Compositional Layering in Alpine Peridotites: Evidence for Pressure Solution Creep in the Mantle

Abstract

Field evidence from the Josephine (southwestern Oregon) and Red Mountain (New Zealand) peridotites indicates that compositional layering in alpine-type peridotites pre-dates emplacement into the crust, and is dissimilar in origin to layering in stratiform intrusions. Field, geochemical, and textural evidence all suggest that the layering formed during anatexis and upward flow of the peridotite in the mantle. A cumulus origin for the layering is rejected as there is strong geochemical evidence to suggest that alpine-type peridotites are the residues of partial fusion in the mantle. Although the layering superficially resembles layering in stratiform intrusions, there are no other features present which suggest a magmatic origin. The orientation of the layering, however, closely resembles that of strain-slip (or crenulation) cleavage in the axial zones of old mountain belts suggesting a deformation related origin. It also appears unlikely that the layering is solely the product of a mechanical segregation accompanying deformation ("flow layering") inasmuch as evidence for prolonged high-temperature creep of the peridotites is found throughout the entire peridotite whereas the layering is not. The most likely mechanism for the origin of the layering appears to be metamorphic differentiation accompanying deformation, pressure solution creep, and anatexis of the peridotite. This suggests that pressure-solution creep may be a principal creep mechanism in areas of ascending athenosphere such as below mid-ocean ridges.