摘要
近年来鞍山地区发现了一系列具异常稀土组成的太古宙长英质岩石,包括花岗质岩石和变质沉积岩两大类。稀土模式可划分为5种类型:Ⅰ—轻稀土相对富集出现强正铕异常;Ⅱ—轻稀土相对富集无明显铕异常;Ⅲ—轻稀土相对富集出现负铕异常;Ⅳ—轻稀土相对亏损出现负铕异常;Ⅴ—中稀土相对亏损出现强负铕异常。高SiO_2,低ΣFeO、MgO、CaO和TiO_2,稀土总量低是它们的共同特征。岩石铕异常和钡异常之间存在明显的相关关系。部分岩石出现无意义t_(DM)年龄。本文重点讨论了类型Ⅳ和Ⅴ两类岩石的成因。它们在中国华南及世界其他地区也有分布,但多为年轻花岗岩。鞍山地区的这类岩石形成于太古宙阶段,部分甚至可能形成于中太古代,这一现象在全球范围内也十分的罕见。它们的存在表明该区至少在太古宙晚期就具有与同类年轻花岗岩形成类似的地质和物理化学条件。与其具有很长地质演化历史相吻合。
Anshan, one of the most important areas in the North China craton, underwent a long geological process in the Archaean from 3. 8 Ga to 2. 5 Ga. Recently we have found a lot of felsic rocks with anomalous REE compositions, including different kinds of granitic rocks (veins) and meta-sedimentary rocks, in the area. They generally occur in small scales and are formed in different stages of the Archaean. Their REE patterns can be subdivided into 5 types:(1) LREE enrichment, with a positive Eu anomaly; (2) LREE enrichment, without obvious Eu anomaly; (3) LREE enrichment, with a negative Eu anomaly; (4) LREE depletion, with anegative Eu anomaly; (5) MREE enrichment, with a strong negative Eu anomaly. All of the rocks are characterized by a high content of SiO_2 and low contents of ΣFeO, MgO, CaO, TiO_2 and ΣREE. Some of them give meaningless t_(DM) ages of the Nd isotope. The discovery of the rocks with REE patterns of types Ⅳ and Ⅴ is of especially important significance. Granitic rocks with the same REE patterns, mostly being young granites, have been found in the South China platform and other areas of the world and show genetic relationships with W, Sn, Nb and Ta mineral deposits. They are considered to be highly evolved rocks formed by strong fractional crystallization and hydrothermal interactions. Their existence in Anshan indicates that at least in the late Archaean, the area was in geological and physical-chemical conditions similar to those under which the young granites were formed. It is consistent with the view that the Anshan area underwent a long geological evolution history in the Archaean. It is also interesting to note that there is a clear linear relationship between the Eu anomaly and the Ba anomaly in rocks with anomalous REE patterns, a fact that has been identified in many felsic materials elsewhere.
出处
《地质论评》
CAS
CSCD
北大核心
2002年第S1期45-52,共8页
Geological Review
基金
国土资源部重点项目(编号DKD2001020-3和9501116)
地质行业科学技术发展基金项目(编号HY979816)
国家自然科学基金项目(编号49473172)资助成果
