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

Abstract

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°C, amphibole becomes 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 ∼25 vol. % melt between 875 and 925°C. At 15 kbar and 875°C the melt fraction is ∼22 vol. %, amphibole is present in trace amounts, and biotite constitutes ∼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°C. At 20 kbar the melt fraction is only ∼5 vol. % at 850°C, amphibole is transformed to omphacite and biotite constitutes ∼5% of the mode. This result shows that the fertility decreases from 15 to 20 kbar at 850°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.