Cu-Fe-Zn Isotopic Compositions Of Pyroxenites And Granulites From The Northern Margin Of The North China Craton And Their Geological Significance

The lower continental crust is an important place for the continental crust growth and differentiation and crust-mantle material exchanges,and it is also a major reservoir for some metal elements（e.g.,Cu）on the Earth.Crustal sediment melts from subducted slab would interact with the lower crustal rocks or stagnate at the crust-mantle boundary,thereby modifying the chemical composition of the lower continental crust.Meanwhile,incorporating oxidized crustal sediment components into the lower continental crust would affect its oxygen fugacity and the stability of multivalent metal elements（e.g.,Cu）,and how Cu is mobilized and migrated in the lower crust is still a matter of debate.Therefore,quantifying chemical composition of the lower continental crust and tracing crustal sediment component from subducted slab are the basis for studying the elemental and isotopic behaviors during lower crustal processes,and are also an indispensable part of exploring the lower continental crust evolution.This study presents petrography,conventional geochemistry,and Cu-Fe-Zn stable isotopic analyses for pyroxenite and granulite xenoliths from the Hannuoba basalts and the nearby Archean granulite terrains from the north margin of the North China Craton to constraint the Zn-Fe isotopic compositions of the lower continental crust and reveal the sediment melt activity in the crust-mantle boundary and the Cu mobilization in the lower continental crust.This study presents high precision Zn-Fe stable isotope analyses for lower crustal rocks（lower crustal xenoliths and granulite terrains）from the northern margin of the North China Craton to understand the behavior of Zn-Fe isotopes during deep crustal processes and the Zn-Fe isotopic compositions of the lower continental crust.The lower crustal xenoliths range in composition from mafic to felsic.Theδ⁶⁶Zn andδ⁵⁶Fe values of the lower crustal xenoliths vary from-0.17‰–0.38‰and 0–0.20‰,respectively.Theδ⁶⁶Zn andδ⁵⁶Fe values of garnet-bearing mafic granulites vary with the Fe O_（T）and V contents,which are likely results of the accumulation of Fe-Ti oxides.The averageδ⁶⁶Zn andδ⁵⁶Fe of the lower crust are estimated to be 0.29±0.02‰（95%SE）and 0.10±0.03‰（95%SE）using lower crustal xenoliths from the Neogene Hannuoba basalts,respectively.Theseδ⁶⁶Zn andδ⁵⁶Fe values are similar to the estimated values(δ⁶⁶Zn:0.28±0.04‰;δ⁵⁶Fe:0.07±0.05‰)obtained from the granulites from Archean terrains,suggesting that there is no significant difference in the Zn-Fe isotopic compositions between the Archean and present-day lower continental crust.Combining theδ⁶⁶Zn andδ⁵⁶Fe data of the lower crustal xenoliths and granulite terrains and different weighting methods,the Zn-Fe isotopic compositions of the lower continental crust are estimated to be 0.28±0.04‰（95%SE）and 0.10±0.05‰（95%SE）,respectively.The study of the Zn-Fe isotopic compositions of lower continental crust lays a foundation for the use of Zn-Fe isotopes to trace material exchange and element cycling between different Earth’s reservoirs.This study integrates Sr-Nd-Zn-Fe isotopic compositions of clinopyroxenites（crust-mantle boundary）and a"glassy"xenolith from the Hannuoba basalt to trace the activity of slab-derived sediment melts in the crust-mantle boundary.The bulk-rock compositions of clinopyroxenites have highly variableδ⁶⁶Zn values（0.04‰–0.46‰）and show excellent correlations with Ba/Th,Rb/La,Th/Nb,K/U ratios,and generate arrays that trend towards a composition similar to the"glassy"xenolith.The"glassy"xenolith has a highly unradiogenic Nd isotopic composition(¹⁴³Nd/¹⁴⁴Nd:0.510991)and a highδ⁶⁶Zn value（0.43‰）.This evidence implies that the"glassy"xenolith may represent a preserved sediment melt formed by the melting of recycled carbonates and terrigenous sediments with upper continental crust components,and which may also be responsible for the metasomatism of clinopyroxenite xenoliths.Clinopyroxenite xenoliths have constant low Cr（8.3–71.9 ppm）and Ni（14.9–34.5 ppm）contents,and no correlation with Ba/Th ratios was observed.The clinopyroxenes from clinopyroxenite xenoliths have radiogenic Sr isotope compositions（0.71027–0.73326）,and core（high）-rim（low）variation of ⁸⁷Sr/⁸⁶Sr ratio is observed in some clinopyroxene crystals.These features indicate that pre-metasomatized clinopyroxenites were crystallized from basaltic melts produced by melting of a mantle source enriched by recycled components from the upper continental crust.The geochemical evidence from these clinopyroxenite and"sediment melt"xenoliths points to the presence of recycled upper continental crust components in the continental lithosphere.Therefore,Zn isotopes can be used to trace the activity of slab-derived sediment melts in the crust-mantle boundary.This study analyzed the Cu elemental and isotopic compositions of well-characterized lower crustal xenoliths from the north margin of the North China Craton to decode Cu mobilization in the lower continental crust.The mafic lower crustal rocks have experienced varying degrees of sulfide accumulation,with Cu contents ranging from 15 to 112 ppm.The majority of lower crustal xenoliths have variable and generally low（compared to the Bulk Silicate Earth）δ⁶⁵Cu values ranging from-3.17‰to 0.13‰（n=24）.Many intragranular and interstitial sulfide grains in these lower crustal xenoliths were partially dissolved and oxidized along the mineral fractures and boundaries.Oxidative dissolution of sulfide will preferentially release⁶⁵Cu into the liquids,leaving the residual sulfides enriched in the light isotope,implying that oxidized melt-rock interactions could account for the light Cu isotopic compositions of the lower crustal xenoliths.Sulfide accumulation and oxidative dissolution are responsible for the enrichment,mobilization,and transfer of Cu in the lower continental crust beneath cratonic margins.This study constraints the Zn-Fe isotopic compositions of the lower continental crust,elaborates the application of Zn-Fe isotopes to trace the activity of slab-derived sediment melts in the crust-mantle boundary,and decodes the Cu mobilization in lower continental crust from a Cu isotope perspective.The results in this thesis could provide some essential data support for the evolution of lower crustal composition and the mobilization of metal elements and lay a part of the foundation for the application of Cu-Fe-Zn isotopes in tracing high-temperature geological processes.