AGS / Geology / Stratigraphy / Igneous Rocks
Summary of Igneous Rocks
The majority of exposures of igneous rocks in Arkansas are present in Pulaski, Saline, Hot Spring, and Garland Counties. Scattered small peridotite-carbonatite dikes, breccia pipes, and sills are exposed in the Arkansas Valley, and a single outcrop of igneous rock with ultramafic affinities is known in the Boston Mountains. The total mapped area of the various exposures encompasses less than 15 square miles.
The largest of these bodies is a partially buried batholith composed of varieties of nepheline syenite that was exposed by erosion in both Pulaski and Saline Counties (Granite Mountain and Saline County intrusions). The Magnet Cove ring-dike and the Potash Sulphur Springs complex in Hot Spring and Garland Counties, respectively, are more mafic in composition, contain a greater variety of rock types, and had different structural styles of intrusion than the Pulaski/Saline County batholith. Several small explosive vents containing diamondiferous lamproite breccia and magmatic lamproite (formerly termed kimberlite breccia and peridotite, respectively) are present in Pike County. Numerous mafic and alkalic dikes are present in the eastern Ouachitas.
All of the rocks mentioned above that have been dated by stratigraphic or isotopic methods are Cretaceous in age, ranging from 87 to 105 million years old. Small bodies of metagabbro and serpentine/soapstone exposed in Saline and Pulaski Counties represent altered ultramafic rock units which date from the late Precambrian Era (~one billion years). These bodies were emplaced into the Ouachita Mountains sedimentary sequence as solid masses during the Paleozoic Era.