Fluid-present disequilibrium melting in Neoarchean arc-related migmatites of Daeijak Island, western Gyeonggi Massif, Korea

摘要

The melting process of meta-igneous rocks was investigated via field, petrographic and geochemical analyses of the Neoarchean (-2.51 Ga) migmatite complex in Daeijak Island, western Gyeonggi Massif. This complex consists primarily of garnet-free amphibolites and tonalitic migmatites, both of which contain hornblende, plagio-clase and quartz as major constituents. Neosomes and leucosomes in the migmatite have dioritic-tonalitic and tonalitic-trondhjemitic compositions, respectively. Compositions of hornblende (X_(Fe) = 0.39-0.42) and plagio-clase (An_(24-27)) vary little between the neosomes and leucosomes. The amphibolites show distinct depletions in Nb, Ta, Zr, and Ti relative to large ion lithophile elements, suggesting an arc-related origin for their basaltic protolith. Leucosomes have lower contents of K2O, MgO, FeO*, TiO2, Zr, Rb, and rare earth elements (REE) than amphibolites and neosomes, but are higher in SiO2, Na2O, and Sr contents. Leucosomes and neosomes have positive [(Eu/Eu*)_N = 132-7.26] and negative (0.71-0.97) Eu anomalies, respectively, which are attributed to the variable degree of plagioclase fractionation during the partial melting. The P-T condition for the migmatite formation was estimated to be -700-730 °C and 4.7-5.5 kbar, primarily based on the hornblende-plagiodase thermobarometry and phase equilibria. Various lines of textural evidence, such as the channel flow of melt along migmatitic layers and the segregation of melt into shear bands or boudin necks suggest a syn-deformation crystallization of melt Chemical disequilibrium in migmatites is documented not only by petro-graphic and geochemical data but also by the REE modeling between melt product and source rock. Disequilibrium process is most likely attributed to the rapidity of melt extraction or migration, compared to chemical diffusion rate. In summary, the fluid-present disequilibrium melting of dioritic-tonalitic protoliths has produced tonalitic-trondhjemitic leucosomes in a dynamic arc environment prevalent in the Neoarchean crust of the Korean Peninsula as well as the Shandong Peninsula, North China Craton.