The Origin of Bimodal Volcanic Suites: A Study of Newberry Volcano, Oregon

双峰火山作用的起源一直是火成岩岩石学中一个特别持久的问题。该项目旨在了解造成双峰玄武岩-流纹岩火山套的岩石学过程。一些岩石成因研究已经假设这样的火山套是在一个单一的、大的、长寿命的岩浆房中形成的，需要复杂的AFC过程来解释双峰套的成分变化。本文认为，双峰套是由基性岩浆多次注入地壳而形成的。这些体积小的天体导致地壳部分熔融。玄武岩-流纹岩岩浆产于一系列小而不相连的储层中。在短时间内大面积喷发的熔岩可能仅与它们共同的无水成因有关。基性岩浆的反复注入，以及随后的地壳部分融合和熔融结晶，将使长寿火山系统下的地壳特征发生显著变化。最近的地球物理研究和良好的地质约束使得这座火山非常适合测试这个提出的模型。全新世熔岩将被用来测试这个火山套件在小的不相连的岩浆室中的生成。然后，通过比较全新世和更新世火山组的差异来检验地壳蒸馏模型的适用性，以确定熔岩特征的时间变化是否与预测相符。该模型的证实将极大地影响岩石学家对岩浆房的认识，以及对时空相关熔岩地球化学数据的定量建模。

The origin of bimodal volcanism has been a particularly persistent problem in igneous petrology. This project seeks to understand the petrologic processes responsible for bimodal basalt-rhyolite volcanic suites. Some petrogenetic studies have assumed such volcanic suites formed by mechanisms operating in a single, large, long-lived magma chamber, where complex AFC processes are required to explain the compositional variations in bimodal suites. Here it is suggested that bimodal suites are produced by repeated injection of mafic magma into the crust. These small volume bodies cause partial melting of the crust. Basalt-rhyolite magmas are generated in a series of small, unconnected reservoirs. Lavas erupted over a large area within a short time may be related only by their common anatectic origin. Repeated injection of mafic magma, and subsequent crustal partial fusion and melt crystallization will generate significant changes in the character of the crust beneath long-lived volcanic systems. Recent geophysical studies and excellent geologic constraints make this volcano ideal for testing this proposed model. Holocene lavas will be used to test the generation of this volcanic suite in small unconnected magma chambers. Then the applicability of the crustal distillation model will be tested by comparing differences in the Holocene and Pleistocene volcanic suites to determine if temporal changes in lava characteristics agree with predictions. Confirmation of this model could dramatically affect petrologists' perceptions of magma chambers and quantitative modelling of geochemical data from temporally and spatially related lavas.