甘肃北山造山带双尖山金矿区正长花岗岩锆石U-Pb年龄和Hf同位素特征及地质意义

Zircon U-Pb Ages and Hf Isotopic Characteristics and Their Geological Significances of Syenogranite in the Shuangjianshan Gold Deposit in the Beishan Orogenic Belt,Gansu Province

为厘清甘肃北山造山带双尖山金矿区花岗岩体的形成时代及其地质意义,对该区的正长花岗岩进行了LA-ICP-MS锆石U-Pb定年和原位锆石Lu-Hf同位素分析。结果表明,岩体成岩年龄为(317.6±2.5)Ma,锆石Ti温度计显示其结晶温度为706.5~865.7℃,Ce^(Ⅳ)/Ce^(Ⅲ)比值为1.38~21.55,锆石稀土配分曲线表现为重稀土富集的左倾模式,具强Ce正异常、弱Eu负异常,以上特征表明双尖山正长花岗质岩浆结晶过程为相对开放的震荡环境,期间可能伴随局部不均匀与其他不同来源和温度的岩浆或围岩的混合或混染,但结晶成岩过程仍主体受控于原始岩浆;正长花岗岩中锆石的ε_(Hf)(t)为6.63~9.77,平均值为7.97,t_(DM2)为705~906 Ma,结合锆石微量元素特征推断岩体应形成于陆缘弧背景,可能为红石山洋向南侧明水-旱山板块俯冲、诱发了起源于亏损地幔的新元古代年轻地壳物质(如北山杂岩)为基底的陆缘边界地体部分熔融产生的岩浆活动的产物。

In order to explore the formation age and geological significance of the granite in the Shuangjianshan gold deposit in the Beishan orogenic belt,in this paper,the U-Pb dating and in situ Lu-Hf isotopic analysis of zircons from syenogranite in the Shuangjianshan gold deposit have been carried out.The results indicate that the diagenetic age of the syenogranite body is(317.6±2.5)Ma.Zircon Ti thermometer shows that the main crystallization temperatures of zircons vary from 706.5℃to 865.7℃.The Ce^(Ⅳ)/Ce^(Ⅲ)ratios of zircons vary from 1.38 to 21.55.The REE distribution patterns of zircons show the LREE depletion and HREE enrichment REE patterns,with the apparent positive Eu anomaly and weak negative Ce anomaly.The above characteristics indicate that the crystallization process of the Shuangjianshan syenogranitic magma was in a relatively open and oscillating environment,with accompanied locally inhomogeneous mixing or contamination of different sourced other magmas or surrounding rocks under different temperatures.However,the magmatic crystallization and diagenetic process was still mainly controlled by the original magma.Zircons’ε_(Hf)(t)values vary from 6.63 to 9.77 with the average value of 7.97,and their t_(DM2) ages vary from 705 Ma to 906 Ma.Combined with characteristics of trace elements of zircons,it is suggested that the pluton was formed in the background of continental marginal arc,and could be the product of magmatic activity generated by the partial melting of continental marginal terraina whose basement was composed of Neoproterozoic young crustal materials(such as the Beishan complex),which was originated from the depleted mantle,due to the subduction of the Hongshishan oceanic plate to the Mingshui-Hanshan plate in south side.