Zircon Composition at Different Stages of the Variscan Orogeny
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To assess the age and origin of the metasedimentary migmatites in southernmost Sweden and their relationships with the Mesoproterozoic granitoid magmatism in the area, we have dated migmatite zircon using the secondary ion mass spectrometry U–Pb method. The studied metasedimentary migmatites, here called the Nöteboda migmatites, occur along the southwestern boundary of the 1442 Ma Tåghusa granitoid intrusion in southeastern Skåne. They contain the mineral assemblage garnet+biotite ± muscovite+cordierite+sillimanite+quartz+plagioclase+K-feldspar and were formed during a retrograde evolution from c. 750–720°C and 6 kbar (peak conditions) to c. 675°C and 4 kbar. Zircon is characterized by detrital cores surrounded by U-rich rims and overgrowths. Separate rounded metamorphic grains also exist. The age probability–density distribution peaks for detrital zircon are at c. 1700, 1670, 1650, 1610, 1570 and 1530 Ma. These ages suggest Gothian orogenic rocks in the present west as the most probable principal source. Sedimentation occurred after c. 1530 Ma, the age of the youngest detrital zircon, indicating the existence of a previously unknown period of Mesoproterozoic sedimentation in southernmost Sweden. A homogeneous zircon overgrowth yielded a concordant 207Pb/206Pb age of 1439 ± 5 Ma, which dates the migmatization and is close to the age of the Tåghusa intrusion. We conclude that the burial of the sediments down to c. 20 km, their metamorphism and progressive migmatization took place concurrently with granitic magmatism, NE–SW compression, folding and shearing of the crust between 1460 and 1440 Ma during the Danopolonian orogeny.
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The Orlica-Śnieżnik dome comprises large orthogneiss bodies interbedded with amphiblite-grade metasediments and minor metavolcanics. New U-Pb and Pb-Pb SHRIMP zircon ages for two major gneiss units of the dome, the Śnieżnik and Gierałtów gneiss, yielded similar ages of ca. 500 Ma. This is interpreted to reflect the magmatic crystallization age from the same or similar igneous precursors, in agreement with the geochemical characteristics of these rocks. Some zircon cores in both gneisses, interpreted to be inherited xenocrysts, have ages of ca. 530–540 Ma, and, additionally, of ca. 565 Ma and 2.6 Ga in the Śnieżnik gneiss. Igneous grains in both gneiss types have high-U rims, which are dark under cathodoluminescence. They are much better developed in the Gierałtów gneiss and they yield a well-defined weighted mean U-Pb age of 342 ± 6 Ma. These high-U rims are interpreted to have grown close to the peak of HT metamorphism which is responsible for the migmatitic texture of the Gierałtów gneiss. The Visean HT–LP metamorphism in the Orlica-Śnieżnik dome is interpreted as a result of rapid uplift and decompression due to overthrusting of high grade rocks over the Moravo-Silesian nappe pile. Our data support geodynamic models that ascribe a predominant influence in the tectonic evolution of the West Sudetes to the Variscan orogenic events. This is suggested by the inheritance of zircon xenocrysts from the Cadomian basement and by the Late Cambrian–Early Ordovician magmatic event, both typical of the Armorican terrane assemblage, as well as by the Early Carboniferous age of the metamorphism. Le dôme d'Orlica-Śnieżnik est constitué de grands corps d'orthogneiss intercalés avec des sédiments métamorphisés dans le faciès amphibolite et des roches métavolcaniques en quantité mineure. De nouvelles analyses U-Pb et Pb-Pb, à l'aide de la sonde ionique « SHRIMP », mettent en évidence un âge commun d'environ 500 Ma pour les deux unités principales (Śnieżnik et Gierałtów). Cet âge est interprété comme celui de la cristallisation magmatique des protolithes ignés de ces roches. Quelques cœurs de 530–540 Ma ont été décelés dans les deux types de gneiss, ainsi que dans les gneiss de Śnieżnik seulement, des cœurs âgés d'environ 565 Ma et 2,6 Ga. Ces âges anciens sont interprétés en termes de xénocristaux hérités. Dans les deux types de gneiss, les images de cathodoluminescence des zircons magmatiques montrent des surcroissances riches en U. Ces surcroissances sont plus développées dans les gneiss de Gierałtów, où elles permettent de mesurer un âge de 342 ± 6 Ma (Viséen), interprété comme celui du climax du métamorphisme de HT et de la migmatisation des gneiss de Gierałtów. Cet épisode métamorphique de HT–BP du dôme d'Orlica-Śnieżnik est interprété comme le résultat de la remontée rapide et de la décompression causées par le charriage des roches de haut degré métamorphique sur l'empilement d'unités des nappes Moravo-Silesiennes. Nos données confirment les modèles géodynamiques qui attribuent une importance prédominante aux événements orogéniques Varisques dans l`évolution tectonique des Sudètes Occidentales. La présence de xenocristaux de zircon d'âge cadomien et l'âge Cambrien supérieur–Ordovien inférieur de l'épisode magmatique sont deux caractéristiques typiques du « terrane » Armoricain, de même que le métamorphisme du Carbonifère inférieur.
