MAFIC INTRUSIONS IN THE ST. FRANCOIS MOUNTAINS OF SOUTHEAST MISSOURI: A STUDY OF THE PETROLOGY, GEOCHEMISTRY, GEOCHRONOLOGY, AND TECTONIC EMPLACEMENT

Basement rock units exposed in the St. Francois Mountains of southeast Missouri represent a major episode of crustal growth during the Precambrian. These unique exposures provide a window into the continental crust showing composition and configuration. Igneous rocks exposed in this area include several different types of granite, silicic volcanic rocks, intermediate volcanic rocks, granitic gneiss, and small mafic intrusions. The purpose of this study was to examine the mafic intrusions and obtain chemical, isotopic, and petrologic data in order to understand their origin and timing of emplacement. A total of six igneous samples were collected and analyzed including four diabase dikes, a trachyte, and granite. Four of the samples were mafic in composition with silica content ranging from 43.7 to 48.7%, one was intermediate at 58.8%, and the last felsic with 69.7% SiO2. Petrographic analyses of the mafic samples indicate that they contain plagioclase as subhedral laths (50-60%) that are partially weathered and opaque minerals including magnetite and hematite (5-15%). The trachyte exhibited small phenocrysts of alkali feldspar contained within a very fine groundmass. Sample SF-M1 exhibits spherical structures often containing hematite and commonly mantled with magnetite. Sm-Nd isotopic analysis of each of these samples resulted in a range of epsilon (t) values and Nd-model ages. Epsilon (t) values and Nd-model ages are calculated assuming a crystallization age of 1470 Ma (million years) based on U-Pb ages that have been previously determined for granite and rhyolite units within the area. Epsilon values at the proposed time of crystallization are strongly positive ranging from +3.69 to +4.86. Model ages are determined using the depleted mantle model curve and range from 1.43 to 1.56 Ga (billion years). Since the epsilon (t) values are high, they suggest a relatively juvenile source for the magma. The Nd-model ages, interpreted as the crustal residence age, are approximately the same as those of the felsic units and therefore, it is possible to conclude that the intrusion of these mafic dikes occurred shortly after the eruption of the volcanic units and the emplacement of the associated granitic plutons. The emplacement of mafic dikes into granitic crustal rocks suggests an extensional tectonic environment.