Rapid extraction of discrete magma batches from a large differentiating magma chamber: the Central Plateau Member rhyolites, Yellowstone Caldera, Wyoming
Rapid extraction of discrete magma batches from a large differentiating magma chamber: the Central Plateau Member rhyolites, Yellowstone Caldera, Wyoming
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从大型分异岩浆房中快速提取离散岩浆批次：怀俄明州黄石火山口的中央高原段流纹岩
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发表时间：
2010
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通讯作者：
J. Stix
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作者：
G. Girard;J. Stix
中央高原段流纹岩的喷发年代为173 ~ 70 ka，是黄石公园最年轻的火山内流纹岩。它们主要由体积非常大的熔岩流组成，总体积约为600 km 3。它们的喷发口定义了两个与区域断层对齐的NNW向线性构造。我们提出了新的全岩，玻璃和矿物分析，并提出了这些韵律的岩石成因和火山构造模型。在整个破火山口范围内，Nb、Y、HREE等元素在时间上呈富集状态，Sr、Ba、Ce/Yb等元素在时间上呈亏损状态。同时，单斜辉石的镁含量降低，石英中的Ti含量降低。相比之下，石英在所有的韵律是圆形的，并承担长期的玻璃再入，这表明加热。根据这些数据和观察，我们提出中央高原段韵律的形成过程如下。位于破火山口西南部的Mallard Lake Resurgent Dome下方的低δ 18 O流纹岩原岩在约250 ka时开始熔融。重复的加热脉冲导致熔化前沿径向扩张，在大部分破火山口下方形成了一个巨大的晶体糊。糊状物能够分化，但由于其高结晶度和粘度而不会喷发。进一步输入的热量和岩浆增加了熔融的程度，形成了贫结晶的岩浆批次，这些岩浆批次在几百到几千年后通过区域断层喷发，形成中央高原段流纹。
The Central Plateau Member rhyolites have been erupted between 173 and 70 ka and are the youngest Yellowstone intracaldera rhyolites. They mostly comprise very voluminous lava flows totaling ~600 km3 in volume. Their eruptive vents define two NNW-trending lineaments which are aligned with regional faults. We present new whole rock, glass, and mineral analyses and propose a petrogenetic and volcano-tectonic model for these rhyolites. At a caldera-wide scale, there is a temporal enrichment in elements such as Nb, Y and HREE, and a depletion in Sr, Ba, and Ce/Yb. Simultaneously, clinopyroxene becomes less magnesian while Ti contents in quartz decrease. By contrast, quartz in all rhyolites is rounded and bears long glass re-entrants, suggesting heating. Based on these data and observations, we propose that the Central Plateau Member rhyolites have been generated as follows. A hydrothermally altered low-δ18O rhyolitic protolith beneath the Mallard Lake Resurgent Dome in the southwestern part of the caldera started to melt at ~250 ka. Repeated heating pulses caused the melting front to expand radially, and a large crystal mush formed beneath much of the caldera. The mush was able to differentiate but not erupt due to its high crystallinity and viscosity. Further inputs of heat and silicic magma in this mush increased the degree of melting, forming crystal-poor magma batches which erupted a few hundred to a few thousand years later through regional faults to form the Central Plateau Member rhyolites.