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 degrees 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 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.