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_Journal of Structural Geology 32 (2010) 1147-1157 Rotation of Irregular Staurolite Porphyroblasts in a Simple Shear Dominated Shear Zone Controlled by Initial Growth Orientation and Aspect Ratio Jochen E. Mezger Institut für Geowissenschaften, Martin-Luther-Universität Halle-Wittenberg, 06120 Halle, Germany Article History: Received 17 January 2010; Received in revised form 25 June 2010; Accepted 4 July 2010; Available online 14 July 2010 Keywords: Staurolite porphyroblasts Porphyroblast rotation Best-fit ellipse Shear strain Shear zone Abstract In a mica schist unit of the Bossét dome (Central Pyrenees, Spain), idioblastic interkinematic staurolite porphyroblasts with straight inclusion trails have rotated with respect to the main schistosity in the same sense by different amounts. Orientation data of 600 porphyroblasts were obtained from planes parallel to stretching lineation (xz), perpendicular to lineation (yz) and parallel to schistosity (xy). Interpenetrating growth twinning resulted in highly irregular shapes with aspect ratios ranging from 1 to 5. Maximum rotation of 120° was observed in blasts with inclusion trails oriented orthogonal to the mineral long axis, implying a shear strain of γ ≈ 4.3 in simple shear flow. Absence of back rotation is indicative of dominant simple shear flow (WK ≈ 0.8-0.9). Elongated porphyroblasts (RA > 3) with inclusion trails parallel to the long axis have rotated very little, because they were already in a stable position when shearing commenced. Rotation of staurolite porphyroblasts was possible because decoupling between blasts and matrix was weak and no significant strain partitioning had developed. The wide variety of shapes with different aspect ratios formed by simple blasts and interpenetrating growth twins of staurolite are potential shear strain and vorticity gauges. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The kinematic behaviour of porphyroblasts has been the focus of interest for anyone working in metamorphic terranes, ever since Zwart (1962) recognized the interaction of porphyroblast growth and deformation. While the discussion about whether rigid porphyroblasts in deforming ductile rocks can rotate at all or not with respect to an external reference frame is still ongoing (Bons et al., 2009; Fay et al., 2008; Lister and Forster, 2009), the majority of research published acknowledges the possibility of porphyroblast rotation (e.g. Robyr et al., 2009 for a recent study). Minerals whose kinematic behaviours are studied include garnet (Robyr et al., 2009; Schoneveld, 1977; Williams and Jiang, 1999), staurolite (Busa and Gray, 1992; Johnson et al., 2006; Mezger and Passchier, 2003), biotite (Holcombe and Little, 2001; Miyake, 1993), and plagioclase (Johnson, 2009). Linking ductile deformation to metamorphism allows distinction of individual tectonic and metamorphic events and establishment of their relative chronological order (Mezger and Passchier, 2003). Under certain conditions, strain rates and vorticity can be deduced from rotated porphyroblasts (Biermeier and Stowe, 2003; Holcombe and Little, 2001; Johnson et al., 2009). A characteristic mineral of amphibolite facies regional metamorphism is staurolite, commonly forming euhedral poikiloblastic porphyroblasts, which can contain inclusion trails that preserve earlier fabrics. In contrast to the equant habit of garnets, staurolite porphyroblasts possess elongated prismatic shapes of various aspect ratios, often forming a variety of interpenetrating growth twins, such as crosses (Ribbe, 1982). The stability field of staurolite is also restricted roughly to medium pressure-medium temperature conditions commonly associated with regional metamorphism resulting from crustal thickening during earlier orogenic stages. Widespread magmatism occurs in later phases of the development of many orogens, evident from late-tectonic pluton emplacement. As a consequence, contact metamorphism around the intrusions overprints previously regionally metamorphosed rocks. Newly formed porphyroblasts within the contact aureole, such as andalusite and cordierite, grow at the expense of earlier phases (e.g. staurolite), which suffer resorption along grain boundaries, leading to loss of euhedral shape, possible reduction to relics within the new overgrowing phases, and ultimately to complete absorption (Mezger et al., 2001; Mezger and Passchier, 2003). Interpretation of flow behaviour and deduction of sense of shear from partially consumed phases, having lost their original shape, become difficult (Mezger, 2010). Despite these limitations, idioblastic staurolite crystals in rocks having undergone ductile non-coaxial flow can be observed under specific circumstances. In a shear zone within the Bossét dome of the Central Pyrenees, Spain, favourable whole rock chemistry preserves staurolite in mica schist layers, while in adjacent rocks it is being replaced by andalusite and cordierite associated with younger contact metamorphism (Mezger and Passchier, 2003; Mezger et al., 2004; Mezger, 2010). In the direction of shear, staurolite porphyroblasts with high aspect ratios tend to lie with their long axis parallel to the schistosity plane. Straight inclusion trails oriented at various angles with the external foliation led Mezger and Passchier (2003) to conclude that staurolite crystals rotated, with respect to the main schistosity, in the same sense by different amounts until a stable position parallel to the shear plane was attained. These observations are in accordance with results from analogue and numerical models that show elongated rigid objects become stationary during general flow (Marques and Coelho, 2003; Passchier, 1987; Passchier and Trouw, 2005; ten Grotenhuis et al., 2002) or simple shear (Arbaret et al., 2001; Mancktelow