ALUMINUM-IN-HORNBLENDE GEOBAROMETRY OF THE NORTHERN PART OF THE SALINIAN BLOCK

The Salinian block, located in coastal California between San Luis Obispo and Point Reyes, consists of Mesozoic granitoids, the surrounding Paleozoic and older metasedimentary country rock, and overlying Cenozoic sedimentary cover. These rocks are proposed to have originated as part of the Southern Sierra Nevada batholith but were subsequently displaced westward as the hanging wall of a Cretaceous-aged detachment fault that was the result of the gravitational collapse of the Southern Sierra Nevada related to the shallow subduction. They were then transported northward by dextral slip on the San Andreas and related faults. Evidence for the upper-plate nature of the Salinian block is based partly on a comparison between the high granitoid crystallization pressures found in the Southern Sierra Nevada and lower pressures present in the Santa Lucia and Gabilan sections of the southern portion of the Salinian block. Geobarometric measurements are sparse further north in the granitoid exposures at Ben Lomond, Montara Mountain, Point Reyes, and Bodega Head.

To test if the upper plate hypothesis applies to the northern portion of the Salinian block, we collected spatially-distributed samples from these northern outcrops. Hornblende compositions were measured by electron microprobe, then total structural aluminum was calculated assuming that all iron was ferrous. Crystallization pressures were then estimated using the aluminum-in-hornblende geobarometer of Schmidt (1992), applicable here due to the presence of hornblende, biotite, plagioclase, quartz, orthoclase, sphene, and opaques.

Results from Ben Lomond yield equilibration pressures that range from 3.5 to 5.7 kb. Notably, samples collected along a NW-SE transect in the northern tonalite exhibit a decrease in pressure to the NW. Preliminary single point data from Point Reyes yields a similar pressure of 5.2-5.3 kb, while the pressure at Montara Mountain of 6.8 - 7.6 kb is slightly higher. As a whole, these pressures are unexpectedly high and may indicate the presence of cryptic low-angle detachments within the Salinian block.