Zuni–Bandera volcanism, Rio Grande, USA — Melt formation in garnet- and spinel-facies mantle straddling the asthenosphere–lithosphere boundary

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 230Th 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 (87Sr/86Sr = 0.702986 to 0.70378, 143Nd/144Nd = 0.512712 to 0.512978), while the tholeiite basalts have higher 87Sr/86Sr (0.704725 to 0.706003) and lower 143Nd/144Nd (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.