Deposits of the ca. 110 ka Halarauður eruption of Krafla caldera (reconstructed volume = 7 ± 6 km3 dense rock equivalent) include the only spatter-rich ignimbrite known in Iceland, and an exceptionally rare lava-like basaltic ignimbrite. We present a revised stratigraphy and new whole-rock major-element data set for products of this unusual event, one of only three Quaternary ignimbrite eruptions identified in Iceland. Compositions of Halarauður products span a broad range (50.0–74.6 wt% SiO2), reflecting mixing of rhyolite with underplating basalt. Small-volume, valley-ponded, basal pumice- and spatter-bearing lithic breccias and ignimbrite (rhyolite to andesite) reflect rapid column collapse during early opening of ring-fault vents. A transition to voluminous, regionally dispersed spatter agglomerates (dacite to basaltic andesite) marks an abrupt eruptive intensification, as gas-poor magma was squeezed into a developing ring-fault system by the subsiding chamber roof. Spatial heterogeneities in ascent rates and outgassing through this variably dilated fault system caused coeval formation of collapsing plumes and spatter fountains at separate vents. Spatter was entrained into flows from the more explosive vents, which deposited proximal spatter agglomerates and more distal spatter-bearing ignimbrite. Overlying lava-like ignimbrite deposits (basaltic andesite to basalt) reflect a final opening of vents, as mafic magma from deep levels of the chamber was squeezed through a dilated ring-fault system by the subsiding roof block and erupted at uncharacteristically high mass flux. Development of a mature ring-fault conduit system during early tapping of silicic magma appears to be a prerequisite for the emplacement of welded basaltic ignimbrites, and it should be considered as a possible eruption scenario in basalt-dominated systems where silicic magma has been known to also accumulate. Poor preservation of the Halarauður deposits exemplifies the challenges of studying ignimbrite eruptions in frequently glaciated regions like Iceland, where they may be more common than the geological record suggests.

中文翻译：

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在玄武岩为主的火山口坍塌过程中，异常的流变质向玄武质火成岩过渡：Halflaauður喷发，克拉夫拉（冰岛）

约的存款。110 kaHalarauðurKrafla火山口喷发（重建体积= 7±6 km3致密岩石当量）包括冰岛唯一已知的富含飞溅的火成岩和一个异常稀有的熔岩状玄武岩。我们为这一异常事件的产物提供了修订的地层学和新的全岩石主要元素数据集，这是冰岛确定的仅有的三个第四纪火成岩喷发之一。Halarauður产品的成分范围很广（SiO2为50.0-74.6 wt％），反映了流纹岩与底层玄武岩的混合。小体积，谷底，基底浮石和飞溅的碎屑角砾岩和火成岩（流纹岩到安山岩）反映了在早期打开环形断层喷口时柱体快速塌陷的情况。过渡到大量 区域分散的飞溅团聚体（从针铁矿到玄武岩安山岩）标志着喷发突然增强，因为贫气岩浆被下沉顶板腔挤压成发育中的环断系统。上升速度的空间异质性以及通过这种可变扩张的断层系统的放气导致了在分开的出气口同时形成塌陷的羽状流和飞溅喷泉。飞溅物从更易爆的喷口被带入流中，这些喷口沉积了近端的飞溅物团聚体和远端的带有飞溅物的火成岩。上覆的熔岩状火成岩沉积物（玄武岩安山岩至玄武岩）反映出了通气孔的最终打开，这是由于从机房深处的铁镁质岩浆被膨胀的顶盖断层系统通过扩张的环断系统挤压并以异常高的质量通量喷发。在硅质岩浆的早期开采期间开发成熟的断层导管系统似乎是安置玄武岩火成岩的先决条件，并且应将其视为在已知玄武岩为主的玄武岩为主的系统中可能发生的喷发情况。也积累。Halarauður矿床保存不善，是研究频繁冰川化地区（例如冰岛）火成岩喷发的挑战，在冰岛，冰川喷发可能比地质记录所提示的更为普遍。