Granites in the Franciscan: A PGS Field Trip Oct 9 2005

Introduction: The chief purpose of this field trip is to visit two 'granitoid' components of the Franciscan Complex near Cazadero, CA. One of these components is a multiply metamorphosed quartz diorite exotic block in the King (formerly Kings) Ridge Road olistostrome. The other is an intraFranciscan rhyolite stock, a plutonic body. We will also examine the classic Ward Creek blueschist locale and look at two other interesting exotic blocks in the olistostrorne, as well as the olistostrome matrix. My main theme will be field and geochemical evidence for a second, oceanic arc in the Franciscan Complex. Mileage for Stop 1 is measured from the Cazadero turnoff from River road; all other Stop mileage is measured from the Cazadero store. UTM coordinates are provided for all stops; be sure to use the 1927 Datum. Figure 1 shows where the stops are located. Appendix I contains an overview article on the Franciscan geology of the area and provides background information on the units visited, and a bibliography of most references cited. Appendix II provides a short history of Cazadero. Appendix lll contains copies of GSA Meeting abstracts of presentations by the author and Sonoma State University Geology department students on some of the geologic features discussed. Appendix IV contains relevant geochemical diagrams keyed to the stops. Appendix V contains the chemical data on previously unpublished samples. References not found in Appendix I are given in the short form used in GSA abstracts.