宁夏西吉盆地花岗岩地球化学特征、锆石U-Pb年龄及其地质意义

对北祁连山东段宁夏西吉盆地花岗岩地球化学特征和U-Pb年龄进行研究,并与区域上中酸性岩体进行对比。西吉盆地花岗岩属高钾钙碱性系列,具有富钠、过铝质的特征,LREE/HREE=10.89~11.93,轻稀土元素相对富集,分馏明显,重稀土元素分馏不明显,具有陆缘弧岩石的特点。岩石负Eu异常不明显,轻、重稀土元素分馏明显(La_N/Yb_N=10.90~15.41)。在微量元素组成上,花岗岩富集大离子亲石元素Rb、Th、Pb、La,亏损Sr、Ta、Nb、Ce、Ti元素,Pr、Nd、Sm、Dy弱富集,曲线形态具有造山花岗岩的特征,并具有负Nb异常,属正常大陆弧花岗岩。西吉盆地花岗岩成因类型为Ⅰ型,形成于陆缘弧环境,为板块碰撞造山作用的产物,岩浆来源于下地壳的部分熔融。通过锆石U-Pb定年获得花岗岩结晶年龄为434.3±9.0Ma,为早志留世,属加里东期岩浆侵入活动的产物。西吉盆地花岗岩与北祁连造山带东段南华山—屈吴山一线的花岗闪长岩体及甘肃老虎山闪长岩体同属北祁连岩浆弧带,是同期岩浆活动的产物,与板块俯冲消减作用有关,间接证明了西吉盆地属于北祁连造山带。

Geochemical characteristics and zircon U-Pb ages of the granite in Xiji basin of Ningxia

In this paper, the authors studied geochemical characteristics and U-Pb age of the granite in Xiji basin of Ningxia in North Qilian Mountains and made a comparative research on the acid rocks of this area. The granite in the Xiji basin is a series of high potassium calc-alkaline rocks and shows sodium-rich and aluminium characteristics. LREE/HREE=10.89~11.93, LREE are relatively rich, LREE fractionation is obvious, and HREE earth distillation is not obvious with characteristics of the continental arc rock. The negative anomaly of Eu is not obvious, REE data for the granodiorite show strongly fractionated REE patterns (La_N/Yb_N=10.90~15.41). As for the composition of trace elements, the granite is rich in LILE Rb, Th, Pb and La, but poor in Sr, Ta, Nb, Ce and Ti, with weak enrichment of Nd, Sm and Dy. The curve of trace elements has the characteristics of orogenic granite with Nb negative anomaly, suggesting normal continental arc granite. The genetic type of granite in the Xiji basin is of I type, which formed in the continental arc environment and was the product of the collision of plates. Its magma was derived from the partial melting of the lower crust. U-Pb zircon dating by LA-ICP-MS shows that magma crystallization age is 434.3±9.0Ma, suggesting early adakite and a product of the magma intrusion of Caledonian period. The granite in the Xiji basin, the granodiorite in Nahuashan-Juewushan area of eastern North Qilian Mountain and the diorite of Laohushan area in Gansu Province are the same products as the northern Qilian magmatic arc belt and were formed by lava activity related to the plate subduction. It is thus proved that the Xiji basin belonged to the North Qilian orogenic belt indirectly.