XENOLITH EVIDENCE FOR ISOTHERMAL DECOMPRESSION IN THE CENTRAL ARIZONA LOWER CRUST PRIOR TO MID-TERTIARY DETACHMENT FAULTING

A suite of mid-Tertiary, xenolith-bearing, alkalic volcanic rocks erupted across central Arizona between 30 Ma and 22 Ma. Early eruptions slightly precede the earliest phase of mid-Tertiary metamorphic core complex denudation and lithospheric extension in western/southern Arizona (27-25 Ma). The eruptive centers along the NE edge of the Basin and Range province and in the central Transition Zone lie in the hanging wall of the detachment fault systems that denuded the Arizona/SE California metamorphic core complexes. Lavas are highly potassic (up to 6 % Wt. K₂O) and rich in incompatible trace elements (Ba up to 1900 ppm and Sr up to 1250 ppm), suggesting an enriched mantle melting source. The dominant petrology of xenoliths across the fields is mafic eclogites that are variably retrograded to amphibolite facies. Partially and completely retrograded eclogite xenoliths contain abundant disequilibrium textures, including symplectitic rims around garnet that consist of pargasitic amphibole, plagioclase, and Fe-Ti oxides, symplectitic intergrowths of plagioclase and diopside that replace omphacite, and ilmenite exsolution lamellae and rims in rutile. These textures have been associated with decompression metamorphism in ultra-high pressure metamorphic belts. Mineral chemistry analyses of amphibole, plagioclase, and hornblende indicate local equilibrium temperatures and pressures of ~750-900°C and ~9-10 kb. Prior pyroxene and garnet thermobarometry on the primary eclogitic assemblages suggests a similar range of temperatures but higher pressures; 11 to 20 kb (Manchester, 1989). These results imply that the lower crust represented by xenoliths was undergoing isothermal decompression at the time of xenolith entrainment. It is possible that retrograde metamorphism of xenoliths occurred during magma ascent, but geochemical and textural evidence suggests very limited interaction between xenoliths and host magma, and xenolith settling modeling suggests very fast magma ascent rates; as little as 4 days from the lower crust. Because the earliest eruptions precede core complex denudation, these results suggest that the lower crust was being thinned, perhaps by ductile flow, prior to the initiation of detachment faulting in the upper crust.