Geochronological imaging of an episodically constructed subvolcanic batholith: U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes
Geochronological imaging of an episodically constructed subvolcanic batholith: U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes
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幕式构造的火山岩基的地质年代学成像：安第斯山脉中部高原-普纳火山杂岩锆石年代化学中的 U-Pb
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发表时间：
2016
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影响因子：
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通讯作者：
R. Economos
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作者：
Jamie M. Kern;S. D. de Silva;A. Schmitt;Jason F. Kaiser;A. Rodrigo Iriarte;R. Economos
15个富含晶体的单调中辉橄榄岩和1个贫晶流纹岩的锆石在11-1 Ma阿尔蒂普诺-普纳火山杂岩(APVC)火成岩爆发期间喷发，记录了阿尔蒂普诺-普纳岩浆体(APMB)最浅水平(地表下5-10公里)岩浆史上的多尺度幕式。这一记录揭示了次火山岩基的构造及其岩浆和喷发节奏。用二次离子质谱仪测定的锆石边缘和抛光颗粒内部的750多个U-Pb年龄确定了每个点火沸石的复杂年龄谱，主要是自晶峰和次要的前结晶峰。异晶石很少见。通过汇集最年轻的、分析上无法区分的锆石年龄而获得的加权平均，大多对应于岩浆中锆石的主要结晶年龄。这些岩浆年龄与喷发地层相一致，分为四组，定义了与喷发脉冲相关的不同的岩浆作用脉冲(从老到年轻，从1到4)，但表明每个脉冲中的岩浆构造至少开始了1英里。在火山喷发开始之前。岩浆活动最初分散在APVC的东侧和西侧，但随着活动的增加而扩散到APVC的大部分地区，然后在耀斑高峰期集中在中部。每个脉冲由空间上不同但在时间上排序的岩浆子脉冲组成，它们代表了喷发前岩浆储集层的构造。在大约6~2.5 Ma的耀斑高峰期，三个嵌套的破火山口是主要的喷发地点。它们显示了大致同步的岩浆发展，但在它们后来的喷发历史中存在一些不一致。这些关系被解释为喷发节奏是局部自上而下控制的，而岩浆节奏是一种更系统、更深、更自下而上的特征。在不同的上地壳岩浆源的岩浆历史的同步性暗示了APMB内部更深层次的共同联系。每一块褐云母岩都记录了一个离散岩浆的发育过程。随着时间的推移，单个辉长岩的锆石年龄分布变得更加复杂，反映了前结晶在相继年轻的岩浆中的携带，也证明了APVC中的上地壳同化作用。虽然存在，但异晶石很少见，这表明遗传是有限的。这归因于在APMB中锆石不饱和条件下的基底同化作用比喷发前水平更深，在喷发前水平，前熔体被合并在锆石饱和条件下。个别点火斑岩的岩浆年龄大于40Ar/39Ar喷发年龄。这一差异被解释为APVC岩浆的平均最小Zr饱和熔融-现今寿命、岩浆持续时间或Δ年龄。大约0.4 Ma的平均Δ年龄表明，在APVC岩浆喷发之前，锆石饱和的热化学条件保持了数十万年。这与记录上地壳条件的730-815°C的锆石饱和温度以及显示长期冷却速率下长期岩浆分异的Zr/Hf、Th/U、Eu/Eu*和Ti的窄范围一致，比典型的岩体冷却速率慢一个数量级。总而言之，这些数据都表明，喷发前的岩浆储集层在其漫长的喷发前寿命期间处于热和化学缓冲状态。微量元素的变化表明不同岩浆储集层不同区域的结晶度、熔体比例和熔体成分存在细微差异。与火山岩有关的大量深成岩得到了地球物理数据、超过1000万英里的有限成分范围、岩浆系统的热惯性以及火山口和喷发中心岩浆活动和抬升的证据的支持，这些证据的分布定义了一种复合的、幕式构造的次火山岩基。APVC耀斑的锆石U-Pb年龄揭示的多尺度幕式可以在大陆弧岩浆系统的背景下得到解释。作为一个整体，apvc点火辉石耀斑是一个10百万∼的二次脉冲，岩浆脉冲1至4反射∼2百万的三级脉冲，而个别的沸石锆石光谱定义了
Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 crystal-poor rhyolite ignimbrite erupted during the 11–1 Ma Altiplano-Puna Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in the magmatic history of the shallowest levels (5–10 km beneath the surface) of the Altiplano-Puna Magma Body (APMB). This record reveals the construction of a subvolcanic batholith and its magmatic and eruptive tempo. More than 750 U-Pb ages of zircon rims and interiors of polished grains determined by secondary ion mass spectrometry define complex age spectra for each ignimbrite with a dominant peak of autocrysts and subsidiary antecryst peaks. Xenocrysts are rare. Weighted averages obtained by pooling the youngest analytically indistinguishable zircon ages mostly correspond to the dominant crystallization ages for zircons in the magma. These magmatic ages are consistent with eruptive stratigraphy, and fall into four groups defining distinct pulses (from older to younger, pulses 1 through 4) of magmatism that correlate with eruptive pulses, but indicate that magmatic construction in each pulse initiated at least 1 m.y. before eruptions began. Magmatism was initially distributed diffusely on the eastern and western flanks of the APVC, but spread out