The ⁴⁰ Ar/ ³⁹ Ar ages from the West Sudetes (NE Bohemian Massif): constraints on the Variscan polyphase tectonothermal development

Abstract

The West Sudetes (NE margin of the Bohemian Massif) consist of a complex mosaic of several tectonometamorphic units juxtaposed during the Variscan orogeny. The polyphase Variscan tectonothermal development of the West Sudetes was determined by 4~ ages of single grains and mineral concentrates. Late Famennian (359 Ma) mica ages from the high-grade G6ry Sowie Block suggest continuous uplift after a Late Devonian high temperature-low pressure (HT-LP) event contemporaneous with the end of subduction-related high pressure-low temperature (HP-LT) metamorphism in the East Krkono~e Complex.Mid-Late Devonian high pressure events in the Krkonoge-Jizera Terrane and Orlica-Snieznik Dome are followed by coeval high temperature events between 345 and 335 Ma (Vis6an). The latter are interpreted as consequence of uplift, and decompression during overthrusting of both complexes on their forelands. Subsequent small-to large-scale shear movements dated at around 325-320 Ma (early Namurian) affected the Orlica-Snieznik Dome, Krkono~e-Jizera Terrane. including the Intra-Sudetic Fault. and also the eastern Lusatian Granitoid Complex. They were accompanied by contemporaneous emplacement of the Krkonoge-Jizera pluton. The upper limit of the tectonometamorphic and magmatic activity is dated at 314-312 Ma (Namurian/Westphalian boundary). The final juxtaposition of the diversified tectonometamorphic units, which constitute the West Sudetes, took place in early Namurian times. Downloaded from T h e K r k o n o g e -J i z e r a T e r r a n e

An insight into a gneiss core of the Orlica-Śnieżnik Dome, NE Bohemian Massif: new structural and U–Pb zircon data

“…The lat ter con clu sion pre sum ably also ex tends to other iso tope sys tems stud ied in the OSD rocks. The Rb-Sr phengite and bi o tite ages for gneiss es of the two for ma tions range be tween 335 Ma and ~319 Ma (Borkowska et al, 1990;Lange et al, 2005) and are well com pat i ble with the Ar-Ar ages de ter mined be tween 341 Ma and 320 Ma (Maluski et al, 1995;Marheine et al, 2002;Glascock et al, 2003;Schnei der et al, 2006). These are cool ing ages, yet of ten consid ered to re flect the tim ing of Variscan meta mor phism in the gneiss es.…”

“…Later geo chem i cal and geo chron ol ogi cal data fur nished the base for a hy poth e sis that all gneiss es in the dome came from a sin gle ~500 Ma gra nitic protolith. The orig inal gran ites were to be come sub stan tially trans formed and even tu ally di ver si fied dur ing Variscan de for ma tion and migmatisation be tween 360 Ma and 340 Ma, then cooled at 340-330 Ma (Maluski et al, 1995;Turniak et al, 2000;Lange et al, 2002Lange et al, , 2005Gordon et al, 2005;Bröcker et al, 2009Bröcker et al, , 2010Štípská et al, 2012) and zon ally sheared at ~321 Ma (Marheine et al, 2002).…”

Radiometric Age Analyses of Rocks from the Northern Envelope of the Karkonosze Massif, the Sudetes, Poland: A Comparative Geochronological Study

“…According to these authors the age of the granite intrusion is 480-450 Ma. Zircon dating by U-Pb and Pb-Pb methods slightly moved the age limits and fixed the crystallization of the Izera granite at 514-480 Ma as mentioned above (Korytowski et al, 1993;Oliver et al, 1993;Kröner et al, 1994;Philippe et al, 1995;Marheine et al, 2002). According to Żelaźniewicz et al (2003) dating of zircons from the intraschists gneisses from Gierczyn has shown that their granite protolith intruded into the envelope at 505 Ma, the Świeradów gneisses (diatextites) were partially melted between 513-483 Ma, while leucogranites from Kotlina crystallized/intruded in the interval of 508-483 Ma.…”

“…For the Karkonosze granite they were reported to average at 304 Ma and 290 Ma (K-Ar and Rb-Sr, respectively) that is somehow controversial taking into account the closure temperature of the Rb-Sr system higher than that of the K-Ar system. According to 40 Ar/ 39 Ar determinations of Marheine et al (2002), the shearing activity within the Krknoše-Jizera terrane dated at about 325-320 Ma was accompanied by comtemporary pluton emplacement (Pin et al, 1987;Mierzejewski et al, 1994). The upper limit of the tectonometamorphic and magmatic activity has been estimated at 314-312 Ma ( 40 Ar/ 39 Ar) − the Namurian/Westphalian boundary (Manheine et al, 2002).…”

“…Due to the discussion on its genesis and age relations, numerous isotopic analyses have been also performed -by different methods and for different minerals and rock types (e.g. Borkowska et al, 1980, Jarmołowicz-Szulc, 1984, Pin et al, 1987, Oliver et al, 1993, Marheine et al, 2002, Żelaźniewicz et al, 2003, Machowiak and Armstrong, 2007, Mikulski and Stein, 2007. On the background of the earlier isotopic age determinations, K-Ar dating has been recently conducted aiming at summing up the age data as well as a comparison to the geochronological data obtained within the complex in the same area (Jarmołowicz-Szulc, 1984).…”