INTERPRETATION OF THE DECOMPRESSION PATH OF PARTIALLY MELTED UHP METAPELITES FROM THE NE-GREENLAND CALEDONIDES BY CORRELATION OF OBSERVED ASSEMBLAGES AND COMPOSITIONS OF ZONED GARNET AND OTHER MINERALS WITH ISOCHEMICAL PHASE DIAGRAM SECTIONS

Rare ultrahigh pressure (UHP) metapelites in the NE Greenland Caledonides show extensive evidence of partial melting, which is interpreted to have formed by dehydration melting of phengite on the decompression path from UHP conditions, estimated at 970°C and 3.6 GPa (Lang & Gilotti, 2007). The partially melted metapelites contain garnet megacrysts (to 1.5 cm), which contain mono- and polyminerallic inclusions and preserve significant zoning in grossular (Grs) and to a lesser extent in almandine and pyrope. Interiors of garnet megacrysts have low and uniform X_Grs (~0.05); megacryst rims, especially those in contact with matrix leucosome, and small subhedral garnets in the matrix have much higher Grs content (~0.22). Crystallized leucosomes that make up much of the rock matrix contain two coexisting feldspars, biotite, kyanite, quartz and rutile.

We have calculated isochemical phase diagram sections (pseudosections) with Theriak/ Domino software (de Capitani & Petrakakis, 2010) for two Greenland metapelites using the up-dated Holland & Powell (1998) database 5.5, which includes silicate melt. On diagrams calculated at the low estimated H₂O content of these rocks, likely decompression paths cross the Coes-Qtz boundary before dehydration melting of Phengite, which is consistent with textures. Omphacite is part of the calculated UHP assemblage, although there is no evidence of Omp in the preserved textures. The calculated diagrams display a region where grossular content of garnet increases dramatically and garnet grows along likely decompression paths. Detailed tracking of observed assemblages and compositions of zoned garnet and other minerals in the partially melted UHP metapelites on the calculated diagrams allows us to document their decompression path in P-T space.

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Lang, H.M. & Gilotti, J.A., 2007. Partial melting of metapelites at ultrahigh-pressure conditions, Greenland Caledonides. J. Met. Geol., 25, 129-147.