over much of the APVC as activity waxed before focusing in the central part during the peak of the flare-up. Each pulse consists of spatially distinct but temporally sequenced subpulses of magma that represent the construction of pre-eruptive magma reservoirs. Three nested calderas were the main eruptive loci during the peak of the flare-up from ca. 6 to 2.5 Ma. These show broadly synchronous magmatic development but some discordance in their later eruptive histories. These relations are interpreted to indicate that eruptive tempo is controlled locally from the top down, while magmatic tempo is a more systemic, deeper, bottom-up feature. Synchroneity in magmatic history at distinct upper crustal magmatic foci implicates a shared connection deeper within the APMB. Each ignimbrite records the development of a discrete magma. Zircon age distributions of individual ignimbrites become more complex with time, reflecting the carryover of antecrysts in successively younger magmas and attesting to upper crustal assimilation in the APVC. Although present, xenocrysts are rare, suggesting that inheritance is limited. This is attributed to basement assimilation under zircon-undersaturated conditions deeper in the APMB than the pre-eruptive levels, where antecrysts were incorporated in zircon-saturated conditions. Magmatic ages for individual ignimbrites are older than the 40 Ar/ 39 Ar eruption ages. This difference is interpreted as the average minimum Zr-saturated melt-present lifetime for APVC magmas, the magmatic duration or Δ age. The average Δ age of ca. 0.4 Ma indicates that thermochemical conditions for zircon saturation were maintained for several hundreds of thousands of years prior to eruption of APVC magmas. This is consistent with a narrow range of zircon saturation temperatures of 730–815 °C that record upper crustal conditions and Zr/Hf, Th/U, Eu/Eu*, and Ti that reveal protracted magma differentiation under secular cooling rates an order of magnitude slower than typical pluton cooling rates. In concert, these data all suggest that the pre-eruptive magma reservoirs were perched in a thermally and chemically buffered state during their long pre-eruptive lifetimes. Trace element variations suggest subtle differences in crystallinity, melt fraction, and melt composition within different zones of individual magma reservoirs. Significant volumes of plutonic rocks associated with ignimbrites are supported by geophysical data, the limited compositional range over 10 m.y., the thermal inertia of the magmatic systems, and the evidence of resurgent magmatism and uplift at the calderas and eruptive centers, the distribution of which defines a composite, episodically constructed subvolcanic batholith. The multiscale episodicity revealed by the zircon U-Pb ages of the APVC flare-up can be interpreted in the context of continental arc magmatic systems in general. The APVC ignimbrite flare-up as a whole is a secondary pulse of ∼10 m.y., with magmatic pulses 1 through 4 reflecting tertiary pulses of ∼2 m.y., and the individual ignimbrite zircon spectra defining quaternary pulses of
影响因子：
5.2
作者：
del Potro, Rodrigo;Diez, Mikel;Gottsmann, Joachim
通讯作者：
Gottsmann, Joachim