阿尔泰南缘阿舍勒盆地泥盆纪火山岩中古老锆石的U-Pb年龄、Hf同位素和稀土元素特征及其地质意义

The U-Pb ages,Hf isotope and REE patterns of older zircons from Devonian volcanic rocks in Ashele basin on the southern margin of Altai orogen and its geological significance

阿尔泰造山带南缘发育有NW向斜列的四个泥盆纪海相火山岩沉积盆地,位于北西端的阿舍勒盆地产有阿舍勒大型铜锌矿床,前人对该矿床开展了大量研究,但其容矿火山岩的年龄尚未明确厘定。本文应用LA-ICP-MS锆石U-Pb定年和锆石Hf同位素、微量元素,对阿舍勒铜锌矿区内出露的泥盆纪火山岩进行了研究。由锆石U-Pb定年获得了一组火山岩老锆石谐和年龄为1985±8.9Ma,加权年龄为2005±30Ma,代表了阿尔泰地区前寒武纪结晶基底的形成时代,其Th/U值在0.25～0.90之间,εHf(t)为-1.4～8.5,平均值为3.3;另外一组新锆石年龄为408±8Ma,其εHf(t)为9.3,Th/U值为0.49,具有典型岩浆锆石特征,代表了早泥盆世岩浆活动高峰期一次火山活动时代。老锆石∑REE从最低的1015×10-6变化到最高的3486×10-6,表现为轻稀土亏损,重稀土富集特征;本组锆石不同于正常岩浆锆石的显著特征是部分出现Ce元素负异常。锆石U-Pb年龄、Hf同位素及稀土元素Ce负异常特征综合显示,阿尔泰前寒武纪结晶基底可能形成于Columbia超大陆拼合聚集背景下,其物源来自强还原性地幔物质与氧化性地壳物质的不均匀混合。

There are four Devonian marine volcano-sedimentary basins on the southern margin of Altai orogenic belt. The Ashele large-scale Cu-Zn deposit occurred in the Ashele basin which located in the northwest of Altai,and lots of research works have been down on it,but the age of its ore-hosting volcanic rock has not been clearly determined so far. In this paper,LA-ICP-MS zircon U-Pb dating,zircon Hf isotopes and REE patterns were used for researching on the Devonian volcanic rocks in Ashele Cu-Zn district. The concordant age of 1985±8.9Ma and the weighted age of 2005±30Ma of a group of older zircons from volcanic rocks by the zircon U-Pb dating represent the forming age of the Precambrian crystalline basement in Altai region,their Th/U values range is 0.25~0.90,and the εHf(t) values range is-1.4^+8.5,with an average value of 3.3; Another group of new zircons have the age of 408±8Ma,and have a typical features of magmatic zircons with the εHf(t) value of 9.3 and Th/U value of 0.49,on behalf of the age of a volcanic activity in the peak time of the Early Devonian magmatism. The ∑REE of old zircons changed from the lowest of 1015×10-6 to the highest 3486×10-6,show the characteristics of depleting in LREE and enriching in HREE; the emergence of negative anomalies of Ce element is the salient feature which is different from normal magmatic zircon in this group of zircons. Zircon U-Pb ages,Hf isotopes and Ce negative anomaly implied that Altai Precambrian crystalline basement may be formed in the background of gathering together of Columbia super-continent,and its source material came from the non-uniform mixing of the reducing mantle materials and oxidized crustal materials.