Issue 3: 1996

Abstract

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

  • 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

    • KP. Skjerlie and AD. Johnston2 1Geologisk Institutt, Universitetet I Bergen, Allegaten 41, N-5007 Bergen, Norway, 2Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, USA and To whom correspondence should be addressed

    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 850oC, 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 925oC. At 15 kbar and 875oC the melt fraction is ~22 vol.%, amphibole is present in trace amounts, and biotite constitute ~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 850oC. At 20 kbar the melt fraction is only ~5 vol.% at 850oC, 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 850oC, 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.

    Keywords: amphibole; biotite, dehydration-melting; epidote; metavolcanoclastic rock

    Pages: 661 - 691

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