Abstract: Two types of cumulate gabbros are exposed in Wulatezhongqi area, Inner Mongolia, including amphibole gabbros and olive gabbros. The major petrogenetic minerals in the Wengeng gabbros are olive, pyroxene, plagioclase and hornblend. Based on electron microprobing analysis (EMPA), pyroxene is diopside and plagioclase is bytownite of An86 in amphibole gabbros. They also show geochemical characteristics of enriched in LILE (K，Rb，Sr，Ba), and depleted in HFSE (Nb，Ta，Zr). According to the diagrams of Al₂-TiO₂, Wo-En-Fs and An-Mg′, the amphibole gabbros are suggested to be generated at an island arc. Whereas olivine of the olive gabbros belongs to chrysolite with high Fo74, plagioclase is bytownite of An80, and pyroxene is enstatite. The geochemistry of the olive gabbros is characterized by low SiO₂ and high MgO contents, enriched in LILE (K，Rb，Sr，Ba) and LREE, and unfractionation in HFSE. All above evidences suggest that the olive gabbros are derived from a depleted mantle source in a mid-oceanic ridge or an island arc setting. According to the diagrams of Wo-En-Fs and An-Mg′, the olive gabbros are suggested to be formed in a back-arc basin setting. The formation of Wengeng gabbros may be related to the subduction of the Paleoasian ocean plate. The Wengeng gabbros should be an important lithological record of the Paleoasian ocean closure.

Key words: Wulatezhongqi, gabbro, rock fabric, geochemistry, tectonic setting, Paleoasian ocean

摘要： 内蒙古乌拉特中旗温更地区发育了一套由角闪辉长岩和橄榄辉长岩组成的混杂堆晶岩系，主要造岩矿物为橄榄石、辉石、斜长石及角闪石等。电子探针数据分析表明，角闪辉长岩中的辉石为透辉石区，而斜长石An平均为86，属倍长石。这些岩石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相对亏损。根据Al₂-TiO₂，Wo-En-Fs以及An-Mg′图解，结合其微量元素地球化学特征可以判断角闪辉长岩类形成于岛弧环境。橄榄辉长岩中橄榄石Fo平均为74，属贵橄榄石，斜长石An平均为80，属倍长石，辉石全部落入顽辉石区。这些岩石表现了贫SiO₂高MgO，富集大离子亲石元素K，Rb，Sr，Ba和轻稀土（LREE），个别高场强元素Nb亏损，兼具大洋中脊与岛弧的特征。根据Wo-En-Fs以及An-En图解推测橄榄辉长岩产于弧后盆地环境。温更混杂辉长岩的形成与古亚洲洋板块的俯冲消减作用有关，是研究古亚洲洋演化历史的一个重要岩石学标志。

关键词: 乌拉特中旗, 辉长岩, 岩石结构, 地球化学, 构造环境, 古亚洲洋