Water has become an effective means to trace the mantle source of basaltic magmas recently. To investigate the source of the potassium-rich component in the Xiaogulihe ultrapotassic volcanic rocks of NE China, we measured the water content of clinopyroxene(cpx) phenocrysts by Fourier transform infrared spectrometry and calculated the H2 O content of the equilibrated melts according to the partition coefficients of H2 O between cpx phenocrysts and basaltic melts. The estimated H2 O content of the ‘‘primary'' magmas is low(0.36 wt%–0.50 wt%),within the range of mid-ocean ridge basalts and ocean island basalts, while it is obviously lower than that of backarc basin basalts and island arc basalts. The calculated H2O/Ce ratio of the ‘‘primary'' magmas is about 15, which might be similar to that of the dehydrated sediments(\100), but observably lower than that of the normal depleted mantle(DMM, H2O/Ce = 150–210). The low water content and especially low H2O/Ce ratio of the‘‘primary'' magmas demonstrate that the K-rich component of these ultrapotassic volcanic rocks seems unlikely tooriginate from K-bearing hydrous minerals(such as phlogopite) in metasomatic subcontinental lithospheric mantle.Combined with the low206Pb/204 Pb ratios, moderately high87Sr/86 Sr ratios of the bulk rocks and the high d18 O values of olivine phenocrysts, we suggest that the K-rich component in the mantle source of the Xiaogulihe ultrapotassic volcanic rocks may come from ancient continental-derived sediments which dehydrated significantly during subduction(e.g., dry K-hollandite).
Based on studies of the water content of the early Cretaceous Feixian high-magnesium basalts in the eastern part of the North China Craton(NCC),it has been suggested that the early Cretaceous lithospheric mantle of the eastern NCC was highly hydrous([1,000 ppm,H2O wt.)and that this high water content had significantly reduced the viscosity of the lithospheric mantle and provided a prerequisite for the destruction of the NCC.The eastern part of the NCC had undergone multistage subduction of oceanic plates from the south,north,and east sides since the early Paleozoic,and these events may have caused the strong hydration of the NCC lithospheric mantle.To determine which subduction had contributed most to this hydration,we measured the water contents of the peridotite xenoliths hosted by the early Cretaceous high-magnesium diorites of Fushan in the southcentral part of the Taihang Mountains.Our results demonstrate that the water content of the early Cretaceous lithospheric mantle beneath the south part of the Taihang Mountains was*40 ppm and significantly lower than that of the contemporary lithospheric mantle beneath the eastern part of the NCC.Thus,the hydration of the early Cretaceous lithospheric mantle of the eastern part of the NCC can be ascribed to the subduction of the Pacific plate from the west side.Thus,the main dynamic factor in the destruction of the NCC was likely the subduction of the Pacific plate.
