MAGMA MIXING/MINGLING IN A HETEROGENEOUS MULTI-PULSE MAGMATIC SYSTEM: EVIDENCE FROM JACKASS LAKES PLUTON, CENTRAL SIERRA NEVADA BATHOLITH

The Cretaceous Jackass Lakes pluton (JLP), central Sierra Nevada California, has been the focus of several attempts to determine the emplacement and temporal evolution of this composite, subvolcanic intrusion. The presence of sub-vertical mafic sheets/intrusions within this predominantly granodiorite pluton has led to a disparity between three proposed models (e.g. McNulty et al., 1996; Wiebe, 2000; Pignotta et al., 2003) for chamber construction. We have conducted detailed field mapping in a central domain (~1kmx1km) of the JLP to test the hypotheses of all models, which have been primarily derived from field and structural data. During field investigations within the central domain 8 units were identified, ranging in composition from diorite to leucogranite. Extensive mingling features (e.g., schlieren layers, load casts, flame structures, mafic enclaves) within units and contact relationships between units indicate that magma mingling was an important process throughout the evolution of this magma chamber. The overall compositional variation within the JLP suggests that the chamber formed as a result of multiple pulses of magma at uniform shallow levels. Hornblende-plagioclase thermobarometry was conducted on five samples along an east-west transect of the pluton to determine pressure and temperature at the time of emplacement. Average pressure (from rim points) is 2.8 kbars with average temperature (from rim points) at 670°C; these findings indicate that mingling may have occurred during ascent. Futhermore, petrographic examination has identified a number of disequilibrium features (e.g., complex zonation of plagioclase, corroded plagioclase cores, rapakivi texture potassium feldspar, acicular apatite) resulting from mafic-felsic magma interaction. These observations raise questions regarding the level of interaction of magmas both locally and at the source. Thus characterization of these phases using several geochemical (XRF, ICP-MS, and mass spectrometry) techniques is being used to determine magma source variability and quantify the amount of local mixing. These findings will improve upon proposed chamber construction models for the JLP and provide further insight into the dynamics of magma interactions.