Triassic Subduction of the Paleo-Tethys in northern Tibet, China: Evidence from the geochemical and isotopic characteristics of eclogites and blueschists of the Qiangtang Block

A ∼500 km-long high-pressure metamorphic belt occurs in the Qiangtang Block of northern Tibet. The occurrence of high-pressure metamorphic rocks (eclogite and blueschist) has a significant bearing on the Paleo-Tethys subduction and plate suturing processes. A few studies have been conducted on petrology, mineralogy and geochronology of high-pressure rocks. In this paper we present the results of a geochemical and isotopic investigation on eclogite and blueschist from this belt, and discuss the probable geodynamic implications. Geochemical analyses show that the two rock types, eclogite and blueschist, have grossly similar bulk compositions, but they can be subdivided into two groups based on their TiO2 contents. The low TiO2 (<2.75 wt.%) rocks exhibit REE distribution patterns and trace element abundances similar to enriched mid-ocean ridge basalts (E-MORB), whereas the high TiO2 (>2.75 wt.%) rocks are more akin to ocean island basalts (OIB). The positive εNd(T) values (ranging from 0.4 to 6.3) for all samples suggest that the basaltic protoliths of the eclogites and blueschists were derived from a long-term depleted mantle source. However, their geochemical characteristics require that the source be metasomatized and enriched in large-ion-lithophile and light rare-earth elements shortly before the formation of these basaltic protoliths, like for most OIB and E-MORB of the world. The recognition of oceanic basalts suggests that a Paleozoic ocean should be present in the central Qiangtang Block. This ocean was hypothesized to be the western extension of the Changning–Menglian Paleo-Tethys Ocean in the eastern margin of the Tibetan plateau. The eclogite and blueschist were formed by northward subduction of this Paleozoic ocean in the Triassic.

► The Qiangtang metamorphic belt consists of blueschists and elcogites.
► Eclogites and blueschists show the characters of OIB and E-MORB.
► Eclogites were formed by northward subduction of Paleo-Tethys Ocean.
► They marked a Triassic suture zone between Gondwana-derived block and Laurasia.