MONAZITE RECRYSTALLIZATION IN THE SHEAR ZONE, HWACHEON GRANULITE COMPLEX, KOREA

The recrystallization of monazite in shear zones that cut the Paleoproterozoic (1.87 Ga) Hwacheon granulite complex (HGC) in the central Korean Peninsula is established by its intergrowth with recrystallized retrograde silicates in the host mylonites, and by the overlap of its apparent age with the ⁴⁰Ar/³⁹Ar cooling ages of regional amphibole and muscovite. The HGC consists primarily of garnetiferous leucocratic gneisses, and its monazites record two events, one Paleoproterozoic and the other Triassic. Ion microprobe dating of zircon from a shear zone sample gives a U-Pb age identical to that of granulite-facies peak metamorphism, confirming the protolith of mylonites and the inertness of zircon during mylonite recrystallization. In contrast, monazite, which is ubiquitous in both garnetiferous leucocratic gneisses of the HGC and mylonitic gneisses of the shear zone, is far more reactive than zircon. Monazite in the former coexists with grt + sill + bt + pl + Kfs + qtz + ilm + zircon, and gives a chemical age of ca. 1.83 Ga, suggesting its growth during the cooling stage shortly after Paleoproterozoic granulite-facies metamorphism. On the other hand, monazite grains in mylonitic gneiss coexist with the retrograde assemblage: bt + mus + pl + qtz + zircon + FeS ± (Th,U)SiO₄ ± YPO₄. Monazite grains are commonly zoned in 0.4-14.4wt% ThO₂, 0.0-2.3wt% SiO₂, 23.0-31.8wt% Ce₂O₃, and 0.0-3.3wt% Y₂O₃, and the Y concentration in Triassic ones is about twice as much as that in Paleoproterozoic monazite. Chemical ages estimated from cores and rims of monazite grains cluster around 225 Ma and 205 Ma, respectively. Some monazite grains preserve the resorbed boundaries in their interiors and develop thin overgrowth rims, suggesting partial dissolution subsequent to their growth. Moreover, high Y/Si in overgrowth rims and lower ratio in cores is supportive of the observed resorbed texture of YPO₄. Thus, Triassic chemical ages are likely to represent two distinct episodes producing monazite in the shear zone. Together with the ⁴⁰Ar/³⁹Ar hornblende age of 226 ± 8 Ma and muscovite age of 202 ± 4 Ma from the HGC, our chemical ages suggest crystallization of monazite below its closure temperature and with its host lower amphibolite facies mylonitic assemblage while the whole HGC was being exhumed during the late Triassic.