Petrology and geochronology of 1.48 to 1.45 Ga igneous rocks in the St. Francois Mountains terrane, southeast Missouri

Abstract

The igneous geology of the St. Francois Mountains terrane in southeast Missouri is dominated by the products of 1.48 to 1.45 billion year old volcanic and plutonic magmatism but also includes volumetrically minor, compositionally bimodal contributions added during plutonism between 1.34 and 1.27 billion years ago. The 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are bimodally distributed between volumetrically dominant felsic rocks and volumetrically minor rocks with mafic to intermediate compositions. All of these rocks are ferroan, which like most of their trace element abundances, suggests a genesis associated with farfield intraplate extensional tectonism and decompression-related magmatism. The diversity of compositions among 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane probably reflects mixtures of mantle-derived mafic inputs and low-degree partial melting of more evolved crustal protoliths. Newly determined ages define essentially continuous magmatism during the 30-million-year period between 1.48 and 1.45 billion years ago. The products of this magmatism are essentially coeval, whether intrusive or extrusive or having mafic, intermediate, or felsic compositions. In addition, the iron oxide-apatite (for example, Pea Ridge) and likely the iron oxide-copper gold (Boss) deposits in the St. Francois Mountains terrane have ages coincident with this magmatic episode. Spatial and temporal relations between 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane and the mineral deposits they host suggest the associated magmatic and mineralization processes are also genetically related. Geochemical, petrographic, geochronologic, and terrane-wide physical characteristics of the 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are consistent with an origin involving extension well inboard from the margin of the Laurentian craton, associated mantle upwelling, lower crustal melting in response to mantle-derived thermal inputs, and mixing of mantleand juvenile lower crustal-derived melts. Significant major and trace element compositional dispersion characteristics of these rocks likely reflect midcrustal magma reservoir fractionation of their principal rock-forming minerals. The resultant magmas constitute a series of variably hybridized reservoirs, emplaced at upper levels in the crust, that form a series of plutonic and associated eruptive products. Introduction Mesoproterozoic igneous rocks (fig. 1) constitute the basement beneath a relatively thick (greater than 400 meters [m]) Paleozoic sedimentary rock sequence in the southern midcontinent region of the United States in the St. Francois Mountains terrane of southeast Missouri (Kisvarsanyi, 1981; Tolman and Robertson, 1969). The 1.48 to 1.45 giga-annum (Ga) rocks of southeast Missouri are unique in hosting coeval and cospatial iron oxide-apatite ± rare earth element (IOA) and iron oxide-copper ± gold ± cobalt (IOCG) mineral deposits. These igneous rocks represent a significant component of a broadly early Mesoproterozoic igneous terrane, intruded by a middle Mesoproterozoic bimodal sequence of granite and gabbro at about 1.3 Ga in southeast Missouri, that crops out extensively in North America (Anderson and Bender 1989; Bickford and others, 2015; du Bray and others, 2018a). The petrogenesis and tectonic regime that prevailed during Mesoproterozoic magmatism in the United States remain elusive. Our study focuses on the characteristics of early Mesoproterozoic igneous rocks in the St. Francois Mountains terrane in order to establish their petrogenesis and potential role in formation of associated IOA and IOCG deposits of the Missouri metallogenic province (Kisvarsanyi and Proctor, 1967). Recent investigations by Bickford and others (2015), du Bray and others (2018a), and Frost and Frost (2008, 2011) define the conditions favorable for ferroan granitoid, broadly within-plate magmatism that extends across the North American craton and Ayuso and others (2016) and Day and others (2016) review the factors that fostered approximately 1.4 Ga magmatism specifically in the St. Francois Mountains terrane. However, the tectonic setting that prevailed during Mesoproterozoic magmatism in the St. Francois Mountains terrane is uncertain and relations between 2 Petrology and Geochronology of 1.48 to 1.45 Ga Igneous Rocks in the St. Francois Mountains Terrane, Southeast Missouri