Geochemistry and Tectonic Significance of Basaltic Lavas in the Neoproterozoic Yanbian Group, Southern Sichuan Province, Southwest China

Abstract

The ~840 Ma Yanbian Group on the western margin of the Yangtze block, Southwest China, is a >6 km thick, volcanic-sedimentary sequence consisting of basaltic lavas + associated cherty layers in the lower part, and flysch deposits in the upper part. The lavas have variable bulk-rock compositions: SiO2 ranges from 45.3 to 50.7 wt%, Al2O3 from 12.8 to 16.8 wt%, and TiO2 from 1.3 to 2.9 wt%. Although the incompatible trace elements, including Nb, Ta, Zr, Hf, Ti, and HREE, are similar to those of MORB, the lavas show features of island-arc tholeiites with pronounced enrichments in Rb, Ba, U, Sr, and Pb. They are characterized by low initial 87Sr/86Sr ratios (0.7030 to 0.7040) and strongly depleted εNd(t) values (+3.8 to +8.0). These geochemical features of the basaltic lavas suggest derivation from an N-MORB-type mantle source modified by slab-derived fluids and formation in an extensional back-arc basin setting. This conclusion supports an arc setting along the western margin of the Yangtze block during Neoproterozoic time. Therefore, the western margin of the Yangtze block can be correlated with the northeastern margin of the Greater India plate, both being an active arc above an eastward subduction zone at that time.