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 amp hibolite grade metasedimentary rocks of the Cascades Crystalline Core, Wash ington State. The northern part of the batholith defines a NW-SE trending a ntiformal fold. We present fabric data from that part of the batholith, col lected during field mapping and anisotropy of magnetic susceptibility (AMS) measurements. The significance of the data is discussed in terms of region al tectonic deformation and plate kinematics. The data were collected from rocks with well prssen ed igneous textures and the fabrics therefore formed during magmatic deformation. The AMS provides measurements of the preferre d orientations of Fe-rich minerals (boiotite +/- hornblende +/- traces of p yrrhotite and magnetite) which are consistent with field measurements of th e mesoscopic fabrics defined by plagioclase, biotite and hornblende crystal s. The magnetic fabrics are also consistent with the orientations of folds, mineral fabrics and boudinage structures that record high-temperature subs olidus deformation in the margin of the pluton and in its host rocks. The l ineations are parallel to the stretching direction associated with small in crements of strain that occurred during deformation of the magmatic are, as the batholith was crystallizing and deforming in the tectonic stress field , ca. 93 hla. The fabrics in the Mt. Stuart batholith are used to infer emp lacement in a magmatic are during either 1) plate displacement perpendicula r to a NW-SE trending plate margin, or 2) wrench dominated transpression. I n the second case the analysis suggests a nearly N-S plate vector along the western North American margin during plutonism. The results demonstrate th e potential usefulness of magmatic fabrics in syntectonic plutons for plate tectonic analyses of orogenic belts. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.