Petrology and geochemistry of Jurassic and Cretaceous granites, South Korea
The petrology and geochemistry of four Jurassic and three Cretaceous granitic bodies from four different areas along the Korean peninsula (from NW to SE; younging direction of igneous activity) are presented and discussed. This work is intended to elucidate the nature and origin of selected Jurassic and Cretaceous granites in connection with the tectonic evolution of a continental margin before the opening of the Sea of Japan (64 Ma). The Jurassic Seoul and Anyang granites ("S-type") are alkaline subsolvus equigranular monzogranites (87Sr/86Sr = 0.712) which crystallised under rather unstable conditions as late-tectonic granites. The Seoul and Anyang granites are thought to have been derived from 30% and 10% partial melting of amphibolite facies metasedimentary rocks, respectively [(Ce/Yb)N = 0.4--13.5]. The Jurassic Nonsan and Daejeon granitic rocks ("S-type") are calc-alkaline, subsolvus granodiorite and monzogranite (87Sr/86Sr = 0.710), respectively. These granitic rocks show foliations and other syntectonic features and are considered to have been derived from 60--70% partial melting of the Precambrian basement [(Ce/Yb)N = 63.3]. The heat source to melt a large proportion of the Precambrian rocks was probably a consequence of the closing of the Ogcheon basin at a time of micro continental collision during the Daebo orogeny (mid-late Jurassic). The Cretaceous Palgongsan granite is a typical, calc-alkaline, subsolvus monzogranite, and shows strong characteristics of "I-type" granite by mineralogy and chemical composition. The composition of this granite is consistent with an origin by fractional crystallisation to form a mineralogically and chemically concentrically zoned pattern [(Ce/Yb)N = 7.09]. The Eonyang and Yoocheon granites are also calc-alkaline, subsolvus and subvolcanic granites ("I-type"; 87Sr/86Sr = 0.707). These granites were emplaced at high crustal level with possibly a direct link between the plutons and caldera-centred andesitic volcanicity. The granites and the andesites seem to share a common parental magma which is thought to have been derived from partial melting of a mantle wedge at a consequence of subduction of the Kula-Pacific ridge. On the basis of these results and taking into account geochemical and structural work already published, it is concluded that during the Jurassic and Cretaceous periods in South Korea, granites were first emplaced at a compresssive intraplate boundary and later above a subducting oceanic slab.