The Ondor Sum Group in the central Inner Mongolia is mainly composed of meta-basic volcanics intercalated with ferruginous quartzite and quartz schist,and has been interpreted as slices of oceanic crust or an ophiolite suite of the Early Paleozoic or much older ages.This paper presents new LA-ICP-MS zircon U-Pb dating and geochemical data for the meta-basic volcanics.The results show that zircons in the meta-basic volcanics were derived from complicated sources,most of which may be captured by basic magma from the country rocks or other sources.They yield a large age range from the Late Archean to Early Mesozoic with the youngest age group between 246 and 261 Ma,constraining the protolith of the meta-basic volcanics formed in the Late Permian to Early Triassic.The meta-basic volcanics have an affinity to E-MORB in geochemistry,and also a similarity toward OIB,representing a tectonic setting of limited intra-continental ocean basin.This limited basin might have been related to the continuous extension of the area since the Early Permian and finally closed in the Early Mesozoic.
