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

只有几个关键特征可以让我们很容易地从它的邻居中识别出地球：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，并且似乎对一个能够支持人类生命的星球的进化至关重要。这项研究的重点是花岗岩的起源和侵位，它们形成了稳定的大陆地壳，没有它们，陆地生命就无法进化。一个多世纪以来，花岗岩的起源一直是许多争论的来源，因为面对新的观察，相继的假设存在，修改或放弃。他们通过研究两种目前流行的关于加州内华达山脉基岩演化的模型，参与了这场争论。内华达山脉基岩不仅是美国国家公园系统的“皇冠上的宝石”（约塞米蒂、红杉和国王峡谷）的所在地，而且长期以来一直被地质学家视为花岗岩地壳是如何在板块构造环境中形成的典型例子。俯冲带?。俯冲带是地球表面上一个构造板块下沉到另一个板块下面的区域，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。比如日本、阿留申群岛和马里亚纳群岛，在这种环境下形成了弧形的火山。人们认为花岗岩就是在这些地区形成的。当这些弧形成浮力强、难以俯冲到另一个构造板块下的花岗岩时，它们最终合并形成今天所见的大陆。在这个项目中，研究小组将研究加州内华达山脉基岩中花岗岩形成的两种关键方式：结晶分化和先前存在的上下地壳的部分熔化。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侵入加热，部分熔化直接形成花岗岩熔体。然后，这些花岗岩熔体通过现有的地壳物质上升，积聚在浅层深处。另一种假设是，新形成的玄武岩被直接运送到上地壳，并在那里分化形成花岗岩物质，这些物质现在主导着内华达山脉的景观。这两种假设之间的差异是显著的。在第一种情况下，中生代晚期几乎没有新地壳形成？较老的地壳只是被回收和重新利用。地壳的年龄比这个模型中最年轻的花岗岩的年龄还要老。相反，当新形成的玄武岩熔体直接分化形成花岗岩时，这代表了地壳中又增加了花岗岩。没有比内华达山脉西部的瓜达卢佩火成岩复合体（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 Ma）包含了非常罕见的新形成的玄武岩熔体，这些熔体在GIC花岗岩形成时侵入了地壳：我们的目标是测试这些熔体是否主要引起了原有地壳物质的加热和部分熔融，或者它们是否直接分化形成了花岗岩。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视角，可以看到新形成的玄武岩侵入的地壳。与内华达山脉的其他花岗质岩体不同，它们可以测试先前存在的地壳物质是否为GIC内花岗质岩石的形成提供了合适的源物质。

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.