Magmatic fabrics in batholiths as markers of regional strains and plate kinematics: example of the Cretaceous Mt. Stuart batholith

The Cretaceous Mt. Stuart batholith was syntectonically emplaced within amphibolite grade metasedimentary rocks of the Cascades Crystalline Core, Washington State. The northern part of the batholith defines a NW—SE trending antiformal fold. We present fabric data from that part of the batholith, collected during field mapping and anisotropy of magnetic susceptibility (AMS) measurements. The significance of the data is discussed in terms of regional tectonic deformation and plate kinematics. The data were collected from rocks with well preserved igneous textures and the fabrics therefore formed during magmatic deformation. The AMS provides measurements of the preferred orientations of Fe-rich minerals (biotite ± hornblende ± traces of pyrrhotite and magnetite) which are consistent with field measurements of the mesoscopic fabrics defined by plagioclase, biotite and hornblende crystals. The magnetic fabrics are also consistent with the orientations of folds, mineral fabrics and boudinage structures that record high-temperature subsolidus deformation in the margin of the pluton and in its host rocks. The lineations are parallel to the stretching direction associated with small increments of strain that occurred during deformation of the magmatic arc, as the batholith was crystallizing and deforming in the tectonic stress field, ca. 93 Ma. The fabrics in the Mt. Stuart batholith are used to infer emplacement in a magmatic arc during either 1) plate displacement perpendicular to a NW-SE trending plate margin, or 2) wrench dominated transpression. In the second case the analysis suggests a nearly N-S plate vector along the western North American margin during plutonism. The results demonstrate the potential usefulness of magmatic fabrics in syntectonic plutons for plate tectonic analyses of orogenic belts.