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很容易地将地球与其行星邻居区分开来：水、大陆地壳(由花岗岩构成)和板块构造。这些特征很可能是相互关联的，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，它们构成了稳定的陆壳，如果没有它，陆地生命就不可能进化。一个多世纪以来，花岗岩的起源一直是许多争论的来源，因为在新的观测面前，连续的假说要么被保留，要么被修改，要么被抛弃。他们通过研究加利福尼亚州内华达山脉岩基演化的两个目前流行的模型来参与这场辩论。内华达山脉岩基不仅是国家公园系统(约塞米蒂、红杉和国王峡谷)皇冠上的明珠所在，而且长期以来一直是地质学家在板块构造背景下如何形成花岗岩地壳的典型例子，被称为“俯冲带”。俯冲带是地球表面的一个构造板块下沉到另一个板块之下的区域，在这个过程中既产生了地震，也产生了火山。这些构造区域也被称为弧形，因为这样的背景下形成的火山的弧形图案，如日本、阿留申群岛或马里亚纳群岛。花岗岩被认为是在这些地区形成的。当这样的弧形形成花岗岩时，这些花岗岩是浮力的，很难俯冲到另一个构造板块之下，这些岩石最终会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内华达山脉基岩的两种关键方法：结晶分异和先前存在的下地壳和上地壳的部分熔融。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，受到更多新形成的玄武岩熔体的侵袭加热，部分熔融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，在浅层堆积。另一种假说是，新形成的玄武岩被直接输送到上地壳，并在那里分化形成现在主导内华达山脉地貌的花岗岩物质。这两个假设之间的反差是显著的。在第一种情况下，在中生代晚期几乎没有新的地壳被创造出来--旧的地壳只是被回收和再活化。那么地壳的年龄就比这个模型中最年轻的花岗岩的年龄要早。相反，当新形成的玄武岩熔体直接分异形成花岗岩时，这代表着新的花岗岩添加到地壳中。没有比在内华达山脉西部的瓜达卢佩火成岩(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的暴露，这些熔体在GIC花岗岩形成时侵入地壳：我们的目标是测试这种熔体是主要导致先前存在的地壳物质的加热和部分熔融，还是它们直接分异形成花岗岩。我们研究GIC的另一个好处是，该地区的岩石提供了一个不同寻常的机会，让我们得以一窥新形成的玄武岩侵入的地壳。与内华达山脉的其他花岗岩不同，它们可以测试先前存在的地壳物质是否为GIC内花岗岩的形成提供了合适的物质来源。

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.