SHRIMP U–PB ZIRCON AGES (~1.87 GA) OF MASSIF-TYPE ANORTHOSITES IN THE YEONGNAM MASSIF, KOREA: IMPLICATIONS FOR PALEOPROTEROZOIC MANTLE DELAMINATION IN THE NORTH CHINA CRATON

The delamination of lithospheric mantle is well established in the core of active collisional orogens, and such a process has been proposed in the North China Craton (NCC). We attempt here to understand the mantle delamination in the NCC, possibly associated with the Paleoproterozoic anorthositic magmatism in the Yeongnam Massif, southern Korea. The Sancheong-Hadong (SH) massif-type anorthosite complex (~ 250 km²) has been long known but its petrogenesis, including the emplacement age, is poorly constrained. This complex consists mainly of massive anorthosite, leucogabbro, leuconorite, and hornblende gabbro. They are emplaced into the Precambrian high-grade gneisses, consisting primarily of mafic granulite, charnockitic gneiss, granitic gneiss, and migmatitic gneiss. In order to unravel the petrogenesis of the SH anorthosite in light of the crust-mantle geodynamics, we have dated zircon grains of anorthosites and gneisses, using a sensitive high-resolution ion microprobe (SHRIMP) at Korea Basic Science Institute. The weighted mean ²⁰⁷Pb/²⁰⁶Pb zircon ages of anorthosite and granitic gneiss are 1873 ± 4 Ma (n = 11) and 1875 ± 5 Ma (n = 17), respectively, indicating their coeval emplacement. In addition, mafic granulite and migmatitic gneiss are dated at 1858 ± 5 Ma (n = 30) and 1863 ± 5 Ma (n = 15), respectively, suggesting that high-temperature regional metamorphism and anatexis are associated with the anorthositic magmatism. This Paleoproterozoic event is consistent with the ~1.87 Ga collisional orogenesis which is widespread throughout the Korean Peninsula and the NCC. Moreover, previous workers reported that this Paleoproterozoic event has accompanied the wholesale mantle replacement beneath the NCC. It is thus likely that the anorthositic magmatism is synchronous with the plate-scale collisional orogeny, in contrast to post-orogenic emplacement of the Proterozoic massif-type anorthosites reported worldwide. Taken together, we suggest that (1) the SH anorthosite complex in the Yeongnam Massif is the syn-orogenic product of ~1.87 Ga collision; and (2) this Paleoproterozoic collisional orogeny has accompanied the mantle delamination in the NCC including the Korean Peninsula.