Eclogites from the south Tianshan, NW China:: petrological characteristic and calculated mineral equilibria in the Na2O-CaO-FeO-MgO-Al2-O3-SiO2-H2O system

Eclogites from the south Tianshan, NW China are grouped into two types: glaucophane and hornblende eclogites, composed, respectively, of garnet + omphacite + glaucophane + paragonite + epidote + quartz and garnet + omphacite + hornblende (sensu lato ) + paragonite + epidote + quartz, plus accessory rutile and ilmenite. These eclogites are diverse both in mineral composition and texture not only between the two types but also among the different selected samples within the glaucophane eclogite. Using thermocalc 3.1 and recent models of activity-composition relation for minerals, a P-T projection and a series of P-T pseudosections for specific samples of eclogite have been calculated in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH) with quartz and water taken to be in excess. On the basis of these phase diagrams, the phase relations and P-T conditions are well delineated. The three selected samples of glaucophane eclogite AK05, AK11 and AK17 are estimated to have peak P-T conditions, respectively, of 540-550degreesC at c . 16 kbar, c. 560degreesC at 15-17 kbar and c. 580degreesC at 15-19 kbar, and two samples of hornblende eclogite AK10 and AK30 of 610-630degreesC and 17-18 kbar. Together with H2O-content contours in the related P-T pseudosections and textural relations, both types of eclogite are inferred to show clockwise P-T paths, with the hornblende eclogite being transformed from the glaucophane eclogite assemblage dominantly through increasing temperature.