大别地体超高压变质岩石锆石Lu-Hf同位素研究

Zircon Lu-Hf isotopic compositions of ultra-high pressure metamorphic rocks from Dabie Terrain,China

对大别山南部超高压变质带双河和黄镇地区的榴辉岩、片麻岩和硬玉石英岩中变质锆石进行了原位LA-MC-ICP- MS的Lu-Hf同位素分析。双河和黄镇的榴辉岩及双河的硬玉石英岩有低^(176)Lu/^(177)Hf和低^(176)Hf/^(177)Hf组成,两地的片麻岩有高^(176)Hf/^(177)Hf比和高且分散的^(176)Lu/^(177)Hf组成。锆石Hf同位素分布主要受变质原岩的形成时代控制,增生锆石基本上继承了原岩锆石的Hf同位素特征,既有增生锆石相对有低^(176)Lu/^(177)Hf和高^(176)Hf/^(177)Hf、继承重结晶锆石相对有高^(176)Lu/^(177)Hf和低^(176)Hf/^(177)Hf的特征,也有二者相互重叠没有区别的,它主要受原岩性质和变质过程中锆石遭受的溶蚀程度控制。增生锆石的低Lu/Hf是锆石在变质过程中Lu含量下降和Hf含量增高造成的,增生锆石的高^(176)Hf/^(177)Hf继承自岩石中其它高Lu/Hf比矿物的长期演化。继承锆石的初始Hf同位素组成ε_(Hf)值和亏损地幔模式年龄T_(DM)示踪表明,各超高压变质原岩的时代和成因是复杂的:双河榴辉岩原岩物质源自25亿年的亏损地幔和至少27亿年以上的古老晚太古地壳混合。双河片麻岩原岩年龄相同,但有不同的壳幔混合物源。黄镇榴辉岩原岩主要源于亏损幔源岩浆形成的初生地壳的重循环,很少的地壳混染。黄镇片麻岩和榴辉岩的物源区年龄相同。两地片麻岩原岩物源主要来自弱亏损地幔,存在古老地壳物质和地幔物质的混合。大别地区超高压变质岩锆石的Lu-Hf同位素特征主要反映了7～8亿年和18～19亿年时扬子克拉通北缘地区的岩浆活动特点和大地构造环境。

Abstract Zircon Lu-Hf isotope analyses were carried out for ultra-high pressure（UHP） eclogite, jadeite quartzite and gneiss from Shuanghe , and eclogite and gneiss from Huangzhen, Southern Dabie Terrain by using LA-MC-ICP-MS technique. The results show that eclogites from Shuanghe and Huangzhen and jadeite quartzite have low ^176 Lu/^177 Hf and low ^176 Hf/^177 Hf ratio, whereas gneisses from two localities have high ^176Hf/^177Hf, high and spread ^176Lu/^177Hf compositions. Zircon Hf isotope distributions are mainly depended on the formation age of metamorphic precursor. Metamorphic zircon succeeded basically Hf isotopes of their protolith. Metamorphic growth zircons have relative low ^176Lu/^177 Hf but high ^176 Hf/^177 Hf ratio, inherited and re-crystallized protolith zircons have high ^176Lu/^177 Hf and low ^176 Hf/^177 Hf ratio. In some cases Hf isotope compositions of metamorphic and inherited zircons are undistinguishable within the same range. It mainly controlled by origin of protolith and zircon ablation during metamorphism. The low Lu/Hf ratio of metamorphic growth zircons were caused by the decreasing of Lu and increasing of Hf, and the high ^176Hf/^177Hf ratio originated from the addition of high radiogenic ^176Hf from other high Lu/Hf minerals evolution. The initial Hf isotope composition of БHf and the model age of depleted mantle TOM indicate that their age and origin are all different for metamorphic precursor. The source material of eclogites from Shaunghe show the mixture of magma derived from depleted mantle of 2.5Ga with at least 2.7Ga late Archean crust. The granitic gneiss from Shaunghe has the same age, but different degree of mantle and crust mixing. The material for eclogite precursor from Huangzhen was formed mainly from the recycling of juvenile crust, which was derived depleted mantle with few crust contamination. The gneiss and eclogite from Huangzhen shares the common source material. The material of gneiss precursor from both localities was derived from mixing of weak depleted mantle and old crust. The zircon Lu-Hf isotope of UHP metamorphic rocks from Dabie orogen mainly reflects magmatic activity character and tectonic setting of north margin of Yantze Craton during 0.7~0.8Ga and 1.8~1.9Ga.