NOTE: The last page of the guidebook, chem data, printed wrong. Download the book PDF below and print out page 29 to stick in your paper copy.
Download the guidebook now as a 29-page PDF file (8.3 MB).
Rolfe, you will remember, gave a talk on these rocks last May 10. http://www.diggles.com/pgs/2005/PGS05-05.html He writes: "I have been studying a complexly metamorphosed quartz diorite block which is, I think, the first granitoid found in the Franciscan, and have also been studying a small trachyte pluton which is probably the first reported intraFranciscan felsic pluton. I will mostly talk about these two units. I gave a poster on the granite block at Cordilleran GSA at Boise this Spring [2004]."
The trip will focus on field relationships of "granitic" units in the Franciscan Complex around Cazadero, CA, but also look at some other features. We will look at:
a >50m block of hornblende-biotite quartz diorite of oceanic island-arc origin (an M-type granite) in an olistostrome. The block has been brecciated once, rehealed, and metamorphosed three times after crystallization. Abundant chemical and isotopic data have been determined on it; see Erickson et al 2004 in GSA Abstracts for the Boise Cordilleran Section meeting for details.
the >100 Ma Little Black Mountain rhyolite stock, an intraFranciscan felsic pluton.
the classic Ward Creek blueschist-metachert sequence, probably the most intensively studied body of metamorphic rocks in the world and the standard for glaucophane-lawsonite facies metamorphism
an exotic block of twice-brecciated lawsonite-glaucophane fels
an exposed contact of a >100 m exotic block of red chert and its sandstone matrix.
We will meet 9:00 AM at the Larkspur Ferry parking lot in Marin County. This is just east of Highway 101 on E. Sir Francis Drake Blvd. Folks down on the Peninsula should gather at 7:70 AM and form ridepools at our usual two places: Geology Corner at Stanford and the flagpole out in front of Topo Map Sales at USGS.
There will be a guidebook provided at the Ferry Terminal and we will try to get a PDF of it posted here online before the trip so you can do your homework.
Rolfe gives overview of the field area
Rolfe lectures by roadcut along Austin Creek
Contact between chert block and sandstone matrix
Examination of roadcut
Finding the contact
Samples from metabasalt - metachert sequence at Stop 2, Ward Creek
Andrew waded then climbed to examine outcrops
Rock heaven at Ward Creek
Here is the glaucophane metamorphite
Rolfe with two of the local landowners
Kat
Andrew and Al check the downstream pillow metabasalt
Good day for field geology
Examining rocks
Examining rocks
Stop 3, explanation of the granitoid block protolith and multiple
metamorphism
Stop 3(a) granitoid
Pilgrimage UP!! to Stop 3(b)
Rolfe is having too much fun busting up the rocks
Examining the Little Black Mountain rhyolite stock
Naomi examines the rhyolite
Ray and others Talkin' the geology
Stop 3(c) - the unfoliated metabasalt breccia
The gang attempts to hide the lawsonite-glaucophane fels metabasalt
block from the Kings River Melange
Rolfe with students Sara and Nathan
Date created: September 12, 2005
Last modified: October 20, 2005
Created by: Mike Diggles, Vice President, PGS.