GRANULITE-FACIES METAMORPHISM IN THE PUNGGI AREA, NORTHEASTERN YEONGNAM MASSIF, KOREA AND ITS TECTONIC IMPLICATIONS FOR EAST ASIA

Abstract

In the Punggi area of the northeastern part of the Yeongnam Massif, one of Precambrian basement terrains in Korean Peninsular, the metamorphic grade increases from the amphibolite- to the granulite-facies zones towards the southeast. The boundary of the granulite-facies zone is defined by the orthopyroxene (Opx)-In reaction in metabasites. As metamorphic grade increases across the Opx-In isograd, the mineral assemblage in calcic metabasites is changed from clinopyroxene-amphibole-plagioclase to orthopyroxene-clinopyroxene-amphibole-plagioclase, and that in alkali metabasites from garnet-orthoamphibole-plagioclase to garnet-orthopyroxene-plagioclase. The metamorphic rocks in the granulite-facies zone were overprinted by upper amphibolite-facies retrograde metamorphism. During retrograde metamorphism, cordierite and spinel formed in the migmatitic gneiss. The P–T conditions of granulite-facies metamorphism are 767–815 °C and 6.0–8.4 kb and the P–T conditions for the retrograde stage are 595–707 °C/4.7–6.4 kb. A U–Pb zircon age of 1904±9 Ma has been obtained from a garnet granitic gneiss and is inferred to date the emplacement of its precursor. This is consistent with the intrusion age of precursors to most orthogneisses in other parts of the Yeongnam Massif. The same sample has yielded a Sm–Nd whole rock-garnet age of 617±38 Ma, which is inferred to represent a late Proterozoic metamorphic event of upper amphibolite facies.