We conducted a detailed petrologic study on six crystal-poor (< 12 vol%) dacites from South Sister volcano (OR) to determine the conditions that favor the formation and extraction (i.e., eruption) of intermediate magmas in a system that features prominent Daly Gaps. We present whole rock compositions for six dacites (63.2–65.1 wt% SiO₂) that form the base of South Sister and six mafic flows (51.3–56.1 wt% SiO₂) that erupted in the peripheral volcanic field. The dacites are saturated in plagioclase + orthopyroxene + clinopyroxene + ilmenite + titanomagnetite; hornblende is not observed. Evaluation of trace element concentrations in whole rock analyses reveals that end member mixing (basalt with rhyolite) is not the primary petrogenetic mechanism for generating the dacites, and comparison of compositions of minerals in the dacites with those equilibrated in experiments suggests that the majority of crystals in the dacites are plausible phenocrysts. Application of multiple thermometers to pyroxene and Fe–Ti oxide compositions in the dacites shows that crystallization in the dacites initiated at elevated temperatures (980–1045 °C) and that modest zonation in the phenocryst assemblage can be attributed to the effects of minor closed system crystallization ± degassing ± cooling. When pre-eruptive temperatures, plagioclase and whole rock compositions from the dacites are incorporated into the plagioclase hygrometer, we obtain maximum pre-eruptive H₂O contents that range from 2.1 to 3.3 wt%; these H₂O contents are consistent with those associated with plagioclase-in curves determined from phase equilibrium experiments. Through review of glasses equilibrated in experimental run products in the literature, we find that dacitic melt compositions with temperatures similar to the South Sister dacites (i.e., 980–1045 °C) are restricted to partial melting experiments on MgO-rich (> 5 wt% MgO) basalts and that dacitic melt compositions most common in partial melting experiments conducted under low H₂O fugacities. Partial melting of basalt resolves the compositional gaps between the basalts and dacites in the South Sister volcanic record because, during partial melting, dacites (1) can be generated without requiring the formation of a compositional intermediate, (2) are produced in eruptible melt fractions (e.g., 20–25%), (3) are favorably erupted if they occur in fluid undersaturated conditions (owing to the positive Clapeyron slope in the absence of a fluid phase). Though the efficiency of partial melting is debated because of the amount of heat required to generate melt, it is a viable process beneath South Sister because of the elevated geothermal gradient, high pre-eruptive temperatures of primitive basalts erupting in the surrounding areas and the recent history of volcanism in the region. Our results suggest that eruption of dacites saturated in two pyroxenes, absent hornblende, may signal partial melting.

我们对南姐妹火山 (OR) 的六块晶体贫乏 (< 12 vol%) 英安岩进行了详细的岩石学研究，以确定有利于在具有突出 Daly 特征的系统中形成和提取（即喷发）中间岩浆的条件差距。六名英安岩（63.2-65.1重量％的SiO我们本整个岩石组合物₂），其形成南姐妹和六个镁铁质流（51.3-56.1重量％的SiO的基部₂) 在外围火山场喷发。英安岩在斜长石 + 斜辉石 + 斜辉石 + 钛铁矿 + 钛磁铁矿中饱和；没有观察到角闪石。对全岩分析中微量元素浓度的评估表明，端元混合（玄武岩与流纹岩）不是产生英安岩的主要成岩机制，将英安岩中的矿物成分与实验中平衡的矿物成分进行比较表明，大多数晶体在英安岩中是似是而非的斑晶。将多个温度计应用于英安岩中辉石和 Fe-Ti 氧化物成分表明英安岩中的结晶在升高的温度 (980-1045 °C) 下开始，并且斑晶组合中的适度分带可归因于微小封闭系统的影响结晶±脱气±冷却。当来自英安岩的喷发前温度、斜长石和全岩成分被纳入斜长石湿度计时，我们获得了最大喷发前 H₂ O含量范围为2.1至3.3 wt%；这些 H ₂ O 含量与从相平衡实验确定的斜长石曲线相关的那些H ₂ O 含量是一致的。通过对文献中实验运行产品中平衡的玻璃的回顾，我们发现温度与南姐妹英安岩相似（即 980-1045 °C）的英安岩熔体成分仅限于对富 MgO（> 5 wt % MgO) 玄武岩和在低 H ₂下进行的部分熔融实验中最常见的英安岩熔体成分哦逃逸。玄武岩的部分熔融解决了南姐妹火山记录中玄武岩和英安岩之间的成分差距，因为在部分熔融期间，英安岩 (1) 可以在不需要形成成分中间体的情况下产生，(2) 以可喷发的熔体部分产生（例如，20-25%）、（3）如果它们发生在流体欠饱和条件下（由于没有流体相时克拉珀龙斜率正值），则它们会被有利地喷发。虽然部分熔化的效率因产生熔体所需的热量而存在争议，但由于地温梯度升高、周围地区喷发的原始玄武岩的喷发前温度高以及最近的该地区的火山活动历史。