GARNET ZONING AS AN INDICATOR OF THE EXTENT OF INTERACTION OF UHP PARTIAL MELT WITH GARNET MEGACRYSTS IN METAPELITES FROM THE NE GREENLAND CALEDONIDES

Rare metapelites in the ultrahigh pressure (UHP) part of the North-East Greenland Caledonides show extensive evidence of partial melting, which is interpreted to have been generated by dehydration melting of phengite at or near UHP conditions. The partially melted metapelites contain garnet megacrysts (to 1.5 cm), which contain mono- and polymineralic inclusions and preserve significant zoning in grossular (Grs) and to a lesser extent in almandine and pyrope. Spessartine is extremely low and shows no detectable zoning in garnet. Interiors of garnet megacrysts that are inclusion free or contain only quartz inclusions have low and very uniform Grs content (X_Grs≈0.05). Garnet megacryst rims, especially those in contact with matrix leucosome, patchy regions of the megacrysts surrounding polymineralic inclusions, and small subhedral garnets in the matrix leucosome have much higher Grs content (0.22). Grs content decreases in outermost rims of small leucosome garnets and garnet megacrysts.

Small garnets scattered throughout the leucosome clearly crystallized from melt, because they share the allotriomorphic granitoid texture of the leucosome. The fact that rims and patchy, high Grs zones of garnet megacrysts have Grs contents similar to those of small leucosome garnets supports the contention that they too formed from melt or recrystallized in the presence of melt. Conditions estimated for melting of these metapelites are in the field of hydrous silicate melt near the second critical endpoint for the pelitic system, so the melt would be expected to have been highly mobile. Evidence that the patchy, high Grs zones in the interior of many garnets were in intimate contact with melt that infiltrated the megacrysts is provided by Na X-ray maps of garnets and their surroundings that highlight feldspar-containing inclusions with similar Na and Ca values to feldspars in the matrix leucosome. We infer that low Grs garnet interiors reflect UHP conditions and higher Grs patches and rims reflect garnet crystallization or recrystallization in the presence of melt. Others (e.g., Storm & Spear, 2005, JMG) have observed similar high Grs garnet rims in rocks with evidence of partial melting and interpreted it similarly.