SHRIMP geochronology and reaction texture of monazite from a retrogressive transitional layer, Hwacheon Granulite Complex, Korea

摘要

Reaction textures, mineral chemistry, and U-Pb SHRIMP geochronology of monazite from the Hwacheon granulite complex were investigated to delineate the timing of granulite-to-amphibolite transition. Reaction textures of apatite and allanite surrounding partially-dissolved monazites in the transition zone (HY-23) are similar to those of symplectic hornblende and garnet corona around pyroxenes in mafic granulite at the same outcrop. The texture and chemistry of retrograde rare earth element-bearing minerals indicate that disequilibrium in metasomatic reactions and the availability of fluid during the amphibolitization contributed to the variable degrees of transition. The U-Pb age of dissolved monazite was dated at 1861 ± 20 Ma, which is indistinguishable from 1873 ± 17 Ma of unaltered monazite. On the other hand, the crystallization age of symplectic allanite is interpreted to be either Devonian or Triassic, based on Th-Pb and U-Pb ages, respectively. Thus, the synchroneity between accessory mineral growth and its microstructure such as corona or symplectite needs to be carefully assessed for unraveling the nature of tectonometamorphic events.