Calculated Phase Relations in the System NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O) for High-Pressure Metapelites |
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Authors: | WEI, CHUNJING POWELL, ROGER |
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Affiliation: | 1 MOE KEY LABORATORY OF OROGENIC BELTS AND CRUSTAL EVOLUTION, SCHOOL OF EARTH AND SPACE SCIENCES, PEKING UNIVERSITY, BEIJING 100871, P.R. CHINA 2 SCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, MELBOURNE, VIC. 3010, AUSTRALIA |
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Abstract: | Petrogenetic grids in the system NCKFMASH (Na2OCaOK2OFeOMgOAl2O3SiO2H2O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the PT range 730kbar and 450680°C, for assemblages involving garnet,chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite,paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene,plagioclase, zoisite and lawsonite, with phengite, quartz/coesiteand H2O in excess. These grids, together with calculated compatibilitydiagrams and PT and TXCa and PXCa pseudosectionsfor different bulk-rock compositions, show that incorporationof Ca into the NKFMASH system leads to many of the NKFMASH invariantequilibria moving to lower pressure and/or lower temperature,which results, in most cases, in the stability of jadeite andgarnet being enlarged, but in the reduction of stability ofglaucophane, plagioclase and AFM phases. The effect of Ca onthe stability of paragonite is dependent on mineral assemblageat different PT conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and PTconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in PT and PXCapseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry |
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Keywords: | : phase relations metapelites NCKFMASH THERMOCALC phengite geobarometry |
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