PLAGIOCLASE HALOS AROUND GARNETS: IMPLICATIONS FOR PRESSURE-TEMPERATURE PATHS IN METAPELITES

Textures found in metamorphic rocks provide insight into the metamorphic and tectonic history of the region. Plagioclase halos forming around partially resorbed garnets are observed in metapelites and meta-amphibolites encompassing the globe. Commonly, these textures are inferred to be the result of decompression based largely on interpretations from petrogenetic grids.

Mineral modes and mineral chemistry of the whole rock, and of the halo, and whole-rock chemistry provide data for determination of P-T conditions and for understanding halo formation. Samples displaying halo textures contain the mineral assemblages of quartz, plagioclase, biotite, muscovite, garnet and sillimanite, with minor K-feldspar, ilmenite, graphite, apatite, zircon and monazite. Varying degrees of garnet resorption are indicated by rims of plagioclase, quartz, and biotite. The halo segregation represents between 15% and 25% of the samples and contains all of the garnet, most of the quartz, < 50% of the plagioclase, and < 20% of the biotite. The matrix contains melanosomes of foliated biotite and parallel as well as oblique lineated sillimanite, and leucosomes of plagioclase and traces of quartz, overprinted by secondary muscovite. Textural analysis indicates two stages of biotite and plagioclase growth.

Phase equilibria modeling using Theriak/Domino with P-T conditions calculated by GBPQ, GB and GASP, which suggest peak metamorphism between 6.5-7 kb (± 0.5 kb) and 700-750⁰C (± 50⁰C), constrain conditions of halo formation. Observations and calculated changes in mineral modes can be explained by three reactions; (1) a prograde reaction Ms + Fsp → Sil + Bt + Grt, (2) a duo of decompression reactions that consumes Grt and produces Sil + Bt + Pl with Bt → Sil + liq, and (3) a retrograde hydration reaction in which Ms reemerges at the expense of Sil, coupled with a secondary reaction between Grt + Fsp + Bt. Most of the halo develops during stage (2) and is linked through material transport with the matrix. We use a local equilibrium, irreversible thermodynamic model to simulate the material transport during the evolution of these rock textures and to evaluate processes responsible for their formation.