A DESCRIPTION OF METAMORPHIC PTT PATHS WITH IMPLICATIONS FOR LOW-P HIGH-T METAMORPHISM

The terms ''clockwise'' and ''anticlockwise'' are well entrenched in t he metamorphic geology literature where they are used to describe, res pectively, decompression or compression of rocks at and after the meta morphic temperature peak. Accordingly, we define the clockwiseness of a PT path, C, as the rate of pressure change at the temperature maximu m, C = dP/dt\(Tmax) and suggest that C may be used to characterise met amorphic PT paths. We illustrate this approach using a simple one-dime nsional kinematic model with a prescribed strain-rate evolution and a thermal model in which the thermal evolution is governed by a cooling intrusion and we consider both the lithostatic and the deviatoric stre ss contributions to the pressure field. In a parallel paper we have sh own that, because of the consideration of deviatoric stresses in this model, PT paths may vary from clockwise to anticlockwise, depending on distance form the intrusion and/or subtle changes of the assumptions of the strain-rate evolution. This result is important as it challenge s many current interpretations of PT paths in particular those from th e low-P high-T metamorphic environment. Here, we use the simple parame ter C to investigate some of the governing controls on the shape varia tions of such paths.