Lithosphere thickness controls continental basalt compositions: An illustration using Cenozoic basalts from eastern China

Abstract

Abstract Recent studies demonstrate that lithosphere thickness variation exerts the primary control on global seafloor basalt compositions. If the mechanism of such control, i.e., the lid effect, is indeed at work, lithosphere thickness variation must also influence basaltic compositions in continental settings. To test this hypothesis, we chose to study Cenozoic basalts in eastern continental China over a distance of ∼260 km along a southeast-to-northwest traverse with a steep topographic gradient (∼500 to ∼1500 m above sea level) mirrored with a steep lithospheric thickness gradient (∼80 to ∼120 km). The basalts erupted on the thinned lithosphere to the east are characterized by lower pressure (e.g., higher Si72, lower Mg72, Fe72, and [Sm/Yb]N; subscript “72” refers to corresponding oxides corrected for fractionation effect to Mg# = 72; N—primitive mantle normalized) and higher extent (e.g., low Ti72, P72, K72, Rb, Ba, Th, and ratios of more- to less-incompatible elements such as [La/Sm]N, Ba/Zr, and Zr/Yb) of melting than basalts erupted on the thickened lithosphere to the west. Importantly, these geochemical parameters all show significant correlations with both lithosphere thickness and topographic elevation. These first-order observations are a straightforward manifestation of the lid effect. Lithospheric contamination and mantle-source compositional variation can indeed contribute to the compositional variability of these continental basalts, but these latter effects are averaged out and are overshadowed by the lid effect. This finding emphasizes the importance of evaluating the lid effect before interpreting the petrogenesis of continental basalts and mantle dynamics. Our results also indicate that the continental surface elevation is isostatically balanced above a mantle depth that is deeper than the lithosphere-asthenosphere boundary.