Carboniferous volcanites in the Bogda orogenic belt of eastern Tianshan: their tectonic implications.

The Lower-Middle Carboniferous marine volcanites in the Bogda Orogenic Belt of Eastern Tianshan show bimodal features and are dominated by soda-rich b asalts and rhyolites with minor dacites and rare andesitic rocks. The basal ts are characterised by modal quartz in a few samples, occasional occurrenc e of olivine phenocrysts, diopsidic and salitic clinopyroxene with higher a luminum (Al2O3 = 4.17 similar to 5.99) and titanium (TiO2 = 2.80 similar to 4.78) contents, albite- and oligoclase-dominated matrices with considerabl e amounts of andesine and labradorite as phenocrysts, most whole-rock CIPW norms containing Di, Ol and Hy with a few containing Qz and Ne, most of the plots on the total alkali vs. SiO2 diagram being located in the subalkalin e area and close to the boundary to the alkaline area, 100Mg/(Mg+Sigma Fe) < 65, Sigma REE= 38.73 similar to 134.--- 48, (La/Lu)(N) = 2.75 similar to 4.03, delta Eu = 0.93 similar to 1.15, moderately enriched in highly incomp atible elements, moderately depleted in Nb and Ti, low contents of Ni and C r, positive epsilon(Nd) and epsilon(Sr) values. These characteristics indic ate that basalts in the Bogda belt belong to olivine tholeiites overall, bu t had diverse evolution to alkaline basalts and quartz tholeiites. These ba salts were produced by an olivine-tholeiitic or picritic magma which origin ated from a depleted mantle and subsequently differentiated and was then co ntaminated by the crust. The bimodal volcanism represented by such basalts with accompanying dacites and rhyolites argues for the Carboniferous contin ental rift environment of the Bogda oregenic belt.