大兴安岭中生代花岗岩类的地球化学

Geochemistry of Mesozoic granites in Da Hinggan Ling ranges

大兴安岭中生代花岗岩根据微量元素地球化学特征划分为高锶花岗岩类和低锶花岗岩类,前者富集Ba、Sr、Ti,而后者强烈亏损这些元素而富集大离子亲石元素和高场强元素。高锶花岗岩类主要由石英闪长岩、英云闪长岩和花岗闪长岩组成,属于I型花岗岩;低锶花岗岩类由二长花岗岩、正长花岗岩、碱长花岗岩和碱性花岗岩组成,二长花岗岩-正长花岗岩-碱长花岗岩也属于I型花岗岩,碱性花岗岩为A_1型花岗岩。这两类花岗岩均显示ε_(Nd)(t)正值、^(87)Sr/^(86)Sr低值以及较低的Nd模式年龄。高锶与低锶花岗岩类地球化学差异性表明,高锶花岗岩起源于相对亏损的幔源岩浆的分异作用,而低锶花岗岩类的源区与显生宙地壳增生时期起源于地幔的年轻地壳物质有关,即起源于富集型幔源基性岩石的部分熔融。大兴安岭中生代花岗岩与流纹岩之间地球化学相似性以及与玄武岩类的相关性表明,它们是统一的构造-岩浆体系的产物,共同制约于古亚洲洋闭合后的大陆伸展的构造环境和闭合期间壳幔相互作用形成的地幔源区。

The Mesozoic granites in Da Hinggan Ling Ranges can be divided into high-Sr granite and low-Sr granite according to the geochemical characteristics of trace elements. The former is enriched in Ba, Sr and Ti elements, while the latter is intensely depleted in the above elements, but enriched in large-ion-lithophile elements and high field intensity elements. The high-Sr granites belong to I-type granite and include quartz diorite, tonalite and granodiorite. The low-Sr granites are composed of monzonitic granite, syenogranite, alkali-feldspar granite which are attributed to I-type granite and alkali granite which is attributed to A(1)-type granite. Both types of granites show positive epsilon(Nd) (t) value, low initial Sr-87/Sr-86 value and relatively low Nd-mode age. Such values may indicate that their source regions are related to mantle-source matters formed during Phanerozoic crustal accretion. The geochemical differences between high-Sr granites and low-Sr granites show that high-Sr granites were derived from differentiation of relatively DM-source magma and low-Sr granites were originated from partial melting of EM-source basic rocks. The geochemical similarity of the Mesozoic granites and rhyolites in Da Hinggan Ling Ranges and their relativity to basaltic rocks suggest that they resulted from a unitive tectonic-magma system. They are attributed to the crust-mantle interaction during closure of the Paleo-Asian Ocean and subsequent continental extension.