Major- and trace-element compositions of Cenozoic basalts in eastern China: Petrogenesis and mantle source

Abstract

The studied Cenozoic basalts from eastern China have SiO 2 contents ranging from 38.63 to 55.24 wt%, and Na 2O + K 2O from 3.1 to 9.4 wt%. They may be divided into three main types — tholeiitic, alkalic and potassic magmas. Variable trace-element abundances, together with the diverse major-element compositions, suggest that their origins involve both partial melting of mantle rocks at different depths, and subsequent fractional crystallization of clinopyroxene and olivine. Variations in the ratios of highly incompatible elements of the basalts in eastern China demonstrate two mantle components. The enriched mantle component, primarily represented by the basalts in NE China, show high LILE/Nb, Ba/Th, Ba/U, Th/U and low La/Ba ratios, typical of the Walvis Bay Ridge, Gough and Tristan da Cunha oceanic island basalts (OIB). These trace-element features support the identification of EM1 in their mantle source previously made by G.H. Xie and coworkers using Sr, Nd and Pb isotopes. This Dupal mantle source is inferred to be lithospheric and related to metasomatism during subduction, rather than due to the direct incorporation of continental crustal material into the mantle. The second component is represented by basalts from SE China which have the trace-element signatures of normal OIB, i.e. they do not have a Dupal anomaly. It appears most likely that upwelling of the asthenosphere and magma ascent induced the melting of overlying heterogeneous mantle lithosphere to generate the Cenozoic basalts of eastern China.