Coesite-bearing eclogite in Henan Province, central China; detailed petrography, glaucophane stability and PT-path

Coesite and polycrystalline quartz pseudomorphs after coesite are described in garnet and kyanite in eclogite in the Hongan block of the Triassic Qinling-Dabie collision zone between the Sino-Korean and Yangtze cratons in central China. The coesite-bearing eclogites occur as 4-8 m thick layers in gneiss and are rich in Al (sub 2) O (sub 3) , MgO and SiO (sub 2) compared to common basalts; they have experienced at least four stages of recrystallization. The pre-eclogite stage I assemblage is found as inclusions in garnet cores and consists of garnet + katophorite/taramite + quartz + paragonite. The peak eclogite stage II assemblage is garnet + coesite + kyanite + omphacite + phengite + rutile +- talc +- glaucophane. Decompression stage III assemblage is garnet + quartz + omphacite + kyanite + phengite + paragonite + epidote + barroisite + ilmenite. The final retrograde stage IV assemblage consists of albite + hornblende +- Ca-clinopyroxene +- taramite/pargasite + quartz +- margarite + ilmenite. Phase equilibria and geothermobarometers were used to constrain P-T conditions for the four stages; the coesite-bearing eclogite shows a clockwise P-T path with a peak P of approx 31 kbar at 620 +- 30 degrees C. The coexistence of glaucophane with kyanite and omphacite, and the existence of talc and omphacite inclusions in barroisite, provides natural evidence of relatively high-T and high-P stability for glaucophane. EPMA results are tabulated for the various silicates in these rocks. The tectonic implications for the geology of the region in central China are discussed.