Mimicking syntectonic growth: cordierite overgrowth of earlier rotated staurolite (cas 12182-56-8) porphyroblasts, strain caps and deflected foliation

Add time:08/01/2019    Source:sciencedirect.com

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 Bossòst dome of the Central Pyrenees, staurolite (cas 12182-56-8) 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 deflection 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 deflection results in curved inclusion trails that mimic syntectonic growth of cordierite. Actually, the larger cordierites statically overgrew rotated staurolites during post-tectonic contact metamorphism.

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