Newly discovered eclogites from the Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China): mineralogy, geochemistry, geochronology, and tectonic implications

Abstract

ABSTRACTWe report the occurrence, mineralogy, whole-rock geochemistry, and geochronology of two types of eclogites newly discovered in the western Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China). Type 1 eclogites contain a peak metamorphic mineral assemblage of garnet + clinopyroxene whereas 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 depleted in Al2O3 contents and Mg numbers. P–T estimates for both types of 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 (OIB), erupted at ~242 Ma. 40Ar/39Ar analyses of hornblende from the retrograded eclogites indicate that the Gaize 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.