Paragenesis and chemical characteristics of the celsian−hyalophane−K-feldspar series and associated Ba-Cr micas in barite-bearing strata of the Mesoarchaean Ghattihosahalli Belt, Western Dharwar Craton, South India

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 °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 ↔Cls 55 , ~350 °C) which probably closes at lower amphibolite facies conditions (X c ~Cls 75; T c ~550 °C). White micas are solid solutions of the end-members muscovite, ganterite (Ba 0.5 K 0.5 )Al 2 (Al 1.5 Si 2.5 )O 10 (OH) 2 , paragonite, celadonite with a significant substitution of [VI] Al 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 [XII] K + [IV] Si ↔ [XII] Ba + [IV] Al 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. [IV] Si 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 ~550 °C, consistent with P-T estimates for assemblages of metapelites (500–550 °C, 4–5 kb).