甘肃北山地区“牛圈子蛇绿岩”属性再研究

Restudy on the attribute of Niujuanzi Ophiolite in Beishan Mountains,Gansu Province

牛圈子蛇绿岩被认为是甘肃北山构造单元划分的重要依据,但是其蛇绿岩属性的真实性一直没有得到有效的验证。本次研究工作通过1∶5万地质调查和剖面研究,查明其岩石组合、接触关系及野外产出特征;通过对辉长岩岩石地球化学测试与分析,讨论其形成的大地构造背景。研究认为原划牛圈子蛇绿岩岩石组合缺失严重、火山熔岩极不发育、缺失超基性岩、没有混杂堆积特征,没有“岩块+基质”的物质组成样式,也没有深海沉积的记录;同时,辉长岩具有明显壳幔混源的岩石地球化学特征,与N-MORB和OIB的微量元素特征差异较大。综合研究认为前人划分的牛圈子蛇绿岩可能不是典型的蛇绿岩。

The Niujuanzi ophiolite determined by previous studies is regarded as an important evidence for the division of Beishan tectonic units in Gansu Province,NW China,but the authenticity of its petrological properties has not been effectively verified.Detail field investigations with 1∶50000 scale have allowed us to revisit the rock association,contact relationship and field occurrence.Meanwhile,geochemistry of Gabbro are also analyzed and discussed.The results show that the rock assemblages developed in the area may be the products of plutonism rather than a typical ophiolite.Evidences are listed as follows:①The pseudo-sequence of ophiolite rock association is seriously missing in Niujuanzi tectonic belt.②Volcanic lava developed with few and sparse in local section of the belt.③Ultrabasic rock including abyssal peridotites are missing.④There is no any characteristics of mélange zone.⑤There is no material components that resemble those style of'rock block and matrix'of a typical suture zone.⑥There is no record of deep-sea deposition such as radiolarian bedded chert.⑦On the contrary,gabbro has obvious geochemical characteristics of crust—mantle mixed source,which is quite different from the trace element characteristics of N-MORB and OIB.Thus,the so-called Niujuanzi ophiolite is a suite of complex pluton mainly including gabbro intruded into the surrounding rocks,the Misjudgment of the tectonic nature of which could attribute to the relatively low and gentle topography and serious coverage of loose deposits in the Beishan Mountain.