The Sacatone Springs gneissic tonalite is located within the Tule Mountain roof pendant in the eastern zone of the Peninsular Ranges batholith (PRB), and outcrops near the middle of the 94 Ma La Posta pluton. It has some features similar to those of western-zone rocks, and others similar to those of eastern-zone rocks. The purpose of this study is to determine whether it is an eastern- or western-type rock of the PRB. Well-developed foliation, abundant mafic inclusions, and subhedral to anhedral mafic minerals are found in both western-zone plutons and the Sacatone Springs tonalite. However, the Sacatone Springs tonalite has primary sphene and no magnetite, which are characteristics of eastern-zone plutons. Simply by analyzing the mineralogical and textural features, a realistic conclusion cannot be drawn. Geochemical data suggest that the Sacatone Springs tonalite is an eastern-type rock. Relatively high Sr content, negligible Eu anomalies, and relative depletion in the heavy rare-earth elements are characteristics of eastern-zone rocks and the Sacatone Springs tonalite. The tonalite samples fit nicely into the linear trend displayed by La Posta rocks on trace element variation diagrams. They also fit into the La Posta range on rare-earth element plots. The Sacatone Springs tonalite appears to be an eastern-type rock, and it is likely a remnant border facies of the La Posta pluton.
