Clasts of Variscan high‐grade rocks within Upper Viséan conglomerates – constraints on exhumation history from petrology and U‐Pb chronology

Abstract

In this paper, U-Pb zircon, monazite and rutile data for crystalline rocks deposited as clasts in the Upper Vise´an conglomerates at the eastern margin of the Bohemian Massif are reported. U-Pb data of spherical zircon from three different granulite clasts yielded a mean age of 339.0 ± 0.7 Ma (±2r), while oval and spherical grains of another granulite pebble define a slightly younger date of 337.1 ± 1.1 Ma. These ages are interpreted as dating granulite facies metamorphism. Thermochronology and the derived pressure-temperature (P-T) path of the granulite pebbles reflect two-stage exhumation of the granulites. Near-to-isothermal decompression from at least 44 km to mid-crustal depths of around 22 km was followed by a near-isobaric cooling stage based on reaction textures and geothermobarometry. Minimum average exhumation rate corresponds to 2.8-4.3 mm year )1 . The extensive medium-pressure/ high-temperature overprint on granulite assemblages is dated by U-Pb in monazite at c. 333 Ma. This thermal event probably has a close link to generation and emplacement of voluminous Moldanubian granites, including the cordierite granite present in clasts. This granite was emplaced at mid-crustal levels at 331 ± 3 Ma (U-Pb monazite), whereas the U-Pb zircon ages record only a previous magmatic event at c. 378 Ma. Eclogites and garnet peridotites normally associated with high-pressure granulites are absent in the clasts but exotic subvolcanic and volcanic members of the ultrapotassic igneous rock series (durbachites) of the Bohemian Massif have been found in the clasts. It is therefore assumed that the clasts deposited in the Upper Vise´an conglomerates sampled a structurally higher tectonic unit than the one that corresponds to the present denudation level of the Moldanubicum of the Bohemian Massif. The strong medium-temperature overprint on granulites dated at c. 333 Ma is attributed to the relatively small size of the entirely eroded bodies compared with the presently exposed granulites.

Vertical extrusion and horizontal channel flow of orogenic lower crust: key exhumation mechanisms in large hot orogens?

“…comm.). Pebbles of granulite and Mg‐rich syenite (durbachite) in the foreland conglomerates have yielded similar information from U–Pb dating of zircon, where two groups of ages occur at c. 340 and c. 325 Ma, the latter being consistent with the stratigraphic age of 330–320 Ma (Kotková et al. , 2007).…”

Challenges in high‐pressure granulite metamorphism in the era of pseudosections: reaction textures, compositional zoning and tectonic interpretation with examples from the Bohemian Massif

“…Medaris et al. , 1990, 2006), evidence of deposition of boulders of eroded 340 Ma granulite within 10 Myr of this peak (Kotková et al. , 2007), and also in studies of zircon by in situ methods which reveal that texturally HP, and low‐pressure zircon from the same hand specimen are of the same age within the error ( c. 1 Myr) of the measurement (Kröner et al.…”

“…These conventional measurements were repeated by more modern evaporation and ion microprobe (SHRIMP) analyses of zircon as well as other methods with essentially the same result: 340 Ma for the granulite event (Aftalion et al. , 1989; Quadt, 1993; Kotková et al. , 1996, 2007; Becker, 1997; Kröner & Willner, 1998; Kröner et al.…”

Provenance study of Permian non-marine sandstones and conglomerates of the Krkonoše Piedmont Basin (Czech Republic): exotic marine limestone pebbles, heavy minerals and garnet composition

“…In garnets 3 and 23 a weak prograde zonality can be observed. There is a slight increase in xMg from the Moldanubian garnets were recycled from Carboniferous, it can be inferred that the Moldanubian Zone was exposed to erosion during the Late Carboniferous, as indicated by the data from Kulm in central Moravia (Dvořák 1973, Hartley & Otava 2001, Kotková et al 2007.…”