Granitic Rocks of Pt. Reyes

In the far distance past (110 million years ago) molten rocks formed (sometimes invading and literally "cooking" the prexisting rocks and creating the peninsula's metamorphic rocks) the pink exposures of Granitic rocks seen in the geologic map. Unlike the molten rocks we see erupting at volcanic sites today, the geologic evidence indicates that these rocks cooled very slowly deep inside the Earth's interior, perhaps under a long vanished island or mountain arc of volcanoes.

Geologic Map Description

Granitic Rocks (the ones colored pink on the map above)

White to light gray, medium- to coarse-grained granitic rock ranging in composition from quartz diorite through grano-diorite to quartz monzonite; contains variably abundant dikes. and masses of aplite, alaskite, and pegmatite.

More background info on the granitic rocks

The grassy and wooded areas of the Pt. Reyes peninsula are developed on deeply weathered sections and, in these areas, exposures are poor. Shoreline exposures provide the best views of unaltered samples of these rocks and the best locations are identified on the map above.

From Tomales Point north the granitic basement consists of a hornblende-biotite quartz diorite, and a larger area of a much more felsic rock rich in K-feldspar. The contact between these two granitic types (there are metamorphic rocks sandwiched between them) can be seen at McClures Beach and is reported to be one of the few contacts between two granitic types visible in the Salinian block.

Age of the granitic rocks-

The earliest attempts to establish absolute age dates from the Salinian block were produced using the Potassium-Argon age dating technique. K/AR age dates throughout the Salinian block, including samples from the Pt. Reyes granites, have run in the range of 70 to 90 million years. More recent work in the Salinian block with Rubidium-Strontium and Uranium-Lead age dating techniques indicate an older origin, in the range of 100-110 million years before present. Work with other age dating techniques, including fission track age dating tools, has provided information that following the rock's origin, (most likely 100-110 million years b.p.), they were subjected to conditions which apparently reset the Potassium-Argon age dates to the younger date.

Sierran granites or an "exotic" origin?

For a long time geologists pointed to the origins of the Salinian granites by tracing back along the San Andreas Fault to the Tehachapi Mountains where the first granites can be found on the East side of the San Andreas. Recent studies however indicate that 60 million years ago Pt. Reyes was attached to the west of Monterey, California where similar Salinian granitic rocks are common. Fault movement along a large, largely offshore, fault of the San Andreas System is believed responsible for moving Pt. Reyes from this location. Support for the non-Sierran, "exotic" nature of the Salinian block comes from geochemical studies of the southern Sierra and Salinian granitics which indicate some large discrepancies in the two areas. One interesting discrepency is that the correlative rocks in the Southern Sierra, Tehachapi Mountains are sites of intense mineralization, gold, silver, tungsten and antimony having been productive of several millions of dollars. The Salinian block on the other hand is quite barren of mineralization to the same degree.

The specific location where these granites formed is still uncertain.


Up section to the non-conformity


Other information on the WWW about Granites:

Rob's Granite Page Here's a great example of a PHd candidate putting his research on the Internet! His page also has pointers to other information sources on Granites.