Pb and Nd isotopic constraints on Paleoproterozoic crustal evolution of the northeastern Yeongnam massif, South Korea

We report Pb isotopic ages and Nd isotopic signatures of Paleoproterozoic b asement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea. The PbSL (lead step-leaching) garnet data of the Wonnam group (Preca mbrian metasediments) yield a (207)pb/(206)pb age 1840 +/- 26 Ma, which can be regarded as the timing of amphibolite to upper amphibolite facies metam orphism and associated garnet growth. Whole rock data for the Pyeonghae gne iss intruding the Wonnam group give a (207)pb/(206)pb age of 2093 +/- 86 Ma , denying the possibility of a direct link between the intrusion of the Pye onghae gneiss and the regional metamorphism of the Wonnam group. Our result s confirm the significance of the 2.1 Ga and 1.8 Ga episodes that have been broadly constrained in the Yeongnam massif. The depleted mantle Nd model a ges of metasedimentary rocks from the Wonnam group (2.63-2.47 Ga) are sligh tly younger than those of the Pyeonghae gneiss samples (2.71-2.57 Ga). This Nd isotopic signature also precludes a direct derivation of the Pyeonghae gneiss from the Wonnam Group, instead implying the presence and involvement of the older, probably late Archean crustal materials during the 2.1 Ga ma gmatism in the northeastern Yeongnam massif. Compiled Pb and Nd isotope dat a from the Yeongnam and Gyeonggi massifs suggest a similar geologic history for them, arguing against the conventional idea that the Gyeonggi and Yeon gnam massifs are separate continental blocks respectively correlated to the South and North China blocks. The whole rock Pb isotope data of basement r ocks from the two massifs form a well defined (207)pb/(206)pb linearity of around 2.0 Ga, suggesting their common crustal evolution process for the pa st two billion years. A broad coincidence of major tectonic episodes in the two massifs is confirmed by reviewed geochronological data. The Nd model a ges of basement rocks from the two massifs support a probable existence of Archean crusts in South Korea. The Nd model ages, both Archean and Proteroz oic, of the Gyeonggi and Yeongnam massifs agree with neither those of the N orth China block (predominantly Archean) nor those of the South China block (predominantly Proterozoic). Our compiled isotope data together with recen t estimation for the age of the Honam shear zone appear to refute the prese nce of suture zone between the two South Korean Precambrian massifs, which leaves the Imjingang belt as the possible suture zone. (C) 2000 Elsevier Sc ience B.V. All rights reserved.