The Distribution Of Water Content In The Early Cretaceous Lithospheric Mantle Of The North China Craton And Implications For Its Destruction

It has been suggested that the longevity of craton is related in part to the low water content of their deep mantle roots,which leads to a much higher viscosity than that of the underlying asthenosphere.While,the removal of cratonic roots is thought to be closely related to hydration of the lowermost lithospheric mantle.The North China Craton(NCC)is generally considered to be representative of "destroyed craton".The previous investigations of the water content have focused mainly on the marginal areas of the eastern part of the NCC(e.g.,Yixian,Wulanhada,Laiyang and Feixian),which yielded a range of 2.2-4.3 wt.%for the water content of primary melts.However,the state of hydration in the interior region remains unclear.To further understanding and verifying the role of water in destruction of the NCC,it is important to figure out the interior water content of the eastern part of the NCC.Here,we present whole-rock geochemical and Sr-Nd-Pb isotope data for the early Cretaceous mafic intrusions in the interior of the eastern part of the NCC and calculate the water contents of their corresponding primary melts.We also estimate the water contents of the lithosphere mantle that produced these melts.The mafic rocks exhibit arc-like incompatible-trace-elements signatures and enriched Sr-Nd isotopic compositions.The water contents of clinopyroxene phenocrysts,as measured by Fourier transform infrared spectrometry(FTIR),vary from 37 to 472 ppm.The water contents of the corresponding primary melts,which were calculated using the clinopyroxene water contents and the clinopyroxene/melt partitioning coefficient of water,range from 0.85 to 2.07 wt.%.The estimated water contents of the mantle source are at least 571-1214 ppm and are considerably higher than the H2O contents(<10 ppm)of Kaapvaal peridotite xenoliths.H2O/Ce of source of our samples higher than that of MORB and OIB,positive related to bulk-rock(87Sr/86Sr)i and negative related to bulk-rock(206Pb/204Pb)i.The most hydrated sample has the lowest(206Pbp/204Pb)i(<16.6),which implies it came from a very ancient source,a possible explanation is the ancient sediments in the mantle transition zone.Combining the new data with previous results,we show that the lowermost lithospheric mantle beneath the eastern part of the NCC in both its marginal regions was hydrated during the early Cretaceous,which underlines the role of water in the thinning of the lithospheric mantle.