Looping P-T paths and high-T, low-P middle crustal metamorphism: Proterozoic evolution of the southwestern United States

Proterozoic rocks of the southwestern United States exhibit a variety of tectonometamorphic histories that can all be explained by a characteristic looping style of pressure-temperature (P-T) path. The path is fundamentally clockwise. However, because the rocks were cooled isobarically at 3 kbar after tectonism, the cooling path crosses the prograde path, leading to a geometry with a counterclockwise character. The absolute size of the loop in the P-T path varies across the region because of different amounts of overthickening (and resulting exhumation) during the contractional event, combined with variable amounts of heating from syntectonic plutons. This style of looping P-T path may be characteristic of high-T, low-P metamorphic terranes and middle crustal tectonism, particularly where syntectonic plutons contribute to elevated geotherms and polymetamorphism and where rocks become stabilized and reside for long periods at middle-crustal depths before being exhumed during a later tectonic event.