THERMAL GRADIENTS AND MAGMA HOST ROCK EQUILIBRATION INFERRED FROM INCLUSIONS IN THE RING PLUTONS OF THE BUTLER HILL CALDERA, ST. FRANCOIS MOUNTAINS, MISSOURI

The Mesoproterozoic St. Francois Mountains contain at least three caldera complexes (Lake Killarney, Butler Hill, and Taum Sauk) and associated volcanic rocks, and four distinct types of granitoid intrusions as part of the Eastern Granite-Rhyolite province along the eastern margin of Laurentia. These rocks represent two pulses of magmatic activity: (1) an older 1.50-1.44 Ga episode of caldera forming volcanism and associated granitic intrusion; and (2) a younger 1.33-1.28 Ga episode of bimodal intrusions. Caldera-forming volcanic rocks are generally rhyolite ash flow tuffs with interbedded volcaniclastic sediments and volcanogenic breccias. Intrusive rocks associated with the first phase of magmatic activity are divided into four sub-types: (1) the Butler Hill/Breadtray-type, (2) the Silvermines-type, (3) the Slabtown-type granitoids; and (4) mafic rocks of the Silver Mines Mafic Group. The younger bimodal gabbroic- granitic intrusions are represented by the Graniteville-types granites and the Skrainka Mafic Group.

Here, we present a preliminary summary of geochronological, geochemical, textural, and microstructural data of two ring plutons of the Butler Hill caldera, the Silvermines and Knoblick granites. We present new U-Pb zircon ages of the granites and rhyolites combined with feldspar, hornblende and glass trace element chemistry collected by LA-ICPMS. Our focus of this study is on the inclusions and xenoliths hosted within the granites to understand the effect of wall rock melting on the magma chemistry at the boundary and seek to understand melting processes during intrusion. We suggest the thermodynamic state and equilibration between host rock and magma is obtained at different times for the two granitic intrusions. Timing of equilibration is directly related to the size and shape of the intrusion, and the presence of recharge indicators such as quenched magmatic inclusions or mixing zones within the body.