Potassic volcanic rocks in NE China: Geochemical constraints on mantle source and magma genesis

Potassic volcanic rocks from the Wudalianchi, Erkeshan and Keluo (WEK) fields in NE China are located between the Mesozoic Songliao Basin and the Palaeozoic Xing'am Mountains fold belt. These rocks erupted during three main eruptive episodes-Miocene (9{dot operator}6-7{dot operator}0 Ma), Pleistocene (0{dot operator}56-0{dot operator}13 Ma) and Recent (AD 1719-1721)-and are subdivided into three types-olivine leucitite, leucite basanite and trachybasalt-on the basis of modal composition. In comparison with Cenozoic alkaline basalts from East China that are similar to oceanic island basalts (OIBs), WEK volcanic rocks are lower in Al₂O₃, CaO, Fe₂O3 and Sc, but higher in K₂O (3{dot operator}5-7{dot operator}1 wt %), K₂O/Na₂O (>1) and incompatible elements. High ⁸⁷Sr/⁸⁶Sr (0{dot operator}7050-0{dot operator}7056), low ¹⁴³Nd/¹⁴⁴Nd (0{dot operator}51238-0{dot operator}51250) and ²⁰⁶Pb/²⁰⁴Pb (17{dot operator}06-16{dot operator}61) ratios also distinguish them from oceanic and Chinese basalts. Trace element and isotope data indicate that a post-Archaean subcontinental lithospheric mantle source similar to the postulated EM1 component (enriched mantle with low ^l43Nd/¹⁴⁴Nd and moderate high ⁸⁷Sr/⁸⁶Sr) must have played a significant role in magma generation. The source rock is considered to be refractory phlogopite-bearing garnet peridotite heterogeneously enriched in both large ion lithophile elements and light rare earth elements by ancient metasomatism during Proterozoic times. This source may have mixed recently with OIB-like melts, but has not been modified by subduction of the Kula-Pacific plate. Primitive WEK potassic magma was generated by a low degree of partial melting, initiated by an extensional phase beginning in the late Tertiary, at pressures of 20-45 kbar and in the presence of mixed volatile components of H₂O, CO₂ and halogens.