Nd isotopic study of Upper Cambrian conodonts from Korea and implications for early Paleozoic paleogeography

Abstract

Correlation of the Korean Peninsula with the neighboring South and North China blocks has been one of the most important and long debated issues in East Asian tectonics. The eastward extension of the Chinese collisional belt between the South and North China blocks into the Korean Peninsula is still in dispute. The lower Paleozoic sedimentary rocks in South Korea, the Joseon Supergroup, accumulated in the Okcheon belt which has been proposed as the suture zone or the block boundary between the South and North China blocks in Korea. The Joseon Supergroup comprises five different lithostratigraphic units, and has been proposed to represent two tectonostratigraphic blocks bounded by the Honam Shear Zone (HSZ) in the Okcheon belt; among them, the Yeongweol Unit in the western part of the HSZ is correlated with South China and the Duwibong Unit in the eastern part of the HSZ with North China. To test the above geodynamic interpretation, this study analyzed the Nd isotopic composition of Upper Cambrian conodonts from the Duwibong and Yeongweol units. The difference of the ɛ Nd ( T) values between the Upper Cambrian Duwibong and Yeongweol units is about 3 ɛ units. Compared to a wide range of the ɛ Nd (0) values in the modern oceanic basins, this slight difference can be regarded as a variation in a same paleo-watermass. The ɛ Nd ( T) values of the two units are consistent with coeval values from North China, and this suggests that the Duwibong and Yeongweol units shared the same oceanic watermass with the North China. Considering current early Paleozoic paleogeographic reconstructions, the Nd isotopic signatures of North China, Duwibong and Yeongweol units may be independent of Laurentia and Baltica and thus can be indicative of another unique oceanic watermass. In contrast to the previous tectonic interpretations, the same paleoceanographic signatures shown by the Duwibong and Yeongweol units and North China and recent developments suggest that the Yeongweol Unit in the early Paleozoic Joseon Supergroup is not correlated with South China but correlated with North China, and the Korean Peninsula and North China block may have been included in a larger continental block, the Sino-Korean block. The collisional belt between the South and North China blocks may not extend into the Korean Peninsula.