Rubidium-Strontium Fractionation Domains in the Peninsular Ranges Batholith and Their Implications for Magmatic Arc Evolution

Abstract

The northern 600 km of this Cretaceous batholith, comprised of hundreds of diverse plutons, appears to consist of 12-15 domains. Each domain is a region where rocks of all types (within our limited sampling) show a regular linear covariation of Rb and Sr concentrations. This regularity displays different concentration levels and a different slope and is accompanied by a distinct set of isotopic systematics in each domain. The domains do not disrupt the remarkable zonation of initial Sr ratios in the batholith (Early and Silver, 1973). No independent field or petrographic recognition of the Rb-Sr characterized entities has been mode. Very similar rocks occur in adjacent domains. Domains are equant or elongate NNW, up to 200 km long and 30 km wide, parallel to regional tectonic grain. Each domain is interpreted as a region of magmas sampled or differentiated from a mantle source reservoir possessing characteristic trace element levels and Sr isotopic properties. An apparent age of reservoir formation and isolation prior to fractionation and crustal emplacement and on apparent initial Sr ratio at the time of reservoir isolation can be derived for each domain. There is no strong correlation between reservoir apparent ages and initial ratios. Mixing systems involving older granitic crust with primitive mantle seem precluded. We identify similar domains characterized by R-Sr systematics in other batholithic and volcanic complexes. We infer these domains to be fundamental loci of chemistry and energy from which magmatic arcs are constructed.