Collaborative Research: From Gabbros to Granites - An Investigation of Arc-Scale Differentiation at the Guadalupe Igneous Complex, Sierra Nevada, CA

合作研究：从辉长岩到花岗岩 - 加利福尼亚州内华达山脉瓜达卢佩火成岩杂岩弧尺度分异研究

• 批准号: 1250219
• 负责人: Scott Paterson
• 金额: $ 12.79万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-01 至 2016-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1250219&HistoricalAwards=false
• 关键词: Collaborative; Research; Gabbros; Granites; Investigation

Just a few key features allow Earth to be easily recognized from its planetary neighbors: water, continental crust (made of granitic rocks) and plate tectonics. These features are likely interlinked, and appear to be essential for the evolution of a planet that can support human life. This study focuses on the origin and emplacement of granitic rocks, which form the stable continental crust, without which terrestrial life could not have evolved. The origin of granite has been the source of much debate for more than a century, as successive hypotheses survive, are modified, or abandoned in the face of new observations. They weigh in on the debate by examining two currently popular models for the evolution of the Sierra Nevada Batholith in California, which is home not only to the crown jewels of the National Park System (Yosemite, Sequoia and Kings Canyon), but which has long served geologists as a type example of how granitic crust is formed in the plate tectonic setting known as a 'subduction zone'. Subduction zones are regions of Earth's surface where one tectonic plate sinks beneath another, in the process generating both earthquakes and volcanoes. These tectonic regions are also referred to as 'arcs', for the arcuate patterns of volcanoes that such a setting develops, such as the Japanese, Aleutian or Marianas Islands. It is in these regions that granites are thought to form. When such arcs develop granitic rocks, which are buoyant and difficult to subduct beneath another tectonic plate, these eventually amalgamate to form the continents as seen today.In this project, the team will investigate the two key means by which granitic rocks of the Sierra Nevada Batholith, California are thought to be created: crystallization differentiation, and partial melting of pre-existing lower and upper crust. Recent work has suggested that earlier-formed crust, which consists mostly of basalt and overlying sediments, is heated by the intrusion of more newly-formed basaltic melts, and partially melted to form granitic melts directly. These granitic melts then rise through existing crustal materials to accumulate at shallow depths. An alternative hypothesis is that newly formed basalts are transported to the upper crust directly, and there differentiate to form the granitic materials that now dominate the landscape of the Sierra Nevada. The contrasts between these two hypotheses are significant. In the first case, little new crust is being created during Late Mesozoic time?older crust is simply being recycled and remobilized. The age of the crust then is then older than the age of the youngest granites in this model. In contrast, when newly formed basaltic melts differentiate directly to form granite, then this represents a new addition of granite to the crust. There is no better place to test these models than at the Guadalupe Igneous Complex (GIC) of the western Sierra Nevada. This Jurassic-age pluton (~151 Ma) contains very rare exposures of the newly-formed basaltic melts that intruded the crust at the time that the GIC granites formed: our goal is to test whether such melts primarily caused heating and partial melting of pre-existing crustal materials, or whether they differentiated directly to form granite. Another advantage to our study of the GIC is that the rocks in this region provide an unusual glimpse into the crust into which the newly formed basalts were intruded. Unlike other granitic plutons of the Sierra Nevada, they can test whether pre-existing crustal materials provide an appropriate source material for the formation of granitic rocks within the GIC.

仅仅几个关键特征就可以让地球很容易地从它的行星邻居中识别出来：水，大陆地壳（由花岗岩组成）和板块构造。这些特征很可能是相互关联的，似乎是一个可以支持人类生活的行星进化所必需的。这项研究的重点是花岗岩的起源和侵位，花岗岩形成了稳定的大陆地壳，没有它，陆地生命就不可能进化。花岗岩的起源是世纪以来争论的焦点，在新的观测结果面前，相继的假说被保留下来，被修改，或被抛弃。他们通过研究两种目前流行的加州内华达州山脉岩石演化模型来参与辩论，该模型不仅是国家公园系统（约塞米蒂，红杉和国王峡谷）皇冠上的宝石，而且长期以来一直为地质学家提供花岗岩地壳如何在称为“俯冲带”的板块构造环境中形成的典型例子。俯冲带是地球表面的一个板块下沉到另一个板块之下的区域，在这个过程中会产生地震和火山。这些构造区域也被称为“弧”，因为这样的环境发展出弧形的火山模式，例如日本，阿留申群岛或马里亚纳群岛。正是在这些地区，花岗岩被认为是形成。当这样的弧发展成花岗岩时，这些花岗岩具有浮力，难以俯冲到另一个构造板块之下，最终合并形成今天所看到的大陆。在这个项目中，研究小组将研究加州的内华达州山脉的花岗岩被认为是通过两种关键手段形成的：结晶分异和预先存在的上地壳和下地壳的部分熔融。最近的研究表明，早期形成的地壳，主要由玄武岩和上覆沉积物组成，被更多新形成的玄武岩熔体的侵入加热，部分熔融直接形成花岗岩熔体。这些花岗岩熔体然后通过现有的地壳物质上升，在浅层积累。另一种假设是，新形成的玄武岩被直接输送到上地壳，并在那里分化形成花岗岩物质，现在主导着内华达州的景观。这两种假设之间的对比是显著的。在第一种情况下，在晚中生代几乎没有新的地壳产生？老地壳只是被循环再利用在这个模型中，地壳的年龄比最年轻的花岗岩的年龄要大。相反，当新形成的玄武质熔体直接分化形成花岗岩时，这代表了地壳中新增加的花岗岩。没有比在西内华达州的瓜达尔卡尔特火成复合体（GIC）更好的地方来测试这些模型了。这个侏罗纪时代的岩体（~151马）包含非常罕见的暴露的新形成的玄武岩熔体侵入地壳的时候，GIC花岗岩形成：我们的目标是测试这种熔体是否主要导致加热和部分熔融的预先存在的地壳材料，或者他们是否直接分化形成花岗岩。我们研究GIC的另一个优势是，该地区的岩石提供了一个不寻常的一瞥，新形成的玄武岩侵入到地壳中。与内华达州的其他花岗岩岩体不同，它们可以测试预先存在的地壳物质是否为GIC内花岗岩的形成提供了适当的源物质。