Chemical and Pb isotope composition of olivine-hosted melt inclusions from the Hannuoba basalts, North China Craton: Implications for petrogenesis and mantle source

Abstract

Melt inclusions and their host olivine provide unique information about the nature, distribution and scale of mantle source heterogeneity. We present the first analyses of the chemical and Pb isotope compositions of melt inclusions and their host olivine from the Cenozoic Hannuoba basalts, North China Craton, which contains coexisting suites of alkali and tholeiitic basalts. There is limited variation in major and trace element composition, but significant Pb isotopic variation, in the tholeiitic samples. This contrasts with the substantial variation in major and trace elements, but limited Pb isotopic variation, in the alkali basalt samples. Based on the results of the major- and trace-element modeling, the compositional variation of the alkali basalts may be primarily attributed to garnet pyroxenite melting, with only a small input from garnet peridotite melts. The garnet–pyroxenite component involved in the genesis of the alkali basalts, of metasomatic origin, may have formed in the lithosphere. The wide range of isotopic composition of the tholeiitic basalts and remarkably limited variability of major and trace element composition is argued to be characteristic of the source region, which is a mixture of peridotite with small amounts of pyroxenite transformed from recycled ancient oceanic crust and sediment. The presence of recycled oceanic crust in the mantle source and the low–velocity anomaly observed beneath the Taihang Mountains, located 50km to the southeast of the Hannuoba region, supports a plume model for their origin.