The La Posta pluton is the largest mapped intrusion of the Peninsular Ranges batholith and covers an area of about 1600 km2 that stretches out from the U.S. side of the border into northern Baj a California on the Mexican side of the border. Different models have been presented in order to explain how large volume granitoids bodies such as the La Posta pluton were generated and whether they were emplaced in a single magmatic event or over more than one event. The purpose of this project is to try and help constrain possible models for generation and emplacement of the La Posta pluton by studying compositional variation over large areas. In order to accomplish this task, two sets of samples from the La Posta pluton were collected. The set analyzed for this thesis came from the U.S. side of the border from along Old Hwy 80 and are relatively closely spaced. The second set came from the Mexican side of the border and are more widely spaced. Whole rock major and trace element concentrations were determined for all samples by X-Ray Fluorescence. The whole rock XRF data demonstrates significant regional variation in the composition of the La Posta pluton to the north and south of the International border. Samples to the nmih of the border range narrowly in Si02 content from ~65 to 71 weight percent and have consistently higher strontium contents than samples south of the border.