Foliation boudinage

Foliation boudinage is a form of boudinage that develops in foliated rocks independent of lithology contrast. This paper describes foliation boudins from the Çine Massif in SW Turkey and the Furka Pass–Urseren Zone in central Switzerland. Four common types of foliation boudin structures can be distinguished in the field, named after vein geometries in their boudin necks in sections normal to the boudin axis: lozenge-, crescent-, X- and double crescent-type. The boudin necks are mostly filled with massive quartz in large single crystals, commonly associated with tourmaline, feldspar and biotite and in some cases with chlorite spherulites. The presence of blocky crystals and chlorite spherulites suggests that these veins formed as open, fluid-filled cavities during the initiation and development of foliation boudin structures, even in ductilely deforming gneiss at a depth of mid-crustal levels (7–10 kbar). The presence of cavities allowed the formation of closed fishmouth structures that are typical for many foliation boudins. The geometry of foliation boudin structures mainly depends on initial fracture orientation, propagation of the fracture during further deformation, and flow type in the wall rock.