Staurolite is a characteristic mineral phase of amphibolite facies, medium pressure, regional metamorphism in pelitic rocks. Its common occurrence and phenoblastic appearance make it a potentially useful kinematic indicator. Inclusion trails are not uncommon in staurolite porphyroblasts, consisting of quartz or graphite, preserving an older foliation. Most inclusion trails observed in staurolite are straight or folded. Curved, sigmoidal or spiral-shaped inclusions, prominently found in garnets are very rarely observed. This suggests that staurolites are generally not formed synkinematically, but nucleate between deformation phases, i.e. are formed interkinematically. While garnets are generally spherical and grow concentrically, staurolites form elongated prisms and commonly show growth twinning of various shapes.
In the Axial Zone of the Variscan Pyrenees Cambro-Ordovician metasediments often occur as staurolite-garnet-andalusite-cordierite schist, with staurolite being metastable, occurring as relict anhedral inclusions in cordierite. At the top of the Bossòst Dome in the central Pyrenees staurolite-bearing schists are strongly deformed in a flat-lying extensional shear zone (Mezger & Passchier 2003). Simple shear fabrics associated with the shear zone are overprinted, just as staurolite is being consumed, by younger contact metamorphism (growth of cordierite) and associated pure shear fabrics related to the intrusion of late-Variscan plutons. A fate which is common for earlier regional metamorphic phases during prograde metamorphism with a clockwise PTt-path.Euhedral staurolite porphyroblasts are preserved in rocks with insufficient Mg to form cordierite (Mezger et al. 2004). There staurolite porphyroblasts display asymmetric fabrics, e.g. deflection of foliation, mica pressure caps. Inclusion trails are tilted at various angles with respect to the external foliation (Se). This study focuses on the correlation between the aspect ratio of staurolite porphyroblasts, the orientation of the inclusion trails (internal foliation Si and the orientation of the length axes with respect to Se.
From north of the village of Arres, in the Val d'Aran, Spain (a popular Tour de France traverse), a total of 274 staurolite porphyroblasts from 14 thin section parallel to mineral lineation and 185 from 11 thin sections perpendicular to mineral lineation were analysed. For each individual porphyroblast the orientation of Si with respect to Se was measured, as well as the average length of the Si. Since growth twins can display complex shapes, determination of the length axis (LA) and the aspect ratio were done with the help of the software Scion Image 4.0.3.2 (PC version of NIH Image, http://rsb.info.nih.gov/nih image/). The program constructs an ellipse from the porphyroblast shape and calculates the length and orientation of the ellipse length axis (used as the porphyroblast LA) and the aspect ratio of length and short axis, which is used as the kinematic aspect ratio of the staurolite. An important parameter is the ratio between length of LA and the length of Si (LA/LSi). In case of parallelism between grain LA and Si it equals 1. If Si is perpendicular to the LA of the grain, this ratio is at its maximum and equals the aspect ratio of the porphyroblast. In sections parallel to lineation almost all show discernable Si, always straight. In contrast in only 110 of 185 staurolites perpendicular to lineation Si could be observed. In most aspects the fabric symmetry in sections parallel and perpendicular to lineation differ. Aspect ratios of porphyroblasts parallel to lineation are slightly smaller (1,68) than those from staurolites perpendicular to lineation. Length axes orientation of staurolites parallel to lineation are relatively evenly distributed, with slight predominance of those at angles less than 30° to Se. Perpendicular to lineation maxima exist at low (less than 20°) and high (more than 70°) angles. Orientation of Si has distinct maximum for each section orientation and averages 49° parallel and 21° perpendicular to lineation.
Important for the interpretation of the porphyroblast symmetry is the assumption that staurolite porphyroblasts grew at random orientation during a period of kinematic quiescence prior to deformation. This is supported by the lack of correlation between the angle LA-Si with the aspect ratio and with the angle of Si-Se, as well as the non-correlation between the angles of length axis-Se and Si-Se. The ratio of LA length/Si length increases with increasing Si-Se and La-Se angles in staurolites parallel lineation, especially for those with high aspect ratios. This suggests that elongated staurolites have rotated up to 90° (highest Si-Se angle and high LA/LSi) to attain stable positions. In contrast, staurolites from sections perpendicular to lineation do not show this correlation. Especially useful is correlation between LA-Si and LA-Se angles, which show a distinct asymmetry for staurolites parallel lineation (see Figure). Staurolites with low aspect ratios (less than 1.5) display continuous positive correlation, implying continuous rotation of up to 180° (or more). LA of elongated porphyroblasts cluster at low angles with the external foliation. Those with low LA-Si angles probably did not rotate at all, or very little, while those with high LA-Si angles rotated up to 90° to obtain a stable position.
Staurolite porphyroblasts can rotate, as evident from different angles of inclusion trails to external foliation. The amount of rotation depends on the shape of the blast. If it is rounded -in the plane of the thin section; it could be elongated perpendicular to the shear direction- it can rotate more than 180°, though not possible to determine from straight inclusion trails. Porphyroblasts elongated in the direction of shearing either rotate little or 90° to obtain stable positions, similar to the orientation of mica fish (ten Grotenhuis 2000). If staurolite porphyroblasts are preserved and not overprinted by later metamorphic events, they can serve as important shear sense indicators for middle to lower crustal shear zones.
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