ORIGINAL PAPER Polyphase strain caps Tamer Abu-Alam & Kurt Stüwe Received: 6 October 2010 / Accepted: 22 November 2010 / Published online: 22 December 2010 # Springer-Verlag 2010 Abstract Strain caps are one of a series of microstructures that typically form during deformation of a softer matrix around hard objects. However—in contrast to other microstructures around porphyroblasts, for example pressure shadows—strain caps are rarely described in the literature. Here we describe strain caps with particular focus on strain caps associated with growth of a new phase, not elsewhere present in the paragenesis. Examples from foliated, amphibolite facies, metapelitic schists from Alaska, Sinai and Bhutan are discussed. All examples show chlorite growth exclusively in strain caps formed around porphyroblasts. Porphyroblasts around which the strain caps grow are muscovite, staurolite and garnet, respectively. In all of these examples strain caps formed synkinematically, but the chlorite grew statically at a later stage. Three mechanisms can explain the formation of new phases in the strain cap region: (a) the strain cap region may have experienced different P-T conditions from the matrix; (b) the strain cap region has a different effective bulk composition from the surrounding matrix; (c) fluid flow that is preferentially focused parallel to the foliation planes causing only local adjustment to retrograde metamorphism in the strain cap region. We show that the third hypothesis is the most preferable mechanism. Indeed, the absence of chlorite outside the strain cap region allows a quantification of the amount of fluid that infiltrated the rock. It is shown that for Bhutan sample about 8.5 mole% more water must have been added to the rock during fluid infiltration to cause the strain cap formation. Introduction Strain caps are one of a series of microstructures that typically form during deformation of a softer matrix around hard objects (e.g. porphyroblasts and-clasts). As such, they can be used to infer aspects of the stress and strain fields around the porphyroblasts. Other microstructural elements around pophyroblasts include their strain- and pressure shadows, as well as the line that effectively separates the flow around the porphyroblast from the “eddy-flow” in the strain shadow: the separatrix (Fig. 1a). Some of these microstructural elements bear characteristic information on the vorticity of the stress field and have—therefore— received a lot of attention in the literature. For example, the orientation of a pressure shadow with respect to the flow lines is a direct consequence of the ratio of pure- to simple shear and can thus be used to estimate the vorticity (Fig. 1b, c and d). A GEOREF search lists more than 36 entries for the term “pressure shadow” or “strain shadow” in the title of a paper (e.g. Takagi and Ito 1988; Etchecopar and Malavieille 1987; Tenczer et al. 2001). In contrast, strain caps are barely described outside their definition (e.g. Passchier and Trouw 1996, 2005), possibly because strain caps are considered to merely reflect the denser flow lines around the porphyroblast without much additional signifi- cance for the orientation of the stress field. Typically, strain caps are evidenced by a region that is depleted in quartz and enriched in micas occurring on opposite sides of the rigid body, in the quarters orthogonal to the strain shadow. In one of the few studies that have discussed strain caps, Trouw et al. (2008) showed this orthogonal relationship between strain cap and strain shadow may be responsible for the orthogonal arrangement of spiral inclusion trails in garnet. They argue that synkinematically grown garnet typically grows preferentially in direction of the strain cap Editorial handling: J. Raith T. Abu-Alam (*) : K. Stüwe Institut für Erdwissenschaften, Universität Graz, Universitätsplatz 2, 8010 Graz, Austria e-mail: tamer.abu-alam@uni-graz.at Miner Petrol (2011) 101:1–19 DOI 10.1007/s00710-010-0141-7