依兰地区原黑龙江群碱性玄武质火山碎屑岩的发现及地球动力学意义探讨

The discovery of alkaline basaltic pyroclastic rocks in original Heilongjiang Group of Yilan area and its geodynamic significance

在依兰地区原黑龙江群中发现了一套火山碎屑岩，它由岩屑和细粒的火山碎屑物组成，岩屑呈透镜状、焰舌状等，细粒火山碎屑物发生了强烈变形。二者矿物组成相似，均由钠质闪石、钠长石、绿帘石以及少量绿泥石组成，但各矿物含量及特征差别较大。火山碎屑岩岩屑的地球化学特征表明，岩屑为碱性玄武岩，LREE／HREE=3．87～4．24，（La／Yb）N=7．20～8．12，Eu^＊／Eu=1．03～1．09，稀土元素配分曲线为右倾型。岩屑微量元素与MORB相比，富集sr、K、Rb、Ba等不相容元素，Yb含量略低于MORB，初步确定岩屑为形成于大洋板内洋岛环境的碱性玄武岩。在岩浆喷出尚未完全凝固之前，大洋板块内的热点再次活动，早期形成的碱性玄武岩破碎，形成塑性-半塑性岩屑，并被火山碎屑物胶结，它们为大洋板内“轴外火山作用”形成的洋岛型火山碎屑岩。这一发现为恢复该地区的古构造环境、确定原黑龙江群的构造属性提供了新的线索。

In this paper, a set of pyroclastic rocks was discovered in the original Heilongjiang Group of Yilan area. They are composed of debris and fine-grained pyroclastics. The debris is in the lenticular and flame forms, whereas the fine-grained pyroclastics have been highly deformed. With similar composition, they are comprised of sodium amphibole, albite, epidote and minor amounts of chlorite; nevertheless, there is an obvious difference between them in mineral content and mineral characteristics. Geochemical characteristics of pyroclastic rocks suggest that the debris belongs to alkaline basalt, with its LREE/HREE ratio between 3.87 and 4.24, its （La/Yb）n between 7.20 and 8.12 , its Eu^＊/Eu ratio between 1.03 and 1.09, and its REE distribution partition curve being of the right-oblique type. Compared to things of MORB, the trace elements of pyroclastics are more enriched in such incompatible elements as Sr, K, Rb and Ba. Yb is slightly lower than that in MORB. In the Zr - Zr/Y diagram and TiO2 - MnO × 10 - P2O5 × 10 diagram, points of all samples are plotted in the oceanic-island basalt and interpolated area. It is considered that pyroclastics are alkaline basalts formed in an ocean island environment within the oceanic plate. Before the complete coagulation of these oceanic-island alkaline basahs, the hotspot in the oceanic plate became active again and broke the previously-formed alkaline basahs into plastic-semiplastic debris, which was then cemented by pyroclastics. The pyroclastics are ocean island type pyroplastics formed by ＂off-axis volcanism＂ within the oceanic plate. This discovery has provided some new clues to the paleotectonic environment and to the determination of tectonic attributes of the original Heilongjiang Group.