Petrology,Geochemistry and Tectonic Implications of the Wengeng Gabbros in Wulatezhongqi Area,Inner Mongolia
ZHAO Lei, WU Tairan, LUO Hongling, HE Yuankai Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University, Beijing 100871
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 Al_Z-TiO_2, 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 SiO_2 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.