GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 78-2
Presentation Time: 9:00 AM-5:30 PM


HARPER, David R. and DEANGELIS, Michael T., Department of Earth Sciences, University of Arkansas at Little Rock, 2801 S. University Ave., Little Rock, AR 72204,

The Magnet Cove Alkaline Intrusive Complex contains several silica-undersaturated igneous rock types (e.g. nepheline syenite, ijolite, carbonatite) that form a concentric ring map pattern approximately 4.6 square miles in area. These rings, which are likely the result of several nearly contemporaneous magma injection events during the mid Cretaceous, become increasingly silica-undersaturated from rim to core, and have been previously mapped as separate geologic units. The outer ring contains nepheline syenite, the intermediate ring contains both garnet ijolite and garnet biotite ijolite, and the core contains carbonatite. Though the detailed modal mineralogy differs somewhat between the silicate (i.e. syenite and ijolite) rock types, they all have in common the presence of mica group minerals.

The purpose of this study is to examine and characterize the diversity of mica group minerals found in the silica-undersaturated rocks of Magnet Cove. Syenite and ijolite rock samples were collected from several locations within the complex, and thin sections were prepared for petrographic and electron microscope analysis using facilities and equipment at the UALR Rock Preparation Laboratory. Overall mineralogy from these samples indicates the presence of potassium feldspar, plagioclase feldspar, several feldspathoid minerals (nepheline, sodalite, altered leucite), amphiboles, pyroxenes (primarily aegerine and aegerine-augite), black Ti-bearing garnets (melanite, schorlomite), and various opaque minerals (e.g. magnetite, pyrite). Previously, micas in these rocks have been labeled simply as “biotite”. However, the ranges of color (yellowish-brown to bluish-green), crystal size (millimeter to several centimeters in diameter), and crystal habit (clusters of euhedral grains) in hand sample and variable pleochroism, ranging interference colors, reaction coronas, and zoning in thin section indicate a more interesting and complex chemical history.