Silicic calderas can evacuate 100 to > 1000 km³ of rhyolitic products in a matter of days to months leading to questions on pre-eruptive melt generation and accumulation. Whereas silicic plutonic units may provide information on the igneous evolution of crystal-mush bodies, their connection with volcanic units remains enigmatic. In the Ivrea-Verbano Zone of the southern Alps, the plumbing system of a Permian rhyolitic caldera is exposed to a depth of about 25 km in tilted crustal blocks. The upper-crustal segment of this magmatic system (a.k.a. Sesia Magmatic System) is represented by the Valle Mosso pluton (VMP). The VMP is a ∼ 260 km³ composite silicic intrusion ranging from quartz-monzonite to high-silica leucogranite (∼ 67 to 77 wt % SiO₂), which intrudes into roughly coeval rhyolitic products of the >15 km diameter Sesia Caldera. In the caldera field, the emplacement of a large, crystal-rich rhyolite ignimbrite(s) (> 400 km³) is followed by eruption of minor volumes (1-10 km³) of crystal-poor rhyolite. Here, we compare silicic plutonic and volcanic units of the Sesia Magmatic System through a combination of geochemical (XRF, ICP-MS and EMP analyses) and petrological (rhyolite-MELTS, trace element and diffusion modeling) tools to explore their connection. Textural and compositional features shared by both VMP and crystal-rich ignimbrites imply thermal rejuvenation of crystal-mush as the mechanism to create large volumes of eruptible rhyolitic magma. Bulk-rock composition of crystal-rich rhyolite erupted during the caldera collapse overlaps that of the bulk VMP. Quartz and plagioclase from these two units show resorbed cores and inverse zoning, with Ti- and anorthite-rich rims, respectively. This indicates crystallization temperatures in rims > 60 °C higher than cores (780 - 820 vs. ∼ 720 °C), if temperature is the sole parameter responsible for zonation, suggesting heating and partial dissolution of the crystal-framework. Decrease in crystallinity associated with thermal energy input was calculated through rhyolite-MELTS and indicates lowering of the mush crystal fraction below the rheological lock-up threshold, which likely promoted eruptive activity. Also, after the climatic eruption, Si-rich melts in the Sesia Magmatic System were produced by extraction of interstitial melt from un-erupted, largely crystalline mush. Regarding both textures and chemical variations, we interpret the deep quartz-monzonite unit of the VMP as a compacted silicic cumulate. Fractionated melts extracted from this unit were emplaced as leucogranite cupola atop the VMP, generating the final internal architecture of the silicic intrusion, or alternatively erupted as minor post-caldera, crystal-poor rhyolite. Ti-in-quartz diffusion profiles in thermally rejuvenated units of the Sesia Magmatic System demonstrate that the process of reheating, mobilization and eruption of crystal mush took place rapidly (c. 10¹-10² yr). A protracted cooling history is instead recorded in the diffusion timescales of quartz from the silicic cumulate units (c. 10⁴-10⁶ yr). These longer timescales encompass the duration of evolved melt extraction from the cumulate residue. We argue that the VMP preserves a complex record of pre-eruptive processes, which span mechanisms and timescales universally identified in volcanic systems and are consistent with recently proposed numerical models.

中文翻译：

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Sesia岩浆系统（意大利南部阿尔卑斯山）的二叠纪深部和火山岩中记录的结晶糊状结晶和熔体提取的时标和机理

硅质破火山口可以在数天至数月内疏散100至> 1000 km ³的流纹岩产品，从而引发有关喷发前熔液生成和积聚的问题。硅质的古生代单元可能提供有关结晶糊状体火成演化的信息，但它们与火山单元的联系仍然是莫名其妙的。在阿尔卑斯山南部的伊夫里亚-韦尔巴诺地区，二叠纪流纹破火山口的管道系统在倾斜的地壳中暴露于约25 km的深度。岩浆系统（又名Sesia岩浆系统）的上地壳部分由Valle Mosso岩体（VMP）代表。VMP是一个〜260 km ^3的复合硅质侵入体，范围从石英蒙脱石到高硅白云石（〜67至77 wt％SiO ₂），会侵入直径大于15 km的Sesia破火山口的粗略流传的流纹岩产品。在破火山口气田中，一个大的，富含晶体的流纹岩火成岩（> 400 km ³）被安置，随后喷出了少量（1-10 km ^3））晶体贫瘠的流纹岩。在这里，我们通过地球化学（XRF，ICP-MS和EMP分析）和岩石学（流纹岩-MELTS，痕量元素和扩散模型）工具的组合，比较了Sesia岩浆系统的硅质火山岩和火山岩单元，以探索它们的联系。VMP和富含晶体的火成岩岩共同拥有的质地和成分特征暗示了结晶糊的热再生是产生大量可爆发流纹岩浆的机制。破火山口塌陷期间喷出的富含晶体的流纹岩的块岩成分与块状VMP重叠。来自这两个单元的石英和斜长石显示出重吸收的岩心和反分区，分别具有富钛和钙长石的边缘。这表明轮辋中的结晶温度比核心高60°C（780-820 vs.〜720°C），如果温度是负责分区的唯一参数，则表明晶体框架受热和部分溶解。通过流纹岩-MELTS计算出与热能输入相关的结晶度降低，这表明糊状结晶部分降低到流变锁定阈值以下，这可能促进了喷发活性。同样，在气候喷发之后，通过从未喷发的，大部分为晶体的糊状物中提取间隙熔体，在Sesia岩浆系统中产生了富硅熔体。关于质地和化学变化，我们将VMP的深层石英蒙脱石单元解释为压实的硅质堆积物。从该装置中提取的分馏熔体放置在VMP顶部，作为无色花岗岩冲天炉，形成了硅质侵入体的最终内部构造，或爆发为破火山口后，晶体贫乏的流纹岩。Sesia Magmatic System的热再生单元中的石英中Ti的扩散曲线表明，晶体糊状物的重新加热，动员和喷发过程迅速发生（C。10 ¹ -10 ²年）。延长的冷却历史，而不是记录在石英中的扩散时间尺度从硅酸累积单元（角10 ⁴ -10 ⁶岁）。这些较长的时间范围包括从累积残渣中析出熔体的持续时间。我们认为，VMP保留了喷发前过程的复杂记录，这些记录涵盖了火山系统中普遍确定的机制和时标，并且与最近提出的数值模型一致。