Numerous leucogranite plutons are scattered throughout the western zone of the Peninsular Ranges batholith (PRB) east of San Diego, California. They are all weakly to moderately foliated, contain biotite and minor amounts of hornblende, have an - 2:1 plagioclase to alkali feldspar ratio, and contain sericitized plagioclase cores, myrmekite and euhedral to subhedral zircons. The rocks range between 71 and 78 weight % Si02, are metaluminous to weakly peraluminous, and are calcic with an alkali-lime index of -63. Most of the major- and trace-element variation diagrams from 19 separate bodies east of San Diego, California, have tightly constrained patterns with curvilinear trends that suggest a common source and/or fractionation mechanism. However, the Iron Index (FeO/FeO + MgO) has a greater variation than would be expected if the rocks were comagmatic. In addition, the zircon U-Pb ages available to date for these discrete plutons fall between 111 Ma and 122 Ma, and suggest that although these rock units formed within the same magmatic episode, the 11 million year span does not support a comagmatic origin. REE patterns are consistent with other reported analyses in the western zone and have moderate LREE enrichment from about 45- l OOX chondritic values, moderate to strong negative europium (Eu) anomalies, and relatively flat HREE from about 5-15X chondritic values. These patterns imply that hornblende and plagioclase were present in the source region, and the wide range in europium anomalies suggests a variable oxidation state in the source region, as well. The leucogranites are reported to be among the youngest magmatic units in the western zone of the PRB. Recent studies, however, suggest that the reported ages of 111 Ma - 122 Ma may be up to 10 Ma older than spatially associated tonalites and gabbros. The REE and trace-element data indicate that the leucogranite plutons were derived from separate but similar magmas having a common plagioclase amphibole (± garnet?) source region. Two possible sources are subducted amphibolite-grade oceanic crust and the abundant amphibole-rich gabbroic rocks found in the western zone of the PRB. However, the relative age relationships between the gabbroic rocks and the leucogranite plutons are ambiguous, and the REE patterns of the leucogranites cannot be derived by partial melting models of the gabbroic rocks. Preliminary ages for the gabbros indicate that they are younger than the leucogranites. Two petrogenetic models are put forth for the ongm of the leucogranites. The first, a two-dimensional model, implies that the location of the subduction plane at the onset of magmatism must be at some shallower level to first create the older leucogranites . The subduction angle then steepens to a point below the "wet" mantle solidus to generate the gabbros and subsequent tonalites. The second is a three-dimensional model that suggests that the curvature of the Earth 's surface, along with the curvature of the arcuate trench will force a subducting slab into a more constricted space at depth. This constriction is taken up by buckling of the subducted slab at a depth below 100 km. This model will allow for a variety of rock compositions from gabbro through tonalite to leucogranite to be derived during subduction, depending on the location of the magma body on the buckling subducted slab, and whether it is above or below the "wet" mantle solidus. The ages of the varying plutons may be explained similarly, and therefore the most likely situation is one where there is a range of ages throughout which gabbro, tonalite and leucogranite plutons were being produced within the PRB.