Late-Variscan magmatism revisited: new implications from Pb-evaporation zircon ages on the emplacement of redwitzites and granites in NE Bavaria

Abstract

Pb-evaporation zircon analyses coupled with a detailed cathodoluminescence (CL) study on the complete series of granitoids from the northern Oberpfalz, NE Bavaria, provide new evidence for the commencement and timing of late-Variscan magmatism. All granitoids analysed in this study were dated before by Rb-Sr and/or K-Ar methods. Investigated samples comprise medium-grained, I-type dioritic rocks (redwitzites), I/Stype granites (Leuchtenberg, Marktredwitz (G1), Zainhammer) and S-type granites (Falkenberg, Liebenstein, Mitterteich, Friedenfels, Steinwald, Flossenbürg, Bärnau). The zircon evaporation technique reveals three groups of 207 Pb/ 206 Pb ages which are interpreted to represent magmatic crystallisation: (1) ages of 324-321 Ma are found in all analysed redwitzites and in almost all I/S-type granites; (2) the granites of Falkenberg and Liebenstein yield ages of ~315 Ma; (3) ages between 312 and 310 Ma are recorded in the Mitterteich, Friedenfels, Steinwald and Flossenbürg granites. Titanite crystals from different redwitzite bodies yield conventional U-Pb ages of 325-322 Ma, identical to the Pb-evaporation zircon data of these rocks. The S-type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages, indicating that some of the cores must have lost their radiogenic Pb. From the geochronological data, we infer that metamorphic conditions of the Variscan crust produced different granite types at different times. The data support a model involving an early period of mantle-related magmatism which postdates the final convergence stage of the Variscan orogen. This magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis, which is probably related to post-collisional extension of the thickened Variscan crust.

Provenance of the Lower Triassic Bunter Sandstone Formation: implications for distribution and architecture of aeolian vs. fluvial reservoirs in the North German Basin

“…). An additional 318 Ma age peak in the Volpriehausen Member is in accordance with late‐Variscan magmatism (Siebel et al ., ; Teipel et al ., ). A 300 Ma age peak in the member presumably records the volcanism or magmatism that occurred around the Carboniferous/Permian boundary in the northern part of the Variscan belt (Breitkreuz & Kennedy, ; Anthes & Reischmann, ).…”

Radiogenic heat production of Variscan granites from the Western Bohemian Massif, Germany

“…Late to post-orogenic granites (I-, S-and A-type) intruded this setting over a period of about 60 Myr (326 to 286 Ma; Siebel et al 1997Siebel et al , 2003Siebel and Chen 2010;Breiter 2012). Within all studied areas (Fig.…”

“…Intrusions of dioritic to granodioritic composition (redwitzites, local name for diorites) occurred at c. 323 Ma (Siebel et al 2003), representing the initial phase of late-orogenic magmatism in the Fichtelgebirge (Fig. 1b).…”

“…Granites reflecting this complex history exposed along the western margin of the Bohemian Massif (e.g. Siebel et al 2003;Finger et al 2009;Siebel and Chen 2010;Fig. 1b) are target of this study.…”

Coeval late-Variscan emplacement of granitic rocks: an example from the Regensburg Forest, NE Bavaria

“…The NBS 987 Sr standard yielded a 87 Sr/ 86 Sr ratio of 0.710252 ± 0.000012 (n = 14). Analytical techniques for zircon Pbevaporation (Kober 1986(Kober , 1987 are described in Siebel et al (2003). The U-Pb zircon analyses were carried out by standard isotope dilution methods as described in Chen and Siebel (2004).…”