Application of K-feldspar-jadeite-quartz barometry to eclogite facies metagranites and metapelites in the Sesia Lanzo Zone (Western Alps, Italy)

The eclogite facies assemblage K-feldspar-jadeite-quartz in metagranites an d metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the e quilibration pressure by dilution of the reaction jadeite + quartz = albite . The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm(63)Prp(26)Grs(10))-K-feldspar-plagioclase-biotite +/- sillim anite to the Eo-Alpine high-pressure assemblage garnet (Alm(50)Prp(14)Grs(3 5))-jadeite (Jd(80-97)Di(0-4)Hd(0-8)Acm(0-7))=zoisite-phengite. Plagioclase is replaced by jadeite-zoisite-kyanite-K-feldspar-quartz and biotite is re placed by garnet-phengite or omphacite-kyanite-phengite. Equilibrium was at tained only in local domains in the metapelites and therefore the K-feldspa r-jadeite-quartz (KJQ) barometer was applied only to the plagioclase pseudo morphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calcul ations on the stability of the assemblage K-feldspar-jadeite-quartz using a vailable mixing data for K-feldspar and pyroxene indicate pressures of 15-2 1 kbar (+/- 1.6-1.9 kbar) at 550 +/- 50 degrees C. This barometer yields di rect pressure estimates in high-pressure rocks where pressures are seldom o therwise fixed, although it is sensitive to analytical precision and the ch oice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barome ter is independent of the ratio P-H2O/P-T. The inferred limiting a(H2O) for the assemblage jadeite-kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.