DIFFERENTIATION AND CONTAMINATION IN THE NORTHERN PENINSULAR RANGES BATHOLITH, SOUTHERN CALIFORNIA

Almost 300 rock samples were collected by Baird et al. from the northern Peninsular Ranges Batholith (PRB) which is made up of the western Santa Ana block, the central Perris block, and the eastern San Jacinto block. These samples have been analyzed for major and trace elements by XRF and LA-ICP-MS respectively. CIPW norms were calculated from the major elements in each sample. The major elements were plotted on AFM, TAS, and R1-R2 diagrams, and the norms were plotted on QAP and Ab-An-Or diagrams. The various diagrams bring out different features and aid in combining the samples into approximately fifteen natural groupings. These plots along with the differentiation index (q+or+ab+ne) show the differentiation trends within and between groups.

Major and trace elements and Sr_i were used to compare the effects of differentiation and mixing/contamination from one regional group to another. Differentiation effects were correlated with the concentration of SiO₂, as well as the alkalis (Na, K, Cs), Mo, U, Y, Tb, and negatively with Al, the alkaline earths (Mg, Ca), and the transition elements (Ti, Co, V, Mn, Fe, Sc, Cr). Contamination effects were correlated with Sr_i, as well as the LREE, Nb, Ta, and Zr.

Samples from the southern part of the Santa Ana block with Sr_i < 0.7045 show an essentially uncontaminated differentiation trend from gabbro to granite. SW-to-NE plots across the northern PRB show the effects of increasing distance from the subduction zone and increasing arc maturity. These SW-NE trends are partly modeled by the Sr/Y ratio. The suture zone diagonally bisecting the Perris block divides the PRB into two sectors with a bimodal SiO₂ distribution on the west (mostly <62% and >69%) and an intermediate SiO₂ on the east (mostly 62-69%). The shear zone bisecting the San Jacinto block separates that block into two differentiation series with a higher plag-qtz on the west and a lower plag-qtz on the east. High magnetite/ilmenite ratios occur in high-SiO₂ samples along the shear zone. Samples from the complicated southeastern Perris Block were more difficult to group and showed more local than regional trends. The complications may be due to complex faulting, straddling the suture zone, and hydrothermal alteration. Major and trace elements from the older Santiago Peak volcanics in the northern Santa Ana block are included for comparison with the PRB rocks.