AGE, PETROLOGY AND GEOCHEMISTRY OF A LATE DEVONIAN LONESTONE FROM THE UPPER OHIO SHALE (CLEVELAND MBR.) EAST-CENTRAL KENTUCKY

The provenance of a large granitic lonestone, approximately 1.7 x 1.3 x 0.6 m in size, weighing nearly 3 tons, and recently found embedded within the uppermost Ohio Shale, has garnered much attention. Considering its size, weight, and exotic lithology, the boulder was probably emplaced as an ice-rafted, glacial dropstone. We suggest that the boulder was transported in an iceberg, which originated from alpine glaciers that formed in Neoacadian mountains some 500 km east of its current location. For this to have happened, glaciers must have extended from Neoacadian mountains westward to sea level with icebergs calving off along their western edges. Biostratigraphically, the lonestone-containing shales can be correlated with several probable tillites (diamictites), exposed in the lower portion of the Rockwell Formation in Maryland and Pennsylvania. Palynomorphs collected from shales around the lonestone suggest occurrence in the late Famennian Retispora lepidophyta-V. nitidans (LN) Miospore Biozone. This occurrence correlates well with material collected from the tillites, which occur in the (LN) and preceding R. lepidophyta-Indotriradites explanatus (LE) biozones, both late Famennian in age.

What is not so well known about the boulder is its petrographic and geochemical character. Dated zircons from the boulder indicate an early Neoproterozoic (late Grenvillian; 890±24 Ma) emplacement with an Early Ordovician, Taconian overprint (474±5 Ma). Moreover, petrographic examination of cores from this bolder shows it to be a biotite granite containing quartz, alkali feldspar (microcline), and biotite, which have been slightly metamorphosed into a meta-granitoid rock. Major-element and trace-element geochemistry with a CIPW norm (Smith, pers. commun, 2011) suggests that the boulder is specifically a monzogranite. Several trace-element plots also indicate that the boulder was derived from an anorogenic (Type A), or a within-plate, granite (WPG). This boulder not only reflects the diversity of tectonic environments present during the Grenville orogen, but also the diversity of rocks that are no longer present in the Appalachian Mountains. Were it not for these diamictites and the occasional lonestone like the one described here, any record of these lithologies and events would be completely lost to us.