长白山区二道白河流域早更新世玄武质熔岩的成因

采用岩石化学和同位素分析方法,研究了二道白河流域早更新世玄武质熔岩的成因。玄武质熔岩由钠质拉斑玄武岩和钾质粗面玄武岩、玄武质粗面安山岩组成。它们的REE分配形式比较相近,表明它们来自共同的源区。Sr、Nd、Pb同位素示踪表明,二道白河流域早更新世玄武质熔岩岩浆源区接近于似原始地幔。它们的Mg#=100Mg O/(Mg O+Fe O)低于中国东部新生代玄武岩原始岩浆的Mg#(60~68),Ni(27.76×10-6~200.6×10-6)低于原始地幔,Rb/Sr(0.05~0.09)、Ba/Rb(15.64~264)高于原始地幔,说明这些岩石不是源自原始地幔。玄武质熔岩的DI变化于42~67,具有高Ca、高Sr、Eu正异常,微量元素图解显示玄武岩保留部分熔融趋势,粗面玄武岩、玄武质粗安岩具有结晶分异趋势,岩浆上升过程中发生了不同程度的地壳混染作用。玄武质熔岩的Nb/Ta之比为14.8~15.8,与勘察加半岛深俯冲带火山类似。Nb/Ta-(Na2O-K2O)关系图解显示研究区玄武质岩浆的形成与俯冲板片的部分熔融有关。

Origin of Early Pleistocene basaltic lavas in the Erdaobaihe River basin, Changbaishan region.

The origin of Early Pleistocene basaltic lavas in the Erdaobaihe River basin is studied on the basis of petrochemistry and isotopic analysis. The results show that the basaltic lavas are composed of potassium trachybasalt, basaltic trachyandesite and soudium tholeiite. The lavas have similar REE patterns, suggesting that they came from the same source. The Sr, Nd and Pb isotopic tracers indicate that the magma source of the basaltic lavas is mostly close to primitive mantle. Their Mg^#(=100Mg²⁺/(Mg²⁺+Fe²⁺)<60) are lower than that (=60~68) of the primary magma of the Early Pleistocene basalts in eastern China. Their Ni contents (27.76×10^-6~200.6×10^-6) are less than that of primary mantle, whereas their Rb/Sr (0.05~0.09) and Ba/Rb ratios (15.64~264) are greater than those of the primary mantle. All lines of evidence indicate that these lavas were originated from evolved magma. The parental magma is potassium trachybasaltic magma. The DI value of the lavas varies from 42 to 67, and the lavas are characterized by high Ca and Sr contents, and positive Eu anomaly in REE patterns. Trace element plots show that the basalts retain the trend of partial meting, while the trachybasalt and basaltic trachyandesite exhibit the trend of crystal fractionation. It is likely that different degree of crustal contamination has occurred during the process of magmas ascending. The Nb/Ta ratio of the basaltic rocks (14.8~15.8) in this region is similar to that of basaltic rocks from the Kamchatka deep subduction zone. Nb/Ta-(Na₂O-K₂O) plots indicate that the generation of the basaltic magma is related to the partial meting of subducted slab.