Separating metamorphic events in the Fosdick migmatite-granite complex, West Antarctica

The Fosdick migmatitegranite complex in West Antarctica records evidence for two high-temperature metamorphic events, the first during the DevonianCarboniferous and the second during the Cretaceous. The conditions of each high-temperature metamorphic event, both of which involved melting and multiple melt-loss events, are investigated using phase equilibria modelling during successive melt-loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS SmNd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the DevonianCarboniferous (M1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M1 metamorphism (similar to 820-870 degrees C, 7.511.5 kbar) and the formation of garnetsillimanitebiotitemelt-bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M1 conditions. The Cretaceous (M2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse-grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS SmNd ages for garnet are similar at 10-299 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M1 garnet constrains M2 conditions to similar to 830-870 degrees C and similar to 67.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M2 event, which would facilitate overprinting of M1 compositions during the M2 prograde metamorphism. Measured garnet compositions and SmNd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the SmNd system during the Cretaceous M1 overprint. The c. 10299 Ma garnet SmNd closure ages correspond to cooling below 700 degrees C during the rapid exhumation of the Fosdick migmatitegranite complex.