Early Paleozoic metamorphism and anatexis of lower retro−/back-arc crust in the western Central Qilian belt, NE Tibet

Abstract

The tectonic-thermal mechanism of crustal melting provides vital information on continental differentiation and orogenic evolution. This study aims to understand the evolution and tectonic setting of newly discovered lower crustal leucosome-rich granulites of the Qilian Orogen, NW China. Tonalitic leucosome within garnet amphibolite occurs in net-like arrays or as stringers and has diffuse boundaries with the host, with enriched LILE, and high Sr and Sr/Y ratios, similar to that of adakite, formed by water-fluxed melting. However, granitic leucosome within garnet granitic gneiss shows sharp contacts with the host, and has low Sr, Ba, K and Pb but high Rb, Rb/Sr, with negative Eu anomalies, consistent with dehydration melting. Petrographic observations, geochemical data and pseudosection modelling suggest that garnet amphibolite experienced water-fluxed melting and garnet granitic gneiss underwent dehydration melting, respectively. LA-ICP-MS zircon and titanite dating indicate the anatectic event proceeded at ∼480–470 Ma. Phase equilibrium modelling suggests the garnet amphibolite and garnet granitic gneiss share a similar P-T path involving decompression heating from ∼12.1 kbar/711–746 °C to reach peak temperatures of 798–836 °C at 10.4 kbar, and subsequently decompression cooling to 4.6–7.6 kbar/600–672 °C. The calculated P-T conditions suggest Barrovian-type metamorphism with thermal gradients of 24.2 °C/km for the Central Qilian lower crust. We interpret a retro−/back-arc setting resulting from the southward subduction of the North Qilian oceanic lithosphere.