FIELD CONSTRAINTS ON MAGMA, HOST ROCK, AND XENOLITH RHEOLOGY DURING PLUTON EMPLACEMENT

Detailed mapping of magmatic fabrics (e.g. foliations and lineations) surrounding xenoliths provides a tool to evaluate the timing and kinematics of magmatic fabric formation and the relative rheologies of pluton-host rock systems. We have mapped the southwestern quarter of the 98 Ma Jackass Lakes Pluton where the intrusion is characterized by a medium to coarse-grained biotite granodiorite with accessory hornblende. Locally, the JLP intrudes 98-101 Ma volcanic rocks of the Minarets Caldera sequence. The JLP includes NW-trending mafic enclave swarms, felsic dikes, schlieren bands, and metavolcanic xenoliths. Magmatic foliations generally display weak to moderate mineral alignment with a mean orientation of N14W/83E.

Volumetrically significant xenolith ‘graveyards’ are observed in the Lady Lake and Stanford Lake basins; locally, these blocks account for greater than 60% of the exposure. Xenolith sizes range from fragments less than 1 m wide to large blocks measuring 75 m x 30 m. Metamorphic foliations within the xenoliths are subparallel to magmatic foliations in the granodiorite. Generally, the xenoliths are elongate parallel to the magmatic foliations, however some appear to have rotated in the pluton. Dikes of JLP (up to 50 cm thick) intrude the xenoliths and are folded about axial planes subparallel to the magmatic foliations; some of these dikes contain axial planar magmatic foliations parallel to those found within the main body of the JLP. These axial planar foliations are continuous with plastic foliations in the enclosing xenoliths. Little to no deflection of magmatic fabrics around the blocks is observed. Even where blocks can be shown to have rotated, there appears to be no change in the magmatic foliations around the xenoliths.

These observations are consistent with 1.) regional coaxial shortening during pluton emplacement, and 2.) a low viscosity contrast between the xenoliths and magma during fabric formation and folding. If these conclusions are correct, they imply that the crystallizing magma had a yield strength high enough to support xenolith capture, yet remained fluid enough to form the observed magmatic fabrics.