Petrogenesis and tectonic implications of Upper Triassic appinite dykes in the East Kunlun orogenic belt, northern Tibetan Plateau

Abstract

This paper presents geochronological, mineralogical, geochemical, and Sr-Nd isotopic data for recently identified Upper Triassic appinite dyke swarms in the East Kunlun orogenic belt (EKOB), northern Tibetan Plateau. Zircon U-Pb isotopic analyses using SHRIMP and LA-ICP-MS techniques yield 206Pb/238U ages of 226.1±1.9Ma and 226.4±3.5Ma, which can be interpreted as the crystallization ages of the appinite dykes. All the samples are characterized by high Al2O3 contents, enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and strong depletions in Nb, Ta, Ti and P. These characteristics are comparable to those of subduction-related calc-alkaline high-Al basalts. Delayed plagioclase nucleation caused by relatively high-pressure conditions and high H2O contents is the most likely interpretation of the high Al contents. The geochemical and isotopic variations suggest that the appinite dykes were derived from partial melting of a mixed mantle source consisting of enriched lithospheric mantle and MORB-type asthenospheric mantle and had experienced some degree of crustal contamination during migration through continental crust. The appinite dykes are associated with previously documented, contemporaneous high-Ba-Sr and/or high-Sr/Y granites (225–223Ma) derived from thickened mafic lower crust. These rocks provide important constraints on the compositional variations in the orogenic lithospheric mantle during the Late Triassic and provide evidence of a post-collisional environment associated with lithospheric delamination or slab break-off.