THE GEOCHEMISTRY OF THE CHALK HILLS MEMBER OF THE CATAHOULA FORMATION OF LOUISIANA AND TEXAS, U.S.A

The Catahoula Formation is found throughout eastern Louisiana and Texas and is composed of layers of fine to coarse-grained sandstones, shale, and volcanic ash. This research focuses specifically on providing a petrographic and geochemical description of the Chalk Hills Member of the Catahoula Formation, which is a volcanic ash. The ash is composed of bubble shards, the majority being hydrated with a minority remaining glassy, and less than five percent subhedral to euhedral sanidine crystals. A TAS plot indicates that the ash is dacitic in origin. Trace element concentrations of the ash are: Ba (68.3 ppm), Rb (141ppm), Th (71.77 ppm), K (6.29 ppm), Nb (53.54 ppm), Ta (8.96 ppm), La (108.4 ppm), Ce (227.3 ppm), Sr (31 ppm), Nd (106.58 ppm), Sm (23.48 ppm), Zr (864 ppm), Hf (27.53 ppm), Ti (0.2 ppm), Tb (3.67 ppm), Y (110.1 ppm), and Yb (9.83 ppm). Spider diagram comparisons with EMORB, OIB, and upper and lower crust show strong negative K, Sr, and Ti and positive Th anomalies. They also indicate that the Catahoula ash is close to OIB in composition (ash/OIB ratio between 1 and 10), with the exception of the negative and positive anomalies mentioned above. This may indicate a magmatic plume-sourced eruptive center as the possible source of the ash. Rare earth concentrations are: La (108.4 ppm), Ce (227.3 ppm), Pr (28.5 ppm), Nd (106.58 ppm), Sm (23.48 ppm), Eu (.97 ppm), Gd (22.67 ppm), Tb (3.67 ppm), Dy (20.66 ppm), Ho (4.06 ppm), Er (10.5 ppm), Yb (9.83 ppm), and Lu (1.37 ppm). These values are also very close to OIB. The REE trend shows a strong negative Eu trend, which, coupled with the negative K and Sr anomalies in the spider diagrams, suggests a strong effect of plagioclase/sanidine crystallization on the evolution of the magma. In addition, a comparison of the ash/OIB REE trends against elemental partitioning in rhyolites (geochemically close to dacites) produces trends that nearly mirror each other, suggesting that plagioclase fractionation was the dominant process in the final magmatic evolution of the sample.