Orogenic Andesites in an Anorogenic Floodbasalt Province: The Role of the Crust in Generating the Calc-alkaline Trend

俄勒冈州的地质历史以多次火山喷发为特征，这些火山喷发使俄勒冈州覆盖着一层层的火山岩。一个特别活跃的火山时期是大约1500万年前，俄勒冈州东南部的斯滕斯山附近、俄勒冈州东北部的沃洛山脉和俄勒冈州东部的其他地方的裂缝裂开，喷发出大量的玄武岩熔岩，统称为哥伦比亚河玄武岩群（CRBG）。CRBG的所有熔岩都是一种化学类型，称为拉斑岩。然而，在由CRBG喷发地点确定的地理中心，喷发产生了草莓火山，它的成分是一种不同的钙碱性火山。具有钙碱性成分的熔岩是喀斯喀特山脉等火山弧的特征，喀斯喀特山脉沿着西海岸从加利福尼亚北部延伸到加拿大。所有的火山弧都位于构造板块会聚的板块边界。在收敛的板块边界处，钙碱性火山岩的形成主要与地幔深处（~ 40km深度）的作用有关。钙碱性草莓火山位于一个非收敛的板块环境中，处于一个由拉斑熔岩主导的时期，代表了当时地幔提供的岩浆，这表明必须存在其他方式来产生这种成分。本研究将探讨仅在地壳（~40km深度）中发生的较浅过程是否以及如何将钙碱性成分作为一级过程施加。化学、矿物学和同位素数据将作为研究这个问题的手段。换句话说，本研究将探讨地壳在非俯冲、拉斑洪水玄武岩省形成钙碱性倾向和相关造山带安山岩的作用。这种设置将最大限度地减少玄武岩类型从地幔传递到地壳的不确定性，从而排除玄武岩安山岩和安山岩/英安岩的钙碱性特征是从直接从地幔源获得钙碱性特征的原始玄武岩转化而来的可能性。临界时间窗为~17 ~ 15ma；在此期间，CRBG的主要体积（~90%）从周边的裂缝喷发到草莓火山场，草莓火山场在此期间喷发了拉斑玄武岩、钙碱性玄武岩安山岩、安山岩和英安岩，偶尔也喷发了拉斑中间岩和流纹岩。暴露在这个火山场内部和周围的地壳岩石提供了获得地壳成分数据的机会，草莓火山的岩浆通过地壳并可能与地壳相互作用。本研究的目的是综合研究地壳相互作用（AFC、同化、混合）与环境拉斑岩倾向的分离对钙碱性倾向发育的控制作用。这对于确定钙碱性倾向的液体下降线早期地壳相互作用的影响尤其重要，这一点目前还没有得到很好的理解。在草莓火山的钙碱性组中观察到的钙碱性特征与在阿拉斯加奥古斯丁火山等非常富水的火山弧环境中观察到的钙碱性特征一样强烈，因此突出了为什么这项研究不仅对理解非俯冲背景下的造山安山岩，而且对火山弧的造山岩具有重要意义。

The geological past of Oregon is characterized by many volcanic eruptions that covered Oregon with layers and layers of volcanic rocks. One particularly volcanically active period was around 15 million years ago when fissures in the vicinity of Steens Mountain in SE Oregon, in the Wallow Mountains, NE Oregon, and other places in eastern Oregon opened to erupt massive amounts of basalt lavas collectively known as Columbia River Basalt Group (CRBG). All lavas of the CRBG are of one chemical type known as tholeiitic. However, in the geographic center defined by the eruptive sites of the CRBG, eruptions gave rise to the Strawberry Volcanics that are compositionally of a different kind known as calc-alkaline. Lavas with calc-alkaline compositions are characteristic for volcanic arcs like the Cascades Mountains that stretch along the West Coast from N. California into Canada. All volcanic arcs are situated at convergent plate boundaries where tectonic plates converge. The generation of volcanic rocks with calc-alkaline composition at convergent plate boundaries has been principally tied to processes going on deep in the mantle ( ~40 km depth). The calc-alkaline Strawberry Volcanics situated within a non-convergent plate setting and within a period dominated by tholeiitic lavas representing the magma supplied from the mantle at that time suggest that other ways must exist to produce such compositions. This study will investigate whether and how shallower processes going on solely in the earth crust ( ~40km depth) can impose calc-alkaline composition as first order process. Chemical, mineralogical and isotopic data will be used as means to research this question.In other words, this study will investigate the role of the crust in generating the calc-alkaline trend and associated orogenic-like andesites in a non-subduction, tholeiitic flood basalt province. This setting will minimize the uncertainty of what type of basalt is delivered from the mantle to the crust thus allowing to exclude that calc-alkaline signature of basaltic andesites and andesites/dacites was translated from a primitive basalt which acquired its calc-alkaline signature directly from the mantle source. The critical time window is ~17 to 15 Ma; that's when the dominant volume (~90%) of the CRBG erupted from fissures around the periphery to the Strawberry Volcanic Field that itself erupted during that time tholeitic basalt, calc-alkaline basaltic andesite, andesites and dacites, occasional tholeiitic intermediates, and rhyolites. Exposures of crustal rocks in and around this volcanic field provide the opportunity to obtain compositional data of the crust through which magmas of the Strawberry Volcanics passed and with which they may have interacted. The goal of this research is to comprehensively study the effect of crustal interactions (AFC, assimilation, mixing) in controlling the development of the calc-alkaline trend by divergence from the ambient tholeiitic trend. This is particularly important for determining the effect of crustal interactions during the early part of the liquid line of descent of the calc-alkaline trend that is not well understood. The calk-alkaline signature observed in the calc-alkaline suite of the Strawberry Volcanics is as strong as one observes in a very water-rich volcanic arc environment like Augustine volcano, Alaska and thus highlights why this study holds important implications not only for the understanding of orogenic andesites of non-subduction settings but also of those of volcanic arcs.