Geographic Variation of Rare Earth Fractionations in Plutonic Rocks Across the Peninsular Ranges Batholith, Southern California

Abstract

Rare earth element patterns of plutonic rocks (chondrite- normolized) across the Peninsular Ranges batholith vary systematically west to east, transverse to the long axis and structural trends and parallel to known asymmetries in petrologic, geochronologic, and Sr isotopic properties. The abundant tonolites in the western region are characterized by slight light REE enrichment relative to heavy REE. Eastward, patterns become more highly fractionated, showing light REE enrichment and heavy REE depletion. The slope of patterns at any geographic location are largely independent of rock type. However, locally, certain silica- saturated and undersaturated gabbros may show divergent trends. The relative fractionation among the middle and heavy REE indicates differentiation processes involving the observed major phases such as hornblende and plagioclase are not important in producing the basic geographic trends. They may be reflected in more local lithologic variations. The pronounced geographic zonation in REE character correlates strongly with Sr concentration, suggesting a common mechanism for their origin. The systematic nature of these fractionations, their correlation with previously reported Sr isotopic data, and their lock of correlation with lithology, argues against significant upper-crustal contamination with old cratonic components. These features appear to originate in heterogeneous deep-seated sources whose sampling has been selectively activated at a convergent plate boundary.