Ultrahigh‐Temperature Metamorphism and Near‐Isobaric Cooling of Garnet‐Orthopyroxene Granulite Xenoliths in the Sancheong Anorthosite, Yeongnam Massif, Korea

ABSTRACT Elevated heat flow associated with mafic magmatism in accretionary orogens has often been proposed as a driving mechanism for (ultra)high‐temperature (UHT) metamorphism and anatexis. The Sancheong–Hadong complex, located in the southern Yeongnam Massif, Korea, consists of a ca. 1.87–1.86 Ga anorthosite–mangerite–charnockite–granite (AMCG) suite hosted by granulite‐facies gneisses. We present new evidence for UHT metamorphism and near‐isobaric cooling in garnet‐orthopyroxene granulite xenoliths entrained in the Sancheong anorthosite. These xenoliths record partial melting, typified by leucosomes containing peritectic garnet and orthopyroxene. The development of garnet coronas or garnet‐quartz symplectites at the orthopyroxene‐plagioclase interface in the granulite reflects near‐isobaric cooling under a low f H2O condition. P–T pseudosection modelling of a garnet‐orthopyroxene granulite reveals peak metamorphic conditions of 900°C–930°C and ~8.0 kbar, followed by near‐isobaric cooling and melt crystallization at 800°C–830°C and ~7.4 kbar. Such a UHT condition is further supported by zircon crystallization temperatures of ~900 ± 30°C, estimated from Ti‐in‐zircon thermometry. Zircons in the granulite and leucosome are distinctly zoned into three domains (cores, inner rims and outer rims) that reflect successive recrystallization under high‐temperature conditions. U(–Th)–Pb dating of zircon and monazite from two garnet‐orthopyroxene granulites and one leucosome yields weighted mean ages of 1861 ± 5 Ma ( n = 9) and 1863 ± 12 Ma ( n = 3), respectively, coeval with the emplacement of anorthosite at 1861 ± 2 Ma. This result suggests an almost complete resetting of the U–Pb systematics during the transient UHT event at ~1860 Ma. The δ 18 O values of zircon in the granulite are uniform at 8.15 ± 0.11‰ ( n = 83). Together with the zircon ɛ Hf (t) values ranging from −7.0 to −0.3, such an elevated δ 18 O value indicates a derivation from crustal sources. Zircon rims in the leucosome yield slightly lower δ 18 O values (7.67 ± 0.10‰; n = 17) which are consistent with 7.52 ± 0.14‰ ( n = 20) estimated from monazites of the granulite. In contrast, monazite from the leucosome has an even lower δ 18 O value of 6.57 ± 0.43‰. These results suggest that δ 18 O values decrease during cooling and melt crystallization, most likely due to the influx of lower δ 18 O juvenile fluids and/or melts during AMCG emplacement. Taken together, the lower crustal xenoliths of the southern Yeongnam Massif underwent UHT metamorphism and anatexis at ~1860 Ma, accompanied by anorthosite emplacement, within the context of a prolonged melt‐bearing system during the late stage of the Paleoproterozoic hot orogenesis in the North China Craton.