Petrology and chemistry of the Green Acres gabbro complex near Winchester, Riverside County, California

Abstract

The Cretaceous Green Acres layered igneous complex, northeast of Winchester, California, is composed of a suite of olivine- and hornblende-bearing gabbros in the Peninsular Ranges batholith within the Perris tectonic block. A consistent mineral assemblage is observed throughout the complex, but there is considerable textural and modal heterogeneity. Both preclude a consistent set of principles based on appearance and mineralogy on which to delineate map units. Distinct changes in the chemistry of olivine, pyroxene, and hornblende, however, serve to define discrete mappable units, and the complex has been divided into five geochemical map units on this basis. Limited whole-rock data show the Green Acres complex is chemically comparable to other Peninsular Ranges batholith gabbroic rocks, and rare earth element (REE) concentrations and patterns are typical of magmas generated in convergent margin settings. For the complex as a whole, olivine is Fo 80–35 , plagioclase is An 100–64 , clinopyroxene is Wo 49–41 En 48–38 Fs 18–6 and Wo 36–26 En 65–42 Fs 30–8 , and orthopyroxene is Wo 5–0 En 78–42 Fs 50–21 , where Fo is forsterite, An is anorthite, Wo is wollastonite, En is enstatite, and Fs is ferrosilite. The Mg/(Mg + ΣFe) atomic ratio in hornblende ranges from 0.84 to 0.50. Magmatic lineations and modal and textural layering are prevalent throughout the complex. Mineral chemistry does not change in any systematic way within and between layers in any map unit. Although the strike of layering varies, in any map unit at any given location it is the same in all units irrespective of intrusive order. Thin dikes, typically late-stage hornblende gabbro, commonly intrude parallel to layering. The strikes of magmatic lineations and modal layers are consistent with the populations of strikes of fabrics in the metamorphic basement as well as tectonic features in surrounding, postgabbro granitic rocks. These relations imply that the regional state of stress at the time of gabbro emplacement played a role in layer formation in conjunction with thermal and hydraulic pressure perturbations.