MEDDLING WITH METAMORPHIC MODELING

Four aspects of recent modeling are central to future work. These are (i) nature, duration and location of the heat source, (ii) which events along Pressure-Temperature (P-T) paths are recorded and when, (iii) what is reset by events later than Tmax and when, (iv) the former relationships between samples from the landscape. (i) Often mineral geothermobarometers used for medium (M)P-MT Barrovian metamorphic terrains indicate no substantial crustal thickening prior to and during continental collision, but relatively high temperatures compared to accepted geotherms. This requires substantial additional heat sources at crustal depths, indicating either a marked local increase in heat flow from underlying mantle, or greatly enhanced internal heat generation. Furthermore we need to investigate how discrepancies between models and deduced PT-paths from natural orogens reflect the initial and boundary conditions used in current computer codes for models of regional deformation and metamorphism. Also we need to discover whether there are systematic correlations between deduced pressure and actual tectonic thickness, and between deduced P and T gradients and position within an orogen. (ii) Rocks with slower equilibration will show P-T-t (time) path segments rather than recording only a single P-T point equilibration condition. Modeling has shown that the exhumation rate of metamorphic terrains determines whether or not the maximum temperature (Tmax) is achieved at the burial depth (zmax) or at some time later in the decompression history. Extrusional exhumation is certainly faster than erosion-controlled exhumation. (iii) Some cation systems used in exchange reactions for geothermometry and in continuous reactions for geobarometry, continuously adjust their chemistries by diffusion at temperatures after Tmax. (iv) Recent geochronological work indicates that metamorphic events are short (10 to 20 Ma) compared to overall orogenic history (perhaps 100 Ma), and that age provinces are about 25 km in size. This suggests that whole terrains are not necessarily exhumed as a single unit. We need to decipher how the differential exhumation may have reset isotherms, P-T gradients and some types of isograd reactions. D-markers (deformational episodes) are as important along PTtD-paths as are geochronological markers.