Giant Intrusive Volcanic Breccia Complex in the Dos Cabezas Mountains, Arizona

The Dos Cabezas mountains are a medium-sized range outcropping in southeastern Arizona near the town of Willcox. They are dominantly a Precambrian complex composed of 10 granitoid plutons and 3 multi-km² terranes of Pinal Schist and Bear Canyon Series. (Erickson, 1969 and 1993). This Precambrian complex is overlain by two deformed Mesozoic ignimbrites, and all these older-units are intruded by several Mesozoic plutons and mid-Tertiary dikes. The oldest ignimbrite is 67 Ma; I judge this is the approximate age of the slightly younger breccia system as well. Paleozoic and Mesozoic sedimentary units typical of southeast Arizona are also found (Erickson, 1969). All these units are cut by strands of the regional WNW striking Apache Pass fault system. These preexistent units are cut by innumerable irregularly shaped intrusive breccia bodies with sizes ranging from small 1-5 m dikes and pods to bodies many kilometers across. These are roughly classified here by matrix color into green, purple, and white groups. The green breccias are by far the most abundant of these. In their post-Precambrian development, active faults forming in the Apache Pass tectonic system extended into the center of the Dos Cabezas range, where, I hypothesize they outlined blocks of crust up to several kilometers across. These blocks became partly or wholly detached from their walls and they sank into the fluidized main green breccia body. The largest green breccia body contains four giant blocks of wall units up to 3 km in maximum dimension, wholly detached from their original location and probably sunk from their original level; these are each partly surrounded by fields of blocks in breccia matrix grading down from multi-kilometer size to hand specimen size and then down to dust. I call such areas block fields. The intrusive contacts of the breccia bodies with their wall units have ~ 1 km of relief, and are commonly well exposed. The breccia bodies were intruded into their walls and roofs as masses of fluidized fragments transported upwards by gases rising vigorously from deeper-seated magma. The giant blocks in the breccia complex were sinking in the fluidized beds when fluidization ceased, in the same manner as similar giant blocks in kimberlite breccia pipes in Africa (McCallum, 1985). There are no surficial breccia eruptive units preserved in the breccia complex (Shawe and Snyder, 1988 and Shawe, 1985). The green breccia alone has been autometamorphosed under hornblende-hornfels facies conditions; abundant metamorphic epidote colors it green. Presence of a ~ 1 km² exposure of typical green breccia in the Circle Hills (Erickson, 1988), 20 km west of the Dos Cabezas mountains, suggests the breccia terrain may be much more widely distributed than can presently be demonstrated.