Collaborative Research: A Detrital Zircon Record of California Arc Magmatism

与构造板块的会聚和俯冲有关的岩浆活动通常会在盖层板块及其地下管道系统中产生一系列火山杂岩。鉴于火山系统和相关的侵入火成岩形成时间超过数千万年，通常导致大型火山建筑物不断受到侵蚀和埋藏，由于地质记录中的巨大空白，重建火山系统的完整时间和地球化学演化可能很困难。在这项研究中，主要研究人员使用了一种在沉积岩中常见的微量矿物，以帮助填补我们对加利福尼亚州内华达山脉火山弧的火山岩和侵入岩地质演化的了解空白。这项研究的对象是锆石，这是一种通常在火成岩结晶时形成的矿物，其化学结构包含可以通过同位素方法精确测年的放射性元素。此外，其他化学元素提供了一种揭示岩浆系统化学演化的手段。在河流体系中，锆石是一种特殊的抗化学和机械破坏的矿物，它通常保存在沉积岩中。它在许多沉积岩中的丰度确保了它可以通过分析火山岩和火成岩的化学成分和年代来提供早已被侵蚀的火山岩和火成岩的记录。因此，对这种锆石的分析提供了一种手段，可以用来重建火山弧缺失部分的历史，这些信息可以用来揭开火山岩形成区域的构造和地球动力学演化的细节。除了研究的科学目标外，该项目还为重要的社会成果做出了贡献，包括对STEM学科的本科生进行培训和指导，以及扩大代表不足的群体对科学的参与。该项目代表着不同类型的学术机构和教育使命之间的四部分合作。这项研究的目标是将碎屑锆石的年龄和微量元素地球化学结合起来，作为中生代科迪勒岩浆弧系演化的替代记录。锆石是多种火成岩中的一种副矿物，在热液蚀变和沉积过程中不易重结晶。碎屑状锆石可以产生岩浆系统的时间整合记录。来自原地火成岩的锆石和来自原地火成岩的碎屑锆石能够记录长期岩浆环境中熔体成分的精确年龄和变化，与原地火成岩套的研究相结合，碎屑锆石记录可能提供对部分侵蚀和未完全剥离的岩浆系统中垂直和长期变化的相对完整和详细的了解。这项研究中的主要研究人员将分析弧源碎屑锆石群体，以开发以下碎屑替代记录：(1)该弧的年龄和岩石学不对称性，它是在可变的基底上构造的；(2)通过弧岩浆作用中的脉冲和间歇作用，岩浆的地球化学变异性。根据对来自弧后前陆盆地的初始数据集的观察，主要研究人员假设，弧源碎屑锆石可以提供科迪勒弧全寿命期间平均熔体成分演变的时间受控记录。他们将从科迪勒弧弧的弧前、弧内和弧后区域的一系列地层剖面中研究碎屑锆石。来自这些剖面的碎屑锆石的铀-铅(U-Pb)同位素年龄和微量元素地球化学将使他们能够评估弧状岩浆活动的纬度和经度变化，而锆石地球化学的长期变化将反过来帮助他们检验关于岩浆脉冲和间歇期间平均熔体成分的假说。这些锆石年龄和地球化学记录也将使我们能够将岩浆脉冲与构造事件联系起来，例如弧内和弧下的地壳增厚事件。因此，这些锆石地球化学数据将成为一个有用的新工具，既可以描述这个长寿命弧系的岩石构造演化，也可以为碎屑状锆石源区的解释增加新的地球化学约束。

Magmatic activity associated with the convergence and subduction of tectonic plates typically results in an array of volcanic complexes in the overriding plate and their subterranean plumbing systems. Given that volcanic systems and related intrusive igneous rocks form over tens of millions of years and commonly result in large volcanic edifices that are constantly subjected to erosion and burial, reconstructing the complete timing and geochemical evolution of volcanic systems can be difficult due to large gaps in the geologic record. In this study, the principal investigators are using a trace mineral commonly found in sedimentary rocks to help fill in gaps in our understanding of the geologic evolution of the volcanic and intrusive rocks of the Sierra Nevada volcanic arc of California. The object of this study is zircon, a mineral that commonly forms when igneous crystallize and its chemical structure contains radioactive elements that can be precisely dated by isotopic methods. In addition, other chemical elements provide a means by which the chemical evolution of the magmatic system may be revealed. Zircon is an unusually resistant mineral to chemical and mechanical breakdown in fluvial systems and it is commonly preserved in sedimentary rocks. Its abundance in many sedimentary rocks ensures that it can provide a record of volcanic and igneous rocks that have long since been eroded away by analyzing their chemistry and dating them. Analysis of such zircons thus provides a means by which the history of missing parts of volcanic arcs may be reconstructed, and this information can be used to unravel details of the tectonic and geodynamic evolution of the region in which the volcanic rocks were formed. In addition to the scientific objectives of the research, the project is contributing to important societal outcomes, including the training and mentoring of undergraduate students in a STEM discipline, as well as broadening of participation of underrepresented groups in science. The project represents a four-part collaboration between academic institutions of diverse type and educational mission.The goal of this study is to combine age and trace element geochemistry of detrital zircons as proxy records for the evolution of the Mesozoic Cordilleran magmatic arc system. Zircon is an accessory mineral in a wide variety of igneous rocks, and is resistant to recrystallization during hydrothermal alteration and sedimentation. Detrital zircon can yield time-integrated records of magmatic systems. Zircon from in situ igneous rocks and detrital zircons derived from them together have the capability of recording both precise ages and variations in melt compositions in a long-lived magmatic environment, and when paired with studies of in situ igneous rock suites, detrital zircon records may provide a relatively more complete and detailed understanding of vertical and secular variations in partially eroded and incompletely exhumed magmatic systems. The principal investigators in this study will analyze populations of arc-derived detrital zircons in order to develop detrital proxy records of (1) the age and petrologic asymmetry of this arc that was constructed across a variable basement and (2) the geochemical variability of magmas through pulses and lulls in arc magmatism. Based on observations from an initial data set derived from a retroarc foreland basin, the principal investigators hypothesize that arc-derived detrital zircons can provide a temporally-controlled record of the evolution of average melt compositions over the full life span of the Cordilleran arc. They will study detrital zircon from a series of stratigraphic sections from within the forearc, intra-arc, and retroarc regions of the Cordilleran arc. Uranium-Lead (U-Pb) isotopic dates and trace element geochemistry of detrital zircons from these sections will allow them to evaluate the latitudinal and longitudinal variations in arc magmatism, and secular variations in zircon geochemistry will in turn help them test hypotheses regarding average melt compositions during magmatic pulses and lulls. These zircon age and geochemical records will also allow us to link magmatic pulses to tectonic events, such as episodes of crustal thickening within and beneath the arc. Thus these zircon geochemical data will constitute a useful new tool, both by describing the petrotectonic evolution of this long-lived arc system and by adding new geochemical constraints to detrital zircon provenance interpretations.