内蒙古北大山杂岩体成因及其地质意义

Petrogenesis and geological significance of the Beidashan complex rockmass,Inner Mongolia

内蒙古北大山岩体是大兴安岭南段成矿带中的一个重要杂岩体。LA-ICP-MS锆石U-Pb测年结果表明,北大山岩体中的斑状石英二长闪长岩、石英二长斑岩和二长花岗岩分别形成于(142.6±1.8)Ma、(141.1±19)Ma、(140.4±1.6)Ma,属早白垩世早期的产物。北大山杂岩体由早至晚,岩石地球化学特征有规律的演化,主要表现为:SiO_2含量逐渐增高,碱度也增高,而钙镁和铝质渐趋降低;稀土元素中弱铕亏损演变为强铕亏损;微量元素中Rb、Nb、Ta含量升高,而Ba、Sr、P、Ti逐渐降低。主、微量元素地球化学特征总体显示该杂岩体为钙碱性—碱性、准铝质—弱过铝质A型花岗岩,其成因可能为在岩石圈伸展环境下,幔源岩浆的底侵促使上覆的先存地壳发生部分熔融形成花岗质岩浆,岩浆源区与壳幔混熔作用有关,并且该作用形成杂岩体中早期含幔源包体的石英二长闪长岩和晚期富挥发分的二长花岗岩。

The Beidashan granite, located in Chifeng area, Inner Mongolia, is an important complex rockmass in the southern of the Da Hinggan Mountains metallogenic belt. The U-Pb dating results of zircon by LA-ICP-MS method show that the porphyritic quartz monzodiorite, quartz-monzonite porphyry and monzogranite in the Beidashan complex rockmass were formed at（142.6±1.8）Ma,（141.1±1.9）Ma and（140.4±1.6）Ma respectively. The geochemical characteristics of Beidashan complex rockmass have a regular evolution from early to late stage as followings：the content of SiO2 increased gradually as well as that of alkalinity, while contents of calcium-magnesium and aluminum gradually reduced. Weak δEu depletion evolved into strong δEu depletion in the chondrite normalized REE patterns. The contents of Rb, Nb, Ta increased , but Ba, Sr, P and Ti decreased gradually. The rocks belong to calc alkaline-alkaline and quasi aluminous-weakly peraluminous A-type granites. Its genesis might be ascribed to the underplating of the mantle-derived magma which caused younger crust partial melting to form granitic magma within the lithosphere extension environment, and its magma source are related to the crust-mantle mixed re-melting. In addition, the quartz monzodiorite which contain mantle enclave in the early time and the rich volatile monzogranite in the late stage was also formed.