A description of metamorphic PTt paths with implications for low- P high- T metamorphism

Abstract

The terms “clockwise” and “anticlockwise” are well entrenched in the metamorphic geology literature where they are used to describe, respectively, decompression or compression of rocks at and after the metamorphic temperature peak. Accordingly, we define the clockwiseness of a PT path, C, as the rate of pressure change at the temperature maximum, C = dP dt | T max and suggest that C may be used to characterise metamorphic PT paths. We illustrate this approach using a simple one-dimensional 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 stress contributions to the pressure field. In a parallel paper we have shown 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 challenges many current interpretations of PT paths in particular those from the low- P high- T metamorphic environment. Here, we use the simple parameter C to investigate some of the governing controls on the shape variations of such paths.