Ophiolites in the Xing’an-Mongolia Orogenic Belt of the Southeastern CAOB: Implications for the Closure Model and Location of the Paleo-Asian Ocean

The Xing’an-Mongolia Orogenic Belt (XMOB) is located in the southeastern segment of the Central Asian Orogenic Belt (CAOB). Three ophiolite belts are recognized in XMOB: the northern Erenhot-Hegenshan ophiolite belt (EHOB), the middle Solonker ophiolite belt (SOB) and the southern Xar Moron ophiolite belt (XOB). Here we report new zircon U–Pb ages for ophiolite fragments from the Jarud Banner area of the SOB and the Xingshuwa area of the XOB. A gabbro sample and a metabasalt sample from the Jarud Banner ophiolite yield weighted mean 206Pb/238U ages of 272.4 ± 4.2 Ma (MSWD = 0.68) and 271.7 ± 3.8 Ma (MSWD = 0.41), respectively, with inherited zircon populations of 517–469 Ma and 338–321 Ma, indicating that the SOB formed in the Middle Permian (ca. 272 Ma) within a forearc setting. A gabbro sample from the Xingshuwa ophiolite yields a weighted mean 206Pb/238U age of 256.9 ± 2.5 Ma (MSWD = 1.4), constraining the formation of the XOB to the Late Permian within a backarc setting. Formation ages of the three ophiolite belts become progressively younger from north to south. Integrated with the bidirectional subduction tectonic model proposed in the XMOB, we conclude that continuous bidirectional subduction of the Paleo-Asian Ocean (PAO) along the SOB generated the EHOB and XOB successively in a backarc extensional setting, with final closure of the main PAO basin during the Late Paleozoic to Early Triassic.