Protolith of eclogites in the north Qaidam and Altun UHP terrane, NW China: Earlier oceanic crust?

Abstract

An early Paleozoic ultrahigh pressure metamorphic belt occurs in the north Qaidam–Altun mountains and was offset about 400 km southwestward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high TiO 2 (2–5 wt%), medium TiO 2 (1–2 wt%) and low TiO 2 (<1 wt%). Geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments, including mid-ocean ridge basalt, island arc basalt and ocean island basalt. The Nd isotopes of these rocks are similar to those of MORB, characterized by mainly positive and minor negative εNd(0) values, providing further evidence that the eclogite protoliths were ocean floor basalts to which minor crustal components were added during subduction. Geochronological data indicate that the UHPM occurred about 500–440 Ma ago, and that there were two eclogite protoliths with ages of 800–750 and ∼1000 Ma, respectively. Geochemically and isotopically, the eclogites are similar to basaltic rocks in Luliangshan, north Qaidam mountains, which have Sr and Nd ages of 768±39 and 780±22 Ma. The age and composition of these volcanic rocks suggest that they were one type of the protoliths for the eclogites. A later series of calc-alkaline, island arc volcanic rocks was formed at about 500 Ma ago, essentially contemporaneous with the UHP metamorphism. We propose the following tectonic model for the evolution of the north Qaidam UHP belt. At about 1000 Ma ago, a number of continents were amalgamated to form the Rodinian continent in this general area, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800–750 Ma ago to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses in the north Qaidam mountains. In the early Paleozoic another Qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 Ma ago and the subduction-related granites (470–450 Ma). Once the oceanic lithosphere was consumed, the subsequent continent–continent collision led to deep subduction of continental crust, which was tectonically mixed with the eclogites, to cause the ultrahigh pressure metamorphism.