Abstract

An intrinsic limitation of studying microstructures in thin section is that their spatial (three-dimensional, 3-D) distribution, shape, and orientation have to be inferred by combining 2-D data from different sections. This procedure always involves some degree of interpretation that in some cases can be ambiguous. Recent advances in high-resolution X-ray computed microtomography have made possible the direct imaging in 3-D of volumes of rock to centimeter scale. This rapidly evolving technology is nondestructive and provides a holistic approach of microstructural analysis that eliminates interpretative procedures associated with 2-D methods. Spatial images can be generated through any part of the rock sample and used as virtual petrographic sections. Our application of this technique to an oriented drill core sample from the classic Orijärvi metamorphic region of southern Finland reveals a number of in situ 3-D aspects, including: (1) the spatial distribution and shape of andalusite porphyroblasts, (2) the geometry of a matrix foliation anastomosing around the porphyroblasts, (3) a millimeter-scale compositional layering that controlled the oscillation of porphyroblasts and sulfide mineralization, and (4) distinct inclusion trail patterns characterizing porphyroblast core versus rim zones. The combined data indicate that the steeply dipping bedding-subparallel foliation that characterizes the Orijärvi area formed by bulk north-south crustal shortening and associated vertical stretching.

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