Description
The eastern portion of the Peninsular Ranges batholith (PRB) in San Diego County and northern Baja California is dominated by the La Posta pluton which is characterized by a relatively homogeneous composition of rocks broadly similar in composition to trondhjemite-tonalite-granodiorite (TTG) suites of Archean cratons (Kimbrough et al., 2001). The La Posta pluton has an enormous outcrop area and is divided into western ( ~ 1200 km2) and eastern ( ~400 km2) outcrops areas that were emplaced at different crystal depths, 2.3 ± 0.2 kbar and 4.7 ± 0.2 kbar, respectively (Symons et al., 2003). The purpose of this study is to determine whole rock major and trace element composition of widely spaced La Posta pluton samples to search for chemical variations and to see if the data can help distinguish between prevailing theories about emplacement of large continental margin batholiths. This study focused on the larger western "lobe" of the pluton in Baja California. This area was a prime choice due to its close proximity, good accessible outcrops, and lack of analytical data. Most work up to now has been done in the Southern California portion of the pluton. Forty-one samples were analyzed. There is significant compositional variation that may reflect different depths for the source of the magmas as well as shallow level crystal fractionation processes. The Baja California samples are also characterized by lower strontium contents relative to samples north of the border along Interstate 8 which suggest regional variations in the composition of the pluton not previously reported. Incremental filling of the magma chamber by separate batches of magma may be required. Local planar discontinuities in the pluton defined by differences in grain size and modal composition may reflect incremental emplacement of magma batches.
