Tectonic settings of continental crust formation: Insights from Pb isotopes in feldspar inclusions in zircon
Tectonic settings of continental crust formation: Insights from Pb isotopes in feldspar inclusions in zircon
复制标题
DOI：
10.1130/g38117.1
复制
发表时间：
2016-10
期刊：
影响因子：
5.8
通讯作者：
Hélène Delavault;B. Dhuime;C. Hawkesworth;Peter A. Cawood;H. Marschall
中科院分区：
文献类型：
--
作者：
Hélène Delavault;B. Dhuime;C. Hawkesworth;Peter A. Cawood;H. Marschall
大多数地壳岩石都是从先前存在的地壳中衍生出来的，因此新生成的（幼年的）大陆地壳的组成，以及其形成的构造环境，仍然很难确定，特别是在地球演化的第一个十亿年。现代原始幔源岩浆具有不同的U/Pb比值，这取决于它们是在板内（平均U/Pb = 0.37）还是在俯冲环境（平均U/Pb = 0.10）。因此，U/Pb比值可以作为一个代理的构造环境中，幼年大陆地壳的产生。本文提出了一种基于离子探针分析锆石中碱性长石和斜长石包裹体中Pb同位素的新方法，以追溯数十亿至数十亿年前形成的幼年大陆地壳的U/Pb比值。Pb同位素数据用于计算时间积分U/Pb比值（即，~（238）U/~（204）Pb = μ）。这些时间积分比率反映了幼年大陆地壳从地幔中提取出来时的组成，因此它们可以用作该地壳形成的构造背景的代理。两个测试样品与元古宙Hf模式年龄和古生代结晶年龄的长石包裹体测得的铅同位素比值重叠的分析误差内的每个样品。来自南极洲的样品Z7.3.1具有Pb同位素比值（平均206 Pb/204 Pb = 16.88 ± 0.08，1σ），表明其来源于低U/Pb比值（0.11）的烃源岩，与俯冲相关背景下的烃源岩相似。来自澳大利亚的Temora 2样品具有更高的放射成因Pb同位素比值（平均206 Pb/204 Pb = 19.11 ± 0.23，1σ），表明其来源具有更高的U/Pb比值（100.36），类似于板内环境产生的岩浆。分析具有一系列Hf模式年龄的碎屑群（例如，锆石及其包裹体是其母岩浆的唯一档案，最终将为我们了解大陆地壳的形成和演化开辟新的途径。
Most crustal rocks derive from preexisting crust, and so the composition of newly generated (juvenile) continental crust, and hence the tectonic settings of its formation, have remained difficult to determine, especially for the first billion years of Earth’s evolution. Modern primitive mantle–derived magmas have distinct U/Pb ratios, depending on whether they are generated in intraplate (mean U/Pb = 0.37) or in subduction settings (mean U/Pb = 0.10). The U/Pb ratio can therefore be used as a proxy for the tectonic settings in which juvenile continental crust is generated. This paper presents a new way to see back to the U/Pb ratios of juvenile continental crust that formed hundreds to thousands of millions of years ago, based on ion probe analysis of Pb isotopes in alkali feldspar and plagioclase inclusions within well-dated zircons. Pb isotope data are used to calculate the time-integrated U/Pb ratios (i.e., 238U/204Pb = µ) for the period between the Hf model age and the U-Pb crystallization age of the zircons. These time-integrated ratios reflect the composition of the juvenile continental crust at the time it was extracted from the mantle, and so they can be used as a proxy for the tectonic setting of formation of that crust. Two test samples with Proterozoic Hf model ages and Paleozoic crystallization ages have feldspar inclusions with measured Pb isotope ratios that overlap within analytical error for each sample. Sample Z7.3.1 from Antarctica has Pb isotope ratios (mean 206Pb/204Pb = 16.88 ± 0.08, 1σ) that indicate it was derived from source rocks with low U/Pb ratios (∼0.11), similar to those found in subduction-related settings. Sample Temora 2 from Australia has more radiogenic Pb isotope ratios (mean 206Pb/204Pb = 19.11 ± 0.23, 1σ) indicative of a source with higher U/Pb ratios (∼0.36), similar to magmas generated in intraplate settings. Analysis of detrital populations with a range of Hf model ages (e.g., Hadean to Phanerozoic), and for which zircons and their inclusions represent the only archive of their parent magmas, should ultimately open new avenues to our understanding of the formation and the evolution of the continental crust through time.