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_Journal of Structural Geology 32 (2010) 703-708_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Mimicking syntectonic growth: cordierite overgrowth of earlier rotated staurolite porphyroblasts, strain caps and deformed foliation Jochen E. Mezger* Institut für Geowissenschaften, Martin-Luther-Universität Halle-Wittenberg, von-Seckendorff-Platz 3, 06120 Halle, Germany Article info Article history: Received 15 September 2009 Received in revised form 16 April 2010 Accepted 28 April 2010 Available online 6 May 2010 Keywords: Staurolite Cordierite Syntectonic porphyroblasts Post-tectonic porphyroblasts Intertectonic porphyroblasts Pseudosyntectonic porphyroblasts Abstract Monoclinic shape fabrics and inclusion trail geometries of porphyroblasts are regarded as reliable shear-sense indicators, provided that timing and sequence of growth can be established. In the southern Bossis dome of the Central Pyrenees, staurolite and cordierite porphyroblasts in mica schists contain inclusion trails oblique to, but continuous with the external foliation, indicating porphyroblast rotation. Straight inclusion trails in staurolites record growth between formation of the main schistosity and subsequent shear, i.e., an intertectonic origin. Strain caps and deformation of foliation around porphyroblasts show a distinct asymmetry with uniform sense of shear. Cordierite porphyroblasts are significantly larger than staurolites and contain curved inclusion trails which would suggest syntectonic growth and a similar shear of sense. However, staurolite and cordierite do not belong to the same paragenesis and textural evidence, corrosion of staurolite rims and relict inclusions in cordierites, suggest partial consumption by cordierite. Complete overgrowth of staurolite with preservation of its inclusion trails and adjacent foliation deformation results in curved inclusion trails that mimic syntectonic growth of cordierite. Actually, the larger cordierites statically overgrew rotated staurolites during post-tectonic contact metamorphism. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Recognition of shear-sense is crucial for the reconstruction of the deformational evolution of orogenic core zones. In discrete zones of strong deformation, shear zones s.l., displaced or deformed markers or foliation curvature provide in most cases reasonable information on the direction of shear (Carreras, 2001). Deformation at deeper crustal levels is commonly distributed across broader zones lacking undeformed wall rocks that limit shear zones. There, the most common macroscopic shear-sense indicators are shear band cleavages, shape-preferred orientation of quartz or calcite, and rotated porphyroclasts and porphyroblasts. Porphyroclasts of magmatic origin, e.g., feldspar augen in metagranites, or sedimentary clasts, such as pebbles in metaconglomerates, are recognizable and clearly predate deformation. Metamorphic porphyroblasts are more difficult to interpret, but they have the potential to reveal more comprehensive information, due to sequential growth of metamorphic phases. Timing of their nucleation with respect to observed deformation events distinguishes * Tel.: +49 (0)345 5526120; fax: +49 (0)345 5527220. E-mail address: jochen.mezger@geo.uni-halle.de 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.04.012 pre-, syn-, inter- and post-tectonic porphyroblasts, a concept developed nearly half a century ago by Zwart (1960, 1962), and which until today remains a very powerful, low-cost tool to recognize orogenic events, both in time and space (Hanmer and Passchier, 1990; Passchier and Trouw, 2005). Correct interpretation of sense of shear from porphyroblasts in the field requires a favourable outcrop plane, oriented orthogonal to the schistosity, the inferred flow plane, and parallel to the mineral or stretching lineation, which represents the shear direction (Passchier and Trouw, 2005). The deformation of external foliation around porphyroblasts, inclusions within porphyroblasts preserving older fabrics, and their relation with the external foliation have to be recognized. It has to be taken into account that reactivation of schistosity during subsequent deformation phases may obscure or rotate stretching lineations, so that rotation axes of porphyroblasts are not necessarily oriented perpendicular to lineations anymore (Bell et al., 1995). A further very important aspect, the sequence of growth of the metamorphic minerals involved, is the most difficult to establish in the field. This paper reports a case where interpretation of monoclinic shapes and inclusion trails from field observations yields apparent unequivocal shear-sense and timing of cordierite porphyroblast growth which were refuted later by close inspection of thin sections and thermobarometric calculations. The results emphasize the necessity to examine microstructural and petrological evidence of fabrics involving deformation and metamorphism. Field observations alone, even if they seem to be unequivocal, do not yield sufficient information, and can lead to wrong conclusions. 