Rare Earth Elements Tracing Crustal Evolution Through Time: A Detrital Zircon Study

最近的多条证据表明，类似现代的板块构造过程的开始相当突然地始于太古宙(约30亿年前)，在构造上，地球经历了几个不同的动力学阶段。这里最重要的问题是检验地球构造-岩浆演化中几个阶梯式变化的假设。这种变化应该反映在中间火成岩和长英质火成岩的地球化学成分上，这些岩石是大陆块体的基本组成部分。如果这真的发生了，我们能更好地限制这种变化的年龄吗？它是突然发生的还是逐渐发生的？在我们这个充满活力的星球上，解开构造阶段及其之间的转换对于我们全面理解资源分布以及与矿物和生物领域的其他进化过程的反馈具有重要意义。在该项目中，研究人员将使用碎屑锆石U-Pb年龄和满足以下条件的沉积物和沉积岩颗粒上的痕量元素浓度(重点是稀土元素)：(1)正在排出相当大的大陆块段，(2)包含覆盖大地质时间间隔的锆石，特别是来自前寒武纪的锆石。年龄范围目标将特别集中在哈甸、太古宙和元古界地区。最近的工作表明，在年轻火成岩上测量的全岩地球化学参数与全球和区域地壳厚度呈正相关。La/Yb和其他痕量元素比值与现代俯冲系统中火成岩的地壳厚度进行了对比，并已用于显示少数显生宙造山带地壳厚度随时间的变化。地壳厚度的隐含变化与受现代板块构造影响的大陆地区地壳厚度数千万年的旋回性相一致，也表明大陆地壳最深的部分(无论其厚度)是导致俯冲和碰撞系统中发现的中间火成岩多样性的分异工厂。

Multiple recent lines of evidence suggest that the onset of modern-like plate tectonic processes started rather abruptly in the Archean (about 3 billion years ago) and that tectonically, the Earth has evolved through several distinctive dynamic stages. The overarching question here is to test the hypothesis of several step-like changes in the tectono-magmatic evolution of the Earth. Such change should be reflected in the geochemical composition of intermediate and felsic igneous rocks - the basic building blocks of continental masses. If this did happen, can we better constrain the age of that change, was it sudden or gradual? Unraveling tectonics stages and transitions between them on our dynamic planet has important implications for our overall understanding of resource distribution as well as feedbacks with other evolutionary processes in the mineral and biologic realms. In the project, the researchers will use detrital zircon U-Pb ages and trace elemental concentrations (with a focus on the rare earth elements) on grains from sediments and sedimentary rocks that satisfy the following conditions: (1) are draining sizable segments of the continental mass, and (2) contain zircons encompassing large geologic time intervals, especially from the Precambrian. The age range target will especially focus on the Hadean, Archean and Proterozoic area. Recent work shows that whole rock geochemical parameters measured on young igneous rocks correlate positively with crustal thickness globally and regionally. La/Yb and other trace elemental ratios are calibrated against crustal thickness on intermediate igneous rocks from modern subduction systems and have been used to show variations in crustal thickness over time in a few Phanerozoic orogenic regions. The implied variation in crustal thickness is consistent with the tens of million years cyclicity of crustal thickness in continental areas being affected by modern plate tectonics and is also indicative of the fact that the deepest parts of the continental crust (regardless of its thickness) are the factories of differentiation leading to the diversity of intermediate igneous rocks found in subduction and collision systems.