The Cenozoic Basaltic Rocks of Eastern China: Petrology and Chemical Composition

The Cenozoic volcanicity of eastern China is entirely basaltic and occurred as relatively small eruptions widely dispersed in space and time, closely associated with graben basins and their regional bounding faults. Samples (157) from over 30 sites in eastern China have been studied. They are predominantly alkaline basalts, but vary in composition from olivine nephelinites and leucitites to quartz tholeiites. The majority are aphyric but some contain olivine and clinopyroxene phenocrysts. Whole-rock analyses (X-ray fluorescence) of all samples for the major and 13 trace elements are used, as are the compositions of all the major mineral phases determined by electron microprobe. It is argued that the most primitive basanites, alkali olivine basalts, and olivine tholeiites represent primary or near-primary magmas which were formed by different degrees of partial melting of the upper mantle at different depths. The olivine tholeiites represent larger degrees of partial melting (8–9%) of a spinel peridotite at depths of <66 km. The alkalic basalts carry xenoliths of spinel and garnet peridotite and appear to have been derived by 1–7% partial melting of a garnet lherzolite (50% ol, 25% opx, 15% cpx, 10% garnet) at depths > 79 km. The olivine nephelinite may have formed by even smaller degrees of partial melting. Most flows are not primary; the variations in their compositions are consistent with fractional crystallization from the spectrum of primary parents created by varying degrees of partial melting of a mineralogically heterogeneous source. The tholeiites have fractionated by the removal of clinopyroxene and some olivine; the alkali basalts by the removal of clinopyroxene with a smaller proportion of olivine. The incompatible behavior of Sr implies the absence of plagioclase from any of the fractionating assemblages and, together with the high Al content of the pyroxene phenocrysts, suggests that much of the fractionation occurred at mantle depths and pressures. The Cenozoic magmatism of eastern China is seen as a typical example of volcanism associated with continental extension. That is, small volumes of predominantly alkalic basalts and olivine tholeiites erupted over a prolonged period and associated with extensional basins and their bounding faults. As such, the province is distinct from continental flood basalt provinces.