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Earth and Environmental Sciences pres.

Smith Lecture: Origin of the Mesoproterozoic Igneous Rocks in the St. Francois Mountains, Missouri, USA

Warren Day, USGS

The Mesoproterozoic St. Francois Mountains (SFM) terrane of southeast Missouri is part of a large felsic igneous province that developed along the margin of the Laurentian craton. New geochemical, geochronological, and geophysical data are used to develop an improved model for the origin of the terrane. The terrane formed during two major episodes of igneous activity: (1) an older episode (ca. 1.48–1.44 Ga) of granodiorite to granite intrusive activity accompanied by felsic and subordinate basaltic to andesitic volcanism and associated subvolcanic intrusive activity and (2) a younger episode (ca. 1.33–1.30 Ga) consisting of bimodal granite and gabbro intrusion. The older rocks are predominantly ferroan, subalkaline with tholeiitic affinity and are enriched in Rb, Ba, Th, K, Pb, and light-REEs and depleted in Ta and Nb relative to primitive mantle. Trace element contents are similar to both within-plate, A-type and volcanic arc, I- and S-type granite compositions; however, the Nb and Ta depletions are characteristic of arc magmatism. Nd isotopic data suggest derivation from a mantle source or a mantle-derived juvenile (< 50 m.y.) crust. The younger granitic rocks are highly evolved with trace element abundances similar to within-plate granite.

We suggest that the SFM terrane involved melting of newly formed crust along the margin of the Laurentia as a result of mantle upwelling and underplating of tholeiitic basaltic magma at or near the base of the crust, possibly due to far field subduction processes or extension along the margin of the craton. The mantle-derived magmas generated partial melting and assimilation of the crust that subsequently fractionated in magma chambers at mid-crustal levels. Evidence of the underplating and incursion of the mantle-derived mafic magmas is seen in the regional gravity and aeromagnetic data, with the SFM underlain by dense, highly magnetic units at mid-crustal levels believed to be the mafic precursor magmas and(or) restite. Three-dimensional modeling of magnetic and gravity data coupled with results from a new magnetotelluric survey are yielding new insights into the crustal architecture of the terrane. Deep-seated magmatic systems can be resolved that we believe are the feeders for the near surface volcanic and shallow plutonic rocks and the coeval mineralizing systems. As well, a new high-resolution aeromagnetic survey acquired in August 2019 is yielding new insights as to the subtle complexities of the intrusions throughout the terrane.

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