The Peninsular Ranges Batholith (PRB) contains a series of "La Posta" type plutons that occur only in the eastern zone of the PRB. These plutons are concentrically zoned, single intrusive bodies with gradational internal contacts. The La Posta pluton is the largest of this type and is estimated to encompass an area of approximately 1400 square kilometers. Its mapped rock units vary inward from hornblende-biotite tonalite to largebiotite granodiorite, small-biotite granodiorite, to muscovite-biotite granodiorite in the core zone. A large roof pendant splits the pluton into lithologically, geochemically, and structurally similar eastern and western halves with zircon U-Pb ages of 94 (+/- 1) Ma. The purpose of this study is to use trace mineral variations from a transect across the La Pesta pluton to determine: l. If the eastern and western portions of the pluton are comagmatic and thus consistent in their trace mineralogy, 2. If different structural levels could be exposed on either side of the roof pendant, and3 If the core facies shows any mineralogical evidence of being a separate but related intrusion The portion of the La Pesta pluton examined in this project lies along 1-8 between La Pesta, and Ocotillo, California. Trace mineralogy is consistent across the eastern and western halves of the pluton. Minerals found include: primary sphene (magnetic), secondary sphene (non-magnetic), allanite, epidote, clinizoisite, carbonate-fluorapatite, ferroaxinite, monazite and zircon These select minerals occur throughout the entire pluton, but may be more prominent in certain zones. Primary sphene (magnetic), zircon, and carbonate-fluorapatite occur most abundantly in the margins with lower concentrations toward the core. The presence of epidote primarily on the western half of the La Pasta pluton suggests that different structural levels are exposed on either side of the roof pendant. In addition, the color of primary sphene varies from brown on the west to yellowish-green on the east and may also support differing structural levels. The core facies is commonly identified by the Muscovite-Biotite Facies, but appears to include the eastern Small-Biotite Facies as well. This core region appears to produced by be a separate, but related magmatic event. Trace mineralogy indicates that secondary sphene (non-magnetic), monazite and clinozoisite occur only or primarily in the core region. Ferroaxinite, primary sphene (magnetic), and allanite which typically occur throughout the pluton are absent in the core.