Gila Bend Mts, Maricopa Co., Arizona, USA

Ref.: Lopez, Robert, and Cameron, K. L. (1997) High-Mg andesites from the Gila Bend Mountains, southwestern Arizona; evidence for hydrous melting of lithosphere during Miocene extension. Geological Society of America Bulletin, July 1997: 109(7): 900-914.

Garvie, L.A.J. (2003) Decay-induced biomineralization of the saguaro cactus (Carnegiea gigantean). American Mineralogist: 88: 1879-1888.

Grant, Raymond W., Bideaux, R.A., and Williams, S.A. (2006) Minerals Added to the Arizona List 1995-2005: 4, 5, 6.

The Gila Bend Mountains are a mountain range with a maximum altitude of 1,581 feet (481.89 meters) above sea level.

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. The high-Mg andesites erupted near the end of Miocene extension in the Gila Bend region. 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. (This record excerpt provided courtesy of AGI/GeoRef).