Early‐Middle Triassic high Sr/Y granitoids in the southern Central Asian Orogenic Belt: Implications for ocean closure in accretionary orogens

Abstract

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.

Late Carboniferous to Early Permian adakitic rocks and fractionated I‐type granites in the southern West Junggar terrane, NW China: Implications for the final closure of the Junggar–Balkhash Ocean

“…The current ~45‐km‐thick crust along the WJT (Xu, Han, et al, ) supports this interpretation, because the dehydration melting experiments of garnet‐amphibolite, amphibole‐bearing eclogite, and/or eclogite were verified to produce adakitic melts at depths greater than 40 km (Rapp et al, ). Such a limited thickness of the crust coincides with the case for a typical accretionary orogen (Li et al, ). If it is the case, the potential heat source that induced the melting event could be best owed to underplating of mantle‐derived basaltic magmas, as indicated by the widespread occurrences of Permian mafic dykes in the WJT (e.g., Buckman & Aitchison, ; Gao et al, ; Xu et al, ; Xu, Han, et al, ).…”

“…Adakites were originally defined as SiO 2 ‐rich (>56%) igneous rocks with high Sr/Y and La/Yb ratios and restricted to a “hot” subduction zone, where a young, hot oceanic slab is subducted, resulting in slab melting, as well exemplified by the South American Andean margin (Castillo, ; Defant & Drummond, ; Martin, ). Alternatively, the intermediate to acid, plutonic or volcanic rocks with similar geochemical features, loosely termed adakitic rocks (Moyen, ), can be produced in variable tectonic environments such as syn‐ to post‐collision (e.g., Dokuz et al, ; Guo, Wilson, & Liu, ; Hou, Gao, Qu, Rui, & Mo, ; Inner Mongolia, NE China: Li et al, , ; NE Turkey: Topuz et al, ; NW Iran: Jahangiri, ; and Tibet, SW China: Chung et al, ) and “normal” modern arcs (Philippines: Macpherson, Dreher, & Thirlwall, ). Elucidating the debated tectonic settings of the adakitic rocks in specific areas are essential to constrain models of tectonic evolution (Castillo, ).…”

Final Closure of the Paleo‐Asian Ocean and Onset of Subduction of Paleo‐Pacific Ocean: Constraints From Early Mesozoic Magmatism in Central Southern Jilin Province, NE China

“…Early–Middle Triassic igneous rocks are widespread within the eastern end of the NCC northern margin and include mafic to felsic volcanic and intrusive rocks (Cao et al, ; Li et al, ; Wang, Xu, et al, ; Yang et al, ; Zhang et al, , , ) that form an E–W trending belt (Tang et al, ; Zhang et al, ). In contrast, only minor Early–Middle Triassic granitoids occur along the eastern segment of the CAOB, mainly distributed in Yanbian Area (Tang et al, , and references therein; Wu et al, , and references therein; Yang et al, ; Yang, Wu, Liu, et al, ; Yang, Wu, Wilde, et al, ; Zhang et al, , and references therein).…”

A Late Carboniferous–Early Permian back‐arc basin in the southeastern Central Asian Orogenic Belt: Constraint from sedimentological and geochronological investigations of the Shoushangou Formation in the central Inner Mongolia

“…The Early Palaeozoic igneous rocks, related to the subduction of the PAO crust, are mainly Cambrian to Silurian in age, mainly including the Bainaimiao arc and the Baolidao arc (e.g., Jian et al, , ). Recently, the Late Palaeozoic–Early Mesozoic magmatic activities were well reviewed, and four episodes related the final amalgamation of the CAOB were recognized (e.g., Li et al, , ), including subduction related (300–273 Ma), slab break off (255–250 Ma), intracontinental contraction (251–235 Ma), and post‐orogenic extension (230–200 Ma).…”