The San Rafael Volcanic Field, Utah, is a 4.6 Ma extinct monogenetic field that is found in the Northern Transition Zone of the Colorado Plateau. The field has been eroded, leaving the dikes, conduits, and sills visible. Within the sills we see evidences of immiscibility in the form of an intermediate syenite (~50 wt% SiO2) enclosed in a mafic shonkinite (~48 wt % SiO2). Field relations indicate that sills were formed due to single events (Richardson et al., 2015), which makes in-situ differentiation the process at the origin of both rock types. Geochemical data supports differentiation of syenite and shonkinite from a single melt. The syenites are more enriched in LREE than shonkinites. The rocks are enriched in LREE compared to an OIB source, indicating melting of a hydrated lithosphere interacting with an asthernospheric garnet peridotite. Olivine with a composition of Fo80-90 further support asthernospheric origin, and pyroxenes indicate that depth of crystallization would have begun around 33 Km, indicating that the melt would have pooled at the base of the crust as it traveled, supporting base of the lithosphere origins. Rhyolite-MELTS modeling further supports differentiation within the sills as the formations of feldspars, biotite and hornblende did not occur until ~800m which would have allowed for fractional crystallization to occur, leading to the immiscibility process and resulting formation of syenite and shonkinite.