HETEROGENEOUS GROWTH AND DISSOLUTION OF SILLIMANITE IN MIGMATITES: EVIDENCE FROM CATHODOLUMINESCENCE IMAGING
The aluminous metapelitic migmatites of the eastern Beartooth Mountains are metatexites and diatexites, and have typical metamorphic assemblages of qtz + pl + Kfs + bt + sil +/- grt +/- crd. The peak metamorphic conditions of this ca. 2.8 Ga metamorphism are 750-800C at 5-7 kbar. Melting is considered to be primarily the result of dehydration melting of biotite via reactions such as bt + sil=grt + Kfs + melt and bt + sil + qtz=grt + crd + Kfs + melt. The growth/dissolution patterns interpreted from the sillimanite luminescent zoning are consistent with prograde and retrograde metamorphic reactions such that (1) there was initial sillimanite growth with the dehydration melting of muscovite, (2) sillimanite was partially consumed with the dehydration melting of biotite, (3) sillimanite overgrowth was reestablished with the crystallization of the melt and retrograde operation of the biotite dehydration-melting reactions and (4) late partial sillimanite dissolution with either the appearance of retrogressive muscovite and/or influx of a reactive fluid.