北祁连山东段西吉地区辉长岩的地球化学特征、锆石U-Pb年龄及其意义

Geochemical Characteristics,Zircon U-Pb Ages of the Gabbro in Xiji Region in the Eastern Segment Northern Qilian Mountains and Their Significance

西吉地区辉长岩属于拉斑系列,具有高Al、低碱、低Ti、低P2O5的特征。辉长岩富集大离子亲石元素(LILE) Rb、Sr、K,高场强元素(HFSE) Nb、Ta、Ce、Nd、Sm、Tb、Y、Yb相对亏损,而且Nb、Ta为负异常,具有形成于消减带之上火山弧岩浆的特征。稀土配分曲线为向右缓倾斜的轻稀土富集型,其曲线型式与岛弧、弧后盆地相似,形成于陆缘弧环境。U-Pb测年结果显示,辉长岩LA-ICP-MS锆石U-Pb测年为437. 0±2. 8 Ma,形成时代为早志留世。辉长岩与西吉地区花岗岩具有相同的形成环境和非常接近的锆石U-Pb年龄,二者为同期岩浆活动的产物。结合区域地质资料,认为西吉地区辉长岩体为板块俯冲的产物,晚奥陶世—早志留世时,北祁连山海盆趋于闭合时,洋壳板块俯冲脱水,提供给地幔楔,橄榄岩发生熔融,形成的基性(辉长岩)、酸性(花岗岩)岩体。通过区域对比,认为西吉地区辉长岩与甘肃东部葫芦河群、甘肃景泰老虎山地区的蛇绿岩带中的辉长岩很可能是相同构造环境下构造—岩浆作用的产物。西吉地区可能存在由于构造作用破坏的蛇绿岩套层序的部分组成或杂乱堆积的混杂岩块。

The gabbro of Xiji region belongs to the tholeiitic series,which has the characteristics of high Al,low alkali,low Ti and low P2O5. The gabbro is enriched in large ion lithophile elements( LILE) Rb,Sr,K,and high field strength elements( HFSE) Nb,Ta,Ce,Nd,Sm,Tb,Y and Yb are relatively low,and Nb and Ta are negative anomalies which shows characteristics of volcanic arc magmas formed on subduction zones. The LREE curve enriched REE right inclined,the type of curve is similar to island arc and back arc basin,it formed in the continental margin-arc environment. U-Pb zircon dating shows that LA-ICP-MS U-Pb zircon dating of gabbro was437.0 + 2. 8 Ma,which the age of formation is early silurian. Gabbro and granite of Xiji region has the same formation environment and very close with the zircon U-Pb age,and both of them are the products of magmatic activity during the same period.Combining regional geological information,It is concluded that the gabbro body of Xiji region is the product of plate subduction.During Late Ordovician-Early Silurian,the northern Qilian basin tends to be closed,the oceanic plate subducted and dehydrated,provided to the mantle wedge,peridotite is melted,leading to mantle upwelling and forming basic( gabbro) and acid( granite) pluton.It is believed that gabbro of Xiji region and ophiolite belt in the gabbro of Huluhe group of Eastern Gansu,and Laohushan area of Jingtai in Gansu province is likely to be the product of tectono magmatism in the same tectonic environment. Xiji region may exist because of mixed rock ophiolite tectonic sequence damaged parts or disorderly deposits by means to regional comparison.