HORIZONTAL LENS-LIKE EMPLACEMENT OF THE SAGE HEN FLAT PLUTON, CALIFORNIA

This work addresses the emplacement of the Sage Hen Flat pluton located in the White Mountains of eastern California. The Jurassic age pluton intrudes the western limb of the White Mountain anticline. Pluton contacts on the western margin are concordant and dip shallowly west. Steeply dipping, discordant contacts are observed elsewhere. Locally, the orientation of the surrounding bedding does not deviate from regional strike. For these reasons, the pluton has been described as having a “cookie cutter” geometry. The current exposure level is likely near the roof of the pluton, based on observations of the western contact and the incorporation of small wall rock screens in the exposed granite. Gravity data constrain the floor of the pluton to a maximum depth of 700 m.

Anisotropy of Magnetic Susceptibility (AMS) foliations exhibit a concentric pattern with steeply dipping foliations near the edge of the pluton and shallow foliations near the center. The foliations dip steeper on the east side than on the west side. The lineations are mainly NNW-SSE trending and shallowly plunging. Thin section observations indicate that high-temperature, solid-state fabrics occur around the northern, western, and southern pluton margins while magmatic fabrics are present in the center and eastern portions of the pluton. Mineral zonation is observed, in which hornblende bearing rocks occur only in the northern and southern portions of the pluton while the remaining granite is hornblende absent. The fabrics have been rotated from their original Jurassic orientations because of movement on NS- and NE-trending faults that offset the pluton contact and rotate (< 20º to the east) overlying Miocene basalt flows.

Together, the geometry, fabrics, and faults provide an emplacement history of the Sage Hen Flat pluton. We suggest that the magma was injected into a horizontal crack in the W-dipping Precambrian-Paleozoic strata. The resultant pluton geometry is horizontal and lens-like, with locally discordant contacts. The gravity data indicate that the thickest portion of the pluton is located near its center, which we interpret as the feeder zone. The field relationships, fabrics, and mineral zonation indicate multiple phases of magma injection from the center of the pluton outward, with the youngest magmatism located above the feeder zone.