High water content in Mesozoic primitive basalts of the North China Craton and implications on the destruction of cratonic mantle lithosphere

摘要

It has been suggested that the longevity of cratons (i.e. ancient and stable cores of continents) is related in part to the low water content of their deep mantle roots; this gives them a higher viscosity than the underlying asthenosphere. Consequently, the removal of cratonic roots is expected to be closely connected to the hydration of the lithospheric mantle, but direct evidence for this speculation has been scarce. The eastern part of the North China Craton (NCC) is a clear example of a "destroyed craton". In this study the H_2O content of clinopyroxene phenocrysts was measured in lithospheric mantle-derived high-magnesium basalts of the Feixian area, in the eastern part of the NCC. These lavas erupted in the early Cretaceous (~120Ma), which was the peak time of the NCC destruction. Based on these data, it was estimated that the H_2O content of the lithospheric mantle source of these basalts consists of more than 1000ppm by weight. This water content is much higher than in the source of mid-ocean-ridge basalts (50-200ppm by weight) and also higher than in the Kaapvaal cratonic mantle in South Africa (~120ppm by weight); the latter is still stable after >3 billion years. This study argues that a large amount of water was indeed added to the NCC's lithospheric mantle, probably due to the multi-stage subduction of oceanic plates since the early Paleozoic. This high water content significantly reduced the viscosity contrast between the lithospheric mantle and the underlying asthenosphere, and provided a prerequisite for the removal of the cratonic root of the NCC by reducing its strength.