The Santiago Peak Volcanics of Larsen (1948), forms the western edge of the batholith in southern California. It is composed of unmetamorphosed to slightly metamorphosed volcanic, volcaniclastic, and minor hypabyssal rocks (Kennedy and Peterson, 1975; Herzig, 1991). Volcanic components are chiefly dacitic and andesitic breccia, tuff, and flows with lesser basaltic and rhyolitic compositions. Uranium-lead zircon dating of silicic subunits of the Santiago Peak Volcanics in the type area in the Santa Ana Mountains and in the eastern part of the city of San Diego indicate an Early Cretaceous age (130 to 120 Ma) (Kimbrough and others, 1990; Herzig and Kimbrough, 1991; Anderson, 1991; Herzig, 1991). A sequence of volcaniclastic rocks located about 2.5 km north of the El Cajon quadrangle contains Late Jurassic fossils (Fife and others, 1967; Jones and Miller, 1982). In addition, a volcanic flow from this sequence has been dated by U-Pb zircon method as Late Jurassic (Anderson, 1991). This Late Jurassic volcanic-volcaniclastic sequence appears to underlie the Santiago Peak Volcanics unconformably (C.T. Herzig, oral commun., 1994; Todd and others, 1994a). The age of the Santiago Peak Volcanics is considered to be Early Cretaceous.

Source - Preliminary Geologic Map of the El Cajon 30´ x 60´ Quadrangle, Southern California, Version 1.0, Compiled by V.R. Todd, Open-File Report 2004-1361 Detailed Description of Map Units, version 1.0

References

Anderson, C.L., 1991, Zircon uranium-lead isotopic ages of the Santiago Peak Volcanics and spatially related plutons of the Peninsular Ranges batholith, southern California: M.S. thesis,San Diego State University, 111 p.
Fife, D.L., Minch, J.A., and Crampton, P.J., 1967, Late Jurassic age of the Santiago Peak Volcanics, California: Geological Society of America Bulletin, v. 78, p. 299-304.
Herzig, C.T., 1991, Petrogenetic and tectonic development of the Santiago Peak Volcanics, northern Santa Ana Mountains, California: Unpublished Ph.D. dissertation, University of California, Riverside, Riverside, California
Herzig, C.T., and Kimbrough, D.L., 1991, Early Cretaceous zircon ages prove a non-accretionary origin for the Santiago Peak Volcanics, northern Santa Ana Mountains, California: Geological Society of America Abstracts with Programs, v. 23, no. 2, p. 35.
Jones, D.A., and Miller, R.H., 1982, Jurassic fossils from the Santiago Peak Volcanics, San Diego County, California, in Abbott, P.L., ed., Geologic studies in San Diego: San Diego Association of Geologists Publication, p. 93-103.
Kennedy, M.P., and Peterson, G.L., 1975, Geology of the eastern San Diego metropolitan area, California: California Division of Mines and Geology Bulletin 200, p. 43-56.
Kimbrough, D.L., Anderson, C.L., Glass, S,N,. Kenney, M.D., Thomas, A.D., and Vitello, Theresa, 1990, Early Cretaceous zircon U-Pb ages from the Santiago Peak Volcanics, western Peninsular Ranges batholith, San Diego County, California: Geological Society of America Abstracts with Programs, v. 22, no. 3, p. 35.
Larsen, E.S., Jr., 1948, Batholith and associated rocks of Corona, Elsinore, and San Luis Rey quadrangles, southern California: Geological Society of America Memoir 29, 182 p.
Todd, V.R., Kimbrough, D.L., and Herzig, C.T., 1994a, The Peninsular Ranges batholith from western volcanic arc to eastern mid-crustal intrusive and metamorphic rocks, San Diego County, California, in McGill, S.F., and Ross, T.M., eds., Geological investigations of an active margin: Geological Society of America Cordilleran Section Guidebook, 27th Annual Meeting, San Bernardino, California, Mar., 1994, p. 227-235.