Extremely high-Na adakite-like magmas derived from alkali-rich basaltic underplate: The Late Cretaceous Zhantang andesites in the Huichang Basin, SE China

The Zhantang andesites, which erupted in the Late Cretaceous Huichang Basin of SE China interior, were associated with the Late Mesozoic lithospheric extension and basaltic underplating. They are high-Na (6.59-8.46 wt% Na2O), high-Al trondhjemitic rocks with high Sr and Ba, low Y and HREE, and high Sr/Y and La/Yb ratios, similar to modern adakites. However, they are much higher in Na2O but lower in CaO than adakites. Their epsilonNd(T) values of -2.3 to -3.8 and initial Sr-87/Sr-86 ratios of 0.707 to 0.708 are also significantly different from those of adakites with MORB-like isotopic compositions. These chemical and isotopic features, along with the tectonic setting, suggest that they were not produced by partial melting of subducting slab. The Zhantang adakite-like magmas may be derived from underplated basaltic lower crust, but require a compositionally peculiar basaltic protolith to account for their extremely high-Na and low-Ca feature. They are compositionally comparable to the experimentally produced partial melts of alkali-rich basalt. We therefore interpret them as the products of melting of an alkali-rich basaltic rock.