Evidence for high‐temperature diffusional creep preserved by rapid cooling of lower crust (North Bohemian shear zone, Czech Republic)

Abstract

Volume diffusion and dislocation creep at T ∼ 800 °C led to high finite strain in granulite and orthogneiss of the Ohře crystalline complex (North Bohemian shear zone). Intragranular creep by volume diffusion is indicated by (i) lobate phase boundaries between feldspar and quartz, and (ii) removal of perthite lamellae and precipitation of tiny, aluminium‐rich needles at the margins of K‐feldspar. The striking diffusional‐creep structures imply high interfacial free energy that has been preserved from equilibration as a result of rapid cooling. U–Pb dating of monazite (342 ± 1 Ma) and 40Ar–39Ar dating of muscovite (341 ± 4 Ma) of Kadaň orthogneiss result in a cooling rate of 50 + 25/−17 °C Myr−1. This high value is attributed to collapse‐related ‘elevator‐style’ movements along the North Bohemian shear zone, resulting in the juxtaposition of upper crustal rocks of the Tepla–Barrandian unit against lower crustal rocks of the Erzgebirge crystalline complex.

Pre-Late Carboniferous geology along the contact of the Saxothuringian and Teplá-Barrandian zones in the area covered by younger sediments and volcanics (western Bohemian Massif, Czech Republic)

“…(Kröner and Willner 1998;Tichomirowa et al 2005). Zulauf et al (2002) dated monazites and muscovites (U-Pb and Ar-Ar method, respectively) from migmatitic orthogneisses of this unit and reported the same ages (within error) of 342 ± 1 Ma and 341 ± 4 Ma, respectively. Further to the northwest, the mica cooling ages from metamorphic rocks in the Erzgebirge Complex show mostly ~340 Ma but they become slightly younger in the area of the Nejdek-Eibenstock Pluton and in the northern part of the Sv.…”

Extreme ductility of feldspar aggregates—Melt‐enhanced grain boundary sliding and creep failure: Rheological implications for felsic lower crust

“…[6] The rocks studied belong to a lower crustal hightemperature metamorphic unit that was rapidly exhumed along the major Variscan tectonic boundary in the Bohemian Massif called the North Bohemian Shear Zone (NBSZ of Zulauf et al [2002]). The trend and spatial limits of the NBSZ are not defined in the field, because it is mostly covered by Neogene volcanics and sedimentary sequences ( Figure 1).…”

“…Zulauf et al [2002] proposed that this vertical fabric is related to the rapid vertical motion of lower crustal rocks along the NBSZ and to intense deformation of gneisses and granulites. The UCN orthogneiss shows significantly higher finite strains of feldspars compared to relatively weakly deformed quartz suggesting extreme ductility of feldspars.…”

“…K-feldspar component crystallized from melt probably grew mostly onto the older grains within K-feldspar bands, which would also explain their perthite-free margins [Zulauf et al, 2002]. The crystallization of melt as unlike phases in residual aggregates commonly results in disappearance of the banded rock texture.…”

“…Melt bearing microstructures have been preserved due to rapid cooling that effectively increased quenching of melt in studied rocks [Zulauf et al, 2002]. Deformation mechanisms of constituent mineral phases are investigated using quantitative microstructural analysis.…”