Detrital zircon U-Pb ages of the Jangsan Formation in the northeastern Okcheon belt, Korea and its implications for material source, provenance, and tectonic setting

摘要

The Jangsan Formation distributed in the Taebaeksan Basin, central eastern Korea is unfossiliferous and is composed of quartz arenite. This formation is conventionally believed to be the lowermost stratigraphic unit of the lower Paleozoic sequence deposited on the Yeongnam massif in South Korea, corresponding to Early Cambrian in age. U-Pb dating of detrital zircon grains using a laser ablation inductively coupled mass spectrometer yields ages ranging from Archean to Middle Proterozoic (1738 ±67 Ma to 3058 ±53 Ma). Detrital zircons show no ages younger than 1.8 Ga, suggesting that the Jangsan Formation was deposited at some time after this age, but much prior to ca. 520 Ma, the depositional age of the immediately overlying Myobong Formation. This interpretation is supported by the recent report on the presence of the unconformity between the Jangsan and the Myobong formations (Kim and Lee, 2006). Zircon ages from Jangsan sandstones define two major groups: Archean-age grains with a maximum frequency at about 2.5 Ga and Paleoproterozoic-age grains with maximum frequencies at about 2.1 and 1.8 Ga. The observed zircon age distribution in the Jangsan sandstones may represent that of the Yeongnam massif on which the Jangsan Formation was deposited. Such an age distribution of the Yeongnam massif matches well with that the North China block, suggestive of the Yeongnam massif being a part of the Sino-Korean craton during the Precambrian.