Some Investigations of the Roraima Formation Intrusives in British Guiana

Abstract

A traverse of a noritic gabbro sill (c. 1,000 feet thick) intruding the Roraima Formation showed no cryptic layering, but small-scale fractionation throughout the sill is indicated by the strongly-zoned plagioclase (An_80→40), and orthopyroxene (En_79→54) with outer rims of inverted pigeonite. Olivine (Fo₈₀) is only present in the chilled margin.New evidence on the age of the granites underlying the Roraima Formation is given by the discovery of post-Roraima felsite dykes, with high-quartz phenocrysts.