Field constraints on the removal and incorporation of host rock xenoliths in the Jackass Lakes pluton, central Sierra Nevada, California



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Texas Tech University


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. Mapping has been done of the southwestern quarter of the 98 Ma Jackass Lakes pluton (JLp) where the intrusion is characterized by a medium- to coarse-grained biotite granodiorite with accessory homblende. Locally, the JLp intrudes 98-101 Ma volcanic rocks of the Minarets Caldera sequence. The JLp includes northwest-trending mafîc enclave swarms, felsic dikes, schlieren bands, and metavolcanic xenoliths. Magmatic foliations generally display moderate to strong mineral alignments with a mean orientation of N14W/83E.

Volumetrically significant xenoliths fields 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 several tens of meters wide. Metamorphic foliations within the xenoliths are subparallel to magmatic foliations in the granodiorite. Generally, 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 xenoliths.

These observations are consistent with: (1) magmatic fabric formation that postdates the incorporation of host rock xenoliths; (2) removal of roof and wall rocks of the JLp via a combination of both stoping and in situ diking; (3) regional east-west coaxial shortening during pluton emplacement. If these conclusions are correct, they imply that viscosity of the crystallizing magma was great enough to 'capture' xenoliths, yet still retained enough melt to form observed fabrics.