Distribution of anomalously high K2O volcanic rocks in Arizona: metasomatism at the Picacho Peak detachment fault

Metasomatized Tertiary lavas with anomalously high K₂O and low Na₂O content are distributed within the northwest-trending Miocene extensional terrane of southwestern Arizona. These rocks are common near core-complex–related detachment faults at Picacho Peak and the Harcuvar Mountains and in listric-faulted terrane at the Vulture Mountains. In addition to systematic changes in K₂O and Na₂O, the rocks have been enriched in Zr and depleted in MgO.

Secondary, introduced minerals include orthoclase, quartz, and calcite. Fine-grained, euhedral orthoclase (var. adularia), from 2 to 10 µm, is the dominant potassium mineral.

Metasomatic changes at Picacho Peak are spatially associated with a major detachment fault. Thus, it is interpreted that detachment provided a conduit for hydrothermal fluids that altered the initial chemical composition of the Tertiary volcanics by potassium metasomatism and charged the upper-plate rocks with mineralizing fluids that carried Zr and Ba, along with Au, Ag, and Cu, during detachment 17–18 Ma.