Newly discovered eclogites from the western Bangong-Nujiang Meso-Tethyan suture zone, central Tibet, western China

Abstract:

We report the occurrence, mineralogy, whole-rock geochemistry, and geochronology of two types of eclogites newly discovered in the western Bangong-Nujiang Meso-Tethyan suture zone (central Tibet, western China). Type 1 eclogites contain a peak metamorphic mineral assemblage of garnet + clinopyroxene while type 2 ones are characterized by a peak metamorphic mineral assemblage of garnet + clinopyroxene + rutile. Type 2 clinopyroxene and garnet are relatively more enriched in Mg contents than type 1 ones. Type 2 eclogites are significantly more enriched in TiO₂, P₂O₅, and light rare-earth elements, but more deplete in Al₂O₃ contents and Mg numbers. P–T estimates for both types eclogites are consistent with minimum pressures of 23–25 kbar at 830–920 °C, with the retrograde amphibolite facies assemblage of 15.2–17.5 kbar and 500–732 °C. Zircon U–Pb dating indicates that the protolith of type 1 eclogites, with normal mid-ocean ridge basalt affinity, emplaced at ~260 Ma, whereas the protolith of type 2 eclogites, geochemically familiar with oceanic-island basalt, erupted at ~242 Ma. ⁴⁰Ar/³⁹Ar analyses of hornblende from the retrograded eclogites indicate that the eclogite-bearing high-pressure rocks were exhumed at ~194 Ma. The occurrence of these high-pressure rocks implies the subduction of oceanic crust to a depth of ~85 km along the southern margin of the Qiangtang block during the Late Triassic, and the Bangong-Nujiang Meso-Tethyan ocean could have lasted over 160 Myr.