242-22 Decoupled behavior of zircon and monazite in the recording of Paleoproterozoic anatexis and Permian–Triassic reworking in the Gaya Anorthosite Complex, Yeongnam Massif, Korea

Abstract:

The timing and mechanisms of reactivation in ancient continental crust are often difficult to constrain, because zircon and monazite respond differently to magmatism, metamorphism, and deformation. To evaluate how these accessory minerals record polyphase crustal reworking, we investigated Paleoproterozoic migmatitic gneisses and Permian granitic rocks exposed along a ductile shear zone in the Gaya Anorthosite Complex, Yeongnam Massif, Korea. This complex consists primarily of ca. 1.86 Ga anorthosite and migmatitic gneisses, which are intruded by Permian granitoids. Following the emplacement of the latter, all of the lithologies were affected by ductile deformation, yielding a mylonitic-to-ultramylonitic shear zone. Phase equilibrium modeling indicates that the migmatitic gneiss underwent partial melting at ~670–760 °C and 3–5 kbar via muscovite dehydration melting. Sensitive high-resolution ion microprobe (SHRIMP) U(–Th)–Pb analyses of zircon and monazite permitted us to identify three distinct tectonometamorphic episodes. Zircons in migmatitic gneiss commonly preserve the sedimentary protolith ages of > 2.0 Ga but lack metamorphic overgrowth rims. In contrast, monazite grains in this paragneiss not only record Paleoproterozoic anatexis at ~1.86 Ga, but also Triassic metamorphism at 229 ± 2 Ma. On the other hand, zircon and monazite grains from granitic and mylonitic rocks yielded Late Permian to Early Triassic ages of ca. 263–246 Ma, which are interpreted as dating the emplacement of granite and the associated contact metamorphic overprint. This is in marked contrast to monazite grains, which only record Late Triassic ages of ca. 231–227 Ma, interpreted as corresponding to the time of ductile shearing. These petrochronological records suggest that the behavior of zircon and monazite is decoupled during repeated crustal reworking associated with the transition from continental collision to arc settings at the margin of the Sino-Korean craton. Zircon largely preserves inherited sedimentary and magmatic ages, whereas monazite records multiple episodes of recrystallization in association with metamorphism and deformation. In addition, our preliminary study indicates that δ¹⁸O values of monazite increased from 7.27 ‰ to 9.29 ‰ during the Triassic shearing event. This suggests that mylonitic recrystallization was facilitated by externally derived fluids. Our results demonstrate that integrated zircon–monazite petrochronology provides a robust framework for determining the timing and mechanisms of the successive reactivation of Paleoproterozoic continental crust, and highlights the contrasting but complementary behavior of these accessory minerals in polymetamorphic terranes.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-17705

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