OCCURRENCE OF MINERAL-FILLED MICROFRACTURES IN IGNEOUS AND METAMORPHIC ROCKS AS DETERMINED BY SEM-BASED CATHODOLUMINESCENCE IMAGING

Microfractures lined with substrate-controlled mineral fill (primarily quartz but also feldspar and calcite) are common, if not ubiquitous, features in igneous and metamorphic rocks. SEM-based cathodoluminescence systems (scanned CL) allow improved visualization of these microfractures, which are typically filled with minerals similar to their host grains and are therefore obscure to invisible using light microscopy. Microfractures in these rocks tend to cut across multiple grains and form in sets of similar orientation. Multiple generations of microfractures are not uncommon. Temperatures of formation for various microfracture generations vary from metamorphic to diagenetic conditions. Previous studies had recognized some high-temperature microfractures in igneous and metamorphic rocks, but lower temperature microfractures are present as well. Microfractures may provide important fluid pathways during metamorphism or diagenetic zone alteration.

Microfracture-filling minerals observed in this study appear to form by sealing (crystallization of minerals from a fluid), rather than healing (fill by intracrystalline diffusion). Fluid inclusion planes, which are commonly associated with these fractures, are narrower than the fracture aperture visible in CL images. In some larger fractures, multiple fluid inclusion planes in the fill result from progressive opening (crack-seal texture).

Microfracture densities are highly variable. Samples from structurally complex areas tend to have higher densities. In some highly deformed samples, microfractures define a penetrative fabric. In many cases, microfractures are related to macrofractures having similar orientation and appearance.

Determination of the temperature at which the microfractures formed is nontrivial. Clues include fracture morphology, minerals cut by the microfractures, cross-cutting relationships with structural elements in the rocks, the type of mineral fill, and the CL characteristics of the fill. In metamorphic rocks, microfractures run the gamut from those relict from the protolith (premetamorphic) to those formed under submetamorphic conditions during uplift. Fluid inclusions in the microfracture fill provide an opportunity for qualitative determination of conditions during deformation.