SPACE-TIME EVOLUTION OF MESOPROTEROZOIC GRANITES FROM THE ST. FRANCOIS MOUNTAIN TERRANE, SOUTHEAST MISSOURI: INSIGHTS FROM HORNBLENDE AND ZIRCON

The midcontinent Granite-Rhyolite province (1.5-1.34 Ga) refers to a region in the central United States that is characterized by the presence of Mesoproterozoic-aged granite and rhyolite rocks. The Eastern Granite-Rhyolite Province (EGRP) formed primarily in eastern Laurentia approximately 1.5 -1.44 Ga. The Southern Granite-Rhyolite Province (SGRP) formed in western Laurentia between 1.44 -1.34 Ga. At the intersection of these two provinces are the St. Francois Mountain terrane, which is the only basement exposure where the two provinces overlap. The granites are anorogenic and geochemically resemble an intracratonic setting formed during the breakup of the supercontinent Columbia. However, the physicochemical conditions (pressure, temperature, and oxygen fugacity (ƒO2) remain unknown. Here, we measure major and trace element contents in hornblende in polished thin sections of four EGRP granite samples that are related to a single caldera forming event. We seek to determine the physicochemical conditions of magmatic emplacement of the 1.463 ± 0.010 Ga “Ring-type” Silvemine granite plutons associated with caldera forming volcanism in the St. Francois terrane. Combined with U-Pb zircon ages and chemistry, we use this data to determine the emplacement conditions of the granites and compare these conditions to the younger granites of the SGRP. We also model the compositions of the melts in equilibrium with hornblende and zircon to determine the magmatic evolution of the granitic melts.