INVESTIGATING EMPLACEMENT RELATED PLUTON STRUCTURES IN THE JACKASS LAKES PLUTON, SIERRA NEVADA, CALIFORNIA

The Jackass Lakes pluton (JLP) is a 98-97 Ma, 175 km2 pluton in the central Sierra Nevada batholith. It is ideal for evaluating pluton emplacement histories as it contains coeval volcanic and porphyry units as roof host rocks, older (Illilouette) pluton wall rocks, and internal “pulsing” usually not found together in other plutons. McNulty et al. (1996) interpreted the JLP to have grown largely via sheeted dike intrusions, while Pignotta et al. (2010) argued for complex irregular intrusions with mixing and mingling of internal pulses, stoping, and host rock downward return flow. A combination of map and outcrop level observations looks at contact relations between metavolcanic, porphyry, and plutonic units, including internal contacts, mineral fabrics, and presence and distribution of host rock xenoliths. This is coupled with microstructure and Electron Backscatter Diffraction (EBSD) analysis to quantify differences in preferred orientations of different minerals to test if they record emplacement related flow. Preliminary observations include: 1) microscale ductile strain in the Illilouette pluton only right at the straight western contact due to minor? JLP induced downward return flow, 2) metavolcanic pendants with shallow JLP contacts constricted to higher elevations with extensive stoped block fields indicating stoping along roof contacts, 3) mapped JLP phases are not sheet-like but separated by wide gradational contacts indicating large-scale mixing and mingling, 4) JLP-scale NNW striking mineral fabrics overprinting internal contacts indicating regional strain, and 5) local meso- and microscale solid state deformation with kinematic indicators showing magmatic fabric parallel dextral shearing. It is not clear if the shear indicating solid state deformation is related to a local regional shear zone (Krueger and Yoshinobu, 2018), or emplacement; or if shearing started operating during syn-emplacement of the JLP. EBSD analysis in progress will determine strain intensity and orientations of especially higher temperature magmatic minerals (e.g. plagioclase) to test for contact-parallel, emplacement-related internal magma flow. Our current conclusion is that the JLP followed a rather complicated growth history by a variety of emplacement processes as proposed by Pignotta et al. (2010).