GARNET GROWTH DURING AMPHIBOLITE ANATEXIS - IMPLICATIONS OF A GARNETIFEROUS RESTITE

Formation of a garnet clinopyroxenite restite may have important physi cochemical consequences for deep crustal and mantle processes. We have experimentally simulated the dehydration-melting of amphibolite, and report the modal and compositional evoltuion of the mafic granulitic m ineral assemblage (the restite), with an emphasis on garnet. A natural , low-K, calcic amphibolite was powdered and run in sealed, gold capsu les in a piston-cylinder apparatus at 10 kbar, 750-1000-degrees-C, 1 t o 9-day runs, and fO2 approximately Ni-NiO. Garnet first appears in 8- day runs at 825 +/- 25-degrees-C. Above 850-degrees-C, clinopyroxene a nd poikilitic garnet comprise most of the restite. Garnet peaks at 950 -degrees-C (at approximately 32 vol.% of the material within the capsu le, or 41 vol.% of the restite assemblage) and disappears between 975- 1000-degrees-C. There is no cross-over of Fe-Mg partitioning between c oexisting liquids and garnets. Liquids remain Mg-poorer than garnets t hroughout the melting interval but do increase in Mg# with increasing temperature; thus the hotter liquids may be less likely to react with mantle wedge peridotite. The modes are consistent with REE patterns of Archean tonalitic-trondhjemitic rocks generated by the partial meltin g of garnet-bearing mafic crust. A garnet clinopyroxenite with 41 vol. % garnet has a density of 3.5 g/cm3, Which may induce lower crustal de lamination in hot, nonthickened, amphibolitic crust. This crust is sei smically indistinguishable from lithosphere or mantle. Because garnet growth traps water-bearing hornblende inclusions, the restite may cont ain up to 0.3 wt.% water. Thus, delivery of poikilitic garnet to the m antle, possibly by delamination or by subduction, may be an important mechanism for adding water to the mantle.