Direct Melting of Foundered Lower Continental Crust in NE China: A Case Study of Potassic Wudalianchi Basalts
Abstract:Foundering of mafic lower continental crust (LCC) is often invoked to account for the relatively silicic composition of the bulk crust. However, the fate of this foundered material in the mantle is not well understood. Here we give geochemical evidence, including Sr-Nd-Pb-Hf isotope data, for historic Wudalianchi potassic basalts from NE China, to show that these basalts with extreme EM1 signatures can be direct melts derived from foundered LCC. They are characterized by linear trends between major elements, isotopes and trace elements ratios, suggesting mixing of a high-Si, low-Mg component with a low-Si, high-Mg one. The two end-members are nearly indistinguishable on a trace element spidergram, and both exhibit relative depletion of Th, U, Nb, Ta and Ti, and positive anomalies of Ba, K, Pb. The small range of absolute abundances of trace elements and the absence of Sr and Eu anomalies argue against significant crustal assimilation, which is further confirmed by a negative correlation between Nb/Th ratios and εHf. As a result, we suggest that these chemical features were inherited from foundered mafic lower crustal materials.
Foundered mafic LCC will transform into garnet pyroxenites in the mantle. Experiments have shown that silica-excess and silica-deficient pyroxenites at high pressures can generate the major element compositions of both end-members of Wudalianchi basalts. Quantitative modeling of garnet pyroxenite melting with LCC as starting compositions can reproduce the trace element pattern of Wudalianchi lavas. Therefore, we suggest Wudalianchi lavas to be direct melts of foundered LCC, without requiring reaction between melt and peridotite. Correlations of decreasing 206Pb/204Pb with increasing SiO2 have been observed in Cenozoic basalts from North and NE China, suggesting that the involvement of foundered mafic LCC has played a more important role than previously thought for the genesis of this intra-plate magmatism.