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Geological Society, London, Special Publications; 2000; v. 179; p. 323-336;
DOI: 10.1144/GSL.SP.2000.179.01.19
© 2000 Geological Society of London

White mica 40Ar/39Ar ages of Erzgebirge metamorphic rocks: simulating the chronological results by a model of Variscan crustal imbrication

Olaf Werner & Hans J. Lippolt

Laboratorium für Geochronologie, Ruprecht-Karls-Universität, INF 234, D-69120 Heidelberg, Germany olaf.werner{at}urz.uni-heidelberg.de

In the Erzgebirge, which is part of the Saxo-Thuringian Mid-European Variscides, an 40Ar/39Ar study was performed on white mica and hornblende separates from 68 metamorphic rocks of varying lithologies. Two groups of late Early Carboniferous cooling ages of 340 ± 2 Ma and 329.7 ± 1.5 Ma are distinguished by evaluating spectra shapes. A third group of rocks yielded intermediate argon ages, presumably related either to thermal rejuvenation or to neoformation of white mica during a reheating process 330 Ma ago. A model that explains rejuvenation is proposed. The observed age patterns and the shape of the age spectra cannot be simply explained by assuming a single uplift and exhumation process set in motion by extensive forces. Chronological and structural arguments favour the idea that Variscan crustal imbrication took place during the mineral cooling phase. A tectonic process that intercalated, at about 20 km depth, a unit of cool rocks into a much hotter environment suitably models the chronological results. This is shown by a simulation of the age record. Temperature profiles are calculated for both intercalation and subsequent uplift models. Rocks in these profiles, which preserved their 340 Ma signature, are distinguished from those heated 330 Ma ago by contact with adjacent hotter rocks and reset isotopically to varying degrees. The agreement between analytical results and modelled ages favours the crustal imbrication hypothesis as explanation for the age distribution throughout all lithologies.





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