摘要
拉脊山火山岩带位于祁连造山带中部 ,通过对该区早古生代基性火山岩系统的地球化学研究 ,揭示该区早古生代地幔的性质及其地幔域的构造归属 .研究表明 ,拉脊山基性火山岩可以分为两类 :Ⅰ类为大陆板内碱性玄武岩 ,其稀土元素组成模式为轻稀土富集型 ,并具有明显的Nb、Ta负异常 ,而Zr、Hf无明显的负异常 ;Ⅱ类为与地幔柱活动有关的拉斑玄武岩 ,具有洋岛玄武岩 (OIB)特征 .稀土元素组成模式同样表现为轻稀土富集型 ,但其富集程度比Ⅰ类基性火山岩的富集程度弱 ,无Nb、Ta、Zr和Hf负异常 .基性火山岩的Sr、Nd、Pb同位素组成特征显示 ,基性火山岩的地幔源区具有亏损地幔 (DM )和第二类富集地幔 (EMⅡ )混合的特点 ,而第二类富集地幔端元 (EMⅡ )占主导地位 ,亏损地幔 (DM )物质混入的程度较低 ;并具有Dupal异常的同位素特征 .通过与华北南缘、北秦岭和扬子北缘西段地幔的Pb同位素组成相比 ,表明拉脊山造山带古地幔与北秦岭、南秦岭西段和扬子北缘西段地幔的Pb同位素组成相似 .进而表明拉脊山造山带古地幔属于扬子型富放射性成因铅地幔 。
The Early Paleozoic Lajishan volcanic rock belt is located in the middle part of the Qilian orogenic belt. In order to reveal the mantle nature and tectonic affinity of the Lajishan belt, this paper presents a study of the major-trace elements and Pb-Sr-Nd isotopes of the Lajishan basalts. Based on major and trace elemental data, the Lajishan basalts can be divided into two groups: continental alkaline basalts (Group I) and tholeiites basalts (Group II). Group I displays LREE enrichment, distinct Nb and Ta negative anomalies and negligible Zr and Hf anomalies. Group II is characterized by typical OIB basalts with LREE enrichment and no Nb, Ta, Zr and Hf anomalies. The formation of Group II is related to mantle plume. Sr-Nd-Pb isotopic compositions of basalts indicate that both Group I and Group II were derived from a mixing source between DM and EM II mantle end-members, and have Dupal Pb isotope anomaly. A regional comparison shows that the high radiogenic Pb isotopic composition of the Lajishan basalts is similar to that of the Yangtze block mantle, and is distinctly from that of the North China mantle. This implies that the Lajishan paleo-mantle has an affinity to the Yangtze mantle instead of a part of the south margin of the North China block.
出处
《地球科学（中国地质大学学报）》
EI
CAS
CSCD
北大核心
2005年第1期61-70,共10页
Earth Science-Journal of China University of Geosciences
基金
国家自然科学基金重大项目 (No .40 2 34 0 5 2 )
