Carbonatite associated with ultramafic diatremes in the Avon Volcanic District, Missouri, USA: Field, petrographic, and geochemical constraints Ethan J. Shavers a,b , Abduwasit Ghulam a, ⁎, John Encarnacion b , David L. Bridges c , P. Benjamin Luetkemeyer b a Center for Sustainability, Saint Louis University, 3694 West Pine Mall, Saint Louis, MO 63108, USA b Department of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Blvd, Saint Louis, MO 63108, USA c Missouri University of Science and Technology, Rolla, MO 65409, USA abstract article info Article history: Received 9 October 2015 Accepted 6 February 2016 Available online 18 February 2016 Here we report field, petrographic, and geochemical analyses of the southeast Missouri Avon Volcanic District intrusive rocks and present the first combined textural and geochemical evidence for the presence of a primary magmatic carbonatite phase among ultramafic dikes, pipes, and diatremes of olivine melilitite, alnöite, and calciocarbonatite. The δ 13 C VPDB values measured for primary calciocarbonatite as well as carbonates in olivine melilitite and alnöite rocks range from -3.8‰ to -8.2‰, which are within the typical range of mantle values and are distinct from values of the carbonate country rocks, 0.0‰ to -1.3‰. The carbonate oxygen isotope com- positions for the intrusive lithologies are in the range of 21.5‰ to 26.2‰ (VSMOW), consistent with post- emplacement low temperature hydrothermal alteration or kinetic fractionation effects associated with decompression and devolatilization. Metasomatized country rock and breccia-contaminated igneous lithologies have carbonate δ 13 C VPDB values gradational between primary carbonatite values and country rock values. Unaltered sedimentary dolomite breccia and mafic spheroids entrained by calciocarbonatite and the lack of microstratigraphic crystal growth typical of carbonate replacement, also exclude the possi- bility of hydrothermal replacement as the cause of the magmatic-textured carbonates. Rare earth element (REE) patterns for the alnöite, olivine melilitite, and carbonatite are similar to each other with strong light REE enrichment and heavy REE depletion relative to MORB. These patterns are distinct from those of coun- try rock rhyolite and sedimentary carbonate. These data suggest that rocks of the Avon Volcanic District represent a single ultramafic-carbonatite intrusive complex possibly derived from a single mantle source. © 2016 Elsevier B.V. All rights reserved. Keywords: Calciocarbonatite Alnöite Olivine melilitite Avon Missouri Diatreme Lamprophyre 1. Introduction There is a recognized genetic correlation between alkaline, ultra- mafic, and carbonatite (AUC) rocks and intracontinental thermal per- turbations, especially related to failed rifts and mantle upwelling zones (Heaman et al., 2004; Jelsma et al., 2009). Such conditions allow for small degree melting of deep, and often metasomatized, subcratonic mantle. Small degree melting may concentrate incompatible elements in the magmas leading to economically important deposits. These types of intrusions are also the main conveyors of diamonds to Earth's surface. North America has several failed rift zones and possible hot spot tracks (Chu et al., 2013) associated with several enigmatic and poorly- studied AUC intrusions (Fig. 1a). The southern midcontinent region is cut roughly west to east by the Lower Midcontinent Rift System which, from west to east, includes the Southern Oklahoma Aulacogen, Reelfoot Rift, Rough Creek–Shawneetown Fault Zone and Rome Trough (Lowe, 1985). Discovery of ultramafic intrusive complexes in North America is ongoing (Baranoski et al., 2007; Pell et al., 2013). These discoveries are im- portant due to increasing strategic need for rare metals that are often enriched in AUCs. Around the Reelfoot Rift and Rough Creek Fault Zone, intrusives with possible economic potential, be it diamonds at Prairie Creek Arkansas (Southern Oklahoma Aulacogen) or fluorite near Hicks Dome (Fig. 1b), have received some attention from researchers (Morris, 1987; Plumlee et al., 1995). The greatest volume of work done on this re- gion has been related to the Reelfoot Rift, as well as other nearby fault zones and their reactivation (Crone et al., 1985; Csontos et al., 2008; Dart and Swolfs, 1998; Marshak and Paulsen, 1996; Nelson et al., 1985; Pratt, 2012). Carbonatite complex research, in general, is a young discipline. Following the important work on the recognition of carbonatites as pri- mary magmas in the 1960s (Wyllie and Tuttle, 1960), there has been progress in the understanding of carbonatites (Deines and Gold, 1973; Heinrich, 1966: Woolley and Kjarsgaard, 2008), yet much remains un- clear. Mitchell (2005) postulates that one reason for this is the “consid- eration of all carbonatites as a single rock-type or of a derivation from a Lithos 248–251 (2016) 506–516 Abbreviations: AUC, alkaline, ultramafic, and carbonatite; AVD, Avon Volcanic District. ⁎ Corresponding author. E-mail addresses: eshaver1@slu.edu (E.J. Shavers), awulamu@slu.edu (A. Ghulam), encarnjp@slu.edu (J. Encarnacion), dbridges@mst.edu (D.L. Bridges), luetkepb@slu.edu (P.B. Luetkemeyer). http://dx.doi.org/10.1016/j.lithos.2016.02.005 0024-4937/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos