MAGMA CHAMBER CONSTRUCTION AND EVOLUTION: CONSTRAINTS FROM THE JACKASS LAKES PLUTON, CENTRAL SIERRA NEVADA

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. The Jackass Lakes pluton (JLP), located in the central SNB, is a 98 Ma composite intrusion that provides an excellent field laboratory to evaluate arc evolution. Presently there are two models proposed for the emplacement and temporal evolution of 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 (McNulty et al., 1996). The second, using internal sheets and associated magmatic structures as evidence, 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 Wiebe (1999, 2000). However, our studies suggest that the JLP: (a) has not been significantly tilted; (b) preserves large metavolcanic pendants representing sub-horizontal roof contacts ; (c) exhibits a vertical petrologic stratification within the chamber with multiple intrusions of mafic material at lower levels in the chamber; (d) the latter do not represent paleo-floors, nor are they pervasive sheets/dikes; and (e) host rock was displaced during chamber construction by magmatic wedging, return flow, and stoping.

These observations have specific implications for the evolution of the JLP and other magmatic systems including: (1) construction of plutons first by numerous sheet like intrusions (preserved along eastern margin and in pendants), followed by larger, irregular pulses of magma (preserved within the pluton); (2) operation of multiple host rock material transfer processes during chamber construction (e.g. ductile flow, downward flow, stoping); (3) significance of tilting which does not seem to be supported by the distribution of stoped blocks within the pluton (derived locally from pendants) and the sub-horizontal roof contact with blocks preserved below them; and (4) magmatic fabrics record an increment of the regional strain-field late in the crystallization history of the pluton and do not reflect emplacement.