DETAILED GEOLOGIC MAPPING OF THE JACKASS LAKES PLUTON AND HOST ROCK SYSTEM, SIERRA NEVADA, CALIFORNIA, REVEAL NEW INSIGHTS INTO ITS MAGMATIC EVOLUTION

The Sierra Nevada batholith, California, is one of the best exposed continental magmatic arcs on Earth. It presents geologists with unique opportunities to better understand pluton emplacement and evolution through time. Geologic maps in the Sierra Nevada are often not available beyond the 1:62,000 USGS (15 minute) quadrangle maps, therefore remapping the rock units in greater detail adds important information about crustal processes involved in pluton emplacement and evolution.

The Jackass Lakes pluton (JLP) is a 98-97 Ma, 175 km2 pluton located in the Ansel Adams wilderness of the northern Sierra National Forest in the east-central part of the batholith. It is predominantly granodiorite to diorite in composition. Peck (1980) mapped the JLP as a broad package with an assemblage of metavolcanic and porphyry leucogranite units as host rocks, which he interpreted to be volcanic ejecta and subvolcanic intrusives sourced from the JLP itself. To study this complex further, two major areas of over 40 km2 in the central western parts of a remote area of the JLP involving graduate and undergraduate students were mapped during the summers of 2023 and 2024 at 1:10,000 scale as part of two USGS EDMAP funded projects. These mapping efforts were synthesized with previous mapping done by Pignotta et al. (2010) and Krueger and Yoshinobu (2018) to create a compilation map of the pluton.

Our new mapping, with support from petrography, U-Pb zircon geochrononology and element and isotope whole rock geochemistry reveals that the JLP consists of several more plutonic phases than previously mapped that range in petrologic composition from diorite, granodiorite, tonalite, and leucogranite and are highly irregular in shape. The cross cutting relations show the older plutonic phases in particular have many xenolith fields of stoped metavolcanic host rocks and inclusions of mafic enclaves that are strained with the dominant NNW striking mineral fabric. Plutonic phases transition into one another and/or mingled along largely steeply dipping contacts and internally with quartz-diorite magmas to form enclave swarms. These contacts are overprinted by the mineral fabric. These observations show both magma fractionation and co-mingling behavior during high strain at the time of crystallization.