Influence Of Melt-peridotite Interactions On Deformation And Seismic Properties Of The Upper Mantle Beneath The North China Craton

Melt-peridotite interaction(also termed as mantle metasomatism in a broad sense)is one of the most common and important geological processes in the Earth’s upper mantle.It can change not only the chemical compositions of rocks and minerals,but also the microstructures of rocks and modal contents of minerals,which in turn affect many physical and chemical properties,including deformation and seismic properties,of the upper mantle.The North China Craton(NCC)represents one of the typical examples of ancient cratons that had been destroyed in the world.The lithospheric mantle beneath the NCC had been widely modified by multiple episodes of melt of various chemical compositions.However,the influence of the melt-peridotite interactions on deformation and seismic properties of the upper mantle beneath the North China Craton had received less attention.In this study,we investigated in detail the petrography,microstructures,whole-rock and mineral chemistries,and crystallographic preferred orientation of minerals of mantle xenoliths entrained by the Cenozoic basalts from Damaping(26 samples)and Changle(24 samples)of the NCC by means of optical microscopy,X-ray fluorescence spectrometer,inductively coupled plasma-mass spectrometry(ICP-MS),electron microprobe analyzer(EPMA),laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)and electron backscattering diffraction(EBSD).We also calculated theoretically the seismic properties of our samples and the lithospheric mantle.1)Based on the intensity of foliation and lineation,the Damaping peridotites can be classified into two microstructural groups: weakly to non-foliated and strongly foliated.Microstructural observations show that coarse-grained olivine partly replaced by fine-grained,strain-free pyroxene and vice versa is common in both groups of samples.In addition,fine strain-free pyroxene grains often occur as chains or films along olivine boundaries or at triple junctions.The whole-rock Ca O and Al2O3 contents both decrease linearly with increasing Mg O contents,which is inconsistent with the theoretical partial melting trend but consistent with the refertilization trend.The rare earth elements(REE)of whole rock and clinopyroxene are characterized by slightly LREE-depleted,flat,and LREE-enriched patterns.Additionally,most of the samples show higher clinopyroxene/orthopyroxene modal ratios at a given olivine mode than those predicted by the theoretical partial melting models.All these results indicate that the Damaping peridotites had interacted with multiple episodes of melt of various chemical compositions.Compared with the weakly to non-foliated samples,the strongly foliated ones have a larger amount of fine-grained(<1 mm)olivine and pyroxene grains,and a lower degree of intracrystalline plastic deformation,pointing to higher degrees of melt-peridotite interactions in the latter.EBSD results show that the[010]-fiber CPO is the most common for olivine in the studied samples.The fabric strength of olivine in the strongly foliated samples is weaker than that in weakly to nonfoliated ones.This was attributed to higher degree of annealing and non-dislocation creep deformation in the former than in the latter.The fabric strength and pattern are nearly identical in the coarse-grained and the fine-grained olivine/orthopyroxene from each sample,indicating that the last deformation postdated the latest melt-peridotite interactions.The calculated seismic properties show that the maximum P-wave propagation anisotropy and the maximum S-wave polarization anisotropy decrease with increasing degree of melt-peridotite interactions.Our calculations also suggest that if foliation and lineation are vertical and horizontal,respectively,the measured SKS splitting parameters can be well explained by the “fossil” anisotropy frozen in the lithospheric mantle,with no need to invoke asthenospheric flow as a source of the anisotropy.2)Microstructural observations on the Changle mantle xenoliths show that coarsegrained orthopyroxene is frequently replaced partially by fine-grained,unstrained olivine,and vice versa,and fine strain-free pyroxene grains often occur as chains or films along olivine boundaries or at triple junctions.In addition,clinopyroxene grains from wehrlites even form megacrysts enclosing olivine grains.The clinopyroxene has slightly LREE-depleted,flat,or LREE-enriched REE patterns.Most of the samples have higher clinopyroxene/orthopyroxene modal ratios at a given olivine mode than those predicted by the theoretical partial melting models.All these results indicate that the Changle mantle xenoliths had experienced multiple melt-peridotite interactions with different melts.Moreover,the wehrlites and the olivine clinopyroxenite recorded a higher degree of melt-peridotite interaction than other samples.EBSD results show that olivine develops significant axial-[010] type CPOs in most of the lherzolites and harzburgites,while olivine and pyroxene from the wehrlites and olivine clinopyroxenite develop random fabric,pointing to that higher degrees of melt-peridotite interactions weakened the CPO strength in the latter.The last stage of deformation was suggested to postdate or be synchronous with the latest melt-peridotite interactions based on the observations that the fabric strengths of coarse-and fine-grained olivine in the lherzolites and harzburgites are different.Calculations of seismic properties show that the seismic anisotropies decrease with increasing degrees of melt-peridotite interactions,and the intensities of P-wave propagation anisotropy and the maximum S-wave polarization anisotropy are correlated with the olivine fabric strength,but uncorrelated with olivine mode and CPO symmetry.The estimated delay time for the lithospheric mantle is much lower than those of geophysical observations,suggesting that the observed seismic anisotropy should mainly originate from the asthenosphere mantle.3)Our results show that the Changle xenoliths are very similar to Damaping peridotites in terms of microstructures and geochemistry.However,the Changle xenoliths are richer in olivine and have higher clinopyroxene/orthopyroxene model ratios than the Damaping peridotites.In addition,the Changle xenoliths have a larger number of fine-grained minerals,a lower density of intracrystalline deformation in minerals,and straighter grain boundaries than the Damaping peridotites.These data all point to that the former had experienced a higher degree of melt-peridotite interactions than the latter.The mineral CPO strengths of the Changle xenoliths are weaker than those of the weakly to non-foliated samples but similar to those of the strongly foliated samples from Damaping.This further indicates that a higher degree of melt-peridotite interactions can reduce the mineral fabric strength.Our data show that the degree of melt-peridotites interactions in the North China Craton gradually increases from the Trans-North China Craton to the Eastern Block,which is consistent with the higher degree of lithosphere thinning in the latter than in the former.