Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study

Abstract

The Papoose Flat pluton of eastern California is commonly cited in the geologic literature as a classic example of a “forcibly” emplaced pluton, although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial. Reexamination of this Late Cretaceous (83 Ma) pluton, using a combination of new structural, petrographic, microstructural, and anisotropy of magnetic susceptibility (AMS) data on the pluton–wall-rock system, has shown that the pluton is an inclined and internally zoned tabular structure, assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12–15 km. Initial pluton formation involved magma ascent in a vertical west- northwest–striking feeder dike, which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying Cambrian metasedimentary rocks, leading to formation of a southwest-dipping sill. Subsequent sill inflation, accompanied by horizontal infilling from the feeder dike at the base of the sill, resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof, facilitated by thermal weakening as the wall-rock temperatures progressively rose during emplacement of successive magma pulses. Cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton. We have been unable to document any regional-scale structures (e.g., of equivalence to similar-age synplutonic strike-slip shear zones in the Sierra Nevada batholith to the west) that may have controlled emplacement of the Papoose Flat pluton. However, this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time; indeed, tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level. Simple thermal modeling, using microstructural and thermobarometric data, indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr. This rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional- scale deformation.