4:45 PM - 5:00 PM
[SIT28-12] Thermal and kinematic history of subcontinental lithosphere: Mantle xenoliths from Colorado plateau and Rio Grande rift
Keywords:Thermal and kinematic history of lithosphere, mantle xenoliths, geothermobarometer
We examined five samples from Thumb, three garnet peridotites and two spinel peridotites, and five spinel peridotite samples from Potrillo. All garnet grains in the Thumb samples shows extensive decomposition via reaction with olivine into aggregates consisting of spinel and pyroxenes (±plagioclase) with diverse size change, suggesting decompression. Clinopyroxene, orthopyroxene, and olivine in most of peridotites from Thumb and Potrillo shows chemical zoning, suggesting that all xenoliths recorded transient state. We extensively examined Ca zoning in olivine and orthopyroxene using a diffusion model. We clarified minerals, components, and their position in grains, representing a chemical equilibrium with specification of the corresponding events or stages for appropriate geobarometry. We were able to estimate pressure and temperature changes with specification of anchor “equilibrium stages” for Thumb in the Colorado plateau. The deepest xenolith (~3GPa) underwent decompression >>~100 years before the eruption with insignificant sign of heating or cooling. The shallowest xenoliths (~ 2GPa) underwent a decompression by ~2GPa from the garnet stability field accompanied by slow earlier heating and later cooling (~105 years), which was followed by a isobaric short heating event for >>~100 yrs before eruption. There are two distinctive xenoliths groups in Potrillo in the Rio Grande rift. One group giving higher pressure (~1.6GPa) in the spinel stability field underwent a heating event after a slow cooling event. The other group giving lower pressure (~0.3GPa) in the plagioclase stability field underwent a decompression and heating event.
We proposed the model of lithosphere-asthenosphere interaction beneath the Colorado plateau - Rio Grande rift area on the basis of our xenolith study. Before the activity of Navajo volcanic field including Thumb at ~25Ma, a thick cratonic lithosphere was subjected to active upwelling beneath the Colorado plateau, which heated the lithosphere to thin it by thermal and chemical erosion. The lithosphere thinning drove uplift of the entire lithosphere to from highland of the Colorado plateau. In later time before the magma activity of the Rio Grande rift ~0.08Ma, eastern periphery of the lithosphere beneath Colorado plateau, where the lithosphere was thinner, was streched to cause passive upwelling of asthenospheric mantle to form low-lying topography of the Rio Grande rift.