延边开山屯花岗闪长岩-石英闪长岩体年代学、地球化学特征及对古太平洋板块俯冲作用时限的制约

Chronology,geochemical characteristics of granodiorite-quartz diorite pluton in Kaishantun,Yanbian area and its constrains to the beginning of Paleo-Pacific Plate subduction

近年来古太平洋构造域的构造演化备受学者关注。本文报道的延边开山屯地区花岗闪长岩-石英闪长岩体LAICP-MS U-Pb年龄表明其形成时间为早侏罗世早期(198±1Ma),所采样品可根据Zr/Hf值分为高Zr/Hf值组花岗闪长岩和低Zr/Hf值组石英闪长岩。高Zr/Hf值组花岗闪长岩起源深度浅,富集Rb、Th、U、K等大离子亲石元素(LILEs),贫Nb、Ta、Ti等高场强元素(HFSEs),具壳源岩浆的特点。低Zr/Hf值组为壳源岩浆与来自深部的亏损地幔岩浆混合而成,岩石亏损Nb、Ta、Zr、Hf、Ti等高场强元素,具有典型的弧型岩浆地球化学特征。岩体中存在细粒闪长质包体,镜下可见针柱状磷灰石。开山屯岩体属钙碱性系列岩石,结合前人资料,认为其与该地区同时代火成岩组成北-东向分布的早侏罗世活动大陆边缘型火成岩带,而位于该带西侧的小兴安岭-张广才岭地区存在同时代弧后拉张带,两者构成典型的大陆弧与弧后拉张带模型,共同揭示了早侏罗世早期古太平洋板块对东北地区的俯冲作用。

In recent years, the evolution history of Paleo-Pacific tectonic domain attracted the attention of many researchers. The zircon LA-ICP-MS U-Pb ages of granodiorite-quartz diorite pluton in Kaishantun, Yanbian area indicates that it formed in early Early Jurassic （ 198 ± 1Ma）. According to the ratio of Zr/Hf, we divided the samples into two groups ： granodiorite with high Zr/Hf ratio and quartz diorite with low Zr/Hf ratio. Magma source of high Zr/Hf ratio group is from crust. Trace element compositions of this group are enrichment in large ionic lithophile elements （LILEs）, such as Rb, Th, U, K, and depletion in high field strength elements （HFSEs）, Nb, Ta, Ti. Magma source of low Zr/Hf ratio group is the mixed product of magma from crust and magma from depleted mantle. Samples of quartz diorite are strong negative with HFSEs （Nb, Ta, Zr, Hf, Ti）, also have the typical geochemistry characteristics of arc-type magma. There are a lot of fine-grained dioritie enclaves in the rock mass. Under microscope, we can see needle like apatite on the thin section. The Kaishantu rock mass belong to cale-alkaline series. Combined with previous research, we found the north-east direction Early Jurassic active continental margin igneous belt. In the west of the belt, there is coeval back arc extensional zone in Xiao Hinggan Mountains-Zhangguangcailing area. The distribution of the two belts make a typical model of active continental margin arc and back arc extensional belt in a subduetion zone, reveal the subduction of Paleo-Pacific plate to NE China in early Early Jurassic.