COALIFIED TREE XENOLITHS WITHIN THE CRETACEOUS AMERICAN-BLACK LICK LAMPROITE, SOUTHWESTERN ARKANSAS

Arkansas is famous for its diamond-bearing lamproites. These lamproites intruded Early Cretaceous and older rocks approximately 100 Ma. Previous reports document that these lamproites include rare crustal xenoliths of shale, sandstone, quartzite, gabbro, amphibolite, granite, granodiorite and gneiss, and rare mantle xenoliths of dunite, harzburgite, websterite, eclogite and spinel and garnet lherzolite. The American-Black Lick diatreme intruded into the Early Cretaceous Trinity Formation. Of particular interest in this diatreme is the inclusion of coalified wood. The coalified wood is encased in brecciated lamproite. The coalified wood that we have observed is compressed, and some of the logs are over 22 cm wide, 35 cm long, and 13 cm thick. None of the coalified wood is enclosed in sedimentary rock matrix. The compressed and coalified wood that is encased in lamproite is very similar to compressed and coalified wood that occurs in the Trinity Formation. This leads us to suggest that coalified wood in the American-Black Lick lamproite are coal xenoliths derived from the Trinity Formation. It is possible that the coalified wood represents trees living at the surface that were somehow encased in the lamproite. However, at this time we think that the laterally compressed nature of the coalified wood best supports a Trinity coalified wood xenolith interpretation.