The 2.9-Ma Hotokezawa Ignimbrite, which was ejected from the Aizu caldera cluster in the northeast Japan arc, is a typical monotonous intermediate ignimbrite, with 40–50 vol% crystals and an eruptive volume of >140 km³ dense-rock equivalent. This ignimbrite filled Hiwada caldera and was deformed by post-caldera plutonic intrusions that formed a resurgent dome. The Hotokezawa Ignimbrite is a calc-alkaline, medium-K dacite to rhyolite with SiO₂ contents of 67.9–71.3 wt%, and has homogeneous trace-element abundances and Sr–Nd isotopic ratios. These geochemical features suggest that the Hotokezawa magma was formed by partial melting of amphibolitic crustal rocks. This crystal-rich magma did not appear during the post-caldera stage. Therefore, it is plausible that the chamber of eruptible magma was emptied by the caldera-forming eruption. In contrast, post-caldera plutonic rocks exhibit a variety of compositions and have a clear SiO₂ gap corresponding to the caldera-forming magma: the early pluton (tonalite) and later ones (quartz porphyry, granite porphyries, and granite) contain 62.0–66.6 and 71.2–76.5 wt% SiO₂, respectively. The tonalite and the Hotokezawa Ignimbrite form a continuous trend in their major-element variations. The Sr–Nd isotopic ratios of the ignimbrite and tonalite overlap, but those of the porphyries and granite are more enriched. The early tonalite represents the more basic part of the Hiwada caldera system that was held in small pockets separate from the main magma chamber, because its trace-element abundances are varied and distinct from those of the Hotokezawa Ignimbrite. The distinct compositional change from the Hotokezawa Ignimbrite to the late porphyries and granite indicates that the partial melting crust generating felsic magma was renewed by the subsequent intrusion of the mantle melts. The new felsic magma ascended through subsidence-related faults into the shallow caldera system and emplaced as laccoliths forming the resurgent dome.

中文翻译：

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日本东北部上新世日和田火山口形成火山口和再生岩浆的地球化学：单调中间系统的一个例子

2.9-Ma Hotokezawa Ignimbrite 是从日本东北弧会津火山口星团喷出的典型单调中间熔凝岩，具有 40-50 vol% 的晶体和 >140 km ³致密岩石当量的喷发体积。这块熔岩填满了日和田火山口，并被火山口后的深成侵入体变形，形成了一个复活的圆顶。Hotokezawa Ignimbrite 是一种钙碱性、中 K 英安岩到流纹岩，含有 SiO ₂含量为 67.9-71.3 wt%，具有均匀的微量元素丰度和 Sr-Nd 同位素比率。这些地球化学特征表明Hotokezawa 岩浆是由角闪质地壳岩石的部分熔融形成的。这种富含晶体的岩浆在后火山口阶段没有出现。因此，可喷发岩浆室被火山口形成喷发清空是合理的。相比之下，火山口后深成岩表现出多种成分，并具有与形成火山口的岩浆相对应的明显 SiO ₂间隙：早期岩体（斑岩）和后期岩体（石英斑岩、花岗岩斑岩和花岗岩）含有 62.0– 66.6 和 71.2–76.5 wt% SiO ₂， 分别。Tonalite和Hotokezawa Ignimbrite在其主要元素变化中形成连续趋势。熔凝灰岩和钠长岩的 Sr-Nd 同位素比重叠，但斑岩和花岗岩的 Sr-Nd 同位素比更富集。早期的tonalite代表了Hiwada火山口系统的更基本部分，它被保存在与主岩浆室分开的小口袋中，因为它的微量元素丰度是不同的，并且与Hotokezawa Ignimbrite的不同。从 Hotokezawa Ignimbrite 到晚期斑岩和花岗岩的明显成分变化表明，产生长英质岩浆的部分熔融地壳因随后地幔熔体的侵入而更新。