Geochemical Distinction between Carbonate and Silicate Metasomatism in Generating the Mantle Sources of Alkali Basalts

Crustal metasomatism by subduction is considered as an important mechanism for generating mantle heterogeneity through infiltration of different metasomatic agents into the mantle. As a consequence of the subduction of oceanic crust (including oceanic basaltic rocks and seafloor sediments), both carbonate and silicate metasomatism are expected to occur at the slab-mantle interface in an oceanic subduction channel. This is demonstrated by an integrated study of major and trace elements, stable Mg-isotopes and radiogenic Sr-Nd-Hf isotopes in Cenozoic and Mesozoic alkali basalts from the West Qinling orogen, China. Although the two series of continental basalts show ocean island basalt (OIB)-like trace element distribution patterns and relatively depleted Sr-Nd-Hf isotope compositions, they exhibit differences in other geochemical variables. The Cenozoic basalts have low SiO2, but high CaO and MgO concentrations, and high CaO/Al2O3 ratios but low delta(26) Mg values of -0.54 to -0.32 parts per thousand. They have relatively high abundances of melt-mobile incompatible trace elements such as the light rare earth elements (LREE) and most large ion lithophile elements (LILE), but low abundances of K, Pb, Zr, Hf and Ti, with high (La/Yb)(N) values but low Ti/Eu and Hf/Sm ratios. In contrast, the Mesozoic basalts have relatively high SiO2, but low CaO and MgO concentrations, and low CaO/Al2O3 ratios, but high delta(26) Mg values of -0.35 to -0.21 parts per thousand. They have relatively low abundances of melt-mobile incompatible trace elements such as the LILE and LREE, but are relatively enriched in Zr, Hf and Ti, with low (La/Yb)(N) values but high Ti/Eu and Hf/Sm ratios. These observations indicate a significant difference in the composition of the mantle sources between the two periods of alkali basalt magmatism. Whereas the low delta(26) Mg values of the Cenozoic basalts indicate the involvement of sedimentary carbonate in their mantle source, the high delta(26) Mg values of the Mesozoic basalts point to a primary contribution from a silicate component. Metasomatic reaction of depleted mid-ocean ridge basalt (MORB)-source mantle peridotite with carbonate and silicate melts, respectively, at the slab-mantle interface in the Paleotethyan oceanic subduction channel are responsible for the generation of their mantle sources. Both carbonate and silicate melts have been incorporated into the depleted MORB-source mantle wedge producing 'crustal metasomatism' in an oceanic subduction channel. We suggest that the type of metasomatic agent in the mantle sources is the key to the composition of the resultant alkali basalts.