Fission-track thermochronology of the Ruhla Crystalline Complex: new constraints on the post-Variscan thermal evolution of the NW Saxo-Bohemian Massif

• Published in: Tectonophysics Vol. 324; no. 1; pp. 17 - 35
• Main Authors: Thomson, S.N., Zeh, A.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.09.2000
• Subjects: denudation; fission-track dating; geochronology; inversion tectonics; thermal history; fission-track dating; thermal history; geochronology; denudation; inversion tectonics
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/S0040-1951(00)00113-X

To help clarify the relative and absolute timing of post-Variscan cooling and exhumation processes in the Ruhla Crystalline Complex (RCC) situated in the NW part of the Saxo-Bohemian massif, eight zircon and eleven apatite fission-track ages and nine apatite track length measurements have been obtained. Zircon fission-track ages support an existing model of post-Variscan exhumation of the RCC as three discrete blocks. The eastern and western blocks show cooling from peak metamorphic conditions to below 260±30°C by ca. 300 Ma and exposure by 296 Ma. The central block shows a later cooling to below 260±30°C by ca. 270 Ma and exposure at the surface by ca. 256 Ma. Apatite fission-track data reveal Late Cretaceous accelerated cooling through the apatite partial annealing zone (120 to 60°C). This can be correlated with tectonic inversion at this time. The apatite data also indicate that the rocks of the RCC were at temperatures above 110±10°C before ca. 85 Ma. This requires either an anomalously high palaeo-geothermal gradient of ca. 60°C km −1 during the Late Cretaceous or that the RCC region was originally covered by up to 1400 m of now eroded Jurassic and Cretaceous sedimentary rocks. We favour a combination of both factors, with accelerated erosion of missing overburden during a period of increased heat flow initiated at ca. 85 Ma.