EMPLACEMENT OF THE JACKASS LAKES PLUTON, SIERRA NEVADA BATHOLITH, CALIFORNIA

This study investigates emplacement mechanisms in the Jackass Lakes pluton (JLP), a 98-97 Ma, 175 km2, 10-13 km deep pluton in the central Sierra Nevada with associated volcanic and porphyry units. Two areas of >40 km2 were mapped at 1:10,000 scale as part of two USGS EDMAP projects in 2023 and 2024 and synthesized with previous geologic maps. Mapping, 3D cross section analysis, fabric and strain measurements, petrography, U-Pb zircon dating, and XRF whole rock geochemistry were performed to unravel the lithology and emplacement of the JLP to determine if the JLP represents a sheeted intrusion (McNulty et al., 1996), if faulting/shear zones impacted emplacement, or if the emplacement history was more nuanced (Pignotta et al., 2010). Results show that: (1) most intrusive units have irregular geometries that are not sheet or dike shaped; (2) JLP’s overall mapped shape is roughly rectangular although it is truncated at the N and S ends; (3) the W contact with the c. 100 m.y. Illilouette Creek (IC) pluton is sharp and jagged – IC fabrics are decoupled from the JLP, indicating a brittle, discordant relationship with the JLP and a different strain field prior to JLP emplacement; (4) a large quantity of xenoliths of host and cognate inclusions of JLP record stoping; (5) a single magmatic to mildly solid-state fabric overprints all magmatic and some host units reflecting tectonic strain; (6) gradational and sharp internal magmatic contacts with mullion and lobate structures and widespread enclave swarms indicate significant amounts of mixing/mingling of coeval magmas; (7) large E-W directed strain (50-70% shortening) perpendicular to fabric strike is recorded by several internal markers; 8) metavolcanic pendants and underlying leucocratic porphyries found at high elevations are indicative of melt caps below the pluton roof resulting from filter pressing; (9) lack of synmagmatic faults and pluton wide pervasive shear zones indicate faulting/shearing played little to no role in the JLP’s emplacement. The above suggests that the incrementally grown JLP experienced a complex emplacement history associated with collapse and burial of volcanic units to 10-13 km depths, stoping, internal recycling, regional shortening and vertical thickening, melt loss and sheet wedging to diapiric/mushy interactions between the plutonic phases.