COMPARISON OF DIABASE DIKES FROM AMHERST COUNTY, VA, VIA THIN SECTION AND XRF ANALYSIS

Amherst County, in the Blue Ridge province of Virginia, is underlain by Mesoproterozoic high-grade metamorphosed plutonic rocks such as charnockite and granitic gneiss, that constitute the basement rocks to the Appalachians. Mafic igneous dikes called diabase have intruded the plutonic rocks in many locations. These dikes have two possible origins: 1) feeder dikes to the Neoproterozoic Catoctin Formation, formed during rifting of the supercontinent of Rodinia; or 2) Jurassic dikes associated with the rifting and breakup of the supercontinent of Pangaea.

The common diabase dikes intruding charnockite in the Amherst County area are medium-grained dark gray mafic rocks; however, a sample located in the community of Alto is a black fine- to medium-grained rock with unusually strong magnetic properties. Samples were collected from both the common dark gray diabase dike southeast of Alto and the black, magnetic diabase at Alto. Our goal is to describe the two types of dikes and use their characteristics to distinguish between a Neoproterozoic origin, in which the basalt would display obvious signs of low-grade metamorphism, vs. a Jurassic origin, in which the basalt would be relatively pristine and unmetamorphosed.

The dikes were compared using thin-section petrography. In contrast with the “control” dike, the magnetic dike has an abundance of opaque minerals (likely magnetite and/or ilmenite), accounting for the magnetism. Chlorite, a hydrous mineral, has altered or replaced the pyroxenes as well as many grains in the groundmass. In contrast, the control dike appears more pristine, with unaltered ortho- and clinopyroxenes in a finer groundmass. Geochemically, the magnetic dike is richer in iron and titanium than the control dike, and is also higher in silicon, aluminum, and potassium. Based on these characteristics, the magnetic dike has been affected either by low-grade metamorphism or by localized hydrothermal alteration.