Abstract Masaya is unusual for a basaltic caldera because it formed by piston-subsidence in response to large-volume magma withdrawal by highly explosive eruptions, i.e. in a fashion typical of silicic calderas. The first and most voluminous of the three explosive eruptions formed the 6 ka old basaltic San Antonio Tephra (SAT). This eruption is also unusual in that most of the 9 km3 DRE basaltic magma was discharged by a plinian eruption. The subsequent eruptions of the basaltic Masaya Triple Layer (MTL, 2.1 ka) and the Masaya Tuff/Ticuantepe Lapilli (MT-TIL, 1.9 ka) each discharged 2 km3 DRE magma and enlarged the Masaya caldera. The SAT consists of a lower sequence of alternating scoria lapilli and ash layers, interpreted as an alternation between more or less phreatomagmatically influenced fallout events. These are followed by two prominent well-sorted lapilli layers: the first one formed by a climactic plinian eruption whose column height reached 21–29 km and discharged most of the total erupted mass including about 35 Mt SO2. The second, lithic-rich lapilli layer probably formed by a phreatoplinian event when partial collapse of the magma chamber roof initiated increasing magma-water interaction which ultimately formed the upper sequence of phreatomagmatic cross-bedded surge deposits, accretionary lapilli-rich tuffs and a final fallout of dense lapilli. Phreatomagmatic activity may have been related to disruption of a hydrothermal system reflected in hydrothermally altered lithics, and/or by the caldera floor subsiding closer to the groundwater table. The bulk-rock chemical composition of the SAT is basaltic but the bimodal glass compositions demonstrate mixing of a basaltic with an andesitic melt probably in the conduit during eruption. The SAT basalt differentiated in a reservoir near the MOHO at 20 km depth by fractional crystallization of olivine, plagioclase, and minor clinopyroxene forming a tholeiitic fractionation trend. Minor intermediate-An plagioclase crystallized from the basaltic melt at H2O concentrations of about 2 wt% as measured by FTIR in melt inclusions. However, a key observation is that the melt inclusions are not in equilibrium with the high-An plagioclases hosting them. Re-equilibration of the inclusions requires initially higher water contents (about 5–6 wt%) which also fits the high Ba/La ~ 80 indicating input from the strongly hydrated subducting slab. Therefore, while the SAT magma evolved under hydrous conditions at depth, it was then stored at shallow level long enough to adjust to the low saturation pressure and to precipitate some intermediate-An plagioclase but still preserving its high temperature (around 1100 °C) and phenocryst-poor composition. Large overpressure due to connection to the deep-seated reservoir and water degassing during ascent limited the storage time at shallow level and drove the unusually intense and voluminous plinian-style eruption that facilitated piston-type collapse of the chamber roof.

中文翻译：

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约 6 ka San Antonio Tephra 的玄武质普林期喷发和尼加拉瓜马萨亚火山口的形成

摘要 Masaya 对于玄武岩火山口来说是不寻常的，因为它是由活塞下沉形成的，以响应高度爆炸性喷发的大量岩浆撤回，即以典型的硅质火山口的方式。三个爆发性喷发中的第一个也是最庞大的一次形成了 6 ka 古老的玄武岩 San Antonio Tephra (SAT)。这次喷发也很不寻常，因为大部分 9 平方公里的 DRE 玄武质岩浆是由普林期喷发排出的。随后的玄武质 Masaya Triple Layer (MTL, 2.1 ka) 和 Masaya Tuff/Ticuantepe Lapilli (MT-TIL, 1.9 ka) 的喷发各自排放了 2 km3 DRE 岩浆并扩大了 Masaya 火山口。SAT 由较低序列的交替的火山渣和火山灰层组成，解释为或多或少受潮岩作用影响的沉降事件之间的交替。紧随其后的是两个突出的分类良好的火山岩层：第一个由高潮期普林期喷发形成，其柱高达到 21-29 公里，并排放了大部分总喷发物质，包括约 35 Mt SO2。第二个富含岩屑的火山岩层可能是由岩浆室顶的部分坍塌引发岩浆-水相互作用增加，最终形成了岩浆交错层状涌流沉积物、增生富集凝灰岩和最终致密的火山碎屑的余波。岩浆活动可能与热液系统的破坏有关，反映在热液蚀变的岩性中，和/或火山口底下沉更接近地下水位。SAT 的大块岩石化学成分是玄武岩，但双峰玻璃成分表明玄武岩与安山岩熔体可能在喷发期间在管道中混合。SAT 玄武岩通过橄榄石、斜长石和少量单斜辉石的分馏结晶在 20 公里深度的 MOHO 附近的储层中分化，形成拉斑岩分馏趋势。次要中间体 - 一种斜长石从玄武岩熔体中结晶出来的 H2O 浓度约为 2wt%，通过 FTIR 测量熔体包裹体中的含量。然而，一个关键的观察结果是熔体包裹体与承载它们的高 An 斜长石并不平衡。包裹体的重新平衡最初需要更高的含水量（约 5-6 wt%），这也符合高 Ba/La ~ 80 表明来自强水化俯冲板片的输入。所以，当 SAT 岩浆在深度含水条件下演化时，它在浅层储存足够长的时间以适应低饱和压力并沉淀一些中间斜长石，但仍保持其高温（约 1100°C）和斑晶-组成不佳。由于与深部储层的连接和上升过程中的水脱气造成的巨大超压限制了浅层的储存时间，并推动了异常强烈和大量的普林式喷发，促进了室顶的活塞式坍塌。