Testing models for the incremental growth of magma chambers and coupled displacement of host-rocks during pluton construction with an emphasis on magmatic stoping.

摘要

Plutons are a powerful tool for unraveling orogenesis and crustal growth. They are wonderful, albeit complicated, recorders of arc evolution and often reveal the complex physical, chemical and temporal history of orogenesis and magmatic processes. The Jackass Lakes pluton (JLP), central Sierra Nevada batholith (SNB) records a complex spatial and temporal history of incremental emplacement that provides insight into arc processes. Previous work on the JLP has led to contrasting incremental growth models describing its emplacement and temporal evolution. Detailed field, structural, petrologic and thermobarometric studies suggest an alternative model in which the JLP (1) contains multiple, irregularly shaped intrusions of both felsic and mafic magma that do not represent paleo-floors or dikes, but instead are interpreted as visco-elastic diapirs, (2) pulses were extensively mingled below the level of emplacement with some mingling both between and within intrusions at the emplacement level, (3) records evidence of local magma mixing forming new phases at depth and locally preserved hybrid phases along internal contacts at the emplacement level, (4) has not been tilted significantly during or after emplacement as determined by application of both thermobarometry and using internal magmatic structures that provide younging directions and paleovertical, (5) has magmatic fabrics that record regional and not emplacement related strain, and (6) was emplaced by ductile deformation of its host-rocks, return flow and widespread stoping of the older host-rock and earlier intrusions.; The importance of magmatic stoping during pluton emplacement is a controversial topic but may prove to be especially important in incrementally emplaced systems like the JLP. The JLP records a lengthy history of stoping of both metavolcanic host-rock and of earlier plutonic phases by younger pulses illustrating that internal recycling was important. Stoping in both the JLP and in another pluton, the Mitchell Peak Granodiorite, southern SNB has been studied in detail to illustrate the mechanisms by which stoped blocks form and disintegrate in plutons. These studies suggest that contamination by mechanical breakdown of stoped blocks may be important in magmatic systems.