Vapour-absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases: Implications for anatexis in the deep to very deep continental crust and active continental margins

Dehydration-melting experiments from 10 to 20 kbar were performed on a metavolcanoclastic rock containing (in vol. %) biotite (16), amphibole (15) and epidote (13) in addition to plagioclase and quartz. At 10 and 12.5 kbar traces of biotite and epidote remain at 850 degrees C, amphibole bel ones more abundant, and the melt fraction is 5-10 vol. %. These relationships reflect that the thermal stability of biotite is lowered in the presence of epidote through the dehydration-melting reaction biotite + epidote + quartz = amphibole + garnet + alkali feldspar + melt. Amphibole dehydration-melting produces an additional similar to 25 vol. % melt between 875 and 925 degrees C. At 15 kbar and 875 degrees C the melt fraction is similar to 22 vol. %, amphibole is present in trace amounts, and biotite constitutes similar to 8 vol. %. These relationships suggest that the curves marking biotite- and amphibole-out intersect close to 15 kbar, and that the fertility of the rock increases from 10 to 15 kbar at 850 degrees C. At 20 kbar the melt fraction is only similar to 5 vol. % at 850 degrees C, amphibole is transformed to omphacite and biotite constitutes similar to 5% of the mode. This result shows that the fertility decreases from 15 to 20 kbar at 850 degrees C, mainly because much Na is locked up in omphacite. Along active continental margins, intrusion of hot mantle-derived magmas is common, and melting of metavolcanoclastic rocks may be an important granitoid-forming process. Intersection of the amphibole- and biotite-out reactions between 12.5 and 15 kbar suggests that fusion of biotite- and hornblende-bearing rocks can produce magmas ranging in composition from granitic (biotite dehydration-melting) to granodioritic (amphibole dehydration-melting) in either order depending on pressure.