Stagnant Slab Front Within The Mantle Transition Zone Controls The Formation Of Cenozoic Intracontinental High-Mg Andesites In Northeast Asia

The geochemistry of Cenozoic intracontinental high-Mg andesites (HMAs) in northeast Asia, together with regional geophysical data, offers an opportunity to explore the genetic relationship between the formation of intracontinental HMAs and subduction of the Pacific plate. Compared with primary HMAs in arcs, Cenozoic intracontinental HMAs in northeast Asia have lower Mg# [100 x Mg/(Mg + Fe2+)] values (53-56) and CaO contents (5.8-6.6 wt%), higher alkali (Na2O + K2O) contents (5.15-6.45 wt%), and enriched Sr-Nd-Hf isotopic compositions (Sr-87/Sr-86 = 0.7056-0.7059; epsilon(Nd) = -4.9 to -3.4; epsilon(Hf) = -4.7 to -2.6) as well as lower Pb isotope ratios (Pb-206/Pb-204 = 16.76-19.19; Pb-207/Pb-204 = 15.42-15.45; Pb-208/Pb-204 = 36.71-37.11). These Cenozoic intracontinental HMAs are similar to Cenozoic potassic basalts in northeast China with respect to their Sr-Nd-Pb-Hf isotopic compositions but have higher SiO2 and Al2O3 contents and lower K2O, MgO, and light rare earth element contents. These features indicate that these Cenozoic intracontinental HMAs originated from the mantle, where recycled ancient sediments and water contributed to partial melting of peridotite. Combined with the presence of a large low-resistivity anomaly derived from the mantle transition zone (MTZ) near these intracontinental HMAs, and their occurrence above the stagnant slab front within the MTZ (at 600 km depth) in northeast Asia, we conclude that the stagnant slab front, with high contents of recycled ancient sediments and water, has controlled the formation of Cenozoic intracontinental HMAs in northeast Asia.