LOCATION OF THE GRANULITE-AMPHIBOLITE FACIES TRANSITION IN NORTHWESTERN ARIZONA

The presence of a boundary between Proterozoic granulite-facies and lower grade rocks (amphibolite/greenschist) in northwestern Arizona has long been recognized, however, there is not a consensus on its precise location. Within the Lower Granite Gorge of the Grand Canyon (LGG), this metamorphic transition appears to correspond with the northeast-striking Gneiss Canyon shear zone (GCSZ). However, in the Peacock Mtns., east of the GCSZ, new metamorphic data suggest that the boundary lies considerably southeast of the projection of the GCSZ. Pelitic schists with granulite-facies mineral assemblages (sillimanite + K feldspar + plagioclase + biotite + quartz ± garnet ± cordierite) have been found in the central and southern Peacock Mtns. Petrogenetic grids indicate that these rocks record minimum temperatures of 650°C. Quantitative thermobarometric studies are currently underway to determine absolute temperatures and pressures during metamorphism.

Greenschist-facies pelitic schists are found in the Cottonwood Cliffs, ~13 km due east of the Peacock Mtns. (<8 km across strike). Our findings suggest that the boundary between granulite-facies and lower grade rocks does not coincide with the GCSZ as it appears to in the LGG. The marked decrease in grade may be the result of either 1) an elevation of the geotherm to the west due to extensive plutonism, 2) a normal regional geotherm (20-25°C/km) through a vertically tilted crustal section in which the Peacock Mtns. represent crustal depths 8-10 km greater than the Cottonwood Cliffs, or 3) rocks of different P-T-t-D histories separated by a structural boundary. Options 1) and 2) are difficult to explain in the context of the larger regional distribution of metamorphic rocks. If the metamorphic transition is a result of juxtaposition along a major shear zone, we suggest that this structural boundary must lie between the Peacock Mountains and the Cottonwood Cliffs.