The upper greenschist - lower amphibolite facies, argillaceous to chemical-exhalative metasedimentary sequence of the Mesoarchaean Ghattihosahalli Schist Belt (GHSB), southern India, has been examined with a special focus on the paragenesis and solid solution characteristics of barian feldspars and associated dioctahedral Ba-Cr-bearing micas. Barian feldspars occur as untwinned porphyroblasts in a recrystallized finely banded matrix of barite, quartz and minor white mica. Idioblastic celsian (Cls(98-76)Or(2-20)Ab(1-8)) and hyalophane (Cls(55-39)Or(35-51)Ab(10)) predate the greenschist-facies foliation, whereas xenoblastic hyalophane (Cls(44-35)Or(45-59)Ab(8-17)) and mantles on celsian (Cls(45-35)Or(42-60)Ab(13-5)) as well as xenoblastic barian K-feldspar (Cls(6)Or(90)Ab(2)) postdate the last fabric-defining event. The preservation of extremely complex zoning patterns down to the micron-scale shows that diffusional homogenization did not operate at fluid-present low to medium-grade conditions (350-550 A degrees C, 3-5 kb). Microstructures indicate that at these conditions barian feldspars deform exclusively by brittle fracturing and do not undergo recrystallization. Barian feldspar compositions confirm the positive correlation of Na-content with temperature and the existence of a narrow asymmetric compositional gap (Cls(90-85)a dagger"Cls(55), similar to 350 A degrees C) which probably closes at lower amphibolite facies conditions (X-c similar to Cls(75;) T-c similar to 550 A degrees C). White micas are solid solutions of the end-members muscovite, ganterite (Ba-0.5 K-0.5)Al-2(Al1.5Si2.5)O-10(OH)(2), paragonite, celadonite with a significant substitution of Al-[VI] by Cr. Zoning is a common feature with cores being enriched in Ba. The data document extensive Ba substitution for K from muscovite to ganterite, exclusively controlled by the coupled substitution K-[XII] + Si-[IV] a dagger" Ba-[XII] + Al-[IV] and strongly dependent on bulk composition. The extent of solid solution from (Ms+Gnt) towards paragonite and celadonite end-members is controlled by the miscibility gap in the (Ms+Gnt)-Pg-Cel pseudoternary, with the Pg-substitution depending on temperature and the Cel-substitution on pressure. Si-[IV] values between 3.1 and 3.3 in Ba-poor micas indicate minimum pressures of chemical equilibration in the order of 3-5 kbar, while the most sodian compositions of low-celadonite micas provide an upper temperature estimate of similar to 550 A degrees C, consistent with P-T estimates for assemblages of metapelites (500-550 A degrees C, 4-5 kb).
