414 METAMORPHIC ROCKS/PTt-Paths Figure Schematic PTt path for crustal thickening by instantaneous overthrusting as typical for one dimensional modelling Point A on the prethrusting geotherm undergoes an isothermal pressure increase to B followed by isobaric heating for 20 Ma before erosion is initiated How can the clockwise PTt loop of Figure be used as an aid to understanding the PTt evolution of natural metamorphic rocks? The PTt loop shows a wide range in PT conditions and, if superimposed on a facies diagram, would show an evolution passing through several different facies Mineral reactions and diffusive transport are, however, thermally activated with kinetics following an Arrhenius relationship For this reason, reaction rates increase exponentially with temperature and therefore the peak temperature is likely to be the point where most reaction occurs Cooling after the thermal peak will see a slowing of reaction and material transport such that the peak temperature mineral assemblage is preferentially preserved In addition, the general shape of dehydration reactions means that a higher degree of fluid release, useful for material transport during prograde reaction, will occur for a path of increasing temperature with minor pressure change Once fluids have left the system at the thermal peak, any retrogression to hydrous assemblages during cooling will be hindered by the absence of a free fluid phase thus again favouring preservation of the peak temperature assemblage Combining this information it is possible to predict the most likely determinable PT point for a rock that followed a standard clockwise PTt path, based on preserved mineral assemblage for the temperature peak If, for a single segment of crust, several rocks formerly at different depths are traced, it is possible to plot the loci of their peak temperature points (Figure 6A) These are the most probable PT points that would be determinable for rocks that had followed these particular paths and represents the sort of information available from field geology for a tilted and peneplained regional metamorphic terrane: hence the name metamorphic field gradient (sometimes also piezothermic array) Several important points can be deduced from this plot especially when additional temperature–age (Figure 6B) and depth–age plots (Figure 6C) for the same model are presented Firstly, the metamorphic field gradient defines a PT trend that lies between that of the initial and final geotherms However, the points defining this trend represent different ages, visible from Figures 6B and 6C, and so the curve does not represent any actual, temporary geotherm that occurred during the thermal relaxation Thus, the sequence of preserved metamorphic rocks is not the same as would be predicted, for example, along one of the Figure thermal trends The age difference between the different preserved points is also important If a major deformation event occurred at 25 Ma, this would be syntectonic with respect to rock B but would be pretectonic for rock A and post-tectonic for rock C In addition, the difference in depth of the peak-temperature points (see Figure 6C) is less than the true depth difference, thus leading to an apparent thinning of the crustal section Such thermal models for crustal thickening followed by exhumation, although relatively simple, illustrate some of the fundamental problems inherent in trying to interpret natural metamorphic sequences Contact Metamorphism So far the models presented have considered only the effects of tectonically induced crustal-scale