The Johnson Granite Porphyry (JGP) is located on the eastern margin of the Cretaceous Sierra Nevada batholith, California. This pluton is the youngest and most felsic unit of the Tuolumne Intrusive Suite. The porphyritic texture, fine-grained matrix, and miarolitic cavities within this igneous body led earlier workers to suggest that the JGP was quenched during a rapid pressure release. Using field mapping, gravity inversion, magnetic fabrics, and regional structures, we further hypothesize that the JGP represents the erosional remains of a volcanic conduit.

Detailed field mapping indicates the presence of large blocks of the surrounding Cathedral Peak Granodiorite within the JGP. Brecciation occurs locally at the margin of the pluton indicating high strain rates at the time of emplacement. Foliations and lineations are difficult to observe in the field. Magnetic foliations, determined using the anisotropy of magnetic susceptibility technique, are randomly oriented. A faint pattern is observed in the magnetic lineations, which plunge moderately and symmetrically away from a NS-oriented axis through the center of the JGP. Bulk magnetic susceptibility varies widely throughout the pluton with no clear pattern. The gravity inversion reveals a thin, tabular shape throughout most of the body though a narrow, 2 km deep feeder zone exists on the southwestern margin of the JGP. The random nature of the magnetic fabrics and field structures are indicative of high strain rates, and the shape of the intrusion is broadly consistent with an interpretation of the JGP as a volcanic conduit. Structural features, such as the Bench Canyon shear zone, may control the location of the JGP.