大兴安岭西缘玛尼吐组火山岩成因及构造指示

我国东北地区分布有大量中生代(侏罗纪至白垩纪)火山岩。选取大兴安岭西缘玛尼吐组粗安岩-粗面英安岩-流纹岩为研究对象，研究结果显示它们为高钾钙碱性的火山岩，主量、微量元素含量变化较大，SiO2含量为5671%~7185%，Na2O+K2O含量为592%~1018%，Na2O/K2O比值较高，为078~133，Mg#为306~564。玛尼吐组火山岩的稀土元素配分图和微量元素蛛网图右倾明显，可见明显的Nb、Ta、Ti、Sr和Eu负异常(Nb*=017~071，Sr*=024~115，Eu*=049~077)。稀土总量较高，为200×10-6~949×10-6。结果分析可知，玛尼吐组火山岩源岩与霍林河火山岩源岩相似，均为混合了少量先存古老地壳的新生基性下地壳。新生基性下地壳部分熔融形成安山质岩浆，并经角闪石+斜长石+钾长石+磷灰石+钛磁铁矿等的分离结晶形成了玛尼吐组粗安岩-粗面英安岩-流纹岩。玛尼吐组火山岩显著的岛弧地球化学特征说明其形成于与太平洋俯冲有关的岛弧环境。

Petrogenesis and Tectonic Implication of Volcanic Rocks from Manitu Formation in the Western Great Xing`an Range

Different tectonic settings have been proposed for the Mesozoic volcanic rocks in northeast China, including the mantle plume, subduction of the Pacific Ocean and post collisional extension associated with the closure of the Mongol Okhotsk ocean. In this paper, we carried out geochemical studies on the andesite dacite rhyolite in Manitu Formation of the western Great Xingan Range (NE China). Samples show variable major and trace element compositions with SiO2=5671%-7185%, Na2O+K2O=592%-1018%, Na2O/K2O=078-133，Mg#=306-564. Chondrite and primitive mantle normalized REE patterns and spidergrams show significant Nb, Ta, Ti, Sr and Eu negative anomalies (Nb*=017-071, Sr*=024-115, Eu*=049-077) and high total REEs contents. These results suggest that the source magmas of the Manitu volcanic rocks were derived from a preexistent juvenile crust and a subordinate recycled crustal component. A fractional assemblage of hornblende+plagioclase+K feldspar+apatite+titanomagnetite was responsible for the systematic elements variations from the andesite to rhyolite. This interpretation is consistent with the tectonic model that the Manitu volcanic rocks were generated in an arc/back arc tectonic setting in response to the Pacific Ocean subduction.