ABSTRACT

Zircon has long been used as the principal mineral for determining the crystallization ages of granitic rocks. However, in leucogranites, accurate age determination is often problematic due to radiation damage from high uranium contents. U–Pb dating of monazite, which is more resistant to metamictization and Pb loss, thus offers a more reliable alternative. Here, I present a case study applying this approach to the Imwon and tourmaline-bearing leucogranites in the northeastern Yeongnam Massif, Korea, which experienced high-temperature metamorphism up to the amphibolite facies. To evaluate the reliability of U–Pb chronometers, I compared monazite and zircon ages from two leucogranites. In the Imwon leucogranite, monazite showing magmatic zoning yields an upper intercept age of 1905 ± 12 Ma, which is interpreted as the magmatic crystallization age. In contrast, the coexisting zircon records ca. 1.86 Ga ages, consistent with the regional metamorphic age indicated by monazite weighted-mean ²⁰⁷Pb/²⁰⁶Pb age of 1865 ± 7 Ma from adjacent psammitic schists. In the tourmaline-bearing leucogranite, magmatically zoned monazite yields a weighted-mean ²⁰⁷Pb/²⁰⁶Pb age of 1860 ± 3 Ma, indicating magmatic crystallization during this metamorphic event. In contrast, zircon from this granite yielded highly scattered U–Pb data and did not define a coherent age because of severe Pb loss. These results demonstrate that, in leucogranites affected by or formed during ca. 1.86 Ga regional metamorphism in the Yeongnam Massif, monazite more faithfully records magmatic crystallization ages than zircon, which is more susceptible to isotopic disturbance and Pb loss. Collectively, our results demonstrate that the Yeongnam Massif experienced two discrete Paleoproterozoic leucogranite-forming events at ca. 1.90 Ga and ca. 1.86 Ga, as revealed by monazite rather than zircon U–Pb geochronology.