山东蓬莱、临朐新生代碱性玄武岩的钕、锶同位素组成

本文报道了鲁东和鲁西新生代碱性玄武岩13个样品的Nd、Sr同位素组成,~(143)Nd/~(144)Nd=0.512967—0.512744,~(87)Sr/(86)Sr=0.70349—0.70450。它们在地质剖面上呈现规律性变化,可能与其地幔源区同位素组成的层状分带有关。鲁西地幔源区具有较鲁东更加亏损的组份。两地地幔源区在演化中都曾发生过地幔交代(或富集)作用,根据玄武岩Nd同位素模式年龄估计地幔交代作用发生的时间为0.45Ga。

THE NEODYMIUM AND STRONTIUM ISOTOPIC COMPOSITIONS OF CENOZOIC ALKALIC BASALTS FROM PENGLAI AND LINGQU, SHANDONG PROVINCE

Cenozoic alkali basaltic rocks at the Wuliqiao-Chishan section of the Penglaiarea, eastern Shandong, and at the Baogushan-Jiaoyeshan-Xiaoyaoshan section ofthe Lingqu area, western Shandong, are composed of olivine nephelines, basanitesand alkali olivine basalts, some of which contain spinel lherzolite xenoliths andmegacrysts. The neodymium and strontium isotopic compositions of 13 samples are reportedin the paper, and their variation ranges are as follows: ~(143)Nd/~(144)Nd=0.512865-0.512771 and ~(87)Sr/~(86)Sr=0.70405-0. 70448 in the Penglai area, eastern Shandong,and ~(143)Nd/~(144)Nd=0.512967-0.512744 and ~(87)Sr/~(86)Sr=0.70349-0.70450 in the Lingjuarea, western Shandong. It is evident that the Sr and Nd isotopic compositions in the mantle sourcesbeneath both areas are heterogenous based on the isotope compositions of alkalibasalts from Penglai and Lingqu. The most depelted mantle component in theLingqu area is similar to PREMA-type mantle end component in the oceanicmantle. This end component is of important significance in the origin of Cenozoiccontinental alkali basalts in eastern China. From the decoupling between the isotopic compositions and the trace elementabundance pattern of the mantale sources beneath both areas, it is inferred thatmantle metasomatism (or trace element enrichment) had occurred in the mantlesources. The time of mantle metasomatism is estimated at 0.45 Ga by means ofthe Nd isotope model age of alkali basalts in both areas.