INVESTIGATING THE AGE AND SIGNIFICANCE OF A RARE OCCURRENCE OF METAGABBRO IN THE OUACHITA MOUNTAINS, CENTRAL ARKANSAS

This study involves the analysis of a greenschist-facies metagabbro located in Central Arkansas, currently believed to be the oldest known rock in the state. A sample of this metagabbro was collected by the Arkansas Geological Survey and given to UA Little Rock for petrological and geochronological investigation. Samples were prepared into thin sections using the UALR Rock Preparation Lab and petrographically examined to determine mineral assemblage. Examination of this rock revealed primary titanaugite, albite, and magnetite with secondary alteration minerals including chlorite, epidote, phlogopite, hematite and small amounts of actinolite and pyrite. Chunks of metagabbro have also been crushed and processed using mineral separation techniques (i.e. jaw crusher, disc mill, water table, and magnetic separation) to collect zircons for geochronology analysis.

This occurrence of metagabbro has received little attention since its first description by Arkansas state Geologist John C. Branner in the late 19^th century, despite the rarity of occurrence of metagabbro in the Ouachita Mountains. Analysis by Morris and Stone (1986) provided a K/Ar whole rock age of 1025 ± 48 Ma; however, the age acquired is only a general estimate, given limitations of the K/Ar dating method. The mineral separation process and the collection of zircons from his rock allows for the acquisition of more precise U/Pb age dates. A modern assessment of the age of the rock, combined with petrological and geochemical analysis, provide insight into the origin and tectonic setting of the metagabbro and the Ouachita Mountains as a whole.