GRL paper: How lithosphere affects potassic basalts (by Jian-Qiang Liu)

The genetic mechanism of potassic basalts has always been one of the hot issues among petrologists. Ph.D. candidate Jian-Qiang Liu in Rockingmantle Group compiled the geochemical data of Cenozoic basalts in NE China, and based on these data well correlations between the lithospheric thickness and chemical compositions of potassic basalts were observed. The paper "Lithospheric thickness controlled compositional variationsin potassic basalts of Northeast China by melt-rock interactions" was published in Geophysical Research Letters. In this work, geochemical data of potassic and ultra-potassic basalts from NE China ( Nuominhe, Keluo, Wudalianchi, and Erkeshan ) were summarized. The potassic rocks show positive correlations of K₂O/Na₂O and Rb/Nb versus ⁸⁷Sr/⁸⁶Sr. Furthermore, these compositional variations in potassic basalts show a direct correspondence to the thickness of underlying lithosphere. For the basalts erupted from the Erkeshan to Nuominhe, the average Sr–Nd isotopic composition becomes less enriched, while MgO contents increase and K₂O/Na₂O along with Rb/Nb ratios decrease with increasing lithospheric thickness, suggesting the influence of lithospheric mantle on compositions of potassic basalts. Based on the occurrence of veined or intergranular phlogopite as the main metasomatic phase in the mantle xenoliths, we proposed a model that the precipitation of phlogopite means that a longer duration of melt-rock reaction under thicker lithosphere would consume more K and Rb and lead to lower K₂O/Na₂O and Rb/Nb ratios in the residual melts, which is consistent with the variations in elemental ratios of the potassic basalts in NE China.

The influence of lithospheric thickness on compositions of potassic basalts