The San Diego prebatholithic rocks were intruded by Middle (?) and Late Jurassic, Early Cretaceous, and Late Cretaceous plutons. Jurassic S-type (Chappell and White, 1974; White and Chappell, 1977) granitoid plutons intruded the Julian Schist and the Rocks of Jacumba Mountains to form an axial belt of metaplutonic rocks within the Peninsular Ranges Batholith. The western limit of Jurassic plutons (I-S line) coincides with a steep magnetic gradient in the northern batholith (Jachens and others, 1991; Jachens, 1992), a gradient that is considered to mark the location of a latest Jurassic to earliest Cretaceous crustal-scale fault (Todd and others, 1994a).Cretaceous I-type felsic to mafic plutons intruded the prebatholithic and Jurassic rocks during two major episodes as the Cretaceous magmatic arc swept eastward across the Late Jurassic continental margin. Plutons of both intrusive episodes, Early Cretaceous to Late Cretaceous, are present in the El Cajon quadrangle. On the west side of the Peninsular Ranges Batholith, Early Cretaceous plutons intruded a Late Jurassic and Early Cretaceous island-arc assemblage; isotopic ages of the Early Cretaceous Santiago Peak Volcanics (Larsen, 1948) range from slightly older than, to coeval with these Early Cretaceous plutons. Early Cretaceous plutons also intruded the western part of the axial belt of Jurassic plutons and their Triassic-Jurassic wallrocks. In the central-eastern part of the batholith, large mid- to Late Cretaceous plutons of trondhjemitic and granodioritic composition (La Posta-type) intruded the Triassic-Jurassic belt, and farther to the east they intruded the Rocks of Jacumba Mountains and miogeoclinal Paleozoic rocks. Small, satellitic mid-Cretaceous trondhjemitic plutons in the southwestern part of the El Cajon quadrangle were apparently precursors of the voluminous eastern-zone La Posta plutons.

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

Chappell, B.W., and White, A.J.R., 1974, Two contrasting granite types: Pacific Geology, v. 8, p. 173-174.
Jachens, R.C., Todd, V.R., Morton, D.M., and Griscom, Andrew, 1991, Constraints on the structural evolution of the Peninsular Ranges batholith, California, from a new aeromagnetic map: Geological Society of America Abstracts with Programs, v. 23, no. 2, p. 38.
Jachens, R.C., 1992, Aeromagnetic map of the El Cajon 1:100,000 scale quadrangle, California: U.S. Geological Survey Open-File report 92-548.
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.
White, Allan J.R., and Chappell, Bruce W., 1977, Ultrametamorphism and granitoid genesis: Tectonophysics, v. 43, p. 7-22.