Geochemical characteristics of amphibole-bearing metamorphic rocks in Sanjiang region,southeastern Tibetan plateau

Abundant and complex metamorphic rocks are widespread in Sanjiang region,southeastern Tibetan plateau.Amphibole-bearing metamorphic rocks,comprising amphibolite and amphibole-bearing plagioclase gneiss,occur as lenses or thin layers within metamorphic supracrustal rocks.Amphibolite could be subdivided into two types: one is a retrogressive product of garnet-pyroxenolite,and the other is a product of mafic protolith which experienced amphibolite-to granulite-facies metamorphism.The amphibolite has an average SiO2 content of ~50.0%,with high-Al,low-Ti,Zr and Zr/Y ratio.The chondrite-normalized REE patterns show enrichment of LREE with slightly negative or no Eu anomalies.These geochemical characteristics are very similar to those of the island-arc basalt,indicating that the protolith of the amphibolites was formed in the subduction-collision tectonic setting.In contrast,the amphibole-bearing plagioclase gneiss has a higher average SiO2 content of ~63.64% with low MgO and TiO2 content,characterized by the enrichment of large ionic lithophile elements(LILE) such as K,Rb,Ba and Th,and the depletion of high field strength elements(HFSE) such as Ti,Zr,Hf,Nb and Ta.The REE patterns are characterized by relatively high fractionations of LREE and HREE with pronounced negative Eu anomalies.These geochemical features are inherited from those of andesitic or andesitic/basaltic rocks,indicating that the protolith of the amphibole-bearing plagioclase gneiss appeared in a plate convergence tectonic setting.