NATURE OF THE MID-TERTIARY LOWER CRUST IN THE TRANSITION ZONE OF ARIZONA AS SUGGESTED BY XENOLITHS AND THEIR HOST LAVA; UHP METAMORPHISM FOLLOWED BY RETROGRESSION AND MELT INTERACTION

A suite of xenolith-bearing, alkalic volcanic rocks erupted across Arizona’s Transition Zone (TZ) from ~30 Ma to 22 Ma, concurrently with onset of Cordilleran tectonic extension and magmatism. The petrology and geochemistry of lavas and xenoliths elucidates the nature of the lower crust during the transition from Laramide contraction to extension. Lavas have up to 6% wt. K2O, are rich in incompatible trace elements (Ba up to 1900 ppm and Sr up to 1250 ppm), are enriched in LREE’s (~200 times chondrite) with steep negative REE slopes, and have no Eu anomaly, all suggesting an enriched-mantle melting source and no plagioclase fractionation. Xenoliths are dominantly variably retrograded eclogite and garnet pyroxenite. In general, lavas bearing biotite and cpx contain the least retrograded xenoliths, and xenoliths in hornblende-bearing lava have been thoroughly retrograded to amphibolite assemblages. Exsolution of cpx lamellae from garnet and of potassium feldspar from cpx indicate ultra-high-pressure metamorphism. These data suggest that the crust beneath the TZ was very thick, possibly >60 km, prior to the onset of mid-Tertiary extension. Last-equilibrated assemblages indicate temperatures >800C prior to xenolith entrainment.

Variability in xenolith mineralogy and chemistry suggests a complex history of metamorphism, metasomatism, and melt interaction since formation of the TZ lower crust, likely in the Proterozoic. Cpx chemistry from the least retrograded eclogite xenoliths, including high Na content, enrichment in mid-REEs, and depletion in H-REEs is consistent with equilibration at eclogite facies. Many retrograded xenoliths contain veins of plagioclase with hercynite and ferro-hogbomite, suggesting that retrogression occurred in the presence of hydrous melt. However, very sharp contacts between coarse plagioclase veins and the host lava and distinct trace element differences suggest that the melt present during retrogression was not the host melt. The trace element pattern (including low Th/U) from one zircon in an amphibolite-retrograded eclogite xenolith is similar to continental magma or granulite metamorphosed in the presence of melt. This may be a Proterozoic igneous zircon that was not modified by metamorphism, or it may have grown during retrogression in the presence of melt.