Mantle-Derived Basanite Features And Their Inclusions From The North Rim Of The Grand Canyon National Monument, Arizona

Abstract

Over one hundred and fifty basanite features are located in a 12.1 square kilometer (4.73 square mile) area on the north rim of the Grand Canyon National Monument, at the southern edge of the Toroweap Valley. Thirty-six of these were analyzed to determine their origin. The features, previously described as "Pressure blisters" (Hamblin and Best, 1970), are believed to be direct mantle orifices that have erupted under an ash or lava cover. Green and red colored peridotite xenoliths of varying dimensions are found enclosed within these features on the Toroweap Valley as well as on the adjacent cinder cone Vulcan's Throne. The unique red coloring, observed primarily in forsteritic olivine (Fo90-92), is believed to be a result of precipitation of a ferric rich phase within individual olivine grains. Results obtained from a theoretical single-pyroxene geothermobarometer (Mercier, 1980) suggest pressures of at least 15 kilobars and temperatures of at least 940°C, corresponding to a depth of origin of at least 52 kilometers for the Toroweap Valley features. A genetic model developed suggests that the basanite lava carrying green colored olivine originated from a magma chamber at least 52 kilometers below the ground surface, transected a shallower magma chamber in which red colored xenoliths are suspended and punctured the crust in the Toroweap Valley.