大兴安岭北部塔河堆晶辉长岩体的形成时代、地球化学特征及其成因

Age, geochemistry and petrogenesis of the cumulate gabbro in Tahe, northern Da Hinggan Mountain.

本文系统地报道了大兴安岭北部塔河辉长岩的主量元素、微量元素和锆石Hf同位素特征,重点讨论了塔河堆晶辉长岩体的岩石成因、原始岩浆的源区特征及其地质意义。研究结果表明,塔河堆晶辉长岩体SiO2含量从42．5％到47．4％,平均为45．2％,Al2O3为15．9-32．9％,平均为23．6％,大于17％,CaO为11．7-17．1％,TiO2<0．5％,P2O5<0．1％,富集LILE, 亏损HFSE,这些地球化学特征类似于活动大陆边缘或岛弧环境的辉长岩。激光ICP-MS锆石年龄测定结果显示,塔河辉长岩形成于333±8 Ma前。负的εHf(t)值(-2--5)表明其源区为富集型岩石圈地幔或下地壳。因此,本文推测,塔河堆晶辉长岩来自于地幔辉石岩的部分熔融,该源区的形成可能与俯冲背景下流体交代作用有关,从而表明在330 Ma左右,该区存在过板块的消减作用,可能与松嫩和兴安地块拼合有关。

In this paper, geochemical data of the major and trace elements, and Hf isotope are reported for the Tahe cumulate gabbro exposed in the northern Da Hinggan Mountain. Basen on these data, its petrogenesis, protolith and tectonic implications are discussed in detail. The Tahe cumulate gabbro, characterized by low SiO2 （42. 59 - 47.4wt. % ） , TiO2 （ 〈 0.5wt. % ） , P2O5 （ 〈 0.1wt. % ） and high Al2O3 （ 15.9 - 32.9 wt. % ）, CaO （ 11.7 - 17. 1 wt. % ）, is enriched in large ion lithophile elements （LILE） and depleted in high field strength elements （HFSE） , showing the similar geochemical characteristics to the gabbros formed in the active continental margin or island arc. Zircon laser ablation （LA）-ICP-MS U-Pb analyses indicated that the Tahe gabbro was emplaced at 333 ± 8 Ma with negative εHf（t） values of -2 - -5. Therefore, it is concluded that the Tahe gabbroic magma was derived from partial melting of the mantle pyroxenite in the enriched lithospheric mantle or lower crust which was metasomatized by fluid released from the subducted slab. Hence, the occurrence of the Tahe cumulate gabbro indicates that a subduction process was conducted beneath this area at - 330Ma, which was considered to be related to the assembly of the Songnen and Xing＇ an blocks.