Ordovician bimodal magmatism in the Ogcheon belt (South Korea): intracontinental rift-related volcanic activity

Abstract

The Ogcheon belt of South Korea has been interpreted to be an oceanic suture zone; nevertheless, all lines of evidence are consistent with an intraplate evolution since early Paleozoic times. The early Paleozoic Ogcheon Supergroup is composed of a Cambrian to early Ordovician quartzite and carbonate platform sequence overlain by a thick volcano-sedimentary sequence; the volcanic rocks have been emplaced in close association with thick debris flow or flysch-like deposits. The metavolcanic complex displays a bimodal association of metabasalts or metadolerites associated with metatrachytes and rare metarhyolites; most of the basic volcanic rocks were emplaced as hypabyssal complexes (sills, dykes and laccoliths) while the acidic volcanic sequences are mainly pyroclastic flow or fall deposits and volcano-derived sedimentary rocks. The scarcity of rocks of intermediate composition, the occurrence of Fe- and Ti-rich basic facies and the high level of incompatible elements are all features of tholeiitic transitional suites. ϵNd values close to zero for basic rocks and trachyte are diagnostic of intracontinental lavas. The negative ϵNd values of the rhyolite suggest that crustal contamination occurred during magma genesis or ascent. Taking into account both the overall characteristics of the sedimentary environment and the petrological and geochemical features of the volcanic component, the sequences appear to have evolved in an intracontinental rift and do not represent oceanic remnants as has been previously interpreted. Thus, the interpretation of the Ogcheon belt as a suture zone is definitely ruled out. Consequently, the limit between early Paleozoic North and South China continents might be located elsewhere, perhaps farther north in the Imjingang belt of central Korea.