佳木斯地块中部二叠纪永清花岗闪长岩的锆石U-Pb年龄、地球化学特征及其地质意义

Zircon U-Pb age and geochemistry of the Permian Yongqing granodiorite intrusion in Jiamusi massif of Northeast China and their implications

位于中国东北佳木斯地块中部的永清花岗闪长岩曾被认为是早古生代麻山群的一部分,LA-ICP-MS锆石U-Th-Pb同位素分析表明其形成时代为265±1 Ma(MSWD=2.1),相当于中二叠世。该岩体主要由半自形到自形的斜长石(30%～35%)、它形的石英(25%～30%)、它形的钾长石(20%～25%)和半自形的黑云母(10～15%)组成。在地球化学上,永清花岗闪长岩是高钾钙碱性到钙碱性系列的过铝质岩石,Sm/Dy值与Dy含量、Sr/Y值与Y含量以及Ce/Sm值与δEu均呈显著的线性相关,暗示在岩体侵位形成过程中同化混染作用有明显表现。考虑同化混染作用的影响,永清花岗闪长岩的母岩浆具有较高的Sr/Y和Sm/Dy值等特征可能反映源区中存在石榴子石,指示源区深度可能大于25 km。这与佳木斯地块南缘和张广才岭地区同期侵入岩的源区深度相对较浅形成鲜明的对照。结合区域研究资料,认为永清花岗闪长岩可能形成于佳木斯地块和松嫩地块及兴凯地块碰撞中或者碰撞后的环境。

The Yongqing granodiorite,located in the inner part of the Jiamusi massif in eastern Northeast China,was previously considered as a part of the Early Paleozoic Mashan complex.LA-ICP-MS zircon U-Th-Pb isotopic analysis yields a middle Permian age of 265 ± 1 Ma （MSWD =2.1）.The granodiorite is composed of subhedral to euhedral plagioclase （30 ％ ～ 35 ％）,anhedral quartz （25 ％ ～ 30 ％）,anhedral K-feldspar （20 ％ ～ 25 ％） and subhedral biotite （10 ％ ～ 15 ％）.Geochemically,the granodiorite is high-K calc-alkaline to calc-alkaline and peraluminous in composition,and shows very good linear correlation between Sm/Dy and Dy,Sr/Y and Y,and Ce/Sm and δEu,probably resulting from assimilation and contamination of some country rocks during the emplacement without melt fractionation.Taking the effects of assimilation and contamination into consideration,the authors hold that the high Sr/Y and Sm/Dy ratios of parental magma of the Yongqing granodiorite might imply the presence of garnet in the source.The depth of the source rocks might have been greater than 25 km,distinctly different from the depths of contemporaneous intrusions in southern Jiamusi massif and the Zhangguangcai Mountain.Combined with previous studies,it is suggested that the Yongqing granodiorite was formed during or after the collision between the Jiamusi massif,Songnen block and Xingkai block.