THE JACKASS LAKES PLUTON: EVIDENCE FOR COMPLEX SPATIAL AND TEMPORAL MAGMA CHAMBER EVOLUTION IN THE SIERRA NEVADA BATHOLITH

Large magmatic arcs like the Sierra Nevada batholith (SNB) are characterized by long and complicated deformational histories and punctuated by short periods of voluminous magmatism. Thus, understanding the physical and chemical evolution of plutons that make up these arcs provides insight into how entire orogens evolve and how continents grow. The Jackass Lakes pluton (JLP), located in the central SNB, is a 98 Ma composite intrusion that preserves field, structural and petrologic evidence of how magma chambers are constructed and how these systems evolve both spatially and temporally.

Previous work on the JLP has led to different models for its emplacement and temporal evolution. The first states that the JLP was emplaced and assembled via vertical diking or sheeting, some downward return flow along the margins of the pluton, and local stoping (McNulty et al.,1996). The second, proposed by Wiebe (2000) uses internal sheets and associated magmatic structures as evidence, and states that sub-vertical mafic sheets represent multiple sub-horizontal floors of an evolving magma chamber, which have been rotated to their present steep dips. However more recent work has led to a third model which suggests the JLP: a) has not been tilted; b) preserves large metavolcanic pendants with sub-horizontal lower contacts suggesting these are roof contacts; c) records numerous intrusions that vary in composition, size and nature and furthermore do not represent paleo-floors, nor are they pervasive sheets/dikes; d) magma chamber experienced extensive magma mingling (between and within pulses) and stoping during its construction; e) preserves evidence for magma mixing (hybridization) at least on a local scale; f) host rock shows evidence for ductile deformation during emplacement; and g) magmatic fabrics record an increment of regional strain not flow in dikes/sheets. This model, based on field, structural, thermobarometric and petrologic studies adheres to the observations made in previous studies but elucidates the significantly more complex spatial and temporal evolution the JLP magma chamber underwent.