变质岩温压条件的确定.pptVIP

* * * * Because intrusions can stall at practically any depth, and orogenic geotherms are quite variable, there is considerable overlap and gradation between the hornfels facies types and the corresponding medium-pressure facies immediately below them on Fig. 25-2 * * We learned that thermodynamic equilibrium is generally maintained during prograde metamorphism, so that relict minerals are more common during the retrograde path, but both types have been observed * * * * * * * * * * Nearly all models indicate that heat flow higher than the normal continental geotherm is required for typical greenschist-amphibolite medium P/T facies series, and that uplift and erosion has a fundamental effect on the geotherm and must be considered in any complete model of metamorphism * * Fig. 25-12 illustrates some examples of modeled P-T-t paths representing common types of metamorphism The paths illustrated are schematic, and numerous variations are possible, depending upon the style of deformation and the rates of thickening, heat transfer, magmatism, and erosion, etc. * * If this assumption is correct, the shape of the typical P-T-t path has some instructive ramifications * * * * * * * * * * 4. Now return to the question of whether the temporal and spatial mineralogical changes associated with progressive metamorphism are the same Fig. 25-14 illustrates the difference between the spatial changes along the metamorphic field gradient and the temporal changes (P-T-t paths) for several rocks along the gradient Note that every rock follows a path involving considerably higher pressures and lower temperatures than the final locus of Tmax-(P at Tmax) points along the metamorphic field gradient suggests Implications for blueschists: most CW paths appear to - blueschists in early stages, but not preserved, because T gradually increases to - med P/T at T(max) where metamorphic imprint developed Perhaps preservation is the main reason that blueschists are relatively rare, not generation