THERMOBAROMETRY AND GEOCHRONOLOGY OF MID-TERTIARY ALKALIC LAVAS, CHINO VALLEY, ARIZONA: RELATION TO METAMORPHIC CORE COMPLEXES

The mid-Tertiary Sullivan Buttes volcanic field (SBVF), located in the Transition Zone of central Arizona, consists of alkalic dome complexes rich in lower-crustal xenoliths. The petrology, thermobarometric and geochronologic analysis on three dome complexes provide important parameters about the evolution of the lithosphere underneath the Transition Zone during the shift from compressional to extensional tectonics at ~30 Ma. The formation of the SBVF began at ~ 27 Ma; the oldest cooling ages in the footwalls of metamorphic core complexes (MCC) ~100 km to the SW are ~26 Ma, suggesting that the SBVF magmas and the MCC low-angle detachment faults developed in the lower/middle crust synchronously. Low-angle detachment faults associated with the Harcuvar, Harquhala, and Buckskin-Rawhide core complexes dip beneath the Transition Zone and exhumed crustal material from depths of ~15-35 km.

Xenoliths include eclogite, amphibolite, quartzo-feldspathic, and garnet-pyroxenite, and range in size from >1 cm to 55 cm. The three compositionally distinct lava domes had coeval eruptions at 23.5 Ma and contain unique assemblages of xenolith lithologies. The eclogite xenoliths present in the biotite-trachyte dome complex display a “breadcrust” texture associated to solid-state thermal expansion suggesting rapid magma ascent to the surface. The crystallization temperatures and pressures for each dome complex are distinctly different indicating that the lavas stalled at different crustal levels. The amphibole trachyte magma chamber formed shallowest at a depth of 14 km while the pyroxene latite chamber formed at 41 km and the biotite trachyte chamber at 51 km. The entrainment of lower-crustal xenoliths from depths as great as 51 km suggests a very thick crust under the Transition Zone during the change in tectonic regimes. The variation in composition, physical characteristics, and xenolith assemblages observed within each dome complex more readily correlates to the depth in which each magma chamber evolved, and is associated with spatial heterogeneity in the lithosphere rather than with temporal changes in the lithospheric structure.