidence, lead us to propose a model of final oceanic contraction in the southern CAOB, resulting in sublinear distribution of high Sr/Y melts along the resultant collision zone, thus defining the onset of postaccretionary processes in the southern CAOB. The identification of collision‐related high Sr/Y granitoids from the southern CAOB not only reveals the magmatic process in response to the final episode of orogenic evolution in the CAOB accretionary collision zone but also constrains how and when an archipelago‐type accretionary orogen is terminated.
A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251–245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen.
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