TSK 11 Göttingen 2006 Neubert et al. Microfabrics and deformation processes in magmatic veins of the Thuringian Forest, Germany Poster Jana Neubert1,2 Sebastiaan van der Klauw3 Jonas Kley2 Introduction The research area is located in the Ruhla-Brotterode crystalline complex in the western part of the Thuringian Forest (Germany), about 20 km south- southwest of Eisenach. The investigated outcrops occur at the eastern and west- ern flanks of the valleys north of the villages Trusetal and Hohleborn. De- formed magmatic veins only occur in the Hohleborn area. Both areas have relative fresh outcropping rocks, due to the steep relief, former quarries and fresh road cuts. According to Obst & Katzung (2000) several periods with the formation of magmatic veins with different chemical composition occur in the Ruhla-Brotterode crystalline com- plex. Presumably older lamprophyric veins and younger doleritic, syenitpor- phyric and granitporphyric veins have been identified (Obst & Katzung 2000). Benek & Schust (1988) already pointed out that some of these magmatic veins have experienced ductile deformation. The subject of this work is the oc- currence of deformed magmatic veins in the Hohleborn area. The contact to their host rocks, their petrography and their microfabrics have been inves- tigated and related to deformation pro- 1 Universidade Federal do Pará, Dep. Ge- ologia, CP 1611, 66017970 Belém, Brazil 2 Friedrich-Schiller-University, IGW, Burgweg 11, 07749 Jena, Germany 3 Ercosplan — Ingenieurgesellschaft Geotechnik und Bergbau mbH, Arnstädter Strasse 28, 99096 Erfurt, Ger- many cesses, which led to a better under- standing of their deformation conditions within the late- to post-variscan devel- opment of the area. Regional geological framework The Ruhla-Brotterode crystalline com- plex is part of the Mid-German Crys- talline High (Fig. 1). The Crystalline High is a 50–70 km broad zone strik- ing NE–SW and forms the NW bor- der of the Variscan Saxothuringian zone (Seidel 1995). During the main phase of the Variscan orogeny, in the lower to middle Carboniferous, the Mid Ger- man Crystalline High is considered to have been part of the active continen- tal margin of the Saxothuringian (mi- cro)continent (Seidel 1995), which over- rode the more northwestern Rhenoher- cynian (micro)continent. This phase continued during upper Carboniferous time with the segmentation of the Variscan orogen through an E–W ex- tensional stage within Central-Europe. The investigated area lies in the south- eastern part of the Crystalline Com- plex. Its rocks mainly consist of para- gneisses, which are intruded and bor- dered by numerous permocarbonifer- ous granites and a diorite (Lützner et al., 1997). According to Zeh et al. (1996), the paragneisses are part of the Truse Formation and represent an accre- tionary wedge. The crystalline rocks are covered by Permotriassic sedimentary rocks and crosscut by numerous E-W- to SSE-NNW-trending, magmatic veins. These veins are of basaltic, andesitic and dacitic to rhyolithic nature (Obst & Katzung, 2000) and appear as simple veins, mixed or combined veins (Mädler & Voigt 1994). U-Pb zircon dating of the veins by Brätz (2000) shows ages be- tween 285±5Ma and 264±7Ma for un- 1 Neubert et al. TSK 11 Göttingen 2006 Figure 1: Position of the Mid-German Crystalline High within the variscan orogen (mod- ified after Hansch & Zeh (1999) deformed veins and ages between 305– 320Ma and 294±4 Ma for the deformed veins. Methodology From 10 oriented samples of deformed veins (2 sections per sample, perpen- dicular to the foliation) as well as for comparison purposes from 10 samples of macroscopically undeformed vein (1 sec- tion per sample) thin sections have been prepared for microscopic structural and petrographical analysis. The sections have been investigated for mineralogical composition, micro-fabrics, deformation structures and deformation intensity trough measurement of length-width- relation of quartz crystals (shape pre- ferred orientation, SPO) and extension- relation of feldspars crystals as well as the crystallographic preferred orienta- tion (LPO) of the quartz crystals with the universal stage. The structural data from veins and host rocks have been evaluated with the program ‘Wintek’ for Figure 2: Stereographic projection (lower hemisphere) of vein-host rock contacts of undeformed, magmatic veins in the re- search area; n = 44. integration of the deformed veins within the late to post-variscan context. Structural Analysis Figure 2 shows the stereographic pro- jection of the contacts of undeformed magmatic veins with their host rocks. Nearly all veins strike E–W to NW–SE 2 TSK 11 Göttingen 2006 Neubert et al. Figure 3: Stereographic projection (lower hemisphere) of vein-host rock contacts of deformed, magmatic veins in the research area; n = 28. and dip steeply with 65–85° to N–NE re- spectively SSW–WSW. In contrast the deformed magmatic veins in Figure 3 strike mostly N–S to NE–SW and dip with 40–65°to ENE to SE. The struc- tural inventory of the host rocks (Fig. 4a & 4b) shows a predominance of N–S to NE–SW striking and 30–75° dipping schistosity S3 and NW–NNE striking 20–60° dipping S2 which both seem to be partly refolded by B4. This leads to the possible directions of 5–35°to ENE–E for B3 respectively 35–60° to ESE–SSE for B4 folding axes. The main rock-forming minerals of the deformed veins own the following char- acteristics. • Feldspars (potassic feldspar & pla- gioclase): microboudinage, serici- tisation, fissures (often filled with antitaxial and syntaxial quartz- and feldspar-fibers), incipient frac- tures, growth rims of quartz par- allel and