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Secular evolution of the lithospheric mantle beneath the eastern North China craton:evidence from peridotitic xenoliths from Late Cretaceous mafic rocks in the Jiaodong region,east-central China

Noriko Kita  Gen Shimoda  Yuichi Morishita  
【摘要】:正The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks.Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major,trace element,and isotopic compositions.The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region.These peridotitic xenoliths are all spinel-facies nodules and two groups,high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features.High-Mg# peridotites have inequigranular textures,high Mg#(up to 92.2) in olivines,and high Cr#(up to 55) in spinels.Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched((La/Yb)_N1) with variable REE concentrations,and have enriched Sr-Nd isotopic compositions(~(87)Sr/~(86)Sr = 0.7046-0.7087;~(143)Nd/~(144)Nd = 0.5121- 0.5126).We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time.The low-Mg# peridotites are equigranular,are typified by low Mg#( 90) in olivines,and by low Cr# ( 12) in spinels.Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12ppm),LREE-depleted REE patterns((La/Yb)_N 1),and depleted Sr-Nd isotopic features,in contrast to the high-Mg# peridotites.These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle.Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas,a highly heterogeneous,secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.

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