Generation of continental intraplate alkaline basalts by edge-driven convection: Insights from the Cenozoic basalts beyond the Big Mantle Wedge

Abstract

Continental intraplate basalts are widespread across Central-East Asia, and their melting mechanisms are poorly known. Herein, we present integrated studies of petrology, elemental-isotope systematics, and thermodynamic modeling of Cenozoic Liangcheng basalts, a representative volcanic field of the vast intracontinental basaltic magmatic province covering Central-East Asia, with the aim of constraining the source characteristics and melting dynamics in this intraplate setting. These basalts have moderate-to-low silica (45.2 to 49.0 wt%), high Fe2O3T (10.0 to 12.2 wt%), and alkali (Na2O + K2O, 4.27 to 7.38 wt%) contents with ocean-island basalt (OIB) -like trace-element patterns and moderately depleted to slightly enriched Sr–Nd–Pb–Hf isotopes (87Sr/86Sr = 0.703924–0.705176, 143Nd/144Nd = 0.512540–0.512861, 206Pb/204Pb = 17.2752–17.9180, 207Pb/204Pb = 15.4860–15.7383, 208Pb/204Pb = 37.6995–38.3599, and 176Hf/177Hf = 0.282852–0.282999). The major elements (e.g., high Fe/Mn, low CaO) and olivine chemistry (e.g., high Ni, Zn/Fe, and low Mn/Zn) favor derivation from a silica-deficient pyroxenite-bearing source, while the trace-element and isotope systematics suggest the involvement of recycled components including oceanic slab and sediment (both terrigenous and pelagic). A grid search with a thermodynamic melting model using incompatible trace elements was carried out to impose quantitative constraints on the mantle source. The modeling results show that the source of the Liangcheng basalt contains 80% primitive mantle peridotite and 20% silica-deficient pyroxenite. This exercise and geobarometric calculation using major elements suggest that melting occurs at a potential temperature of 1380 °C underneath a thinned continental lithosphere with a basal pressure of ∼2GPa, consistent with the source characteristics and melting conditions estimated for many other volcanic fields in the vast Central-East Asia magmatic province. By combining these results with the regional geology and tectonic history, we can suggest that these basalts were formed by decompression melting owing to mantle convection driven by lithospheric thickness variations (edge-driven convection). We suggest that such a melting scenario is ubiquitous beneath Central-East Asia and may have played a vital role in the formation of widespread intraplate continental alkaline basalts.