Processes involved during incremental growth of the Jackass Lakes pluton, central Sierra Nevada batholith

Abstract

The Jackass Lakes pluton (JLP), located in the central Sierra Nevada batholith, is a 98 Ma composite intrusion that preserves field, structural, and petrologic evidence of how incrementally emplaced plutons grow and evolve both spatially and temporally. In contrast to many other Sierra Nevada batholith intrusions, the compositional and textural diversity found within the JLP allows individual increments to be easily discerned in the field. Previous work has resulted in two different incremental emplacement models for the JLP. 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. The second involves replenishment by mafic sheets that may represent originally subhorizontal floors of an evolving magma chamber. We present new data and a model suggesting that the JLP (1) contains multiple, irregularly shaped intrusions of both felsic and mafic material that do not represent dikes or paleofloors; (2) magma increments were extensively mingled, both between and within intrusions; (3) records evidence of magma mixing locally and possibly at the intrusion scale; (4) has not been tilted; (5) has magmatic mineral fabrics that record superimposed regional strain, not emplacement-related strain; (6) preserves large metavolcanic pendants representing subhorizontal roof contacts; and (7) was emplaced by ductile deformation of its host rocks, return flow, and widespread stoping of older host rock and its internal increments. This model, based on field, structural, strain, thermobarometric, and petrologic analyses, elucidates that the JLP construction is considerably more complex spatially and temporally than previous models suggest, and highlights processes involved during incremental emplacement of plutons.