Lithium Isotope Signatures Of Neoarchean-Paleoproterozoic Basaltic Volcanic Rocks From The Southeast Of The North China Craton And Their Geological Significance

The North China Craton is one of the oldest and most extensive cratons in China with the most complete geological information contained in their geological body.The geochemical characteristics of the rocks of the Neoarchean-Paleoproterozoic North China Craton are of great significance for understanding the mantle evolution and the initiation time of plate subduction in the early earth.Herein,the detailed petrological and geochemical analyses were excuted on Neoarchean basalts in Western Shandong area on the southeast of the North China Craton and Paleoproterozoic basaltic andesites in Northern Henan area.The important results of this dissertation were summarized as follows:The whole rock Li isotope research and the major-trace element researches were carried out on the Neoarchean(>2.7 Ga)pillow basalts of the Liuhang Formation of Taishan Group in Western Shandong.The major-trace elemental compositions characteristics of the basalts show that it belongs to Tholeiite.The ratios of the primitive mantle-normalized trace element is ten times as large as primitive mantle,respectively,and the elemental distribution pattern is flat.Similarly,the ratios of the chondrite-normalized REE is also ten times as large as chondrites,and the elemental patterns show also flat.The Li concentration varies from 4.7 to 15.8 ppm,with an average value of 8.4 ppm.The ?7Li value varies from+1.8‰ to+4.2‰,with an average value of+3.0‰.These results indicate that the Li isotopic composition was not affected by the assimilation,partial melting,and crystallization differentiation of the surrounding rock in the later period,and therefore,it can represent the uncontaminated mantle composition(approximately+3‰)of the lower part of the North China Craton at that time.In addition,the overall performance of the Li isotope composition is slightly lower than that of modern mantle values(+4‰)reflecting that the plate subduction did not start at that time.The whole rock Li isotope research and mojor-trace element researches were carried out on the Paleoproterozoic 1.80 Ga basaltic andesites in the Xushan Formation of the Xiong'er Group in the Northern Henan area.As shown in results,the basaltic andesites are rich in large ion lithophile elements and light rare earth elements,indicating that they may originate from the enriched mantle.In addition,the concentration of Li content varies from 13.9 to 23.9 ppm,with an average value of 20.6 ppm.The ?7Li value varies from+4.5‰ to+6.1‰,with an average value of+5.2‰.The Li isotope composition is higher than that of Neoarchean and modern mantle values(+3‰vs.+4‰).After excluding the influence of surrounding rock assimilation,partial melting,and crystallization differentiation on the Li isotope composition,it can be concluded that the higher Li isotope composition relative to the normal mantle may indicate the addition of crust-derived crustal materials in the mantle source area.