Description
The La Posta pluton is part of a great chain of similar intrusions that form from the eastern side of the Peninsular Ranges batholith from San Jacinto to southern Baja California. The La Posta-type intrusions are big. Outcrop areas range mostly from 200 to 1400km2. Large intrusions such as these pose important questions about how the magma is generated and emplaced into the crust and over what time period. The purpose of this study is to investigate compositional variations within the La Posta pluton. The focus is an apparently homogeneous and distinct domain within the La Posta in the northern part of the intrusion between the Carrizo overlook along McCain Valley Rd extending north to Aqua Caliente and the Elsinore Fault zone. La Pasta rocks here are homogeneous non-foliated and nearly free of mafic enclaves. Relative to outcrops immediately to the south, these rocks stand out at bouldery solid outcrops. The rocks also have a very consistent mineralogy and texture in hand sample. The rock is a medium grained leucocratic biotite tonalite/granodiorite. Accessory minerals are distinctive. Prismatic allanite crystals up to ~2cm in length are present in virtually every sample collected for this study. These are most easily spotted on big faces of rocks. Most of the samples for this study were collected in Bow Willow Canyon and Indian Valley. Strontium concentrations from these samples are uniformly high, around 600ppm. The distinctly higher Sr concentrations relative to other parts of the La Posta is consistent with the possibility that these rocks may represent a separate intrusive suite within the La Posta pluton. On the other hand, light rare earth element (LREE) concentrations within the sample suite are highly variable and span nearly the entire range of compositions displayed by samples around the entire La Posta pluton. LREEs are strongly concentrated in allanite. The variation in LREEs may be due to nonreprsentative whole rock powders which do no adequately account for allanite distributions in the rock.
