甘肃北山桥湾北花岗岩体的年代学、地球化学及其地质意义

为加深对北山晚古生代构造演化的认识, 选取位于甘肃北山南带、敦煌地块北缘的桥湾北花岗岩体进行年代学、地球化学研究。桥湾北角闪石花岗岩体锆石LA-ICP MS年龄为303.7±2.4 Ma, ε_Hf(t)为-1.2~5.8, ε_Nd(t)相对较高(-0.40~-0.06), (⁸⁷Sr/⁸⁶Sr)_i =(0.704524~0.705062)。 桥湾北花岗岩体K₂O含量为4.09%~5.58%, 属高钾钙碱性岩石, A/CNK值在0.92~1.04之间, 为I型花岗岩; 微量元素较富集LILE, 相对亏损HFSE, 有明显的Nb, Ta负异常。岩体具相对富集的微弱分馏的轻稀土元素, 重稀土元素分馏不明显, 基本没有Eu异常。由Hf, Nd同位素及地球化学特征判断, 桥湾北花岗岩体为壳幔混合成因, 其形式可以是底侵的幔源岩浆再分异, 上升过程中遭受地壳物质混染。根据区域构造背景及岩石学、地球化学特征, 可以认为桥湾北岩体是北山南带后碰撞过程岩浆活动的产物, 反映出在晚石炭世北山地区的碰撞拼贴已经完成。

Geochronology and Geochemistry of Granite Pluton in the North of Qiaowan,Beishan Mountain,Gansu Province,China,and Its Geological Significance

To deepen the understanding on tectonic evolution of Beishan Moutain,granite pluton which lies in the north of Qiaowan,northern margin of Dunhuang block(south belt of Beishan Mountain),Gansu province,is studied in geochronology and geochemistry.The hornblende granite contains a LA-ICP MS zircon U-Pb aged at 303.7±2.4 Ma.Its εHf(t) value varies from 1.2 to 5.8,εNd(t) value is high(0.40 to 0.06),and(87Sr/86Sr)i(0.704524 to 0.705062) is relatively low.The pluton apparently belongs to sub-alkline and high-K cal-alkaline series(K2O is 4.09% to 5.58%,K2O/Na2O is 1.34 to 2.07) and shows as I-type granite with metaluminous-peraluminous characteristics for A/CNK is between 0.92 and 1.04.The hornblende granite pluton has LILE enrichment,especially rich in Pb,Rb,Th,and U,depleted in HFSE(Nb,Ta) obviously in spidergram.It also has a rich LREE and flat HREE in REE patterns with no or extremely weak Eu anomalies(δEu = 0.67 to 1.17).From the εHf(t),εNd(t) values and geochemical characteristics,it can be concluded that the pluton is a mixture of crustal and mantle derived magma.The mantle derived magma may be directly from anatexis magma in deep mantle and then underplate below the continental crust.Based on regional geology and the geochemical characteristics,the hornblende granite pluton in the north of Qiaowan was formed during post-collision period,which suggests the collision of Beishan Mountain is already finished in late Carboniforous.