Geochemical relationships between anorthosite and associated iron-rich rocks, Laramie Range, Wyoming

Abstract

Rocks enriched in iron oxide and mafic silicates are commonly present as minor volumes of Proterozoic anorthosite complexes. In the Laramie Range, Wyoming, anorthositic rocks, gabbros, and iron oxide ore have been chemically analyzed to determine if the spatial association is a result of genetic relationships between the rock types. Variations in abundances of REE, Th, Sc, and Sr in whole-rock and in mineral separates from anorthositic rocks provide evidence for the presence of trapped intercumulus liquid. Initial 87Sr/86Sr ratios in apatites separated from iron oxide ore (0.70535±0.00004) are analogous to initial 87Sr/86Sr ratios in Laramie Range anorthosite (0.70531 and 0.70537). In addition, REE abundances in calculated parental liquids for both anorthositic rocks and iron ore are similar, providing further evidence for a comagmatic relationship. Trace element and textural characteristics of spatially associated Laramie Range gabbros indicate that they are not mixtures of the trapped liquid and cumulus components which formed anorthositic rocks. It is suggested that gabbros are early differentiation products of a high-Al gabbroic magma which subsequently crystallized large volumes of plagioclase to produce the anorthosite massif.