Miocene andesites and basaltic andesites from the eastern Gila Bend Mountains have high Mg# values (most 66–72), high concentrations of Ni and Cr, and olivine phenocrysts as Mg rich as Fo90. Cores of euhedral olivine phenocrysts in some andesites are in equilibrium with the whole-rock Mg# values, and the compositions of chromian spinel inclusions in olivine phenocrysts are similar to those from boninites and other primitive magnesian silicic volcanic rocks. The Gila Bend lavas are interpreted to have evolved from a variety of primary mantle-derived melts, some as silicic as approximately 56% SiO2. All the lavas have strong lithospheric geochemical characteristics: Ba/Nb ratios range from 60 to 170, initial 87Sr/86Sr is from 0.7063 to 0.7073, and ϵNd is from −5 to −8. Previous experimental studies have established that the generation of primary andesitic magmas from mantle peridotite requires low-pressure (<15 kbar), hydrous conditions, and a refractory source. The high-Mg andesites erupted near the end of Miocene extension in the Gila Bend region. Because of their primitive nature and tectonic setting, the Gila Bend lavas can be used to test three general types of models that have been proposed recently to relate magmatism, extension, and lithospheric geochemical character of mid-Cenozoic volcanic rocks of the Basin and Range. Models that require crustal contamination of the magmas or that produce the magmas by melting of basaltic material within mantle lithosphere are unsuitable for the origin of the Gila Bend lavas. However, some elements of a model involving hydrous melting of mantle lithosphere during extension are consistent with the chemistry and tectonic setting of the lavas. The high-Mg andesites are interpreted to reflect hydrous melting of harzburgite near the base of mantle lithosphere at unusually low pressure. The low pressure was the result of tectonic thinning of the lithosphere during extension, and the source of the volatiles was most likely contemporaneous subduction. Although the Nd isotopic composition of the Gila Bend lavas originated from Proterozoic mantle lithosphere, the Pb isotope ratios mainly reflect the influence of Mesozoic and Cenozoic subduction.

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