山东蓬莱大黑山岛碱性火山岩地球化学特征

GEOCHEMICAL CHARACTERISTICS OF DAHEISHAN ISLAND ALKALIC VOLCANIC ROCKS FROM PENGLAI,SHANDONG PROVINCE

庙岛群岛中的大黑山岛出露碱性火山岩,对研究壳-幔过程及黄、渤海演化具重要意义。文中作者对大黑山岛火山岩的常、微量元素、K-Ar年龄及Nd-Sr-Pb同位素体系进行测试确定。结果表明,岩石属钠质碱性玄武岩系列,岩浆岩喷发可分为8Ma前和8Ma后两个阶段;岩石常量元素含量除K2O、Na2O外差异小;随着火山岩的演化,岩浆源区深度变大;大黑山火山岩Nd-Sr-Pb同位素比值与夏威夷洋岛玄武岩相似,其岩浆的地幔端元组分为PREMA+DM,又以PREMA为主,并有MORB物质混入。

The Daheishan Island（37°58＇10＂N, 120°37＇13＂E） is a small island of the Miaodao Archipelago in Bohai Sea of China. The island features with alkalic volcanic rocks. These rocks bear important information of the interaction between the crust and the mantle in the area and of the geological evolution of Chinese marginal seas. However, the rocks are poorly studied with no citable reference. In this case, the fresh volcanic rocks were sampled from outcrop at the northwest of Daheishan Island on 2001. The contents of major elements and isotopic compositions, and K-Ar ages were analyzed with the Philips PW 1400 X-Ray fluorescence spectrometry（XRF）, Finnigan-MAT261 mass spectrometer and MM1200B inert gases mass spectrometer, at the Institute of Geology and Geophysics （IGG） , Chinese Academy of Sciences （CAS） , on 2002. Geological data of chemical composition, K-Ar age and Nd-Sr-Pb imprints were obtained from these rocks. By analyzing these data, the authors concluded that： the volcanic formation of Daheishan Island belongs to the petrological series of sodic-alkalic basalt, and the volcanism can be divided into two phases（ 〉 8Ma and 〈 8Ma）. Difference in chemical composition is small except for K20 and Na20. The depth of magma chamber became deeper as magmatism developed. Mantle composition of the chamber area included end members of mainly PREMA and then DM in mantle composition. Furthermore, the melting of MORB of the Pacific Plate probably provided the magmatic matter in the chamber area.