The pillow lava of Baiyanggou in Bogda，Xinjiang：geochemical and Sr-Nd-Pb isotopic characteristics

Abstract：The late Carboniferous-Early Permian volcanic rocks in Baiyanggou of northeast Bogda orogenic belt mainly consist of sub-alkaline basalt and andesite, in which large quantities of pillow laves are excellently preserved. Pillow lavas are mainly basic lavas, whose SiO2 content is 46.65% to 52.50%, Al2O3=13.10%～16.44%，MgO=4.62%～7.89%，TiO2=1.23%～2.79%，CaO=7.06% ～13.07%，Na2O=2.76%～4.99%，and K2O=0.07%～0.89%. A group of basic lavas is enriched with large ion lithophile elements (LILE) such as Rb and Ba, and the other group is relatively depleted in LILE. In the N-MORB diagram, high field strength elements (HFSE) have significant negative anomalies of such elements as Nb and Ta. In the chondrite-normalized REE patterns, lavas show right-oblique curve with overall performance of flatness and slight enrichment of light rare earth elements (LREE); δEu=0.89～1.12. The composition of Sr-Nd-Pb isotopes shows that lavas have characteristics of the depleted mantle：εNd(t) =+2.8～+3.1, 143Nd/144Nd(i)=0.512407～0.512422, εSr(t)=-14.0～-17.5, 87Sr/86Sr(i)=0.70293～0.70317, 206Pb/204Pb(i)=17.622～17.835, 207Pb/204Pb(i)=15.374～15.463, and 208Pb/204Pb(i)=38.163～38.339. In the tectonic setting diagram, Baiyanggou volcanic rocks have the characteristics of MORB and WPB. The comprehensive regional geological data and the results of previous studies have led the authors to conclude that Late Carboniferous-Early Permian volcanic rocks of the study area might have been derived from the depleted mantle during the closure of the back-arc basin in the active continental margin of Paleo-Asian Ocean.