STRUCTURAL EVOLUTION OF THE SING PEAK PENDANT: EVIDENCE FOR 98 MA DEFORMATION DURING MAGMA EMPLACEMENT, JACKASS LAKES PLUTON, CENTRAL SIERRA NEVADA, CALIFORNIA

The 98 Ma Jackass Lakes pluton (JLP), central Sierra Nevada, is composed of primarily granodiorite with less abundant leucocratic and dioritic intrusions. Host rocks in the Sing Peak pendant consist of deformed metavolcanic rocks of the mid-Cretaceous Minarets Caldera sequence and less common Jurassic metasedimentary rocks. Structures in the Sing Peak pendant include: a) a penetrative, subvertical, NNW-trending foliation defined by stretched phenocrysts; and b) a ductile shear zone with a shallow plunging (<15 degrees), NNW-trending lineation defined by elongate lithic clasts in a fine-crystalline matrix. This shear zone can be traced into the JLP along the northwestern margin of the pendant. Within the JLP, structures include a well-developed magmatic foliation, defined by elongated crystals of biotite, hornblende and plagioclase phenocrysts that consistently strikes NNW and dips steeply to the west. The magmatic foliation is continuous across compositional zones within the JLP and parallel to the elongated axis of mafic enclaves observed throughout the pluton. Abundant xenoliths (up to 100 m long) of metavolcanic rocks appear to be concentrated near the roof pendant but are also found throughout the pluton. Metamorphic foliations observed within the xenoliths are sub-parallel with those in the host rocks and the magmatic foliation of the pluton. Some xenoliths contain JLP dikes, up to 1 m in width, that have been folded with axial planes striking sub-parallel to the metamorphic foliation in the xenoliths, host rocks and magmatic foliation in the pluton. These observations suggest that the magmatic and metamorphic foliation as well as the folded dikes were formed during chamber construction and probably due to regional deformation. Based on the preliminary lineation data, the ductile shear zone may be the result regional transpression. However, the lateral extent of the shear zone and its relationship to other mid-Cretaceous shear zones in the central Sierra Nevada batholith is presently unknown.