佳木斯地块东部二叠纪方山花岗岩体的锆石U—Pb年代学、地球化学特征及其地质意义

Geological implications of Permian Fangshan granitic rocks in eastern Jiamusi Massif: evidences from U—Pb chronology and geochemistry

本文报道了佳木斯地块东部方山花岗岩体的锆石LA--ICP--MS U--Pb定年结果、全岩地球化学特征,确定了花岗质岩石的形成时代以及大地构造背景。花岗岩的锆石主要呈自形-半自形晶,振荡环带发育,Th/U在0.26~1.16之间,显示其典型的岩浆成因;测年结果表明这些花岗岩形成于早二叠世（278 Ma,277 Ma）。方山花岗岩呈现出弱过铝质高钾钙碱性的地球化学特征,这些岩石轻稀土富集,重稀土亏损,具有Eu负异常,富集大离子亲石元素（Rb、K等）,相对亏损高场强元素（Nb、Ta、P、Zr和Ti）。该区花岗岩地球化学特征与佳木斯东缘二叠纪的美作、青山、楚山、柴河、石场、锦山等岩体相似,体现出火山弧型花岗岩的特征。结合区域研究资料,我们认为佳木斯地块东缘在早二叠世期间为活动大陆边缘。

Zircon LA--ICP--MS U--Pb dating results and the whole rock geochemistry of Fangshan granite in the eastern Jiamusi Massif are reported. The formation age and geodynamics setting of these granitic rocks are identified. Zircons from Fangshan granite rocks are euhedral-subhedral in shap,and display distinctly oscillatory zoning structures. The values of Th / U are 0. 26 ~ 1. 16,indicating a typical magmatic origin. The dating results show that the granitic rocks formed in the Early Permian（ 278 Ma,277 Ma）. The geochemical characteristics of Fangshan granite are weakly peraluminous and high in K calc alkaline series. These rocks are rich in LREE and large lithophile elements（ Rb and K） and depleted in HREE and high field strength elements（ Nb,Ta,P,Zr and Ti）,with Eu anomalies. The geochemical characteristics of the Fangshan granitic rocks are similar to the granitic rocks from the eastern Jiamusi Massif（ e. g. Meizuo,Qingshan,Chushan,Chaihe,Shichang and Jinshan）,representing a feature of volcanic arc granite（ VAG）. Combining with the regional research data,we deduce that the eastern margin of the Jiamusi Massif was active contenental margin during the Early Permian.