Collaborative Research: A Detrital Zircon Record of California Arc Magmatism

与构造板块的汇聚和俯冲相关的岩浆活动通常会在上方板块及其地下管道系统中产生一系列火山复合体。鉴于火山系统和相关的侵入火成岩形成了数千万年，通常会形成不断遭受侵蚀和掩埋的大型火山建筑物，由于地质记录中存在巨大差距，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，主要研究人员使用沉积岩中常见的微量矿物质来帮助填补我们对加利福尼亚内华达山脉火山弧火山岩和侵入岩地质演化的理解空白。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，其化学结构含有放射性元素，可以通过同位素方法精确测定年代。 In addition, other chemical elements provide a means by which the chemical evolution of the magmatic system may be revealed.锆石是一种对河流系统中的化学和机械分解具有异常抵抗力的矿物，通常保存在沉积岩中。它在许多沉积岩中含量丰富，确保它可以通过分析其化学成分和年代测定来提供早已被侵蚀的火山岩和火成岩的记录。因此，对此类锆石的分析提供了一种可以重建火山弧缺失部分的历史的方法，并且该信息可用于揭示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 学科本科生的培训和指导，以及扩大代表性不足的群体对科学的参与。该项目代表了不同类型和教育使命的学术机构之间的四部分合作。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，作为中生代科迪勒拉岩浆弧系统演化的代理记录。锆石是多种火成岩中的副矿物，在热液蚀变和沉积过程中能够抵抗再结晶。 Detrital zircon can yield time-integrated records of magmatic systems.原位火成岩中的锆石和由它们衍生的碎屑锆石能够记录长期岩浆环境中精确的年龄和熔体成分的变化，当与原位火成岩组的研究相结合时，碎屑锆石记录可以为部分侵蚀和不完全挖掘的岩浆系统的垂直和长期变化提供相对更完整和详细的了解。 本研究的主要研究人员将分析弧衍生的碎屑锆石群，以开发碎屑代理记录：(1) 跨可变基底构建的弧的年龄和岩石学不对称性，以及 (2) 弧岩浆活动中的脉冲和间歇导致的岩浆地球化学变异性。根据对弧后前陆盆地的初始数据集的观察，主要研究人员假设弧衍生的碎屑锆石可以提供科迪勒拉弧整个寿命期间平均熔体成分演化的时间控制记录。他们将从科迪勒拉弧的弧前、弧内和弧后区域的一系列地层剖面中研究碎屑锆石。这些剖面中碎屑锆石的铀铅（U-Pb）同位素年代和微量元素地球化学将使他们能够评估弧岩浆活动的纬度和纵向变化，而锆石地球化学的长期变化反过来将帮助他们测试有关岩浆脉冲和平静期间平均熔体成分的假设。这些锆石年龄和地球化学记录还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构成一个有用的新工具，既可以描述这个长寿弧系统的岩石构造演化，又可以为碎屑锆石物源解释添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.