The Aso-4 caldera-forming event (86.4 ± 1.1 ka, VEI-8) is the second largest volcanic eruption Earth experienced in the past 100 ka. The ignimbrite sheets produced during this event are some of the first ever described compositionally zoned pyroclastic flow deposits exhibiting clear compositional, mineralogical and thermal gradients with stratigraphic position. Large quantities of the deposits are composed of crystal-poor, highly evolved juvenile pumices, while late-erupted pyroclastic flows are in many cases dominated by crystal-rich and less silicic scoria. These petrological gradients in the Aso-4 deposits have been linked to extensive magma mixing of two compositionally distinct magmas in a complex upper crustal reservoir. However, new studies on several other zoned ignimbrites suggest that magma mixing alone is not sufficient to fully explain such strong compositional gradients in the deposits. These gradients are expected to be dominantly caused by the recharge-induced reactivation of extracted melt caps and their complementary cumulate in the upper crust. Here, we investigate bulk rock and matrix glass data with detailed analyses of mineral chemistry in order to re-evaluate the Aso-4 deposits in light of these latest developments. Reverse chemical zoning in phenocrysts, Sr enrichment in euhedral rims of plagioclase and the presence of mafic minerals (clinopyroxene, olivine) indicate recharge of hot, mafic magmas shortly prior to eruption, inducing a mixing signature. However, the marked enrichment in some elements in bulk-rock analyses and the presence of highly evolved minerals (some in the form of glomerocrysts) in the late-erupted, crystal-rich units, provide clear evidence for crystal accumulation in these scoria. Mass balance modeling of P₂O₅, Sr and SiO₂ supports the extraction of melt-rich lenses within an upper crustal mush zone, leaving a partly cumulative evolved crystal residue. We therefore propose an origin of the compositionally zoned Aso-4 ignimbrite largely by erupting a heterogeneous upper crustal reservoir, consisting of crystal-poor rhyodacitic melt caps within its associated cumulate mush. This complex reservoir was reactivated by mafic recharge shortly prior to eruption, imparting an additional mixing signature to the deposits.

Aso-4火山口形成事件（86.4±1.1 ka，VEI-8）是过去100 ka经历的第二大火山喷发地球。在此事件中产生的火成岩片是一些最早描述的按成分划分的火山碎屑流沉积物，这些沉积物显示出清晰的成分，矿物学和热梯度以及地层位置。大量的沉积物是由晶体贫乏，高度演化的幼体粉尘组成的，而在许多情况下，晚喷发的火山碎屑流主要由富含晶体和硅含量较低的氧化渣所主导。Aso-4矿床中的这些岩石学梯度与复杂的上地壳储层中两个成分不同的岩浆的广泛岩浆混合有关。然而，对其他几个带状火成岩的新研究表明，仅岩浆混合不足以完全解释矿床中如此强的成分梯度。预计这些梯度主要是由补给引起的提取熔盖的再活化及其在上地壳中的累积堆积引起的。在这里，我们通过对矿物化学的详细分析来研究块状岩石和基体玻璃的数据，以便根据这些最新进展重新评估Aso-4矿床。表晶的反向化学区划，斜长石的真面状边缘中的Sr富集以及镁铁矿质（斜辉石，橄榄石）的存在表明火山喷发前不久，热的镁铁质岩浆会重新充填，从而引起混合特征。然而，大量岩石中某些元素的显着富集以及后期喷发的富含晶体的单元中高度演化的矿物（某些形式为球隐晶）的存在，为这些矿渣中的晶体积累提供了清晰的证据。P的质量平衡模型₂ O ₅，Sr和SiO ₂支持上地壳糊状区中富熔晶状体的提取，留下部分累积的析出晶体残留物。因此，我们主要是通过喷出一个非均质的上地壳储层来提出组成区带的Aso-4火成岩的起源，该储层由晶体稀疏的流纹岩熔盖在其相关的累积糊状物内组成。在喷发前不久，通过镁铁质补给使该复杂的储层活化，为沉积物赋予额外的混合特征。