Comprehensive Refertilization Of The Ancient Lithospheric Mantle

The subcontinental lithospheric mantle(SCLM)records meaningful geological information in deep Earth and has an important role in lithospheric evolution.Therefore,the formation and evolution of SCLM is critical to understanding the processes responsible for the development of Earth's continents.Petrological observations,in situ major and trace element and Sr isotopic data for spinel-facies harzburgite and lherzolite xenoliths from the Siziwangqi region shed light on the genesis of mantle-derived xenoliths and provide further constraints on the nature and evolution of the lithospheric mantle beneath the northwestern North China Craton(NCC).The Siziwangqi peridotites can be divided into high-Mg# peridotites(Fo ? 91)and lowMg# peridotites(Fo < 91),according to olivine forsterite(Fo)contents and textures.The high-Mg# peridotites are harzburgites,which are characterized by medium-to coarse-grained granular textures.The Fo contents of olivines in high-Mg# peridotites general range from 91.0? 91.8 and plot in the field for Proterozoic peridotites.Furthermore,they show higher whole rock Mg#(91.0 ? 91.8),as well as lower Ca O(0.41 ? 1.17 wt.%)and Al₂O₃(0.62 ? 2.03 wt.%)contents.The refractory harzburgites plot in the field for cratonic peridotites.It is implied that there may exist ancient lithospheric mantle in studied area.However,Fo contents in some samples decrease from core to rim and some have slightly LREE-enriched and Ushaped REE patterns and dominantly radiogenic Sr isotopic compositions(0.7031 ? 0.7053).These features indicate that the high-Mg# peridotites are remnants of ancient refractory lithospheric mantle,which then experienced variable refertilization.In contrast,the low-Mg# peridotites are mainly lherzolites and minor harzburgites,which are fine-to medium-grained granular textures.Olivines from the low-Mg# peridotites are characterized by much lower Fo(88.2 ? 90.8),and plot mostly in the field for Phanerozoic lithospheric mantle.Fo contents in some samples decrease from core to rim and newly formed olivines and clinopyroxenes in melt pocket have relatively low Mg#.The low-Mg# peridotites have lower whole rock Mg#(89.2 ? 90.9),and higher Ca O(0.98 ? 3.53 wt.%)and Al₂O₃(1.38 ? 3.85 wt.%)contents,relative to the high-Mg# peridotites.The clinopyroxenes in lowMg# peridotites have similar REE patterns as those in the high-Mg# peridotites,and some grains show increasing LREE contents from core to rim.These clinopyroxenes have relatively depleted Sr isotopic compositions(0.7021 ? 0.7045).The low-Mg# peridotites are interpreted to be the reaction products of high-Mg# peridotites and asthenospheric melts.Pyroxenite xenoliths are mainly websterite with minor olivine websterite,which show medium-to coarse-grained granular textures.Melt pockets with newly formed olivine,clinopyroxene,plagioclase,and spinel are also observed.Olivine in the reaction texture has relatively lower Fo and Ni contents.Otherwise,clinopyroxene has higher Ni content due to inheriting from Olivine.Olivine(86.0 ? 89.7),orthopyroxene(82.9 ? 91.0),and clinopyroxene(83.6 ? 92.0)show relatively higher Mg#.They have similar major elements composition of minerals,REE contents,REE and trace element patterns to Siziwangqi peridotites.Moreover,whole rock compositions of pyroxenite xenoliths fall in the same refertilization trend composed of high and low Mg# peridotites.The clinopyroxenes show variable depletions of Nb,Ta,Zr,Hf,and Ti.They also have relatively high Th,U,and Sr contents and depletions of Rb and Ba.In addition,the clinopyroxenes have low(La/Yb)N(0.2? 1.5),Ca/Al(2.9 ? 9.1),high Ti/Eu(1704–5911)ratios,and dominantly radiogenic Sr isotopic compositions(0.7032 ? 0.7054).It is displayed that negative correlations between⁸⁷Sr/86 Sr and Mg#,Ca/Al,and Sm/Hf ratios,positive correlations between ⁸⁷Sr/86 Sr and Al₂O₃,Na2 O,TiO₂,Sr,Eu contents,Ti/Eu,and Ti/Nb ratios.Morevover,some grains show decreasing ⁸⁷Sr/86 Sr ratios from core to rim.All these features indicate that Siziwangqi pyroxenite are formed by reaction via peridotites and melts derived from subducted PaleoAsian altered oceanic crust.In conclusion,the lithospheric mantle beneath the Siziwangqi region experienced an earlier metasomatic event related to subduction of Paleo-Asian oceanic materials,as evidenced by the relatively high ⁸⁷Sr/86 Sr ratios(up to 0.7053)in high Mg# peridotites.The clinopyroxenes in both high-and low-Mg# peridotites have depletion of Rb,Ba,Pb,Nb and Ta,enrichment of Th and U,low(La/Yb)N(0.1 ? 3.9),and high Ti/Eu(1704 ? 5911)ratios.They also show positive correlations between ⁸⁷Sr/86 Sr and Mg# and Ca/Al,negative correlations between⁸⁷Sr/86 Sr and Na₂O,TiO₂,Ti/Eu,and ?(middle REEs + heavy REEs),and decreasing⁸⁷Sr/86 Sr ratios from core to rim.These observations imply that the lithospheric mantle beneath the Siziwangqi region experienced later multi-stage refertilization by asthenospheric melts which may result in the increase of Li contents in peridotites.Combined with the previous research of mantle-derived xenoliths from the NCC,we conclude that the lithospheric mantle beneath the northwestern NCC has been comprehensively refertilized by multiple stages of peridotite-melt reactions,like other regions of the NCC,which had a significant role in the transformation of the ancient lithospheric mantle.