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 and metapelites from the Sesia‐Lanzo Zone (Western Alps, Italy) records the equilibration 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±sillimanite to the Eo‐Alpine high‐pressure assemblage garnet (Alm 50 Prp 14 Grs 35 )–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 replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K‐feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs 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 calculations on the stability of the assemblage K‐feldspar–jadeite–quartz using available mixing data for K‐feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high‐pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K‐feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O / P T . The inferred limiting a (H 2 O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.