HETEROGENEOUS GROWTH AND DISSOLUTION OF SILLIMANITE IN MIGMATITES: EVIDENCE FROM CATHODOLUMINESCENCE IMAGING

Compositional heterogeneities within coarse-grained sillimanite crystals provide important clues to the growth and dissolution processes and reactions involved in migmatization of aluminous metasediments. SEM-cathodoluminescence (SEM-CL) imaging reveals a heterogeneous distribution of red luminescence within sillimanite from a series of migmatites from the high-grade Archean terrain of the eastern Beartooth Mountains, Montana (USA). The heterogeneous red luminescence is generally consistent with a pattern of (1) nucleation and growth followed by dissolution, (2) further growth, and (3) late dissolution or re-equilibration with a late reactive fluid phase. Sillimanite, whose chemistry is commonly neglected, is particularly useful as a reaction and fluid phase monitor in migmatites because most of the associated minerals homogenize as a consequence of volume diffusion at high grades.

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.