跃进山杂岩中二叠纪变玄武岩的锆石U-Pb年代学、地球化学及其地质意义

那丹哈达增生杂岩为中国东部古太平洋板块俯冲最为可靠的地质记录之一,对限定古大洋板块的西向俯冲具有重要意义。跃进山杂岩位于那丹哈达增生杂岩的增生前锋位置,发育典型的增生杂岩组合。其中变玄武岩类多呈构造透镜体状与硅质岩、片理化泥岩和片岩相伴产出,部分可见变余枕状构造。本文对位于研究区北部勤得利和南部东方红地区的3件变玄武岩样品进行了同位素年代学和地球化学研究。变玄武岩样品中含有数量不等的锆石,多呈自形-半自形,条带结构,结合其较高Th/U比值(0.15~2.4),暗示其具有典型的基性岩岩浆锆石特征。锆石LA-ICP-MS U-Pb年代学证据显示跃进山杂岩中的变玄武岩形成于270±2 Ma~279±4 Ma,表明其原岩形成时代应为早二叠世。地球化学分析显示,跃进山杂岩南、北地区的地球化学组成略有不同,北部勤得利地区的变玄武岩具有富Na2O,贫Mg O,轻度富集轻稀土元素,富集大离子亲石元素Sr和Ba,亏损高场强元素Nb等特征,南部东方红地区变玄武岩具有贫Na2O、K2O和Mg O,亏损轻稀土元素和富集大离子亲石元素Rb、Sr等特征,结合构造判别图解,北部勤得利地区变玄武岩形成于消减带或俯冲带(SSZ)构造背景下,而南部东方红地区变玄武岩显示为N-MORB型,形成于大洋中脊(MOR)环境。结合前人资料及北部勤得利地区岩石的年代学与地球化学研究确定了佳木斯地块与那丹哈达地体之间的缝合线的具体位置为勤得利-二龙山-尖山子乡—线,并对该岩浆岩演化的动力学背景进行了初步分析。

Geochemistry and Zircon U-Pb Age of Permian Metabasalts in the Yuejinshan Complexes and its Tectonic Implications

As the key geological record of Paleo-Pacific Plate subduction in NE China, the Nadanhada accretionary complex is critical to understand the westward subduction of Paleo-Pacific Plate. The Yuejinshan complex is the first stage accretion complex that consists of metamafic-ultramafic rocks. The metabasalts show typical pillow lava structure and occur as structural lenticular along with siliceous rock, schistositized mudstone and schist. In this paper, we present both geochronological and geochemical data for three metabasalt samples in the Qindeli and Dongfanghong areas. Zircons from three metabasalts are euhedral-subhedral and display striped absorption in CL images as well as high Th/U ratios (0.15?2.4), implying magmatic origin. The dating results indicate that metabasalts were formed at 270±2 Ma to 279±4 Ma, at the Early Permian. There are some differences on geochemical characteristics from the north to the south of the Yuejinshan complexes. In the Qingdeli area, metabasalts are characterized by high Na2O, low MgO, high light rare earth elements, enrichment in Sr, Ba, and relative depletion in Nb. Whereas, they are characterized by low Na2O, K2O, MgO and light REE, enrichment in Rb, Sr in the Dongfanghong area. According to the tectonic discrimination diagrams, the metabasalts in the northern Qindeli area was formed under the Supra-Subduction Zone, while the metabasalts in the Dongfanghong area was formed from the type of Middle Ocean Ridge. Combined with data in literature and our results, we suggest that the Jiamusi and Nadanhada accretionary terrains were sutured along the Qindeli?Erlongshan?Jianshanzixiang.