The Whakamaru Magmatic System (Taupō Volcanic Zone, New Zealand), part 2: Evidence from ignimbrite deposits for the pre-eruptive distribution of melt-dominated magma and magma mush
The Whakamaru Magmatic System (Taupō Volcanic Zone, New Zealand), part 2: Evidence from ignimbrite deposits for the pre-eruptive distribution of melt-dominated magma and magma mush
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华卡马鲁岩浆系统（新西兰陶普火山带），第 2 部分：来自火凝灰岩沉积物的证据，表明喷发前以熔体为主的岩浆和岩浆糊的分布
DOI：
10.1016/j.jvolgeores.2024.108013
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发表时间：
2024
影响因子：
2.9
通讯作者：
Gravley, Darren M.
中科院分区：
文献类型：
--
作者：
Harmon, Lydia J.;Smithies, Sarah L.;Gualda, Guilherme A.R.;Gravley, Darren M.
新西兰新西兰华卡马鲁火山矿床的复杂火山学和岩石学迄今为止掩盖了喷发阶段的数量以及这些喷发的相对时间。我们研究了来自多个地点的浮石碎屑，以阐明喷发的相对时间，重点关注为华卡马鲁喷发提供原料的以熔融为主的岩浆体的喷发前条件以及提取这些岩浆的糊状物。成对的全岩和玻璃成分证实了华卡马鲁火山喷发期间喷发的四种岩浆类型（A、B、C、D 型；最初由 Brown 等人，1998 年确定）。利用浮石碎屑的玻璃成分，我们计算了喷发前的储存温度（使用锆石饱和地温测量法）和压力（使用流纹岩-熔体地压测量法）。使用来自浮石碎屑子集的匹配全岩成分，我们计算了从岩浆糊中提取的压力（也使用流纹岩-熔体地质气压测量法）。喷发前储存压力估计了喷发前以熔融为主的岩浆体所在的深度；相反，提取压力估计的是熔体从岩浆中提取以在地壳较浅层形成以熔体为主的岩浆体的深度。岩浆在喷发前储存在浅层（~50-150 MPa）。 B 型和 C 型的萃取压力很好地限制在 155-355 MPa（组合物包括斜长石和石英）。 A 型和 D 型的萃取压力取决于氧逸度 (fO2)，因为萃取组合包括斜长石和斜方辉石（ΔNNO = 0 至 +0.5 时为 170-290 MPa，ΔNNO = +1 至 +1.5 时为 290-360 MPa）。这四种岩浆类型可能代表独立的岩浆体，这些以熔融为主的岩浆体储存得比糊状体浅且与糊状体分开。至少有两种不同的岩浆子系统为华卡马鲁喷发提供了源泉——一个子系统提供了 A 型和 D 型岩浆，而另一个子系统则提供了 B 型和 C 型岩浆。火凝灰岩矿床和相关火山灰岩中记录的岩浆类型分布（Harmon 等人，2024）揭示了四种不同的可绘制火凝灰岩的喷发顺序。火山口以东的火凝灰岩（Rangitaiki ± Te Whaiti）先于火山口以西的华卡马鲁火凝灰岩（sensu stricto）喷发。最年轻的华卡马鲁火山灰凝结岩喷发可能与火山口以西的马努伊火山灰凝灰岩同时沉积在火山口西北部。火凝岩和相关火山灰岩的岩石学数据相结合，表明存在一个复杂的系统，其中包括横向并置的以熔体为主的岩浆以及横向并置的岩浆糊，这些岩浆糊跨越了大部分浅地壳，但有些区域的岩浆浓度较低或完全不存在。这种复杂的喷发前结构可能导致了华卡马鲁火山喷发所观察到的复杂喷发模式。
The complex volcanology and petrology of the Whakamaru volcanic deposits in Aotearoa New Zealand have thus far obscured the number of eruptive phases and the relative timing of these eruption(s). We investigate pumice clasts from multiple localities to elucidate the relative timing of the eruptions, with a focus on the pre-eruptive conditions of the melt-dominated magma bodies that fed the Whakamaru eruptions and on the mushes from which these magmas were extracted. Paired whole-rock and glass compositions confirm four magma types erupted during the Whakamaru eruptions (types A, B, C, D; originally identified by Brown et al., 1998). Using the glass compositions of the pumice clasts, we calculate pre-eruptive storage temperatures (using zircon saturation geothermometry) and pressures (using rhyolite-MELTS geobarometry). Using matching whole-rock compositions from a subset of pumice clasts, we calculate extraction pressures from magma mush (also using rhyolite-MELTS geobarometry). Pre-eruptive storage pressures estimate the depths where melt-dominated magma bodies were located prior to eruption; extraction pressures, in contrast, estimate the depths at which melt was extracted from magma mush to form melt-dominated magma bodies at shallower levels of the crust. Magmas were stored at shallow depths (~50-150 MPa) prior to eruption. Extraction pressures for types B and C are well constrained to 155-355 MPa (with an assemblage including plagioclase and quartz). Extraction pressures for types A and D depend on oxygen fugacity (fO2), as the extraction assemblage includes plagioclase and orthopyroxene (170-290 MPa for ΔNNO = 0 to +0.5 and 290-360 MPa for ΔNNO = +1 to +1.5). The four magma types likely represent independent magma bodies, with these melt-dominated magma bodies stored shallower than and separate from the mush. At least two different magma subsystems fed the Whakamaru eruptions – one subsystem sourced the type A and type D magmas, while the other sourced the type B and type C magmas. The distribution of magma types recorded in the ignimbrite deposits and in the correlated tephras (Harmon et al., 2024) reveal the sequence of eruption for the four different mappable ignimbrites. The ignimbrites to the east of the caldera (Rangitaiki ± Te Whaiti) erupted before the Whakamaru ignimbrite (sensu stricto) to the west of the caldera. The youngest Whakamaru ignimbrite eruptions likely deposited to the northwest of the caldera contemporaneously with the Manunui ignimbrite to the west of the caldera. The combination of petrological data from the ignimbrites and associated tephras suggest a complex system that included laterally juxtaposed melt-dominated magmas as well as laterally juxtaposed magma mushes that spanned much of the shallow crust, but with regions in which magma appeared in low concentration or was entirely absent. This complex pre-eruptive architecture probably contributed to the complex eruptive patterns observed for the Whakamaru eruptions.