Constraints on the origin of historic potassic basalts from northeast China by U–Th disequilibrium data

The only historic eruptions of potassic basalts on the Earth reported so far took place in 1719–1721 AD in the Wudalianchi area of northeast China. These historic potassic basalts are characterized by significant 230Th excesses [(230Th/238U)=1.24–1.33], and low 143Nd/144Nd (εNd=−5.0 to −3.7) and 206Pb/204Pb (16.8–17.1). Strong 230Th excesses, low CaO+Al2O3, and large bulk partition coefficients (D) in Rb, K, and Zr (DRb(DK>DZr>DYb>DSm) inferred from a melt-normalized spider diagram suggest phlogopite-bearing garnet peridotites as dominant source rocks. The presence of garnet peridotites as mantle source, combined with negative εNd in basalts and the geophysically determined lithosphere thickness in the study region (120 km), indicates the generation of these basalts at the depth of 80–120 km in the subcontinental lithospheric mantle. Dynamic melting inversion (DMI) of between-magma trace element concentration ratios and U–Th disequilibrium data suggests low-degree partial melting (5–7%) of a slow upwelling (<1.6 cm/year) mantle source, although extremely low-degree batch partial melting (0.3–0.5%) of a static source is also possible. Source Nd model ages suggest that the source rocks for the Wudalianchi basalts were metasomatized at 1.0 Ga, provided that it has not experienced multistage enrichment history. Lithospheric extension since late Tertiary in the study region was responsible for slow decompression melting of phlogopite-bearing garnet peridotites in deep subcontinental lithospheric mantle. Recent (<350 ka) subduction of the Pacific Plate may have produced lithospheric extension in the study region, but did not directly contribute subduction-related fluids to the source rocks for these potassic basalts because 230Th enrichments are uncharacteristic of melts generated by subduction. The subducted slab may have lost fluids released from subducted sediments before the slab can reach the mantle beneath NE China.