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首页> 外文期刊>Contributions to Mineralogy and Petrology >Magma evolution in the Purico ignimbrite complex, northern Chile: evidence for zoning of a dacitic magma by infection of rhyolitic melts following mafic recharge
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Magma evolution in the Purico ignimbrite complex, northern Chile: evidence for zoning of a dacitic magma by infection of rhyolitic melts following mafic recharge

机译：智利北部普里科火成岩带中的岩浆演化：镁铁质充注后，流纹质熔体被感染，从而对Damagtic岩浆进行分区的证据

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摘要

The 1.3 Ma Purico complex is part of an extensive Neogene-Pleistocene ignimbrite province in the central Andes. Like most other silicic complexes in the province, Purico is dominated by monotonous intermediate ash-flow sheets and has volumetrically minor lava domes. The Purico ignimbrites (total volume 80-100 km~3) are divided into a Lower Purico Ignimbrite (LPI) with two extensive flow units, LPII and LPI II; and a smaller Upper Purico Ignimbrite (UPI) unit. Crystal-rich dacite is the dominant lithology in all the Purico ignimbrites and in the lava domes. It is essentially the only lithology present in the first LPI flow unit (LPI I) and in the Upper Purico Ignimbrite, but the LPI II flow unit is unusual for its compositional diversity. It constitutes a stratigraphic sequence with a basal fall-out deposit containing rhyolitic pumice (68-74 wt percent SiO_2) overlain by ignimbrite with dominant crystal-rich dacitic pumice (64-66 wt percent SiO_2). Rare andesitic and banded pumice (60-61 wt percent SiO_2) are also present in the uppermost part of the flow unit. The different compositional groups of pumice in LPI II flow unit (rhyolite, andesite, dacite) have initial Nd and Sr isotopic compositions that are indistinguishable from each other and from the dominant dacitic pumice (?Nd=-6.7 to -7.2 and ~(87)Sr/~(86)Sr=0.7085-0.7090). However, two lines of evidence show that the andesite, dacite and rhyolite pumices do not represent a simple fractionation series. First, melt inclusions trapped in sequential growth zones of zoned plagioclase grains in the rhyolite record fractionation trends in the melt that diverge from those shown by dacite samples. Second, mineral equilibrium geothermometry reveals that dacites from all ignimbrite flow units and from the domes had relatively uniform and moderate pre-eruptive temperatures (780-800 deg C), whereas the rhyolites and andesites yield consistently higher temperatures (850-950 deg C). Hornblende geobarometry and pressure constraints from H_2O and CO_2 contents in melt inclusions indicate upper crustal (4-8 km) magma storage conditions. The petrologic evidence from the LPI II system thus indicates an anomalously zoned magma chamber with a rhyolitic cap that was hotter than, and chemically unrelated to, the underlying dacite. We suggest that the hotter rhyolite and andesite magmas are both related to an episode of replenishment in the dacitic Purico magma chamber. Rapid and effective crystal fractionation of the fresh andesite produced a hot rhyolitic melt whose low density and viscosity permitted ascent through the chamber without significant thermal and chemical equilibration with the resident dacite. Isotopic and compositional variations in the Purico system are typical of those seen throughout the Neogene ignimbrite complexes of the Central Andes. These characteristics were generated at moderate crustal depths (<30 km) by crustal melting, mixing and homogenization involving mantle-derived basalts. For the Purico system, assimilation of at least 30 percent mantle-derived material is required.
机译：1.3 Ma Purico复合物是安第斯山脉中部广阔的新近纪-更新世火成岩省的一部分。像该省其他大多数硅质综合体一样，Purico以单调的中间灰分流床为主，体积上较小的熔岩穹顶。 Purico易燃物（总体积80-100 km〜3）被分为下部Purico易燃物（LPI），它具有两个扩展的流动单元LPII和LPI II。以及较小的上普里科点火装置（UPI）。在所有Purico火山岩和熔岩穹顶中，富含晶体的da石是主要的岩性。它本质上是第一个LPI流动单元（LPI I）和上Purico Ignimbrite中存在的唯一岩性，但是LPI II流动单元因其成分多样性而与众不同。它构成了一个地层序列，其基础沉积物含有流纹岩浮石（68-74 wt％SiO_2），上面覆盖着由火成岩和富含晶体的主要洋山浮石（64-66 wt％SiO_2）。流动单元的最上部还存在稀有的安山岩和带状浮石（60-61 wt％SiO_2）。 LPI II流动单元中的浮石的不同组成组（流纹岩，安山岩，钠铁矿）具有初始Nd和Sr同位素组成，它们彼此之间以及与主要的菊苣浮石无法区分（ΔNd= -6.7至-7.2和〜（87 Sr /〜（86）Sr = 0.7085-0.7090）。但是，有两点证据表明，安山岩，榴辉岩和流纹岩粉尘并不代表简单的分馏系列。首先，流纹岩中夹杂在斜长石层状带的连续生长区中的熔体夹杂物记录了熔体中的分馏趋势，其与达克特样品显示的趋势不同。其次，矿物平衡地热法显示，所有火成岩流单位和穹顶的隐性芽孢杆菌具有相对均匀和适中的喷发前温度（780-800摄氏度），而流纹岩和安山岩的温度始终一致（850-950摄氏度）。 。 Hornblende地压法和来自熔体包裹体中H_2O和CO_2含量的压力约束指示了上地壳（4-8 km）的岩浆储存条件。因此，LPI II系统的岩石学证据表明，一个异常区域的岩浆腔室带有一个流纹岩盖，该岩心比下面的da榴石更热，并且化学性质与其无关。我们建议，较热的流纹岩和安山岩岩浆都与在达科古纪的Purico岩浆室内的补给过程有关。新鲜安山岩的快速有效的晶体分馏产生了热流纹岩熔体，其低密度和低粘度允许通过腔体上升，而与驻留钠镁石没有明显的热和化学平衡。 Purico系统中的同位素和组成变化是安第斯中部新近纪火成岩复合体的典型特征。这些特征是在中等地壳深度（<30 km）通过地幔融化，混合和均质作用（涉及地幔衍生的玄武岩）而产生的。对于Purico系统，至少需要吸收30％来自地幔的材料。

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