Petrogenesis of the Large-volume Cardones Ignimbrite, Chile; Development and Destabilization of a Complex Magma–Mush System
Petrogenesis of the Large-volume Cardones Ignimbrite, Chile; Development and Destabilization of a Complex Magma–Mush System
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智利大体积 Cardones Ignimbrite 的岩石成因；复杂岩浆-糊状系统的发育和不稳定
DOI：
10.1093/petrology/egx079
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发表时间：
2017
影响因子：
3.9
通讯作者：
J. Blundy
中科院分区：
文献类型：
--
作者：
M. E. van Zalinge;R. Sparks;J. Blundy
21.9 Ma Cardones褐斑岩是智利最北部早中新世Oxaya组的成员，是一种富含晶体(约40vol%)的流纹岩，其额外破火山口厚度可达1000米。通过8个约1公里的钻孔进入岩心，能够在前所未有的空间尺度范围内对整个褐斑岩厚度的浮石进行取样和岩石学特征。矿物化学和模式揭示了贫辉长石和富辉长石两种浮岩类型的存在，代表了两种岩浆，前者的温度(850-750℃)高于后者(770-670℃)。贫辉长石岩浆含角闪岩，具有富巴熔体相(400-1000ppm)，而富辉长岩岩浆具有贫钡熔体相(<；200ppm)，缺乏角闪石。对缺乏闪锌矿的浮岩碎屑中前晶残留物的热压测定表明，流纹质熔体来自地壳中部热区(950-850℃)，在那里潮湿、中等到镁铁质的岩浆停滞和分馏。锆石和斜长石的结构和分带与流纹质熔体在约200Ky期间在6.0~8.7±2.0千米深度的浅部陆续侵位的解释一致。富辉橄榄岩的岩浆成分与贫辉玛瑙岩浆中的熔体相一致，前者可能与后者的残馀熔体分凝作用有关。闪长岩中的高钡和高锶带的成分分别高达28,000 ppm和400 ppm。如此高的浓度需要更多中间岩浆的输入和/或累积岩浆的部分熔融。富辉橄榄岩的岩浆可能形成于贫辉橄榄岩的岩浆之上。然而，不同的浮石类型与褐沸石地层之间没有系统的关系，这表明岩浆系统在喷发前的重大不稳定作用导致了不同的熔融主导的透镜和周围的晶体碎屑的融合和混合，形成了一个大型的非均质喷发岩浆体。
The 21.9 Ma Cardones ignimbrite, a member of the early Miocene Oxaya Formation in northernmost Chile, is a crystal-rich (ca. 40 vol%) rhyolite with an extra-caldera thickness up to 1000 m. Access to core from eight ca. 1 km drill holes enabled sampling and petrological characterisation of pumice throughout the entire thickness of the ignimbrite at an unprecedented range of spatial scales. Mineral chemistry and modes reveal the presence of sanidine-poor and sanidine-rich pumice types, representing two, petrologically distinct magmas with the former being hotter (850-750°C) than the latter (770-670C). The sanidine-poor magma is amphibolebearing and has a Ba-rich melt phase (400-1000 ppm), whereas the sanidine-rich magma has a Ba-poor melt phase (<200 ppm) and lacks amphibole. Thermobarometry of antecrystal remnants in the sanidine-poor pumice clasts indicate derivation of rhyodacitic melts from a middle crustal hot zone (950-850°C) where wet, intermediate to mafic magmas stalled and fractionated. Textures and zoning in zircon and plagioclase are consistent with the interpretation that rhyodacitic melts were episodically emplaced into the shallow crust at 6.0 to 8.7 ± 2.0 km depths over a period of ca. 200 ky. Sanidine-rich magma compositions are consistent with the melt phase in the sanidine-poor magma, hence the former could be derived by residual melt segregation from the latter. High-Ba and high-Sr zones in sanidine have compositions up to 28,000 ppm and 400 ppm respectively. Such high concentrations require input of more intermediate magmas and/or partial melting of cumulative magmas. The sanidine-rich magmas likely formed above of the sanidine-poor magmas. However, there is no systematic relationship between the different pumice types and the ignimbrite stratigraphy, suggesting major pre-eruptive destabilisation of the magmatic system led to the amalgamation and mixing of different, meltdominated lenses and surrounding crystal-mush to form a large heterogeneous eruptible magma body.
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