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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 (Ph.D. thesis (1984) 588). 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 may have postdated silicic activity. The rocks of both suites have a number of incompatible element indices typically associated with subduction zone environments. This suggests that the voluminous silicic magmatism of the St. Francois Mountains, and contemporaneous portions of the Granite–Rhyolite Provinces, originated during subduction along an active continental margin or during post-subduction (orogenic) extensional collapse. We favor the former tectonic setting, but an active margin in which extensional stresses were prevalent in the overriding plate. |
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