大别山超高压榴辉岩和花岗片麻岩中锆石Lu-Hf同位素研究  

Zircon Lu-Hf isotope study of ultrahigh-pressure eclogite and granitic gneiss in the Dabie orogen

作　　者：郑永飞[1] 陈仁旭[1] 张少兵[1] 唐俊[1] 赵子福[1] 吴元保[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院中国科学院壳幔物质与环境重点实验室,合肥230026

出　　处：《岩石学报》2007年第2期317-330,共14页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40334036和40573011)资助成果

摘　　要：对大别山超高压榴辉岩和花岗片麻岩进行了锆石Lu-Hf同位素分析,结果为原岩来源提供了制约,表明扬子陆块在Rodinia超大陆裂解时的裂谷岩浆活动中发生了显著的陆壳生长。对这些锆石的不同区域进行的U-Pb和Lu-Hf同位素分析和比较表明,不同成因的锆石在^(206)Pb/^(238)U年龄、初始Hf同位素组成、Th/U及Lu/Hf比值等方面具有明显的差异。与年龄较老的岩浆核部和幔部相比,年轻的变质增生边具有低的Th/U和Lu/Hf比值但高的ε_(Hf)(t)值。不同成因锆石的Th/U和Lu/Hf比值存在着正相关性,表明变质作用对锆石的U-Th-Pb和Lu-Hf同位素体系有着相似的影响。高级变质作用有时能够引起岩浆锆石增生边^(176)Hf/^(177)Hf比值的显著升高,导致变质新生颗粒或增生边类似于新生地壳的高ε_(Hf)(t)值假象。对榴辉岩和片麻岩锆石核部的分析发现,镁铁质和长英质原岩在大约750Ma左右形成一个双峰式火山岩套,另外包含少量的年龄约为2.15Ga的陆壳。初始Hf同位素组成可分成两组:第一组具有正的ε_(Hf)(t)值,为5.9±0.9～12.9±0.7;第二组ε_(Hf)(t)值在零左右,为-4.3±0.5-2.3±0.3。正的ε_(Hf)(t)值与较年轻的模式年龄相对应,负的ε_(Hf)(t)值与古元古代模式年龄相对应。前者表明,在扬子陆块北缘裂谷岩浆作用将亏损地幔物质加入到大陆地壳中,同时在新元古代中期的裂谷构造带中存在同时期的壳-幔相互作用。因此,在扬子陆块北缘新元古代中期裂谷岩浆活动中,既有新生地壳生长和即时再造,也有古老地壳再造。Abstract Zircon Lu-Hf isotopic analyses were carried out for ultrahigh-pressure eclogite and granitic gneiss from the Dahie orogen. Together with zircon U-Pb dating, the results provide constraints on their protolith origin, with significant implications for growth of continental crust by rift magmatism during breakup of the supercontinent Rodinia. Analyses and comparison of combined U-Pb and Lu- I-If isotopes in different domains of igneous and metamorphic zircons indicate that different origins of zircons show considerable differences in ^206Pb/^238U age, initial Hf isotope ratio, Th/U and Lu/Hf ratios. Metamorphically grown zircons have lower Th/U and Lu/Hf ratios but higher БHf（t） values than igneous core and mantle. There are positive correlations between Th/U and Lu/Hf ratios for metamorphically reworked zircons, suggesting similar effects of eclogite-facies metamorphism on zircon U-Th-Pb and Lu-Hf isotope systems. Therefore, high-grade metamorphism can cause an increase in ^176Hf/^177Hf ratios for metamorphically grown grain or domain, resulting in enhanced Бnf （t） values as if crystallizing from juvenile crust. Hence caution must be exercised when interpreting metamorphic zircon Hf isotope data for the purpose of source tracing. Zircon U-Pb dating for igneous cores from both eclogite and gneiss clearly indicate bimodal magmatism at about 750Ma, with a minor incorporation of juvenile crust in an age of 2.15Ga. Initial Hf isotope ratios can be categorized into two groups, first with positive БHf（t） values of 5.9 ± 0.9 to 12.9 ± 0.7 and second with neutral БHf（t） values of - 4.3 ±0.5 to 2.3 ± 0.3 around zero. The positive Бnf （t） values correspond to young Hf model ages mostly on early Neoproterozoic and late Mesoproterozoic, whereas negative БHf（t） values to Paleoproterozoic Hf model ages. The former implies growth of continental crust by extraction of juvenile crust from the depleted mantle along rifting volcanic margins at middle Neoproterozoic, with contemporaneous crus

关 键 词：大别造山带 榴辉岩 片麻岩 锆石 U-Pb定年 LU-HF同位素 地壳生长 

分 类 号：P588.348[天文地球—岩石学] P597.3[天文地球—地质学]