黑龙江东部完达山地体蛇绿岩形成时代及其构造意义

对出露于完达山造山带的饶河蛇绿岩和跃进山蛇绿岩进行了岩石地球化学以及锆石U-Pb年代学的研究,以便了解其成因、形成时代以及它们所揭示的区域构造背景。饶河蛇绿岩TiO₂含量高,高场强元素Nb、Ta、Hf、Zr等富集,大离子亲石元素Rb、Sr、Ba等相对富集,岩石样品球粒陨石标准化稀土元素配分模式及原始地幔标准化微量元素蛛网图与典型的洋岛玄武岩一致,总体表现出洋岛玄武岩的特征。跃进山蛇绿岩的微量元素特征兼具洋中脊拉斑玄武岩与岛弧拉斑玄武岩的属性,富集大离子亲石元素Rb、Ba、K、Sr等,亏损高场强元素Zr、Hf、Nb、Ta等,无负Eu异常,轻微富集轻稀土元素,形成于弧后环境。测年结果表明:饶河蛇绿岩的年龄为(219.0±2.4)Ma,跃进山蛇绿岩的年龄为(311±11)Ma。结合二者形成的构造环境,在晚石炭世—晚三叠世,黑龙江东部地区在古太平洋板块向西俯冲的背景下经历了洋岛或海山的剥落与拼贴以及“弧-盆”体系演化等复杂的地球动力学过程。

Formation Ages and Tectonic Significance of Ophiolites in Wandashan Terrane of the Eastern Heilongjiang

Rock geochemistry and zircon U-Pb dating are reported for Raohe and Yuejinshan ophiolites in Wandashan orogenic belt in order to understand its origin and age, and reveal their regional tectonic setting. Content of TiO₂ is high, high field strength elements (Nb, Ta, Hf, Zr, etc.) are enriched, large ion lithophile elements (Rb, Sr, Ba, etc.) are relative enriched for Raohe ophiolite; chondrite normalized REE patterns and primitive mantle-normalized trace element spider diagrams of rock are consistent with that of the typical oceanic island basalt (OIB), showing the characteristics of OIB. The characteristics of trace elements of Yuejinshan ophiolite are both mid-ocean ridge tholeiite and island arc tholeiite, large ion lithophile elements (Rb, Ba, K, Sr, etc.) are enriched, high field strength elements (Zr, Hf, Nb, Ta, etc.) are depleted, Eu anomaly is not negative, LREE is enriched lightly, and Yuejinshan ophiolite is formed in back-arc environment. Dating results show the ages of Raohe and Yuejinshan ophiolites are (219.0±2.4)Ma and (311±11)Ma, respectively. Combined with the formation of tectonic environment, the eastern Heilongjiang in Late Paleozoic Late Triassic experiences a series of complicated geodynamic processes in the background of the westward subduction of Paleo-Pacific Plate, including the scaling and combining of oceanic island or seamount and “arc-basin” system evolution.