Chloritoid‐bearing rocks associated with blueschists and eclogites, northern New Caledonia

Abstract

Abstract Chloritoid‐bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the Tertiary high‐pressure metamorphic belt of northern New Caledonia. The typical assemblage of chloritoid‐bearing rocks in the epidote zone is quartzchlorite‐muscovite‐garnet‐chloritoid. In the omphacite zone, epidote is an additional member of the chloritoid‐bearing assemblage. Paragonite is rare, plagioclase was not detected, and rutile and ilmenite are the Fe‐Ti oxide phases. Chloritoid‐glaucophane is not a common assemblage. Chloritoid‐bearing rocks have relatively low (Ca+K+Na)/Al ratios and the chloritoids are relatively Mg‐rich with Mg/ (Mg+Fe) up to about 0.4. A comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560° C in the omphacite zone and a minimum temperature limit near 450° C at 10 kbar. An empirical garnet‐chlorite Fe‐Mg exchange thermometer does not yield consistent results for the higher‐grade rocks, suggesting Ts ranging from 390 to 535° C in the omphacite zone and 420–465° C in the epidote zone. The distribution coefficient KD= (Fe/Mg)ctd/(Fe/Mg)chl for chloritoid and chlorite ranges from 3.9 to 6.4, values which are lower than those (=10) from lower greenschist facies rocks, but are near those of upper greenschist facies and albite‐epidote amphibolite facies.