PETROLOGY AND GEOCHEMISTRY OF THE 1.32 GA GRANITEVILLE GRANITE, ST FRANCOIS MOUNTAINS, SOUTHEAST MISSOURI

Precambrian magmatism in the St Francois Mountains, SE Missouri is marked by shallow intrusion of late-stage ring-pluton granitoid plutons. The shallow intrusion of these granites provides an opportunity to understand the chemical evolution of the Eastern Granite-Rhyolite Province and the timing of crystallization of shallow plutonic rocks. The historically economically significant 1.3 Ga Graniteville Granite, also known as Missouri Red Granite, is the youngest dated granite in the St Francois Mountains and represents the type-granite of the two-mica Graniteville-type granites of the anorogenic 1.33-1.3 Ga bimodal suite. These granites are unrelated to caldera forming magmatism, which ended in the area nearly 100 million years prior. Here, we present a focused petrologic and geochemical study of the Graniteville Granite. We use whole rock trace element geochemistry combined with mineral textures and petrography to explore the chemical variation and emplacement history of the pluton.

Rocks with prominent xenoliths of rhyolite contain 1-3 cm megacrysts of euhedral alkali feldspar, similar to alkali feldspar observed within the xenoliths. Similarly, a preferred orientation of euhedral alkali feldspar is present around xenolith margins. Quartz, plagioclase, biotite and hornblende are anhedral to subhedral, alkali feldspar is dominantly euhedral. Preliminary trace element geochemistry suggests the magma was anorogenic in origin with Nb contents ranging between 29-40 ppm, Ta contents between 3.0-4.5 ppm and Yb contents between 14.0-15.5 ppm. We suggest that the Graniteville Granite is heterogeneous in composition and mineralogy as a direct result of assimilation of the previously intruded Silvermines-type granites in the north, assimilation of the 1.48-1.45 Ga mafic rocks to the southeast and partial melting of the 1.48 Ga rhyolite suite.