THE PORPHYROBLASTS OF THE BOSSòST DOME, CENTRAL PYRENEES: DISCOVERING THE RELATIONSHIP BETWEEN METAMORPHISM AND DEFORMATION

Microtectonics and the concept of syn-tectonic porphyroblast growth have their roots in the meticulous mapping projects that the University of Leiden in the Netherlands undertook in the Central Pyrenees from the late 1950's on. Studying metamorphic rocks of the Bossòst (Garonne) dome, and erroneously cutting thin sections parallel to mineral lineation rather than perpendicular, as it was the custom in those days, Henk Zwart (1924-2012) noticed the interference patterns of foliation and porphyroblasts. The results were published in 1960 and 1962 in the Geologische Rundschau, thus changing the way crystalline geologists looked at metamorphic rocks, and mica schists in particular, ever since. Unlike folds, faults or shear zones, outcrops of regional metamorphic rocks are rarely spectacular. This also holds true to the key outcrop of Zwart's study which is located on a small forestry road along the northern slope of the Aran valley in the Spanish Pyrenees, 800 m northwest of the village of Arres. The rock exposed here is a mica schist with prominent reddish brown idioblastic staurolite crystals up to 6 mm in diameter. Rotation of the blasts can be inferred from deflection and drag folding of schistosity. The plane of view is oriented favourably, parallel to a distinctly recognizable mineral lineation. One can also observe ubiquitous dark centimetre-sized cordierite crystals with rounded or oblate shapes and curved tails that give them a sigmoidal appearance. A casual field geologist would identify them as syn-tectonic porphyroblasts, as markers on the outcrop face attest. Thin sections, however, reveal a different, more complex story. Cordierites overgrew smaller rotated staurolite blasts and the foliation they deflect, defining cordierite as a post-tectonic porphyroblasts overgrowing inter-tectonic staurolites. The cordierite-staurolite schist of the Bossòst dome teaches the important lesson that the obvious is not always correct, and attest to the power of microstructural studies.