USING GEOLOGIC MAPPING AND OTHER TOOLS TO INVESTIGATE EMPLACEMENT MECHANISMS IN THE JACKASS LAKES PLUTON OF THE SIERRA NEVADA, CA

The Jackass Lakes pluton (JLP) is a 98-97 Ma, 175 km² pluton in the central Sierra Nevada batholith that intruded coeval volcanic and leucogranite units. Two ~40 km² areas in the western JLP were mapped at 1:10,000 scale as part of two USGS EDMAP projects in 2023 and 2024. Mapping and outcrop analysis, including 3D cross section analysis, fabric and strain measurements, microscopy, U-Pb zircon dating, and XRF whole rock geochemistry were performed to unravel the emplacement and evolution of the JLP from microstructure to map scale to test following hypothesis:

Does the JLP represent a sheeted intrusion (McNulty et al., 1996), or is the emplacement history more nuanced (Pignotta et al., 2010)?

Our observations include: 1) Pluton wide NW-NNW striking fabrics cut across all contacts indicating regional strain related fabrics; 2) 3D cross sections of the JLP show pendants of leucogranite and volcanics overlying JLP granodiorites and quartz diorite suggest stoping along the pluton roof. The leucogranite at higher elevations might represent melt caps collected from interstitial granodioritic mush; 3) Metavolcanic pendants have shallow to steep JLP contacts constricted to higher elevations with extensive xenolith fields and dike swarms, indicating stoping along contacts; 4) Mapped JLP plutonic phases are not sheeted but irregular and often separated by gradational contacts indicating mixing and mingling between magmas; 5) No microscale ductile strain is preserved in the host Illilouette pluton, showing no record of JLP induced downward return flow; 6) NE striking JLP dikes intruded perpendicular to pluton-wide fabrics and are folded, with enclaves and volcanic clasts elongated parallel to fabrics; Rf phi strain analyses show 20-60% of E-W directed shortening; 7) Microstructures in all units display largely magmatic textures and thermally annealed quartz with evidence of minor dynamic recrystallization; 8) local meso- and microscale solid state deformation with shear-sense indicators show magmatic fabric parallel dextral shearing post-JLP crystallization.

We conclude that the JLP followed a complex incremental growth history via outcrop- to map scale sheet-like to irregular intrusions undergoing local magma mixing facilitated by diapiric ascent and large-scale stoping of overlying volcanics and leucogranites.