Geochemical Characteristics And Significances Of Archean Komatiite In The Western Shandong Province,North China Craton

Komatiites are ultramafic volcanic rocks with distinctive spinifex textures and high MgO(MgO>18 wt%),widely distributed within greenstone belts of Archean craton around the world.In this thesis,komatiites and komatiitic basalts from the Neoarchean Taishan Group in the western Shandong,North China Craton,were analyzed for their whole rock chemical composition,titanite U-Pb dating and trace elements,olivine electron probe.The results are used to reveal their geochemical compositions and ages,to determine the source nature and petrogenesis process and to help our understanding Archean mantle.All the komatiites from Sujiagou,Tianjingyu and Nanbaita have Al2O3/TiO2 ratios higher than 20 and(Gd/Yb)N ratios of 0.73-1.09,which demonstrates that they all belong to aluminum-undepleted komatiite.The olivines in Sujiagou komatiite are surrounded by serpentite.Compared with typical igneous olivines from komatiite,the oliivnes from Sujiagou komatiite is characterized by low Fo values of 76.7-85.0,which is significantly lower than whole-rock Mg#(84.2-87.3).Their higher MnO contents and lower NiO contents than typical igneous olivines indicates that they are of metamorphic origin.Titanite U-Pb dating for Sujiagou komatiite and Tianjingyu komatiitic basalts gave lower intercept age of 2600±57 Ma,2592±64 Ma,respectively.And their Zr-intitanite temperatures are 682-718℃ and 614-634℃,respectively The chemical composition of the titanites indicates they are both metamorphic.The metamorphic age of about 2.6 Ga is consistent with the widely developed magmatism at around 2.6 Ga in the West Shandong area.The large ion lithophile elements(K,Rb,Pb,Ba)of the komatiites show obvious mobilization and a few samples record some degrees of crustal contamination.Theoritical modeling suggests that the mantle potential temperature of komatiite in Sujiagou is 1674±33℃ and the MgO content in the primary magma is 23.727.5wt%.In contrast,the MgO content of the primary magma of the Tianjingyu komatiitic basalts is 19.6-21.0wt%,and the mantle potential temperature is 1577±14℃.The komatiites have a temperature much higher than that of normal Archean mantle,suggesting that the komatiites are derived from a hot mantle plume whereas the komatiitic basalts are derived from the ambient mantle.According to the Al/Ti ratios and partial melting degrees of the primary magma,the formation pressure of komatiite and komatiitic basalts is estimated to be 7.8-9 GPa and 6 GPa,respectively,corresponding to the sublithosperic depth and lithosphere bottom.In this regard,the komatites are the products of high degree partial melting(F=0.48-0.58)in the deeper mantle,while komatiitic basalts are the products of low degree partial melting(F=0.300.33)in relatively shallower depth.Their origin may be linked to the mantle plume.