U-Pb and Ar-Ar dating of minerals from metamorphic and syn-orogenic sedimentary rocks as a key to understanding architecture and evolution of collisional orogens
Zusammenfassung der Projektergebnisse
In the light of continuous attempts at interpreting the Variscan continental collision the study has focussed on the Saxo-Thuringian Zone of the Variscan belt, where a unique association of syn- to late-orogenic sediments records the exhumation of ultrahigh to low-pressure metamorphic rocks and the emplacement of gravitational nappe units. The investigation has aimed at dating formation, metamorphism, and exhumation of basement rocks with a key function for the tectono-metamorphic evolution. The second goal was to determine how the sedimentary detritus monitoring exhumation and denudation of the basement rocks has changed over time in age and provenance. The results are partially surprising and answer long-debated issues, with important implications for the geodynamic evolution. One novel result concerns the Saxon Granulite Massif, where U-Pb and Ar-Ar dating revealed that the Metagabbro/Serpentinite Unit has an Early Cambrian magmatic formation age and underwent Middle to Upper Devonian metamorphism, followed by two cooling events after metamorphism and reheating. The metamorphism thus preceded that in the structural units above and below, albeit all units merged along the high-temperature mylonite zone linking the granulites to the hanging-wall rocks. This result is at odds with the assumption that Variscan metamorphic events dated in the Middle Devonian, Upper Devonian, and Lower Carboniferous occurred each in a different subduction-exhumation channel due to a two-fold retreat of the subduction zone. The depositional age of metasediments from the Schist Cover of this massif just below the faulted contact with nappe sedimentary rocks is younger than Middle/Upper Devonian boundary time, based on detrital U-Pb zircon data. The lowdiversity palynomorph assemblage reported from this and other localities and considered typical of the late Neoproterozoic Cadomian orogenic belt has thus no biostratigraphic significance. A structural unit at the northern flank of the Erzgebirge with garnet-kyanite-staurolite-bearing peak assemblages in mica schist and gneiss has yielded Lower Carboniferous Ar-Ar mica ages similar to those from high-temperature rocks of the Erzgebirge. The consistent ages and heating of this unit to P-T conditions coinciding with the decompression path of the high-temperature rocks suggest a merger of both units at moderate high-pressure conditions. An unexpectedly old Ar-Ar biotite age from two-mica orthogneiss of the Erzgebirge implies that pre-Upper Devonian metamorphic rocks preserved in the late-orogenic Flöha transverse fault zone have overridden the late Viséan exhumed crustal stack of this massif without resetting of the K-Ar system in large biotite. Only a few of the obtained Ar-Ar mineral ages date cooling from Lower Carboniferous metamorphism, whereas many data from the Saxon Granulite Massif and the Erzgebirge cluster into two groups at 325-313 Ma and 307-270 Ma that overlap with two periods of post-Variscan granite-rhyolite magmatism and are due to resetting of metamorphic K-Ar mineral systems during these thermal events. Phyllites from the western Erzgebirge have yielded Rb-Sr mineral isochron ages of 357 Ma and 336-332 Ma corroborating published Ar-Ar white mica ages interpreted as dating cooling from blueschist facies metamorphism and greenschist facies overprinting. These rocks and those from the Garnet-Phyllite Unit farther to the east in the Erzgebirge differ clearly in peak P-T conditions and likely represent separate structural units. U-Pb zircon dating of granite gneiss and amphibole gneiss from the Frankenberg Klippe has revealed Cambro-Ordovician magmatic protolith ages and magma sources in Early Cambrian and Neoproterozoic crustal rocks. Ar-Ar biotite and amphibole ages from Frankenberg and Wildenfels nappe metamorphic rocks indicate cooling and exhumation in the Late Devonian. Despite poor miospore yields and preservation, a palynological study of late-orogenic fan-delta sediments and post-orogenic terrigenous sediments of the Hainichen Basin has disclosed Lower and Mid-Upper Viséan stratigraphic ages, respectively. Detrital white micas from greywackes and sandstones have simple and narrow Ar-Ar age spectra indicating an origin from small catchment areas dominated by basement rocks with uniform Ar cooling ages. For the Hainichen Basin, lag times shorten from 15-20 Ma for the oldest (340 Ma) sediments to nearly zero for the youngest, Upper Viséan sediments, which reflects increased exhumation rates of high-grade sources between the early and the late Viséan. The detrital zircon ages are dominantly Neoproterozoic-Early Palaeozoic, showing that Cadomian and Cambrian magmatic rocks were the main sources of detritus.
