Carboniferous volcanites in the Bogda orogenic belt of eastern Tianshan: their tectonic implications.
东天山博格达造山带石炭纪火山岩及其形成地质环境

The Lower Middle Carboniferous marine volcanites in the Bogda Orogenic Belt of Eastern Tianshan show bimodal features and are dominated by soda rich basalts and rhyolites with minor dacites and rare andesitic rocks. The basalts are characterised by modal quartz in a few samples, occasional occurrence of olivine phenocrysts, diopsidic and salitic clinopyroxene with higher aluminum (Al2O3=4.175.99) and titanium (TiO2=2.804.78) contents, albite and oligoclase dominated matrices with considerable amounts of andesine and labradorite as phenocrysts, most wholerock 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 subalkaline area and close to the boundary to the alkaline area, 100Mg/(Mg Fe)<65, REE=38.73134.48, (La/Lu)N=2.754.03, Eu=0.93~1.15, moderately enriched in highly incompatible elements, moderately depleted in Nb and Ti, low contents of Ni and Cr, positive Nd and Sr values. These characteristics indicate that basalts in the Bogda belt belong to olivine tholeiites overall, but had diverse evolution to alkaline basalts and quartz tholeiites. These basalts were produced by an olivine tholeiitic or picritic magma which originated from a depleted mantle and subsequently differentiated and was then contaminated by the crust. The bimodal volcanism represented by such basalts with accompanying dacites and rhyolites argues for the Carboniferous continental rift environment of the Bogda oregenic belt.