内蒙古白云鄂博地区呼和恩格尔杂岩体的地球化学特征及其构造意义

Geochemistry of Huheengeer Complex,Bayan Obo Region,Inner Mongolia and It's Tectonic Implications

内蒙古白云鄂博地区白音珠日和苏木以南呼和恩格尔附近出露一套杂岩体,由中心向外依次出现角闪橄榄岩、角闪辉长岩和闪长岩,具有环状分布的特征,并以闪长岩为主。主要造岩矿物为橄榄石、辉石、斜长石及角闪石等。根据K-Ar同位素表观年龄,呼和恩格尔杂岩体形成于242～287Ma之间。电子探针数据分析表明,角闪辉长岩中的辉石为透辉石,斜长石An值平均为90.6,属倍长石。杂岩体的各类岩石均表现为大离子亲石元素(LILE)K,Rb,Sr,Ba明显富集,高场强元素(HFSE)Nb,Ta,Zr,Hf相对亏损。根据AlZ-TiO2,An-Mg,Hf/3-Th-Ta以及Hf/3-Th-Nb/16图解,结合其微量元素地球化学特征,可以判断呼和恩格尔杂岩体形成于活动大陆边缘,是二叠纪古亚洲洋板块向华北板块北缘下部俯冲事件的直接反映。

Huheengeer mafic-uhramafic complex is found in Bayan Obo region, Inner Mongolia. Petrographically, the complex shows a concentric zoning in rock distribution： hornblende-peridotite, hornblende-gabbro and diorite from core to margin successively. Characteristics of structure and chemistry of the pluton suggest that the rocks were derived from a gabbroic- dioritic magma by crystal differentiation. The hornblende-peridotite and hornblende-gabbro were formed by cumulation of early crystallized mafic minerals, and the other rocks crystallized from a magma formed after cumulative process by eutectic crystallization. The parental magmas were compositionaeey similar to high-A1 basalt and （or） basaltic andesite. The K-Ar apparent ages of the rocks are between 242-287 Ma. The major rock-forming minerals in the Huheengeer Complex are olivine, pyroxene, plagioclase and hornblende. Based on electron microprobe analysis （EMPA）, pyroxene is diopside, and plagioclase is bytownite （An90.6） in amphibole gabbros. The complex also shows geochemical characteristics of enrichment in LREE, LILE （K, Rb, Sr, Ba）, and depletion in HREE, HFSE （Nb,Ta, Zr, HI）. According to the Alz-TiO2, An-Mg＇, Hf/3-Th-Ta and Hf/3-Th-Nb/16 diagrams, the Huheengeer complex is suggested to be generated at an active continental margin. All the combined information indicates that these plutons are mafic-ultramafic complex of peritotite-diorite type and they should be a record of Paleoasian Ocean plate subduction to the northern side of North China plate in the Permian.