Abstract

The Johnson Granite Porphyry and the Silver Pass Porphyry are two late-stage porphyritic intrusions in the Cretaceous Sierra Nevada Batholith, California. Both intrusions are characterized by fine-grained textures, miarolitic cavities, pegmatite veins, and abundant xenoliths. Microstructures in the Johnson Granite Porphyry record magmatic flow. Magnetic fabrics in the Johnson Granite Porphyry, determined by anisotropy of magnetic susceptibility, show that the poles to magnetic foliations define a NNE-SSW girdle and magnetic lineations plunge shallowly to moderately to the WNW and ESE. A gravity survey and three-dimensional inversion indicate that the Johnson Granite Porphyry is shallow and tabular, extending to 1.5 km depth beneath the southwestern margin. A second gravity anomaly was delineated southeast of the Johnson Granite Porphyry, suggesting the intrusion continues to depth. The Silver Pass Porphyry is deformed by the dextral Rosy Finch shear zone and records a gradient of high- to low-temperature, solid-state fabrics. Field and magnetic foliations in the Silver Pass Porphyry are subvertical and lineations are shallowly plunging.

We propose that regional shear zones controlled the emplacement of these intrusions. The Johnson Granite Porphyry was emplaced in a three-dimensional tension gash associated with the termination of the Bench Canyon shear zone. The Rosy Finch shear zone facilitated the emplacement of the Silver Pass Porphyry, probably as a tension fracture associated with the shear zone. The field observations and magnetic fabrics suggest that the Johnson Granite Porphyry may record an extrusive event, suggesting that volcanism may be related to deformation along active shear zones.

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