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

俄勒冈州过去的地质特征是多次火山爆发，使俄勒冈州覆盖着一层又一层的火山岩。一个特别活跃的火山时期是大约1500万年前，当时俄勒冈州东南部的Steens山附近、俄勒冈州东北部的Wallow山脉以及俄勒冈州东部的其他地方的裂缝打开，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的所有熔岩都属于一种化学类型，称为拉斑玄武岩。然而，在由CRBG的喷发地点定义的地理中心，爆发产生了草莓火山，其成分与钙碱性不同。含有钙碱性成分的熔岩是像喀斯喀特山脉这样的火山弧的特征，喀斯喀特山脉沿着西海岸从北卡罗来纳州延伸到北卡罗来纳州。从加州到加拿大。所有火山弧都位于板块汇聚的板块边界上。在会聚板块边界上生成的具有钙碱性成分的火山岩主要与地幔深处（~40 km深度）发生的过程有关。钙碱性草莓火山位于一个非会聚板块设置和拉斑玄武岩熔岩代表当时从地幔提供的岩浆占主导地位的时期内，表明必须存在其他方式来产生这样的组合物。这项研究将调查是否以及如何在地壳（~ 40公里深度）进行的较浅的过程可以强加钙碱性成分作为一级过程。化学、矿物学和同位素数据将被用作研究这一问题的手段，换句话说，这项研究将探讨地壳在非俯冲拉斑玄武岩溢流玄武岩省中产生钙碱性倾向和相关造山类安山岩的作用。这种设置将最大限度地减少从地幔到地壳的玄武岩类型的不确定性，从而允许排除玄武安山岩和安山岩/英安岩的钙碱性特征是从原始玄武岩转换而来的，原始玄武岩直接从地幔源获得其钙碱性特征。临界时间窗口为~17至15 Ma;这是CRBG的主要体积（~90%）从草莓火山场周边的裂缝中喷发出来的时间，草莓火山场在此期间爆发了拉斑玄武岩，钙碱性玄武安山岩，安山岩和英安岩，偶尔的拉斑玄武岩中间体和流纹岩。在这个火山领域和周围的地壳岩石的曝光提供了机会，以获得地壳的成分数据的草莓火山岩浆通过，并与他们可能有相互作用。本研究的目的是全面研究地壳相互作用（AFC，同化，混合）的影响，在控制钙碱性趋势的发展，从环境拉斑玄武岩趋势的分歧。这对于确定钙碱性趋势下降液线早期地壳相互作用的影响特别重要，而钙碱性趋势下降液线的早期部分尚未得到很好的理解。钙碱性签名中观察到的钙碱性套件的草莓火山是一个非常丰富的水的火山弧环境，如奥古斯丁火山，阿拉斯加，从而突出了为什么这项研究具有重要的意义，不仅了解非俯冲设置的造山安山岩，但也火山弧。

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.