Water Content Of Mesozoic And Cenozoic Basalts Of The North China Craton

The stability of a craton and the genesis of continental basalts have been enthusiastically studied but still hotly debated during the past few decades. The destruction of a craton is expected to be closely associated with the hydration of the lithospheric mantle, but direct evidence for this speculation has been scarce. The eastern part of the North China Craton (NCC), a clear example of a "destroyed craton", has been subducted by multiple oceanic slabs from different directions since early Paleozoic. Thus, the removal of the lithospheric mantle’s root in eastern NCC provides an appropriate case to study the relationship between the stability of the cratonic lithospheric mantle and its water content. The genesis of the Cenozoic continental alkali basalts in the NCC are in large controversy:for the basalts in eastern NCC the recycled lower continental crust, the recycled oceanic crust (most likely the Pacific oceanic slab) and the recycled metasomatized veins in lithospheric mantle had been suggested to be involved in the mantle source; for the basalts in the Taihang Mountains of central NCC, the early work explained them as the production of the interaction between normal mantle derived melts and the old, enriched lithospheric mantle, while the recent Mg isotope study implied the involvement of recycled oceanic carbonate in the source.In this thesis, in order to give new insights into these two problems, three suites of basalt samples were studied:1) the Feixian basalts, which were erupted in the early Cretaceous (～120Ma), the peak time of the NCC’s destruction, and suggested to be derived from the metasomatized lithospheric mantle;2) the same sample suite of the Cenozoic alkali basalts from the Taihang Mountains of central NCC mentioned above;3) the Shandong Cenozoic alkali basalts of eastern NCC, which were suggested to be associated with the recycled continental crust. For the first time, the water content, the major element concentrations, the oxygen isotope compositions, the trace element abundances and the87Sr/86Sr ratios of the clinopyroxene (cpx) phenocrysts in the studied basalts were systematically investigated, by in-situ analytical techniques (e.g. micro-FTIR, SIMS,(MC)LA-ICP-MS). A reverse calculation, based on the water content in cpx and plagioclase (pl)(the water content of pl was measured only when the large and clear crystals were available) and the water partition coefficients between minerals and basaltic melts (e.g. for cpx it is tetrahedral Al3+dependent, for pl it is assigned as0.004), was used to recover the water content of the magma, by which the water content of magma could be calculated with error less than32%.By this phenocryst-based approach, the H2O content of the primary Feixian basaltic magma, was estimated to be3.4±0.7wt.%. The corresponding H2O content of the lithospheric mantle source was estimated to more than1000ppm by weight, which is much higher than both the source of mid-ocean-ridge basalts (MORB) and the Kaapvaal craton (South Africa) which is still stable after>3billion years. This indicates that the NCC’s lithospheric mantle was indeed highly hydrated at the peak time of its destruction. When the water partition coefficient between ol and pyroxenes and the mineral proportions were assumed, the water content of ol in the lithospheric mantle was estimated to be180ppm at least. The calculation of effective viscosity by the power flow law shows that the effective viscosity of this hydrated lithospheric mantle was very close to that of the underlying asthenosphere, which might have provided a prerequisite for the destruction of the NCC.For the Taihang basalts, the calculated water contents (H2O by weight) of the primary magmas range from0.20to1.07wt.%, and the corresponding H2O/Ce ratios correlate well with bulk rock (Nb/La)n (n represent primitive mantle normalization),(Ba/Th)n and εNd. The oxygen isotope ratios (δ18Ov-SMOW) of the cpx phenocrysts range from5.8to7.4%o and the average value for each sample range from6.6to7.0%o, much higher than the value of cpx derived from a normal-mantle (5.6±0.2‰). In-situ trace element and Sr isotope data excluded the possibility that crustal assimilation or contamination during magma ascent was responsible for such high δ18O. In sample JD-1and MAS-1,δ18O of cpx positively correlate with Mg#. In addition, the δ18O correlate well with cpx (Sm/Lu)n, and (Sm/Lu)n also negatively correlate well with (Lu)n, which indicate a magma/cpx mixing in the volcano plumbing system. These new data suggest that the components from an extremely dehydrated recycled upper oceanic crust together with sediments have contributed to the mantle source of the Taihang basalts.For the Shandong basalts (Fuyanshan basalts), the estimated water content of magma varies from0.58to3.89wt.%. These water contents have reach the water content range of island arc basalts. The recoverved water conents are not correlated with bulk rock MgO and La/Yb, and positively correlates with bulk strong imcompabile elements, like Rb. These observations indicate that the hight water contents of Fuyanshan basalts and their variation were not caused by fractional crystallization, magma degassing and partial melting. The positive correlation between H2O/Ce and87Sr/86Sr excluded the possible role of amphibole veins in the lithospheric mantle, because they were usually depleted in87Sr/86Sr, and the role of the recycled Su-Lu eclogites, because they are relatively dry and have low H2O/Ce ratios. Such high water contents in basaltic magmas point to a water-rich component in the mantle source. The δ18O of the cpx phenocrysts vary from3.6%o to6.3‰, and most of them are lower than the value of cpx derived from a normal-mantle. Profile analysis show that they are not caused by water-rock interaction after eruption.The depleted87Sr/86Sr ratios of these cpx phenocrysts indicate that the low δ18O are not result from involvement of Sulu low-δ18O eclogites. Instead, they could be best explained by the involement of recycled lower part of oceanic crust. Because our modeling calculation shows that the decoupling of εHf and sNd could be caused by involvement of marine sediment, which could results in high Ba/Th ratios, positive Sr spikes, and low Ce/Pb ratios for Fuyanshan basalts, we suggest that subducted oceanic sediment in the mantle transition zone (MTZ) is the most likely water source. The systematic differences in water contents and oxygen isotopes among the Taihang basalts and the Shangdong basalts demonstrate that distinct components from the subducted oceanic crust, most likely the Pacific slab, were involved in the mantle source.