锡林浩特杂岩中斜长角闪岩锆石U-Pb年代学及Hf同位素研究

锡林浩特杂岩是一套经历强变形变质作用的变质岩,主要由黑云斜长片麻岩、角闪斜长片麻岩、斜长角闪片麻岩及变粒岩夹透镜状或脉状斜长角闪岩组成。查明其形成年龄、变质年龄对深入认识中亚造山带的形成演化具有重要意义。本次研究对锡林浩特杂岩中斜长角闪岩进行了SHRIMP锆石U-Pb测年和LA-MC-ICP-MS锆石Hf同位素组成分析,给出了锆石SHRIMP U-Pb的加权平均年龄为(316±4)Ma,该年龄代表斜长角闪岩的原岩形成年龄,表明锡林浩特杂岩不是前寒武纪地质体。通过锆石Hf同位素分析,εHf(t)值主要为正值(+4.1～+17.0),揭示其物源可能来自亏损地幔或壳幔混合。根据亏损地幔和古老地壳的锆石Hf同位素进行两端元的混合计算,得到斜长角闪岩原岩岩浆的源区以幔源增生组分为主体(经过计算壳幔混合比例大约1∶2)。部分锆石的模式年龄在1.8～2.5 Ga之间,表明斜长角闪岩的原岩在形成过程中有古元古代地壳物质的加入,通过兴蒙造山带和锡林浩特杂岩中斜长角闪岩的Hf模式年龄对比可知,物源可能来自兴蒙造山带内部。基于Hf同位素的两端元混合计算表明该区在晚古生代存在地壳增生。根据野外观察和室内岩石特征分析,该套杂岩可能是一套经历强变形与变质作用的晚古生代火山-沉积建造。

U-Pb Dating and Hf Isotopic Composition of Zircons in Amphibolite from the Xilinhot Complex

The Xilinhot complex is mainly composed of biotite-plagioclase gneiss,hornblende-plagioclase gneiss,amphibolitic gneiss and granulite with lenticular or vein amphibolite rocks,whose formation and metamorphosed age and petrogenesis have been debating.Accurate determination of its formation and metamorphosed age is of great significance to understanding the formation and evolution of the Hinggan-Mongolia Orogenic Belt.Through zircon SHRIMP U-Pb dating and LA-MC-ICP-MS Hf isotopic analysis on zircons in plagioclase amphibolite from the Xilinhot complex,weighted average value of 206Pb/238U-Pb ages is(316±4) Ma,which indicates the Xilinhot complex should not be Precambrian.According to the field observations and petrography,the complex may be a suite of Late Paleozoic volcanic-sedimentary formation which had been subjected to strong deforma-tion and metamorphism.According to the zircon Hf isotopic compositions of plagioclase amphibolite in the Xilinhot complex,εHf(t) value is positive(+4.1 to +17.0),it should have originated from the depleted mantle or crust-mantle mixing source.A mixing calculating of depleted mantle and ancient crustal end-members based on zircon Hf isotopic compositions shows that depleted mantle-derived accretion component was dominant in the plagioclase amphibolite(after calculation,the crust to mantle mixing ratio is about 1∶ 2).Part of the zircons model ages between 1.8 Ga and 2.5 Ga suggests that Paleoproterozoic crustal materials mixed with depleted mantle-derived component during the formation process of the protolith of the plagioclase amphibolite.Comparison of the Hf model age of the plagioclase amphibolite to those in the Hinggan-Mongolia Orogenic Belt shows that crustal component in the plagioclase amphibolite originated from the Hinggan-Mongolia Orogenic Belt.Almost consistent Hf model ages with those of zircon U-Pb dating suggest that the crustal accretion event took place in Late Palaeozoic.