STRUCTURAL EVOLUTION OF THE SING PEAK PENDANT: IMPLICATIONS FOR MAGMA CHAMBER CONSTRUCTION AND MID-CRETACEOUS DEFORMATION IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

Metamorphosed strata of the Sing Peak pendant, central Sierra Nevada, CA, record structures formed prior to and during the emplacement of the ~98 Ma Jackass Lakes pluton (JLP). The Sing Peak pendant consists of deformed metavolcanic rocks of the 101-98 Ma 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 recrystallized biotite, epidote and amphibole; and b) a dextral sense ductile shear zone with a ~40º-plunging, NNW-trending lineation. The shear zone can be traced into the ~98 Ma JLP along the northeastern and northern margins of the pendant. The Jurassic metasedimentary rocks are locally folded with NNW trending axial planes and the folds are commonly truncated by the Jackass Lakes pluton. Structures within the JLP include a well-developed NNW trending, steeply west dipping magmatic foliation and a moderately north plunging (~40º) lineation defined by elongated biotite and hornblende phenocrysts. The magmatic structures are continuous across compositional zones within the JLP and parallel to flattened mafic enclaves observed within the pluton. Abundant xenoliths (up to 300 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 meter-wide granodioritic/aplitic dikes emanating from the JLP 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 most likely due to regional deformation. Based on the lineation data, the deformation in the ductile shear zone and the Jackass Lakes pluton is the result of regional transpression. Thus, the Jackass Lakes pluton – host rock system is the earliest documented regional dextral transpression in the central Sierra Nevada.