Skip to main content
Article thumbnail
Location of Repository

Clasts of variscan high-grade rocks within Upper Visean conglomerates : constraints on exhumation history from petrology and U-Pb chronology

By J. Kotkova, A. Gerdes, Randall Parrish and M. Novak

Abstract

In this paper, U-Pb zircon, monazite and rutile data for crystalline rocks deposited as clasts in the Upper Viséan 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 (±2σ), 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 Viséan 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. \u

Topics: Earth Sciences
Publisher: Blackwell
Year: 2007
DOI identifier: 10.1111/j.1525-1314.2007.00730.x
OAI identifier: oai:nora.nerc.ac.uk:4149
Download PDF:
Sorry, we are unable to provide the full text but you may find it at the following location(s):
  • http://www.blackwellpublishing... (external link)
  • https://doi.org/10.1111/j.1525... (external link)
  • https://doi.org/10.1111/j.1525... (external link)
  • http://nora.nerc.ac.uk/id/epri... (external link)
  • Suggested articles


    To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.