Simultaneous horst-basin formation and magmatism during Late Variscan transtension: evidence from Ar-40/Ar-39 and Pb-207/Pb-206 geochronology in the Ruhla Crystalline Complex

The Ruhla Crystalline Complex (RCC) is a constituent of the NE-trending Mid -German Crystalline Rise (MGCR) formed during the Variscan Orogeny, Further more, it is part of the NW-trending Franconian Fault system, which caused a n intensive block faulting from the Late Carboniferous/Early Permian onward . In this paper new Ar-40/Ar-39-mica and Pb-207/ Pb-206-zircon dates are pr esented and combined with existing P-T data and the sedimentary record. The se data indicate that the RCC was faulted into three segments which underwe nt different exhumation histories during the Late Carboniferous/Early Permi an, The eastern segment shows Ar-40/Ar-39-biotite data of 336 +/- 4 and 323 +/- 3 Ma. Furthermore, it is intruded by the Thuringian Hauptgranite dated at 337 +/- 4 Ma by the (207)pb/(206)pb Single zircon method. At approximat ely 300 Ma rocks of the eastern segment were finally exposed and, subsequen tly, subsided as part of the Oberhof pull-apart basin, filled by Late Carbo niferous/Early Permian molasse sediments and volcanic rocks (296-285 Ma: Co il 1996), A similar Late Carboniferous evolution is inferred for the wester n segment, since it is also overlain by Upper Carboniferous volcanic rocks. In contrast to the eastern and western segments, distinctly younger intrus ion and cooling ages were recorded for the central segment of the RCC (Ar-4 0/Ar-39 muscovite: 311 +/- 3 Ma: Ar-40/Ar-39 biotite: 293-288 +/- 3Ma) that was intruded by the Trusetal Granite, the Ruhla Granite and Brotterode Dio rite (207pb/206pb Single zircon: 298 +/- 2, 295 +/- 3, 289 +/- 4 Ma. respec tively). These young data are unique in the MGCR and testify that plutonic activity and cooling of basement rocks took place simultaneously with basin formation and volcanism in the eastern and western segments. Overlying Upp er Permian (Zechstein) and Triassic sediments indicate final exposure of th e central segment by approximately 260 Ma, as a part of the Ruhla-Schleusin gen Horst. Combination of these results with P-T data from the contact aure ole of the Trusetal granite indicate that the central segment was unroofed by at least 8.5 km during the Late Carboniferous. The Late Carboniferous/Ea rly Permian horst-basin formation, documented in the RCC, is due to dextral transtensional movements along the NW-trending Franconian fault system. It may have been enhanced by mantle upwelling widespread in Central Europe du ring the Early Permian that also caused intensive magmatism in the Thuringi an Forest region.