et al., 2002; Marques and Coelho, 2001). Various angles of the internal foliation (Si), represented by straight quartz inclusion trails, with the external foliation (Se) indicate that rotation of staurolite porphyroblasts was not uniform and depended on their aspect ratios and their orientation in space with respect to the kinematic frame of the bulk flow prior to the onset of deformation (Holcombe and Little, 2001; Johnson, 2009). To test the conclusions of Mezger and Passchier (2003), which were based on a small number of staurolite porphyroblasts, statistical analyses of a larger data pool were carried out. In this study the aspect ratios and angular relations of crystal long axes and inclusion trails with external schistosity were determined on 628 staurolite porphyroblasts from the Bossét sample locality in sections parallel to mineral lineation and perpendicular to schistosity (xz), orthogonal to mineral lineation and schistosity (xy) and parallel to the schistosity plane (xy). In addition, the obtained aspect ratios and amount of rotation allow estimation of finite strain and vorticity. The results show that porphyroblasts in a simple shear dominated flow can rotate relative to one another. The amount of rotation is controlled largely by initial orientation of the crystal prior to onset of shearing and its shape, i.e. aspect ratio, which varies significantly among staurolite grains. This implies a strong coupling between blast and matrix, and weak strain partitioning around the staurolite porphyroblasts, which originates from the particular lithology of the schist unit. The rheological properties of this staurolite schist are different to those required for the porphyroblast non-rotation model of Bell (1985). 2. Geological Setting The examined sample is a staurolite-mica schist from the Bossét structuralemetamorphic dome of the Axial Zone in the Spanish Central Pyrenees (Fig. 1). The dome is an elongated, 30 km long, roughly E-W-trending structure, formed during the main Variscan deformation phase by the intrusion of Carboniferous leucogranites into Cambro-Ordovician metasedimentary rocks. Granite emplacement was facilitated by NW-ESE-directed extension (D2a) along a flat-lying shear zone which developed during local strain partitioning in an overall NE-S compressive setting (Mezger and Passchier, 2003). Continuous granite emplacement (D2a) and subsequent SS-W-NNE compression (D3) led to the formation of domal shaped doubly plunging antiforms. Southward thrusting of the northern block along the E-W-trending Bossét fault created the present structural constellation: a northern half-dome with an exposed granitic core, and a southern doubly plunging antiform. Two metamorphic phases can be distinguished: an early medium pressure-medium temperature regional metamorphism (5.5 kbar, 580 ± 14°C), recorded in garnet-staurolite schist, and a later high-temperature contact metamorphism, preserved in sillimanite-cordierite-biotite assemblages, locally migmatized, related to the intrusion of the Carboniferous Bossét granite (Mezger et al., 2004). Inclusion trails in staurolite porphyroblasts are straight. In some blasts Si can be traced into the external schistosity, implying static intertectonic growth postdating formation of the main S1 schistosity. Within an approximately 1.5 km thick shear zone in the eastern southern antiform, staurolite inclusion trails oblique to Se indicate uniform top-to-the-SE shearing during D2a. Andalusite and cordierite in the contact aureole grew syntectonically under coaxial conditions (D2b), while older staurolite grains are anhedral or occur as relics within cordierite. 1148 J.E. Mezger Journal of Structural Geology 32 (2010) 1147-1157 Ключевые слова: rigid, aspect ratio, xy, decoupling, yz-plane, simple shear, best-t ellipsis, external, ellipse, johnson, comment, rigid object, equation, regional metamorphism, cartesian coordinate, mezger passchier, metamorphic, mineral lineation, external foliation, schist, elongated, society, small ral, shear plane, ne-grained matrix, behaviour, oriented parallel, wa, main schistosity, layer, staurolite porphyroblasts, elongated shape, journal structural, porphyroblast, axis, foliation, staurolite blast, short, dynamic recrystallization, high, sample, elsevier, bell, porphyroblast non-rotation, parallel, long axis, angle, ?ow, se, ribbe, rotational behaviour, si, boss?st dome, garnet, straight, biotite porphyroblasts, journal metamorphic geology, position, deformation, rotation, plane, grain, tectonophysics, lie, jeffrey, maximum, internal foliation, small angle, larger, partitioning, shear direction, observed, journal structural geology, porphyroblast rotation, journal, stable position, shear zone, staurolite, ax, yz-planes, schistosity, small, long, ral, arbaret, blast, study, staurolites, short axis, inclusion trail, data, ratio, trail, dome, perpendicular, metamorphism, crenulation, initial orientation, biotite, sense, aspect, zone, structural geology, mezger journal, rarely exceed, small number, result, mezger, strain, cordierite, mineral, kinematic behaviour, euhedral shape, zwart, simple, foliation rotation, structural, correlation, geological, main, ra, length, discussion, rock, phase, metamorphic geology, initial, shape, direction, rigid particle, orthogonal, jiang, geology, obliquity, non-coaxial deformation, inclusion, existing schistosity, angular relationship, lineation, oriented, matrix, staurolite crystal, bossst dome, smaller, growth, high angle, xz-plane, staurolite schist, plane parallel, shear strain, boss?st, passchier, holcombe, external schistosity, xzand yz-planes, observation, elongated blast, axial zone, porphyroblasts, passing, crystal, staurolite grain, journal metamorphic, posse, xzand, stretching lineation, large ral, rotated, respect, object, large, orientation, shear