MAFIC INTRUSIONS AND THERMAL OVERPRINTS IN THE SILVERMINES GRANITE, ST. FRANCOIS MOUNTAINS

Recent studies indicate a thermal signature at ca. 400 Ma in the St. Francois Mountains of Southeast Missouri. The timing of this thermal activity is based on Ar-Ar isotopic studies as well as U-Pb isotopic ages. The study area for this research is along the East Fork of the St. Francis River at the Silver Mines Recreation Area where diabase dikes intrude the surrounding granite. Samples of the Silvermines granite and the diabase were collected for isotopic analyses. Gas ages were determined using Ar-Ar methodologies for the diabase groundmass as well as K-feldspar, biotite, and amphibole in the host granite. According to the Ar-Ar data collected, the total gas age for the diabase is 409.8 ± 9.1 Ma with a concordant spectrum age of 433.6 ± 17.1 Ma. There is no evidence of older gas ages from the large diabase dike. Ar-Ar data from the granite is more complex indicating Precambrian ages as well as thermal overprints at ca. 400 Ma with total gas ages ranging from 300 to 455 Ma. In addition to the Ar-Ar analyses, a total of 12 zircon crystals were recovered from the diabase rock sample, four of which were analyzed for U-Pb isotopic data. Two of the zircon fractions resulted in a ^207*Pb/^206*Pb ages of approximately 1475 Ma, similar to those published for the surrounding granite. The other two zircon fractions gave ^206*Pb/²³⁸U and ^207*Pb/²³⁵U ages of 345 and 377 Ma. These ages are significantly younger than the majority of the igneous activity in the region but may be associated with a series of Devonian diatremes to the north and east. The data presented in this study provide some constraints on the timing of thermal activity in the Silver Mines area that may be linked to economic mineralization or a larger-scale event associated with the structural resetting of K-feldspar minerals in the area.