ABSTRACT The eastern Great Smoky Mountains basement complex consists of the following components: (1) ca. 1350–1325 Ma orthogneiss and mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (similar to “pre-Grenville” basement components in the Adirondack, Green Mountain, Hudson Highland, and Shenandoah massifs); (2) ca. 1150 Ma augen orthogneisses and granitic orthogneisses correlating with the Shawinigan phase of Grenville magmatism; and (3) paragneisses (cover rocks) that have either pre- or syn-Grenville (i.e., Mesoproterozoic) versus post-Grenville (Neoproterozoic) depositional ages, and that experienced Taconian metamorphism and migmatization. Mesoproterozoic paragneisses contain major zircon age modes that require a component of Proterozoic crust in the source region. The Neoproterozoic paragneisses exhibit the archetypical “Grenville doublet” in detrital zircon age distributions that matches the age distribution of Ottawan and Shawinigan magmatic/metamorphic events in eastern Laurentia. Most zircon U-Pb age systematics exhibit variable lead loss interpreted to result from high-grade Taconian (ca. 450 Ma) regional metamorphism and migmatization. Neodymium mantle model ages (T DM ) for ortho- and paragneisses range from 1.8 to 1.6 Ga, indicating that all rocks were derived from recycling of Proterozoic crust (i.e., they are not juvenile), which is consistent with Proterozoic detrital zircon ages in pre- to syn-Grenville paragneisses. Lead isotope compositions confirm the presence of an exotic (Amazonian) crustal component in the source region for the protoliths of the pre-Grenville orthogneisses and xenoliths, and that this exotic component was incorporated to varying degrees in the evolution of the basement complex. The oldest age component may represent an Amazonian pre-Grenville analog to the ca. 1.35 Ga native Laurentian crust present in Adirondack and northern Appalachian basement massifs.

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.

Beginning ca. 2.76 Ga, evolution of the Kola-Karelia crust was related to the intracontinental high-temperature metamorphic (up to granulite facies) and magmatic events in combination with formation of the basins related to rifting and infilling with intracontinental volcanic and sedimentary sequences initiated by plume-type processes in the mantle. The geological events corresponding to intracontinental evolution were expressed not only in the formation of new rock associations, juvenile to a significant extent, but also in reworking of previously formed rocks. The age, content, and mode of geological activity are somewhat different in the Kola and the Karelian-Belomorian regions. The Karelian-Belomorian region is oval in plan view. The long axis of this oval extends for 600–700 km in the meridional direction; its maximum width is 400–450 km. The southern part of this oval structure is cut off along the NW-trending boundary with the Paleoproterozoic Svecofennian accretionary orogen. The main constituents of the Karelian-Belomorian region are: epicontinental sequences of greenstone belts (Kostomuksha, Khedozero-Bolsheozero, Gimoly-Sukkozero, Jalonvaara) and paragneiss belts (Hattu, Nurmes); granulite-gneiss complexes and intrusive enderbite-charnockite series; sanukitoid-type granitoid intrusions and lamprophyre dikes, along with migmatization and emplacement of within-plate young granites; and local manifestations of granulite-facies metamorphism superposed on older rocks. Concentric spatial distribution of related geological units is characteristic of the Karelian-Belomorian region. The geometric pattern of the region can be satisfactorily explained assuming initial activity of a mantle plume ca. 2.76 Ga in the central part of the region. A peak of activity was related to the events that occurred ca. 2.74–2.70 Ga. The geochronological data show that a region of high-temperature processes expanded from its center (2.76–2.73 Ga) to the periphery (2.74–2.70 Ga). The concentric character of the tectonic structure was eventually formed as a result of these processes. Widespread high-temperature magmatism and metamorphism in combination with formation of synformal and linear sedimentary basins indicate the setting of anorogenic extension and vigorous influx of extracrustal heat, i.e., a large event related to a mantle plume. In contrast to the Karelian-Belomorian hot region, the coeval Kola region of intracontinental manifestations of high-temperature metamorphism and magmatism is characterized by oval-block geometry. This area, confined to the central part of the Kola Peninsula, extends for 600 km in the northwestern direction, having a width of ~200 km. It is possible that this area extends further to the southeast beneath the platform cover. The main tectonic units are the intracontinental greenstone belts (Sør-Varanger, Titovka, Uraguba, Olenegorsk, Voche-Lambina, Kachalovka, Runijoki–Khikhnajarvi, and Strelna system) in the Inari-Kola microcontinent, the granulite-gneiss Central Kola complex, and the Keivy volcanotectonic paleodepression. Sanukitoid intrusions play a modest role. The Keivy volcanotectonic paleodepression is situated in the eastern Kola Peninsula. Rocks of this tectonic unit are peculiar, and many of them have no obvious analogs in the Fennoscandian Shield or elsewhere. The major Neoarchean amphibolite-gneiss association consists of calc-alkaline to subalkaline garnet-biotite and subalkaline-peralkaline aegirine-arfvedsonite gneisses, as well as biotite-amphibole and amphibole gneisses, amphibolites, and rheomorphic alkali granites. In the western part of the paleodepression, gneisses (metavolcanic rocks) are cut through by small Sakharjok and Kuljok nepheline syenite intrusions. Geochronological estimates characterize two outbursts of magmatic activity separated by a long gap. The early outburst corresponds to magmatic crystallization of calc-alkaline metavolcanic rocks at 2.90–2.87 Ga. The second vigorous outburst documented at 2.68–2.63 Ga corresponds to eruption of subalkaline and subalkaline-peralkaline volcanic rocks, emplacement of alkali and nepheline syenites, and crystallization of gabbro-anorthosite of the Tsaga-Acherjok complex. The duration of the main magmatic phase is ~50 m.y., whereas the preceding gap lasted for ~200 m.y. A model of a volcanotectonic depression largely filled with pyroclastic flows seems plausible to explain pre-metamorphic events. Such manifestations of volcanic activity are inherent to intracontinental domains and related to activity of mantle plumes; similar processes can also develop in the back extensional zone of active continental margins. The synchronism of felsic volcanism and emplacement of the typically intracontinental gabbro-anorthosites form a sound argument in favor of an intracontinental setting for the Keivy paleodepression. The geometry of the Kola region can be satisfactorily explained in terms of mantle-plume activity noted ca. 2.76 Ga in the marginal part of this region; a peak of activity in its central part is related to the events that happened ca. 2.68–2.63 Ga.