Calculation of Garnet Fractionation in Metamorphic Rocks, with Application to a Flat-Top, Y-rich Garnet Population from the Ruhla Crystalline Complex, Central Germany

A mathematical approach is presented for the calculation of the major and trace element fractionation that is caused by growth of zoned garnet in metamorphic rocks. This approach is based on textural and compositional parameters directly obtained from natural examples. It takes into account the mode and composition of all unzoned minerals, as well as the mode, crystal size distribution and zonation patterns of garnet grains of different sizes within a certain rock volume. These parameters can be used to fit functions from which the amount of garnet fractionation at each step of a garnet growth history can be calculated. The approach is tested for two compositionally distinct domains within a single garnet–biotite gneiss sample from the Ruhla Crystalline Complex. This sample contains unusual flat-top garnet grains with Y2O3-rich cores. It is shown that MnO, FeO and Y2O3 are extremely fractionated during garnet growth, but in different ways, and that MnO fractionation does not obey a Rayleigh function. To demonstrate the influence of garnet fractionation on P–T path estimates, quantitative phase diagrams in the model system Na2O– K2O–CaO–MnO–FeO–MgO–Al2O3–TiO2–SiO2–H2O are constructed by means of the computer software THERMOCALC. The good agreement between calculated and observed mineral assemblages and garnet compositions for all fractionation steps indicates that the new approach can be used to infer detailed P–T paths, even for rocks that contain complexly zoned garnet grains. The results indicate that garnet growth in the metapelite under investigation occurred along a linear P–T path from 470 C and 2.7 kbar to 580 C and 8.5 kbar. The results also show that garnet cores with high Y2O3 contents of about 1 wt % nucleated over a temperature interval of c. 90 C, indicating that Y in garnet is relatively insensitive to temperature changes.