甘蒙北山地区奥陶纪火山岩地球化学特征及形成环境探讨

Geochemical Characteristics and Tectonic Settings of the Volcanic Rocks from Beishan Area,Gansu Province and Inner Mongolia,China

对甘蒙(甘肃-内蒙古)北山地区花牛山、洗肠井、伊哈托里及希热哈达出露的奥陶纪火山岩进行了地球化学研究。结果表明:该区火山岩类型主要为安山玄武岩,以拉斑系列为主。花牛山和洗肠井火山岩的稀土元素配分具有弱的轻稀土亏损-平坦、重稀土略富集的特征,高场强元素比值特征与蛇绿岩(N-MORB)一致,形成环境为洋中脊;伊哈托里和希热哈达火山岩具轻稀土富集,高场强元素Nb、Ta、Ti等亏损的特征,类似于岛弧火山岩,但其ThN/NbN(1.00)、Nb/La(<1.00)、Zr/Nb值类似于N-MORB,暗示火山岩遭受了地壳物质的混染作用。伊哈托里和希热哈达火山岩样品w(Zr)>70.00×10-6和Zr/Y>3.00,显示了板内玄武岩的特征。结合区域构造演化分析,认为花牛山和洗肠井火山岩形成于洋中脊环境,而伊哈托里和希热哈达火山岩应属伸展裂解环境的产物。

By studying geochemical characteristics of Ordovician volcanic rocks in the Beishan area of Gansu and Inner Mongolia,the authors consider that the volcanic rocks in this area mainly belong to the andesitic basalt predominated by tholeiite series. The samples collected from Huaniushan and Xichangjing are presented nearly flat-depleted of LREE and the slightly enriched HREE in the chondrite-normalized REE pattern,and their HFSE characteristic is similar to that of the N-MORB ophiolite.The samples from Yihatuoli and Xirehada are enriched in LREE and depleted in HFSE （Nb,Ta,Ti）,which are similar with those of the island-arc volcanic rock.however,they have ratio values of （Th/Nb）N 1 and Nb/La＆lt;1 ,as well as Zr/Nb similar to the the N-MORB,which suggested that the volcanic rocks were contaminated by the crust.The features of Zr＆gt;70.00×10-6 and Zr/Y＆gt;3.00 of the samples are accordant with those of intra-plate basalts.Combined with the research of tectonic evolution in the study area,the authors concluded that Ordovician volcanic rocks in Huaniushan and Xichangjing formed at mid ocean ridge settings,and Ordovician volcanic rocks in Yihatuoli and Xirehada formed at a extensional rifting setting within the plate.