MAGMA-WALL ROCK INTERACTION IN THE ST FRANCOIS MOUNTAINS, MISSOURI: UNRAVELLING MAGMATIC ARCHITECTURE USING MICROSTRUCTURES AND TRACE ELEMENT GEOCHEMISTRY

Upper crustal granitic intrusions display their interaction with wall rock in a variety of ways preserving direct evidence of emplacement, magma flow and assimilation. The St Francois Mountains in southeast Missouri provide excellent exposure of granite-wall rock contacts, which preserve boundary interactions and crystal exchange between the granite and either rhyolite or crystalline basement material. The 1.49 Ga Silvermine-type Slabtown and Knoblick granites intrude the Priday Rhyolite and Grassy Mountain Ignimbrite, respectively. The Slabtown Granite is a porphyritic fine grained hornblende-biotite granite. The Knoblick Granite is a coarse grained granite with clusters of biotite and hornblende exposed. Both granites are intruded by younger Butler Hill-Breadtray-type granites and overlain by the LaMotte Sandstone. Additionally, diabase dikes of the Silvermines Dike Swarm, intrude the 1.48 Ga Silvermines Granite. The dikes contains zircons and alkali feldspars with ages of 1.47 Ga and 1.19 Ga, respectively.

Here, we present a textural and microstructural analysis of intrusion contacts and new U-Pb zircon ages of the rhyolites combined with feldspar and glass trace element chemistry of xenoliths and both the host wall rock and intruding magma. We seek to determine the effect of wall rock melting on the magma chemistry at the boundary and seek to understand the melting processes during intrusion. Observations of both emplacement microstructures and exposed xenoliths in both the granites and the diabase suggest differing thermal gradients for each of the three systems. Equilibration between host rock and intruding magma is obtained at different times and is directly related to the timing of intrusion and thickness of the intruding body. Xenoliths and disaggregated crystals of the rhyolite observed in the Slabtown and Knoblick granites along with irregular and gradational contacts suggest a longer equilibration times in a sustained thermal state. Shear microstructures and changes in glass composition away from the contact between the diabase and the Silvermines Granite suggest that thermal equilibrium was only obtained through rapid solidification of a chilled margin in the diabase which prevented extensive crystal assimilation into the interior of the diabase dike.