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

摘要

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_6Or_(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_(Al_(1.5)Si_(2.5)) O_(10)(OH)_2, paragonite, celadonite with a significant substitution of ^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).