TSK 11 Göttingen 2006 Hauten et al. Feldspar deformation in greenschist facies shear zones (Aar-Massif, Switzerland) Poster Sarah Hauten1 Mark R. Handy1 Christoph Dobmeier1 Granitic gneisses of the Central Aar Granite host a shear zone network that formed at greenschist facies con- ditions. The work area is located in the Bächlital (Grimsel area, Central Switzerland) and was chosen for the analysis of shear zones because of the weakly anisotropic fabric of the host gneisses. Contrary to previous pub- lications (e.g. Choukroune & Gapais, 1983), none of these host rocks are un- deformed. They contain a penetrative foliation (S1) that strikes consistently ENE-WSW with a steep dip of around 70° to the south. This foliation is overprinted by the aforementioned shear zone network, which was the main focus of this study. The granitic gneisses are predominantly equigranular, but locally contain feldspar augen. Albite, quartz, magmatic K-feldspar, biotite, chlorite and epidote make up the rock. Albite is the dominant plagioclase mineral, re- flecting the greenschist facies metamor- phic overprint. Partial chloritization of biotite is associated with the segrega- tion of a Ti-phase. Calcium from the K-feldspar was used for the formation of epidote. The shear zone network comprises major, dm to km long shear zones which are interconnected by mi- nor, cm to dm long shear zones. The major shear zones are oriented predom- inantly NE–SW and have a cm-wide my- lonitic to ultramylonitic centre which 1 Freie Universität Berlin, Department of Eath Sciences, Malteserstr. 74-100, 12249 Berlin is bounded on either side by cm to m long fractures that run parallel to the shear zone centre in intervals of meters to decimeters. The minor shear zones are not preferentially oriented and reach lengths of several centimeters to decime- ters. They do not always show a distinct shear component and sometimes end as cracks before reaching the next major shear zone. Four stages of feldspar deformation dur- ing the shear zone development could be observed in thin section: 1. Discrete, intracrystalline micro- faults form conjugate sets. Al- though some of these microfrac- tures run parallel to the cleavage planes, the majority runs oblique to them. Bent twin lamellae rep- resent the beginning stages of mi- crofracturing. Rotated micropha- coids (fragments bordered by the microfractures) are the end result; 2. Dynamic recrystallisation charac- terized by serrate grain bound- aries and core-mantle structure of feldspar grains. What processes leading to this recrystallisation are unclear and and are still under investigation. Cracks filled with small feldspar grains and pressure shadows of K-feldspar clasts point to pressure solution as a relevant process in this stage. Electron microprobe analysis revealed that some fractures are filled with K- feldspar which is chemically dis- tinct from the host K-feldspar (e.g. higher Ba content). Progressive subgrain rotation is also commonly observed in feldspar grains. Some subgrain aggregates show a crystal- lographic preferred orientation; 3. Recrystallised aggregates become 1 Hauten et al. TSK 11 Göttingen 2006 more prevalent and grains start to creep, leading to a mylonitic fabric; 4. The transition to the fourth, ultra- mylonitic stage is not well defined. The aim of my work is to understand the mechanisms of the feldspar deformation in the four stages that lead to the ul- tramylonitic final stage. The results I have presented in this abstract are pre- liminary. 2