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 basement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea. The PbSL (lead step-leaching) garnet data of the Wonnam group (Precambrian metasediments) yield a (207)pb/(206)pb age 1840 +/- 26 Ma, which can be regarded as the timing of amphibolite to upper amphibolite facies metamorphism and associated garnet growth. Whole rock data for the Pyeonghae gneiss 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 Pyeonghae gneiss and the regional metamorphism of the Wonnam group. Our results 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 ages of metasedimentary rocks from the Wonnam group (2.63-2.47 Ga) are slightly 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 magmatism in the northeastern Yeongnam massif. Compiled Pb and Nd isotope data from the Yeongnam and Gyeonggi massifs suggest a similar geologic history for them, arguing against the conventional idea that the Gyeonggi and Yeongnam massifs are separate continental blocks respectively correlated to the South and North China blocks. The whole rock Pb isotope data of basement rocks 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 past two billion years. A broad coincidence of major tectonic episodes in the two massifs is confirmed by reviewed geochronological data. The Nd model ages of basement rocks from the two massifs support a probable existence of Archean crusts in South Korea. The Nd model ages, both Archean and Proterozoic, of the Gyeonggi and Yeongnam massifs agree with neither those of the North China block (predominantly Archean) nor those of the South China block (predominantly Proterozoic). Our compiled isotope data together with recent estimation for the age of the Honam shear zone appear to refute the presence of suture zone between the two South Korean Precambrian massifs, which leaves the Imjingang belt as the possible suture zone. (C) 2000 Elsevier Science B.V. All rights reserved.