SHRIMP zircon geochronology and geochemistry of the Orlica-Śnieżnik gneisses (Variscan belt of Central Europe) and their tectonic implications
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Abstract:
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 Gierałtow 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 Gierałtow 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 oro- genic 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. © 2000 Éditions scientifiques et médicales Elsevier SASKeywords:
Geochronology
Basement
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Continental Margin
Ultramafic rock
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Rb-Sr age data are reported for late Precambrian to Cambrian igneous and metamorphic rocks from the border region of England and Wales. A felsic intrusion from Stanner Hill defines a good whole-rock isochron of 702 ±8 Ma. Uriconian pyroclastic rocks from the Wrekin area give a whole-rock age of 558± 16 Ma, while the late- or post-Uriconian Ercall granophyre from the same area gives a good whole-rock isochron of 533 ±13 Ma. A biotite cooling age from the nearby pre-volcanic Rushton Schist is consistent with these dates and suggests that the region cooled after the close of Uriconian magmatism at 536 ±8 Ma. All these results relate to widespread igneous/metamorphic activity in latest Precambrian to Lower Cambrian time. The Ercall granophyre and Rushton schist are directly overlain by Lower Cambrian sediments and 533±13Ma is thus a maximum age for the deposition of these rocks.
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Orogeny
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