Abstract:

The Ondor Sum Group (OSG) in western Inner Mongolia can be divided into two parts: the lower part consists of ferrian quartzites, metavolcanic rocks and sericite quartz schists; the upper part includes sericite quartz schists and quartzites mixed with localized carbonate rocks. In SHRIMP and ICPMS geochronometric determination, one SHRIMP peak age of 460±4 Ma and two detrital zircon peak ages of 417 Ma and 444 Ma were acquired from the metavolcanic rocks in lower part and sericite quartz schists in upper part, respectively. These new data, combined with previous zircon dating indicate OSG developed from 500 Ma to 415 Ma. Composition of OSG suggests that it formed in the continental margins of the SongliaoHunshandake block, which is called the “Ondor Sum Ocean” (OSO) in this paper, representing the Paleo Asian Ocean in China during the early Paleozoic. During the earlymiddle Paleozoic, OSO was subducted beneath the north China block in the south and XinganAngin Sum block in the north, respectively; its main part formed the northern and southern accretionary wedges whereas its marginal part became the matrix of northern and southern mélanges. Deformation and metamorphism in OSG recorded the evolution process of the eastern Central Asia Orogenic Belt. Detrital zircon analyses of OSG demonstrate that its Precambrian source area is similar with the lower Paleozoic in Mongolia and different from that in the North China Craton (NCC), indicating there were other provenances for OSG.

Key words: Ondor Sum Group, Central Asia Orogenic Belt, western Inner Mongolia