Experimental Constraints on Partial Melting in the Crust

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Vielzeuf and Holloway (1988) experimentally studied the melting of a metapelite under fluidabsent conditions. At 10 kbar, three main stages of melting were distinguished : I) The first melting occurs below 800~ and corresponds to the breakdown of muscovite via reactions such as Ms + Bt + PI + Qtz + V = M + Als;Ms + PI + Qtz= Bt + Als + Kfs + M. The proportion of melt was small (10-15 vol.%). 2) Between 850 and 875~ there was a dramatic increase in the proportion of melt, changing from about 10% to 50-60%. This step corresponded to the breakdown of biotite (Bt + Als + PI + Qtz = Grt + Kfs + M) and was followed by a plateau along which garnet and sillimanite progressively dissolved in the liquid. 3) The last stage represented the melting of garnet and spinel at very high temperatures (above 1100~ In this scheme, the reaction Bt + Als + P1 + Qtz ~ Grt + Kfs + melt (1) plays a major role. Vielzeuf and Holloway (1988) considered that, at 10 kbar, this reaction occurred at about 850-875~ over a small temperature interval. These experiments indicate a very steep slope for that reaction in the range 7 to 12 kbar. Experimental investigation on related lithologies at 10 kbar by Lc Breton and Thompson (1988) indicated that melting began between 780 and 800~ and was extensive at 850~ Patifio-Douce and Johnston (1989) performed experiments on a natural metapelite. They observed biotite, sillimanitc and quartz coexisting with melt at temperatures up to 975~ and a very progressive increase of the proportion of melt. This is significantly different from the findings of Viclzcuf and Holloway but can be explained by the low amount of plagioclase (4 wt%) in PatifioDoucc and Johnston's starting material. Thus, temperatures between 850 and 900~ seem sufficient to produce a significant proportion of liquid in pelites. Such silicate liquids, or at least a proportion of them, are potentially able to segregate from the source, leaving behind a residue of quartz, garnet, sillimanite, feldspar, and futile. Such a rcstitic residue has the characteristic mineral assemblage of silicic aluminous granulites.