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Abstract Absolute ages of migmatization in the polymetamorphic, parautochthonous basement of the Sveconorwegian Province, Sweden, have been determined using U–Pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses. Migmatite zircon was formed by recrystallization whereas dissolution–reprecipitation and neocrystallization were subordinate. The recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes. It is backscatter electron‐bright, U‐rich (800–4400 ppm) with low Th/U‐ratios (generally 0.01–0.1), unzoned or ‘oscillatory ghost zoned’, and occurs as up to 100 μ m‐thick rims with transitional contacts to cores of protolith zircon. Protolith ages of 1686 ± 12 and 1668 ± 11 Ma were obtained from moderately resorbed, igneous zircon crystals (generally Th/U = 0.5–1.5, U < 300 ppm) in mesosomes; protolith zircon is also present as resorbed cores in the leucosomes. Linkage of folding, synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south‐vergent folding at 976 ± 7 Ma. At a second locality, similar recrystallized zircon rims in leucosome date pre‐Sveconorwegian migmatization at 1425 ± 7 Ma; an upper age bracket of 1394 ± 12 Ma for two overprinting phases of deformation (upright folding along gently SSW‐plunging axes and stretching in ESE) was set by zircon in a folded metagranitic dyke. Lower age brackets for these events were set at 952 ± 7 and 946 ± 8 Ma by zircon in two crosscutting and undeformed granite–pegmatite dykes. Together with previously published data the present results demonstrate: (i) Tectonometamorphic reworking during the Hallandian orogenesis at 1.44–1.42 Ga, resulting in migmatization and formation of a coarse gneissic layering. (ii) Sveconorwegian continent–continent collision at 0.98–0.96 Ga, involving (a) emplacement of an eclogite unit, (b) regional high‐pressure granulite facies metamorphism, (c) southvergent folding, subhorizontal, east–west stretching and migmatization, all of which caused overprint or transposition of older Mesoproterozoic and Sveconorwegian structures. The Sveconorwegian migmatization and folding took place during or shortly after the emplacement of Sveconorwegian eclogite and is interpreted as a result of north–south shortening, synchronous with east–west extension and unroofing during late stages of the continent–continent collision.
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The high-temperature and high-pressure granulites in the internal zone of the Variscan belt are witnesses of deep crustal processes and subsequent exhumation of deeper lithospheric fragments. The Góry Sowie Block granulites in SW Poland and the surrounding gneisses and migmatites contain different inherited zircon age spectra, testifying different sources of their protoliths: mainly Cadomian (ca. 580 Ma) igneous rocks, and early Ordovician (ca. 500 Ma) granitoids (or their reworked products), respectively. The metamorphic spheric and oval zircons in the granulites give two statistically distinct ages of c. 395 and 380 Ma, in excess of experimental uncertainty. These ages may correspond to the high-temperature (HT) and high-pressure (HP) granulite facies metamorphism and subsequent amphibolite facies retrogression. However, essentially isochemical reconstitution of zircons and inheritance of radiogenic Pb cannot yet be excluded. The retrogression in the granulites coincided with the amphibolite facies metamorphism in the surrounding gneisses evidenced by a range of various isotopic ages between 384 and 370 Ma. The new SHRIMP zircon data provide new evidence of the presence of "old" (c. 400–395 Ma) HT-HP granulites in the central European part of the Variscides. The granulites were subsequently exhumed to mid-crustal levels, and tectonically interleaved with gneisses. Continuing uplift and decompression caused migmatization in the surrounding gneisses and partial re-equilibration of the granulites, at around 380 Ma. This high-temperature and medium-pressure (HT-MP) event was followed by rapid uplift and exhumation at c. 360 Ma due to Eo-Variscan orogenic movements.
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