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