Chloritoid-Bearing Mineral Assemblages in High-Pressure Metapelites from the Bughea Complex, Leaota Massif (South Carpathians)

• Published in: Journal of petrology 2009-01, Vol.50 (1), p.103-125
• Main Authors: Negulescu, Elena, Săbău, Gavril, Massonne, Hans-Joachim
• Format: Article
• Language: English
• Subjects: high-pressure metapelite; Mg-rich chloritoid; P–T path; P–T pseudosection; very high-pressure eclogite
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egn075

High-Mg chloritoid (XMg = 0·40–0·47) and relatively high-Mg staurolite (XMg = 0·25–0·28) coexisting with kyanite and garnet were identified in a mica–garnet-rich rock associated with very high-pressure eclogites in the Bughea Complex of the Leaota Massif (South Carpathians). Major and trace element geochemical data for both fresh eclogites and associated rocks which represent a metasomatic or retrograde alteration rind of the eclogites, indicate a pelitic precursor. Magnesian chloritoid was found as inclusions in garnet as part of a chloritoid–kyanite–garnet assemblage which is indicative of high-pressure conditions. The host garnet shows a typically prograde chemical zoning pattern. The chloritoid-bearing assemblage is confined to the inner part of the garnet porphyroblasts, whereas the matrix assemblage in equilibrium with Mg-rich garnet rims has exceeded the thermal stability limit of chloritoid. Pressure–temperature pseudosections for simplified compositions approaching the rock bulk-chemistry show a high-pressure field for the identified chloritoid-bearing assemblage in good agreement with pressure–temperature estimates in the CFMASH and KCFMASH chemical subsystems using analysed mineral compositions. The derived pressure–temperature path is clockwise, indicating overprinting during exhumation from 1·8 GPa and 580°C to 1·15 GPa and 620°C, at a water activity approaching aH2O = 1. These conditions were attained in a subduction mélange indicating transient thermal perturbations of a subduction channel.