IEG 환경지질연구정보센터

Proceedings of the Annual Joint Conference, Mineralogical Society of Korea and Petrological Society of Korea May 31, 2007, Andong, Korea

한국광물학회․한국암석학회 2007년 공동학술발표회 눈문집

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Syn-deformational Anatexis of the Neoarchean Migmatites in Daeijak Island, western Gyeonggi Massif: Evidence for Disequilibrium Melting

Yuyoung Lee

․Moonsup Cho

School of Earth and Environmental Sciences, Seoul National University([email protected])

We report the results of our field and geochemical studies on the Neoarchean (ca.

2.51Ga) migmatite complex in Daeijak Island, western Gyeonggi massif. The migmatite complex consists primarily of deformed amphibolite and migmatite. Mesosomes and leucosomes of stromatic and patchy migmatites have dioritic and tonalitic compositions, respectively. Thin layers of hornblende-rich melanosomes are locally intercalated with leucosomes. Pinch-and-swell structures of magmatic origin are present in some leucosome patches, where random crystals of idiomorphic to hypidiomorphic hornblendes (ca. 5mm) occur. Mineral assemblages of both amphibolites and migmatites consist of hornblende, plagioclase, and quartz although their modes are variable. Hornblendes in stromatic leucosome have anhedral inclusions of plagioclase, quartz and rare biotite. The chessboard extinction and lobate boundary of quartz grains commonly occur in stromatic leucosomes.

Plagioclase grains in mesosome and leucosome of stromatic migmatite have similar compositions (Xan = 0.24–0.27). The P–T conditions estimated from migmatites are 700–

750°C and 3–5kbar, respectively, based on hornblende-plagioclase thermobarometry and mineral phase equilibria. This temperature range is consistent with the occurrence of chessboard extinction in quartz grains. Textural evidence for channelized melt flowage along fractures and melt segregation into boudin necks suggests syn-deformational melting in association with the crystallization of leucosomes.

Leucosomes have lower K2O, MgO, Fe2O3, TiO2, Zr, Rb and total rare earth elements

(REE), but higher SiO2, Na2O and Ba contents than amphibolites and mesosomes. The

latter show slightly to moderately negative Eu anomalies (EuN/EuN* = 0.71–0.97), whereas

the former has strong positive Eu anomalies (EuN/EuN* = 1.32–7.26). Geochemical and

textural features suggest that the disequilibrium melting of stromatic migmatite was

facilitated by the rapid melting and extraction processes. This interpretation is corroborated

by trace element modeling for the disequilibrium melting. Two possible melting reactions

for the leucosome formation are inferred from mineral assemblages and textures: (1) Hbl +

Pl + Qtz + H2O = melt; and (2) Hbl1 + Pl + Qtz + Bt + H2O = Hbl2 + melt. All the

petrographic and geochemical data suggest that the tonalitic leucosomes were formed by

the fluid-present, disequilibrium melting of amphibolites accompanied by high-temperature

deformation.