Zuni–Bandera volcanism, Rio Grande, USA — Melt formation in garnet- and spinel-facies mantle straddling the asthenosphere–lithosphere boundary Timothy J. Peters a, ⁎ , Martin Menzies a , Matthew Thirlwall a , Philip R. Kyle b a Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 OEX, England, United Kingdom b N.M. Bureau of Geology and Mineral Resource, N.M. Institute of Mining and Technology, Socorro, N.M. 87801, USA Received 17 October 2006; accepted 6 August 2007 Available online 16 August 2007 Abstract The Zuni–Bandera Volcanic Field (ZBVF) is a late-Neogene volcanic field on the boundary of the stable Colorado Plateau and the active Rio Grande Rift. Alkalic and tholeiitic magmas have erupted through Proterozoic continental crust with the tholeiitic magmas having undergone shallow-level fractional crystallization of olivine ± clinopyroxene ± spinel. The alkaline–tholeiitic lava flows lack elemental and isotopic correlations usually indicative of concomitant crust assimilation and fractional crystallization (AFC) and appear to have inherited their geochemistry from sub-Moho depths. Consideration of isotopic data, and modelling of REE and U–Th data constrains the mantle-melting history. The tholeiite basalts are primarily spinel-facies mantle melts (95–100% for spinel-facies), whereas the alkali basalts have a much higher proportion of garnet-facies mantle melts (15–25% for all samples except QBO 607 assumed to be 100% garnet-facies). The increased contribution from garnet-facies mantle in the alkali basalts is supported by U–Th isotopic data, where a shift towards 230 Th excess is observed. Since the lithospheric thickness increases from 45–55 km beneath the Rio Grande Rift, to 120–150 km beneath the Colorado Plateau, the ZBVF volcanic rocks are most likely a mixture of asthenosphere-derived (garnet-facies) alkali basalts and lithosphere-derived (spinel-facies) tholeiitic basalts. This is further supported by Sr–Nd isotopic data with alkali basalts having isotopic compositions similar to depleted-mantle values ( 87 Sr/ 86 Sr = 0.702986 to 0.70378, 143 Nd/ 144 Nd = 0.512712 to 0.512978), while the tholeiite basalts have higher 87 Sr/ 86 Sr (0.704725 to 0.706003) and lower 143 Nd/ 144 Nd (0.512379 to 0.512913) typical of basaltic magmas derived from ancient, LREE-enriched lithospheric mantle. Overall, melting is observed to be polybaric in nature, with mixing of melts over an extended depth range with the development of melting columns that span (a) the garnet-to spinel-facies phase boundaries in the mantle, and, (b) the asthenosphere–lithosphere boundary. © 2007 Elsevier B.V. All rights reserved. Keywords: Garnet facies; Spinel facies; Rio Grande rift; Colorado plateau; Zuni–Bandera; Rare Earth Elements 1. Introduction Late Cenozoic lithospheric extension in western North America was accompanied by considerable intra- plate basaltic magmatism, (e.g. Leeman, 1982; Ormerod et al., 1988; Fitton et al., 1991; Baldridge et al., 1991; Available online at www.sciencedirect.com Lithos 102 (2008) 295 – 315 www.elsevier.com/locate/lithos ⁎ Corresponding author. Department of Earth and Environmental Sciences, Vanderbilt University, VU Station B #351805, 2301 Vanderbilt Place, Nashville, TN 37235-1805, USA. Tel.: +1 615 322 2976; fax: +1 615 322 2138. E-mail address: Timothy.J.Peters@Vanderbilt.edu (T.J. Peters). 0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2007.08.006