TECTONIC INSIGHTS PROVIDED BY MESOPROTEROZOIC MAFIC ROCKS OF THE ST. FRANCOIS MOUNTAINS, SOUTHEASTERN MISSOURI

Although Mesoproterozoic silicic rocks are predominant in the St. Francois Mountains of southeastern Missouri, basalts, basaltic andesites and their plutonic equivalents are not uncommon. These mafic rocks fall into two distinct petrologic suites as first discerned by Sylvester (1984). One, the Silver Mines suite, consists of mafic rocks formed contemporaneously with the voluminous silicic rocks. The second suite, the Skrainka suite, originated from mafic magmatism that postdated silicic activity. Even though the two suites have significant petrological and geochemical differences, they do share a number of incompatible element indices associated with subduction zone environments. This suggests that the voluminous silicic magmatism of the St. Francois Mountains, and of the much larger Eastern Granite-Rhyolite Province to which it is but a small part, originated during subduction along an active continental margin or during post-subduction (orogenic) extensional collapse. We tentatively favor the latter scenario.