ISOTOPIC CONNECTIONS IN THE GRANITE AND RHYOLITE PROVINCES OF THE SOUTHERN MIDCONTINENT

Two distinct granite and rhyolite provinces have been identified in the southern midcontinent based on U-Pb isotopic data for the timing of crystallization. Both provinces contain large volumes of virtually undeformed granite and rhyolite, some metamorphosed basement material, and relatively minor amounts of mafic material as diabase dikes and sills. The Eastern Granite and Rhyolite (EGR) province is characterized by granite and rhyolite with crystallization ages of 1470 ± 30 Ma. Exposures of the basement rock units in the EGR province are restricted to the St. Francois Mountains in southeastern Missouri. Similarly, the Southern Granite and Rhyolite (SGR) province is characterized by granite and rhyolite with crystallization ages of 1370 ± 30 Ma. The SGR province is exposed in the Arbuckle Mountains of southern Oklahoma. Although the timing of crystallization for the SGR is approximately 100 Ma after the EGR province, there are similarities in the Sm-Nd isotopic signatures for the units exposed in the Arbuckle Mountains and the St. Francois Mountains. Depleted mantle model ages for basement rocks within the Arbuckle Mountains range from 1.53 to 1.43 Ga with epsilon-t values of +3.2 to +4.1 including samples of granite, granodiorite, granitic gneiss, and amphibolite. Basement units within the St. Francois Mountains range from 1.55 to 1.43 Ga and +4.5 to +4.7 respectively including granite, granitic gneiss, trachyte, and diabase. A comparison of the Sm and Nd isotopic data also indicates similarities between the 147Sm/144Nd and 143Nd/144Nd ratios for the rock units in these separate exposures. Finally, Sm-Nd isotopic data from these two locations can be compared with mafic samples in the subsurface further to the west. Three diabase units analyzed within the Panhandle region of Texas provide similar results with depleted mantle model ages of 1.53 to 1.44 Ga and epsilon-t values of +3.1 to +4.4. When plotting the 147Sm/144Nd and 143Nd/144Nd ratios, these data points lie on the same trend as those from the Arbuckle and St. Francois Mountains. Therefore, even though the timing of crystallization is different for the EGR and SGR provinces, the source material may have been very similar across the southeastern margin of the continent during the Mesoproterozoic.