Influence of subducted carbonate on the composition of basalts in the Paleo-Asian Ocean domain

Abstract

The fate of subducted carbonate of the Paleo-Asian Ocean and its control on the compositional variability of Cenozoic basalts to the west of the Daxing'anling–Taihang Gravity Lineament remain uncertain. Here we report major- and trace-element contents and Zn–Sr–Nd isotopic data for Cenozoic basalts from eastern Mongolia. The studied basalt samples from eastern Mongolia are classified as nephelinite, basanite, alkaline basalt, and tholeiite. They have different Zn–Sr–Nd isotopic compositions (δ66Zn = 0.38‰ ± 0.02‰ to 0.50‰ ± 0.04‰, 87Sr/86Sr = 0.704239–0.705175, εNd = 4.72–5.67) but display trace-element signatures of sedimentary marine carbonate, reflecting mantle heterogeneity and marine carbonate imprint. Compared with the high-Si tholeiites, which were erupted over thinner lithosphere, the nephelinites–basanites–alkaline basalts erupted over thicker lithosphere have higher δ66Zn values, La/Yb, Sm/Yb, and Nb/Y ratios, and Nb contents, indicating that these low-Si basalts were the product of lower-degree melting of carbonated mantle under higher pressure. By combining our data with available data for Cenozoic basalts from central Inner Mongolia, we suggest that the compositions of the nephelinite–basanite–alkaline-basalt–tholeiite suite show systematic spatial variation with respect to the northward subduction zone of the Paleo-Asian Ocean. The tholeiites, which have lower δ66Zn values, SiO2 contents, and Sm/Yb ratios, are dominant near the subduction zone, whereas the basanites and alkaline basalts, which have higher δ66Zn values, Sm/Yb ratios, and SiO2 contents, are dominant in the region far from the subduction zone. On the basis of a mixing model for Sr–Nd–Zn isotopic compositions, we suggest that the mantle source of the basanites and alkaline basalts incorporated a higher proportion of marine carbonate, whereas the tholeiite mantle source incorporated a higher proportion of siliceous sediments. We deduce that with increasing depth of slab subduction, more sedimentary carbonates and lesser siliceous sediments were released from the subducted slab.