摘要
对采自8个火山锥和其它产状的15个火山岩样品和一个幔源包体样品进行了系统的岩石薄片观察,常量元素、微量元素和稀土元素的化学分析,K-Ar定年及氧、锶和钕同位素测定。显微镜下首次发现了贵橄榄石的贯穿双晶,包括四角星状和六角星状双晶。化学分析表明这是一套SiO_2不饱和、高K、Mg而贫Ca,同时特别富集LREE、不相容元素和地幔元素的岩浆。全岩的δ^(18)O值略高于初始地幔,Sr和Nd同位素组成在^(87)Sr/^(86)Sr-^(143)Nd/^(144)Nd图解上,落在第Ⅳ象限地幔排布的延长线上,并且靠近坐标原点,显示初始地幔的特征,K-Ar表现年龄值域在0.03Ma和2.98Ma之间,大多数落在晚更新世。地幔包体表现年龄约为70Ma,中生代的玄武岩年龄在93.3Ma和95.5Ma之间。趋势分析表明年青火山仍有喷发的可能性、显微镜下挑除橄榄石捕虏晶后的对比定年实验给出了有益的提示:我国东北和东部大面积分布的晚新生代橄榄玄武岩过去的定年可能都偏老了。
Fifteen volcanic rock samples and one mantle xenolith sample which were collected from 8 volcanos and other places have been studied systematically, including petrogra-phic investigation, K-Ar dating, major and trace element and REE analysis as well as O, Sr and Nd isotope determination. The chrysolites with the inject twin are found under a microscope firstly in the world, including various crystal stock with the fourling and the six-ling. The chemical compositions show that it is a suite of magma wth a characterized SiO2-unsaturated, high aboundent of K and Mg and poor Ca as well as by much rich incompatible elements, REE (ΣREE=321-1002 ppm, (La/Lu)a= 42.5-76.4) and mantle compatible element also. The values of δ18O (5.8-8.7%0) are a little higher than that of the primary mantle while the isotope compositions of Sr and Nd fell in quadrant Ⅳ along the mantle array extension and closed to the origin of the coordinates on Sr-Nd diogram, it appeared as a primary mantle. K-Ar apparent age of volcanic rocks range from 0.03Ma to 2.98Ma, majority of them belong to Late-Pleistocene. An apparent age of the xenolith is about 70Ma while the oldest basalt is from 93.3Ma to 95.5Ma. The trend analysis suggest that there is a still posibility of eruption. The comparative experiment of dating between the conventional sample and after pick out of olivine sample implied that the dated ages of the Late-Cenozoic olivine-basalts distributed in northeast and east China may be older than the real values.
出处
《北京大学学报（自然科学版）》
CAS
CSCD
北大核心
1992年第6期733-744,共12页
Acta Scientiarum Naturalium Universitatis Pekinensis
