Geochemical Characteristics And Significance Of The Early Neoarchean Metavolcanic Rocks In Western Shandong Province

The early Neoarchean metavolcanic rocks are well preserved in Western Shandong Province?WSP?,which provides a good condition for exploring the evolution of crust and mantle in the North China Craton during the early Neoarchean.A large number of ultrabasic rocks represented by meta-komatiite and basalt-meta-basaltic andesite in the WSP are located in the supracrustal rocks zone to the east of B belt as a whole.The formation age of meta-komatiites and contemporaneous meta basalts-basaltic andesites are constrained to 2704±90 Ma and 2.77-2.71 Ga respectively.The basic rock series can be divided into tholeiites and enriched basalts according to their geochemical characteristics,and the enriched basalts formed earlier than tholeiites.The characteristics of ultrabasic-basic series are consistent with the plume-derived association in the global cratons,and can be explained by partial melting of mantle plume at different depths.According to the different degrees of the contamination among the samples,we believe that this series were formed during the oceanic basin expansion period.DTTGs of the same period with ultrabasic-basic rock series mainly distribute in the east and west sides of B belt.The obvious low MgO,Cr,Ni contents and negative anomalies of Nb,Ta and Ti indicate that they are not the origin of mantle plume,but the product of plate tectonics.Because of the high heat flow value of the crust,there may be a series of low-angle and small-scale thermal subductions.The samples in the east?located on the west side of ultrabasic-basic series?have low K content and high?_Hf?t?,and are not contaminated by the crust.The samples in the west?near the junction of B and C belts?have high K content and extremely negative?_Hf?t?,showing obvious contamination characteristics of the crust.These evidences indicate that the eastern side was intraoceanic subduction,while the western side was oceanic-continental subduction during that time.Previous evidences also prove that there may be an ancient continental nucleus in the west.In addition,a small amount of 2.69-2.60Ga island arc-originated metabasalts were first discovered in the WSP,mainly 2655±13Ma island arc tholeiites?IABs?and 2618±36Ma siliceous high-Mg basalts?SHMBs?.The former distributes in western and northern Dawangzhuang,which has a flat REE distribution pattern,weak negative anomalies of Nb?Ta and Ti elements.Xenocryst zircons and?_Hf?t?lower than the depleted mantle in the same period show the characteristics of crustal contamination.The intrusions of TTGs,quartz diorites and gabbros with high MgO,Cr,Ni and K contents discovered by predecessors and the same research group indicate that the heat flow value decreases and the crust tends to be rigid.The angle and depth of subduction increases gradually,and the subduction oceanic slab and the mantle wedge begin to interact gradually.These samples contaminated by crustal materials indicate that they may be the products of island arc to continental margin.The latter is distributed in the north of Dawangzhuang,southeast of Mengyin and east of Yiyuan.It has obvious characteristics of high LREE content and low Nb,Ta and Ti content,high MgO content?>9%?,low TiO₂ content and no crustal contamination.