ABSTRACT The Gyeonggi Massif, Korea, consists of basement gneisses and supracrustal rocks migmatized to varying degrees. We conducted a petrologic-geochronologic study of the Mount Cheonggye gneisses, located in the western part of the Gyeonggi Massif, and we discuss the crustal evolution of the massif based on our results combined with a compilation of available data from the literature. Mineral assemblages and reaction textures in cordierite-garnet-biotite gneisses suggest a composite pressure-temperature path defined by two clockwise trajectories, M 1 and M 2 . Pseudosection modeling constrains M 1 peak metamorphic conditions as ~10.5 kbar and 840– 860 °C, followed by M 2 recrystallization at 4.5–5.5 kbar and 720–770 °C. Textural relationships of garnet to cordierite and kyanite to plagioclase transitions, as well as the pseudosection analysis, corroborate the clockwise pressure-temperature-time paths in the Gyeonggi Massif. We dated the polyphase metamorphism using sensitive high-resolution ion microprobe (SHRIMP) U-Pb data for zircon and monazite grains from eight samples. Overgrowth rims of zircon in a cordierite-garnet-biotite gneiss and a K-feldspar megacrystic orthogneiss yielded weighted mean 207 Pb/ 206 Pb ages of 1854 ± 9 Ma ( n = 11) and 1852 ± 12 Ma ( n = 19), respectively. This Paleoproterozoic age was reproduced by monazite grains from three cordierite-bearing gneisses dated at ca. 1861–1851 Ma. In contrast, monazite grains from a cordierite-bearing mylonitic gneiss and two biotite gneisses yielded consistent 206 Pb/ 238 U ages ranging from 235 ± 2 Ma ( n = 12) to 231 ± 2 Ma ( n = 15), suggesting a strong Triassic thermal overprint. Finally, we dated a postkinematic granitic dike at ca. 226 Ma, suggesting Late Triassic termination of the orogenesis. Our compilation of SHRIMP U-Pb ages from zircon, monazite, allanite, and titanite available from the literature confirms that the Gyeonggi Massif underwent two distinct thermal events in association with Paleoproterozoic (1.88–1.85 Ga) and Triassic (245–230 Ma) collisional orogenies. In contrast, Mesoproterozoic to Paleozoic thermal episodes are present in the Gyeonggi marginal belt, newly named in this study, where Neoproterozoic (ca. 950–750 Ma) and Paleozoic (ca. 450–430 Ma) ages are prominent in magmatic and detrital zircons. Our tectonic model, exemplified by the Qinling-Gyeonggi microcontinent, suggests that prolonged accretionary tectonics produced arc-related lithologies overlying the Gyeonggi Massif basement rocks. The juxtaposition of these terranes onto the Gyeonggi Massif produced tectonic mixtures with affinities to either the North or South China cratons. On the basis of similarities in zircon age distributions, we further suggest that the Qinling-Gyeonggi microcontinent is built upon basement rocks with North China craton affinity, at least in the Korean Peninsula and extending toward the Japanese Islands.

Abstract U–Th–Pb dated zircons from Western Province paragneisses and orthogneisses were analysed for rare earth element (REE) concentrations, as well as oxygen and hafnium isotopic compositions. Experiments performed in situ using a sensitive high-resolution ion microprobe (SHRIMP) and laser ablation multicollection inductively coupled plasma mass spectrometer (LA-MC-ICPMS) allow better understanding of crustal growth on the Zealandia margin of Gondwana from the micron scale. Paragneiss zircons were probably derived from similar sources to those that supplied the regional Ordovician Greenland Group and correlative southern Australian and Antarctic meta-sedimentary rocks. Detrital zircon grains record variable REE patterns relating to magmatic and metamorphic crystallization processes operating prior to and following Ordovician deposition. δ 18 O and ɛ Hf(T) values trace major phases of juvenile crust formation and subsequent reworking in provenance sources, signifying an increase in the recycling of compositionally diverse, evolved crustal materials through time. Orthogneiss zircons relate to two episodes of magmatism that record similar REE concentration patterns. Devonian zircons have elevated δ 18 O and un-radiogenic ɛ Hf(T) ; Cretaceous zircons record more primitive δ 18 O and radiogenic ɛ Hf(T) . Both orthogneiss suites require thorough mixing of mantle-derived magmas with a component of Greenland Group rocks. The relative proportion of this crustal contamination is c. 20–50% for the Devonian orthogneisses and c. 10–40% for the Cretaceous orthogneisses. Orthogneiss protolith materials were largely hybridized prior to and during zircon crystallization, suggesting that plutonic assembly occurred over restricted structural levels. These results demonstrate the ability of zircon to retain detailed petrogenetic information through amphibolite-facies metamorphism with excellent fidelity. Supplementary material: Analytical methods and data are available at www.geolsoc.org.uk/SUP18755

The Baltimore Gneiss, exposed in antiforms in the eastern Maryland Piedmont, consists of a suite of felsic and mafic gneisses of Mesoproterozoic age. Zircons from the felsic gneisses are complexly zoned, as shown in cathodoluminescence imaging; most zircon grains have multiple overgrowth zones, some of which are adjacent and parallel to elongate cores. Sensitive high-resolution ion microprobe (SHRIMP) analyses of oscillatory-zoned cores indicate that the volcanic protoliths of the felsic gneisses crystallized at ca. 1.25 Ga. These rocks were subsequently affected by at least three Mesoproterozoic growth events, at ca. 1.22, 1.16, and 1.02 Ga. Foliated biotite granite intruded the Baltimore Gneiss metavolcanic sequence at ca. 1075 Ma. The Slaughterhouse Granite (renamed herein) also is Mesoproterozoic, but extremely discordant U-Pb data from high-U, metamict zircons preclude calculating a precise age. The 1.25 Ga rocks of the Baltimore Gneiss are coeval with rocks emplaced in the Grenville Province during the Elzevirian orogeny, and the 1.22 Ga zircon overgrowths are coincident with a later stage of this event. Younger zircon overgrowths formed during the Ottawan phase of the Grenville orogeny. Backscattered electron imaging of titanites from felsic gneisses and foliated biotite granite reveals that many of the grains contain cores, intermediate mantles, and rims. Electron microprobe traverses across zoned grains show regular variations in composition. SHRIMP ages for titanite from the foliated biotite granite are 374 ± 8, 336 ± 8, and 301 ± 12 Ma. The ca. 374 Ma age suggests growth of titanite during a thermal event following the Acadian orogeny, whereas the late Paleozoic titanite growth ages may be due to greenschist-facies replacement reactions associated with Alleghanian metamorphism and deformation.