Rhyolitic Explosive Eruptions of the Central Snake River Plain, Idaho: Investigations of the Lower Cassia Mountains Succession and Surrounding Areas
Rhyolitic Explosive Eruptions of the Central Snake River Plain, Idaho: Investigations of the Lower Cassia Mountains Succession and Surrounding Areas
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爱达荷州中部蛇河平原的流纹质爆炸性喷发：对下决明山序列及周边地区的调查
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发表时间：
2009
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通讯作者：
B. Ellis
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作者：
B. Ellis
本杰明·斯蒂芬·埃利斯（Benjamin Stephen Ellis）美国西北部的斯内克河平原是中新世大量双峰热点火山活动的发生地。在约12.7-6 Ma之间，硅质火山作用产生了一个矿床组合，与其他地方典型的普林尼期和火成岩矿床非常不同，它被称为蛇河（SR）型。爱达荷州南部的决明子山脉含有sr型火成岩，这些火成岩是由复杂的岩浆管道系统产生的，该系统涉及多个岩浆房，这些岩浆房产生了多个斜辉石成分群，这些斜辉石在喷发过程中混合并沉积在一起。从整体岩石、玻璃、长石和斜辉石组成来看，决明子山的辉灰岩逐渐呈现基性上演替，反映出可用于部分结晶的时间在减少，这一点得到了地质年代学的支持。根据斑晶、玻璃和整个岩石组成，新发现的三个“超级喷发”中有两个是决明子山的火成岩；磁极性;基于“增大化现实”技术/ Ar地质年代学;氧同位素;现场数据。喷发量在640至1200公里之间，是有记录以来最大的火山之一。在决明山序列中，新发现的一个矿床代表了有记录以来第一次从蛇河平原中部爆发的流纹岩式呼吸岩浆喷发。细粒、非焊接型矿床整体岩石、玻璃、氧同位素及岩浆温度特征与周围焊接型火成岩相似，认为该异常沉积相代表了上升流纹岩岩浆与近地表水的相互作用。在sr型火山活动中，在短时间内喷发出具有类似化学性质的熔岩和火成岩。熔融包裹体含水量较低，与无水矿物学特征一致，岩浆温度较高。岩浆的挥发性含量（由熔体包裹体记录）不控制喷发方式。sr型火烟煤具有强烈的流变性，这似乎是由于放置温度高而不是卤素含量增加所致。
1 RHYOLITIC EXPLOSIVE ERUPTIONS OF THE CENTRAL SNAKE RIVER PLAIN, IDAHO: INVESTIGATIONS OF THE LOWER CASSIA MOUNTAINS SUCCESSION AND SURROUNDING AREAS Benjamin Stephen Ellis The Snake River Plain of north-western U.S.A. was the site of voluminous, bimodal, hotspot volcanism in the Miocene. Between c. 12.7-6 Ma silicic volcanism produced an association of deposits so different to typical Plinian and ignimbrite deposits elsewhere it has been termed Snake River (SR)-type. The Cassia Mountains of southern Idaho contain SR-type ignimbrites produced from complex and dynamic magmatic plumbing systems involving multiple magma chambers which gave rise to multiple compositional populations of clinopyroxene that mixed during eruption and were deposited together. The Cassia Mountain ignimbrites become progressively more mafic up-succession in terms of whole rock, glass, feldspar and clinopyroxene compositions, reflecting decreasing time available for fractional crystallisation, as supported by geochronology. Two Cassia Mountain ignimbrites are among three newly discovered ‘super-eruptions’ defined on the basis of phenocryst, glass and whole rock compositions; magnetic polarity; Ar/Ar geochronology; oxygen isotopes; and field data. Erupted volumes range between 640 and 1200 km, amongst the largest recorded. Intercalated within the Cassia Mountain succession is a newly discovered deposit representing the first recorded explosive, rhyolitic phreatomagmatic eruption from the central Snake River Plain. The fine-grained, non-welded deposit has similar whole rock, glass, oxygen isotope and magmatic temperature characteristics to the surrounding welded ignimbrites, so the unusual deposit facies are interpreted as representing interaction of rising rhyolitic magma with near-surface water. During SR-type volcanism, lavas and ignimbrites of similar chemistry were erupted within a short time. Water contents of melt inclusions were low in both ignimbrites and lavas, consistent with the anhydrous mineralogy and high inferred magmatic temperature. Volatile contents of the magmas (as recorded by the melt inclusions) did not control eruptive style. The intense rheomorphism which characterises SR-type ignimbrites appears to be due to high emplacement temperatures rather than enhanced halogen contents.