MAGMA TO MYLONITE: COEVAL VOLCANISM, PLUTONISM, VERTICAL DISPLACEMENT AND REGIONAL DEFORMATION IN THE CENTRAL SIERRA NEVADA BATHOLITH

The construction and deformation of batholiths has long been known to be episodic, characterized by rapid and voluminous emplacement of lavas/magmas coupled with local and regional deformation. The Jackass Lakes pluton (JLP) is a superb example of how punctuated construction and deformation of volcano-plutonic systems can be. The JLP exhibits a complex history of incremental construction and within the pluton there are several large volcanic pendants and countless xenoliths with lithologic, structural, geochemical and geochronologic affinities to the Ritter Range pendant located east of the pluton. Six new U-Pb SHRIMP ages from metavolcanic pendants in the JLP reveal that the volcanic ages of these pendants, all ca. 98 Ma, overlap directly with U-Pb ages from the pluton indicating that coeval magmas intruded their own volcanic cover. These ages permit investigation of volcano-plutonic connections to help decipher the geochemical evolution of the system. Furthermore, when coupled with barometric and structural data these ages reveal that volcanism, plutonism and deformation in this region were occurring synchronously. Hornblende-plagioclase thermobarometry from the pluton indicate an emplacement depth of 3 kbar, suggesting rapid vertical displacement of volcanic rocks. Magmatic fabric orientation in the pluton is sympathetic with solid-state fabrics in xenoliths, pendants and host rock and are interpreted to reflect the regional strain field during the latest stages of emplacement. Magmatic fabrics are also contemporaneous with the Bench Canyon Shear Zone located just east of the pluton. Minor and local occurrences of mylonitic fabrics are also observed in the eastern JLP and interpreted to be associated with shear zone deformation. All of these data suggest that in the central Sierra Nevada batholith circa 98 Ma there was a rapid succession of volcanism, vertical displacement and pluton emplacement occurring within a regional strain field recorded in both volcanic and plutonic record.