Die geologische Affinität und prävariszische Entwicklung des Brunovistulischen Mikrokontinents (Tschechische Republik): Nachweis durch U-Pb und Ar-Ar Mineraldatierungen an granitoidische Basementgesteine und Decksedimente
Zusammenfassung der Projektergebnisse
Objective 1. The narrow 590–601 Ma age range for Slavkov Terrane granitoids shows that the main magmatism took place within a short time interval in the Ediacaran and was coeval with that in the Thaya Terrane that has published ages of 568–634 Ma, of which most are between c. 580–605 Ma. In contrast to the Thaya Terrane, the dearth of older, inherited ages in Slavkov Terrane granitoids points to a more juvenile origin for the latter. This is in keeping with published models and recent isotope data that indicate a continental arc setting for the Thaya Terrane and an oceanic primitive arc setting for the Slavkov Terrane. The 740 ± 4 Ma age for felsic metatuff from the Metavolcanic Belt is somewhat older the 725 ± 15 Ma Pb-Pb zircon evaporation minimum age reported by Finger et al. (2000) and the 733 ± 5 Ma pooled age of Hanžl et al. (2019). The ages for metatuffs from three different localities show that the felsic, and by inference, mafic volcanism lasted from at least ca. 740 Ma to ca. 725 Ma and is more than 100 Ma older than the oldest magmatic activity in the BVM namely 655 ± 3 Ma diorite from the Diorite Zone. The large age difference and its published MORB-like geochemistry render it unlikely that it constituted the backarc basin to the Thaya and Slavkov arcs, but instead formed much earlier during break-up of Rodinia. Objective 2. The ages of the felsic tuff beds in the fluviatile red sandstones overlying the Slavkov Terrane (“Basal Clastics”) show that deposition took place around 550 Ma in the late Ediacaran, c. 50 Ma after the main plutonism in the arc terranes. The ages of detrital zircons and white micas and the protolith crystallization ages of granitoid clasts show that nearly all material was derived from local sources that must have been exposed in small catchment areas only. Older detrital zircons are rare. However, the wide 564 Ma to 3.30 Ga range of detrital zircons from a quartzite and a sandstone clast shows that around 550 Ma, Palaeoproterozoic to Archaean continental crust must have been in erosional position nearby. Thus, like the Małopolska Block, the Brunovistulian microcontinent may have been close to Baltica as early as in the Ediacaran. Objective 3. Most available 40Ar/39Ar ages for magmatic micas and amphiboles overlap the zircon crystallization ages showing that, following crystallization of the plutonic and sub-volcanic rocks, cooling through the respective closure temperatures for Ar diffusion general occurred within a few million years and that the rocks were uplifted if not exhumed shortly after the main phase of plutonism. The exception is white mica from amphibolite-facies two mica schist KRÁSNÁ1 from the Upper Silesia Block that appears to have cooled later at ca. 563 Ma, and a white mica from amphibolite-facies metapelites GOTT2 that has an anomalousy young ca. 520 Ma age. The ca. 598 Ma detrital white mica age of sandstone HÁDY7 of the Ediacaran cover of the Slavkov Terrane compare well with the ca. 595-600 Ma cooling ages for the Slavkov Terrane granitoids from which they much have been derived with little or no admixture of much older material. New objectives. Slavkov Terrane anchizone to amphibolite-facies metasediments GOTT2, JABLŮNKA1, ŚLEMIEŃ1, LACHOWICE7. and KRÁSNÁ1 have very simple age spectra that are dominated by a main peaks at around 580-640 Ma; much older zircons are very limited in number. The sedimentary protolith of GOTT2 was clearly derived by material mainly eroded from magmatic rocks such as those of the Slavkov and Thaya Terranes, and perhaps the Diorite Zone (ca. 655 Ma). However, the protoliths of the central and northeastern Slavkov Terrane contained material from younger sources not yet identified among the granitoids. In general, the zircon age spectra point to a Gondwanan source area that had a record of the Pan-African orogeny. Thus, not so much a Cadomian provenance but more likely sources such as the Sahara Metacraton or Arabian-Nubian Shield. The detrital zircon data form the lowermost Cambrian (Sub-Holmia) sandstone O-2 of Upper Silesia show a predominant population of Ediacaran zircons that may be linked to a Cadomian orogenic event. Importantly, zircons older than 1 Ga point to sources within the East European Craton. The younger zircons probably reflect docking of the BVM to Baltica during an Ediacaran event in which the distal margin of the craton was involved, supplying much older zircons. Similar to the metasediments of the Slavkov Terrane, detrital zircon ages of low grade metasediment WARYŚ5 and unmetamorphosed sediment ŁĘTOWICE20 of the Małopolska Block show large age peaks around 600 Ma. In contrast, they contain substantial numbers of older, Mesoproterozoic, Early Palaeoproterozoic and Neoarchaean zircons that are rare in the Slavkov Terrane lithologies. The combined evidence shows that the majority of Małopolska Block detrital zircons were derived from a Gondwanan source, but also uniformly point to a source area within Baltica. Hence, the Małopolska Block was already close to, or even docking to, Baltica in the Ediacaran. The detrital zircon age spectrum for siltstone clast TRUS6 from western Ukraine is very similar to those of the Małopolska Block samples, providing evidence that the latter extends further southeastwards under younger cover.