perpendicular to schistos- ity and in strain shadows of crys- tals, undulatory extinction & mi- croclinic & myrmekitic structures (pot. feldspar) or uniform extinc- tion and mechanical twins (plagio- clase). • Quartz: occurs as porphyroclast, matrix-quartz or growth-rims. Por- phyroclast have a strong elonga- tion (extreme length-wide-relation, long axis parallel to schistosity), undulatory extinction, deformation ribbons, subgrain structures (size ca. 1mm), recrystallised grains with highly irregular grain bound- aries (size 10–15µm); as coarse strain-shadow of crystals or vein filling; grain size spectrum: coarse / fine matrix = 15–50µm / 10– 15µm, strain shadow = ca. 50µm, growth rim = 100–300µm. • Biotite: structures indicating slid- ing of layer-packages parallel to cleavage or bending with undula- tory extinction in extreme frac- turing perpendicular to schistosity; single clasts as well as small orien- tated fragments (frequently marks the schistosity together with chlo- rite, hematite, sericite and opaque phases), partly strongly alterated. The evaluation of the quartz-elongation and the stretched feldspars in the Flinn- Diagramm shows that the deformation regime is flattening and that the in- tensity of the quartz deformation (in all directions) is higher than that of the feldspars. The determination of recrystallized grain size of quartz and the calculation of differential stress af- ter Blenkinsop (2000) and Twiss (1977) yielded relative high differential stresses in the range of 95–125MPa for this flat- tening deformation stage. The deforma- tion also resulted in the development of a lattice preferred orientation (LPO) of 3 Neubert et al. TSK 11 Göttingen 2006 Figure 4: a) and b) Stereographic projection (lower hemisphere) of the host rock struc- tural inventory in the research area; a) of outcrop A, n = 28; b) of outcrop B, n = 35. the quartz in the deformed veins. The preferred orientation of the c-axis for isometric and elongate quartz grains is not well defined. Nevertheless the mea- sured quartz c-axes suggest an uniform reorientation with increasing deforma- tion intensity. The c-axis reorientation is to within the plane of the vein internal schistosity. Conclusions The investigation shows deformed an- desitic and rhyolitic veins with quartz- growth rims mostly orientated parallel to the schistosity. This strongly sug- gests that a fluid was available during deformation of the veins. The compari- son with the position of the main schis- tosity in the host rocks indicates that the deformed magmatic veins intruded before or during the last deformation phase of the host rocks (D4). This in- ference is supported by the nearly par- allel orientation of the veins with the shallower schistosity planes of the main schistosity (S3), the formation of ecc- structures in the deformed veins as well as fabrics of similar deformation inten- sity within the younger quartz-veins in the host rocks. The different orientation of deformed and undeformed veins also argues for their formation during differ- ent time episodes with a changed orien- tation of σ3. The sparse age determina- tions for the magmatic veins do not con- tradict this conclusion. The observed micro-fabrics limit the boundary condi- tions during the deformation to a regime with relative high differential stresses of 95–125MPa and low temperatures. The microstructures and the LPO of quartz indicate that the vein-deformation took place between the deformation regimes of low temperature plasticity and dislo- cation creep of quartz. References Benek R & Schust F (1988) Bemerkungen zur partiellen Gefügedeformation in Magmatit- gängen des Ruhlaer Kristallins (Thüringer Wald). Z. dt. Geol. Wiss. 16, 801–816 Blenkinsop, T (2000) Deformation Microstruc- tures and Mechanisms in Minerals and Rocks. Kluwer Academic Publishers Dor- drecht, p 150 4 TSK 11 Göttingen 2006 Neubert et al. Brätz H (2000) Radiometrische Altersdatierun- gen und geochemische Untersuchungen von Orthogneisen, Graniten und Granitpor- phyren aus dem Ruhlaer Kristallin, Mit- teldeutsche Kristallinzone. PhD-Thesis, Julius-Maximilians-University Würzburg, pp 131 Hansch R & Zeh A (1999) Metabasites from the Ruhla Crystalline Complax: Evidence for Distinct Pre-Variscan, Plate-tectonic En- vironments within the Mid-German Crys- talline Rise. Mineralogical Institute of the University Würzburg, pp 25 Lützner H & Seidel G (1997) Ruhlaer Kristallin, Tambacher und Eisenacher Rotliegendbecken (westlicher Thüringer Wald). Schriftenreihe der Deutschen Ge- ologischen Gesellschaft - Exkursionsführer ‘Regionale Geologie von Mitteleuropa’, pp 34 Mädler J & Voigt H (1994) Aufbau, Petro- graphie und Genese eines Systems zusam- mengesetzter Gesteinsgänge am Südrand des Ruhlaer Kristallins westlich Seligenthal / Thüringer Wald (ehemaliger Gieselsberg- Schacht); In: Thüringer Landesanstalt für Geologie — Geowissenschaftliche Mitteilun- gen von Thüringen, Weimar, pp 321 Obst K & Katzung G (2000) Die magma- tischen Gänge am Südrand des Kristallins von Ruhla-Brotterode (Thüringer Wald) - Herkunft der Magmen, Aufstieg und Platz- nahme im variszischen Spannungsfeld. Z. dt. geol. Ges. 151, 441–470 Passchier CW & Trouw RAJ (1998) Microtec- tonics. Springer-Verlag Berlin Heidelberg, pp 289 Seidel G (1995) Geologie von Thüringen. Schweizerbart’sche Verlagsbuchhandlung Stuttgart,pp 555 Twiss RJ (1977) Theory and applicability of a recrystallized grain size palaeopiezometer. In: Pure & Applied Geophysics, 227–244 Zeh A (1996) Die Druck-Temperatur- Deformations-Entwicklung des Ruhlaer Kristallins (Mitteldeutsche Kristallinzone). Schweizerbart’sche Verlagsbuchhandlung Stuttgart, pp 212 5