2. Initial field observations from the southern Bossis dome Mica schists with large, centimetre-sized porphyroblasts of andalusite, staurolite and cordierite, and smaller garnets, crop out in the eastern part of a structural and metamorphic dome in the central Pyrenees. The Bossis dome, also known as the Garonne dome, is one of several structural and metamorphic domes which evolved during the main phase of the Variscan orogeny in the Axial Zone of the Pyrenees. These structures possess a core of pre-Variscan orthogneisses or, as in the case of the Bossis dome, Variscan granite plutons, mantled by Cambrian to Devonian metasedimentary rocks. The development of these Pyrenean dome structures is still a matter of debate, as summarized in detail by Mezger and Passchier (2003) and Mezger (2009). Relevant for the understanding of the problem discussed here, it is sufficient to know that the rocks of the metasedimentary mantle have experienced polyphase metamorphism and deformation, resulting from pre-Variscan regional metamorphism and intrusion of orthogneiss protoliths (Castiñeiras et al., 2008; Denèle et al., 2008), main phase Variscan ductile deformation (Carreras and Capella, 1994), late main phase Variscan plutonism (Gleizes et al., 1997, 1998), late Variscan shear zone development and Alpine deformation and faulting (Lamouroux et al., 1980; McCaig and Miller, 1986; Soula et al., 1986). The mica schist outcrop of particular interest for this study is located along a forestry road on the northeastern side of the Spanish Aran valley, 800 m northwest of the village of Arres and approximately 2.5 km south of Bossis (0 x42°02'6" E, 42°45'40" N; Mezger et al., 2004). A well-developed continuous schistosity is formed by a fine-grained matrix (ca. 200 mm) of aligned muscovite, biotite and elongated quartz. Distributed equally throughout are numerous reddish brown idioblastic staurolite crystals up to 6 mm in diameter. Many of the staurolite porphyroblasts contain interpenetrating twins, whose variable orientations produce a variety of shapes. Rotation of the blasts can be inferred from deformation and drag folding of schistosity. The plane of view is oriented favourably, parallel to a distinctly recognizable mineral lineation. A smaller number of ubiquitous centimetre-sized black minerals, significantly larger than the staurolites – an important fact to keep in mind – with rounded or oblate shapes are identified as cordierite (Fig. 1). Some cordierites possess monoclinic sigmoidal shapes, which were interpreted by previously visiting geologists as d-clasts, evident from labels on the rock face beside marked specimen. A polished rock sample from that outcrop reveals straight inclusion trails in staurolites, variably oriented with respect to the main schistosity, and larger cordierites with sigmoidal inclusion pattern (Fig. 2). The interpretation as outlined in Fig. 3 seems straightforward: staurolite grew intertectonically, overgrowing an existing schistosity, and then rotated clockwise with respect to the main schistosity, as indicated by continuation of the internal foliation Si, preserved as quartz inclusion trails, into the external foliation Se, which is deformed near the staurolite margin. Cordierite, on the other hand, appears to have grown syntectonically, in the same clockwise sense relative to the schistosity, evident from S-shaped inclusion trails, which likewise continue into Se. One can conclude from the fact that in both phases Si is continuous with Se, i.e., inclusion trails in both phases represent the same foliation, staurolite nucleated earlier than Fig. 1. Closeup photographs of the staurolite-cordierite schist along the road outcrop northwest of the village of Arres, Spain. The foliation and lineation of the schist dip moderately (20-30° ESE-SE). The plane of view is perpendicular to schistosity and approximately parallel to the mineral lineation. Cordierite porphyroblasts form dark grey centimetre-sized ellipses, although unequivocal distinction is not easy. (a) Weathering has outlined the deformed foliation around the central crystal which displays a non-stairstepping geometry characteristic of d-type clasts. The grain in the upper right corner (white arrow) has a similar symmetry. The sense of shear derived from such geometry would be dextral. Thin sections reveal that cordierites are porphyroblasts with sigmoidal inclusion patterns, and not d-clasts in the true sense of Passchier and Trouw (2005). However, this does not become obvious when looking at this rock face. (b) Numerous cordierite blasts, some of them displaying similar d-type as in (a), are circled with blue marker, w' Ключевые слова: e, r, o