Description
The Blair Valley tonalite outcrops just north of the La Posta pluton in the eastern zone of the Peninsular Ranges batholith (PRB). It is a 4-km long elongate body of rock that shows strong and steeply dipping foliation as compared to the relatively undeformed hornblende-biotite facies of the La Posta pluton. AU-Pb zircon age of93Ma was determined on a Blair Valley tonalite sample, which makes it essentially the same age as the 94Ma La Posta pluton. Petrographic studies of the Blair Valley tonalite show that it is primarily made up of plagioclase-quartz-biotite-hornblende with minor amounts of sphene-epidote-chlorite and oxide minerals. Comparison between previous petrographic studies of the hornblende-biotite facies of the La Posta pluton and the Blair Valley tonalite show that they are petrographically the same. Geochemical data further suggests that the Blair Valley tonalite is part of the La Posta pluton or at least a La Posta-type pluton. Variation diagrams of nearly all major element data show that the Blair Valley tonalite lies within the same linear trend as that displayed by the La Posta tonalite. An interesting relationship between the Blair Valley tonalite and the La Posta pluton is observed in trace element variation diagrams. In a strontium-silica variation diagram five of the eight samples analyzed plot away at an almost perpendicular angle to the main La Posta trend. A similar yet less obvious pattern is observed with zirconium and rubidium variation diagrams. This pattern suggests that some mixing may have occurred between the La Posta pluton and some other magmatic body. A variation diagram showing all of the PRB geochemical data along with that of the Blair Valley tonalite shows the Blair Valley tonalite merging into the field of older western zone tonalites. This suggests that the Blair Valley tonalite may represent a magmatic mix between the older western zone tonalites and the younger eastern La Posta pluton. Could this relationship represent an east-west magmatic connection in the Peninsular Ranges batholith?
