Flare Up of Hot-Dry-Reduced Ignimbrites Related to Extension in the Cascades Arc: The Deschutes Formation, Central Oregon
Flare Up of Hot-Dry-Reduced Ignimbrites Related to Extension in the Cascades Arc: The Deschutes Formation, Central Oregon
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与喀斯喀特弧延伸相关的热干还原凝结岩的爆发：俄勒冈州中部的德舒特地层
DOI：
10.1093/petrology/egad058
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发表时间：
2023
影响因子：
3.9
通讯作者：
Kent, Adam J
中科院分区：
文献类型：
--
作者：
Pitcher, Bradley W;Grunder, Anita L;Kent, Adam J
火山灰岩爆发是罕见的强烈火山活动时期，在此期间，火山碎屑量和喷发频率比背景活动高出一个数量级以上。调查耀斑和稳定状态的岩浆之间的成分差异提供了关键的限制事件的成岩原因，并可以提供独特的机会，调查大规模的构造或地球动力学过程中的作用弧岩浆。在这项研究中，我们专注于双峰Deschutes形成熔结凝灰岩耀斑的中央俄勒冈州，爆发异常高体积的火山碎屑物质6.25-5.45马从一个新的火山作用轴的瀑布弧。与很少喷发流纹岩的第四纪喀斯喀特弧相比，这一事件的特点是喷发率增加，并且喷发出更多的硅质成分。英安岩是贫晶体（<10%）英安岩至流纹岩（大多数为65-77重量% SiO2）与无水矿物组合和较高的FeO/MgO，Y，Eu/Eu*，MREE和Zr/Sr，表明较干燥的岩浆演化相比，第四纪弧，更类似于那些从弧后高熔岩平原（HLP）省，位于东部。磁铁矿-钛铁矿氧压测定表明，Deschutes组长英质岩浆往往更热，更减少（NNO-1至NNO）比第四纪弧（NNO至NNO + 1.5）。流纹岩熔体的地质压力表明，复杂的存储不同的Deschutes形成岩浆在地壳浅（50-250 MPa），和常见的共同喷发的多个斜长石人口，浮石成分，并组成带状浮石建议不同程度的混合和混合的不同的岩浆。Deschutes岩浆也具有低δ 18 O斜长石，表明部分熔融和同化的热液蚀变浅地壳。微量元素系统学和流纹岩熔融模拟表明，长英质浮石不能产生简单的分馏共同爆发的镁铁质浮石或玄武质熔岩，并需要地壳熔融的起源，微量元素和铅同位素表明，年轻的镁铁质地壳可能是主要的原岩。我们认为部分熔融产生低硅流纹岩熔体（~72 wt.%）它既是最进化的流纹岩的母体，又是产生英安岩到流纹英安岩岩浆的混合端元，并具有异质斜长石群。与第四纪喀斯喀特弧中爆发的主要钙碱性玄武岩不同，德舒特组原生玄武岩大多是低钾拉斑玄武岩，表明减压熔融。这些都是类似的组合物喷发在同期脉冲的低钾拉斑玄武岩火山活动在整个HLP达到了瀑布弧后。我们认为，弧内延伸集中减压熔体从弧后到弧，张应力允许这种高通量的热干还原玄武岩在整个地壳柱，造成部分熔融的镁铁质原岩和生产的热干还原流纹岩熔体。连续韵律中不相容元素的消耗意味着原岩肥力的逐渐消耗。伸展还允许建立一个强大的热液系统，和同化的水热蚀变岩石的岩浆居住在一个浅，复杂的存储网络导致低δ 18 O熔体。我们的研究结果表明，伸展构造在产生一个不寻常的熔结凝灰岩耀斑的干热还原流纹岩岩浆，是非典型的瀑布弧，并可能是一个重要的贡献者在全球范围内的弧耀斑起了不可或缺的作用。
Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclasevalues that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide.