Abstract

The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO₂ group, and a high-Al, high-temperature, low-fO₂ group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.

中文翻译：

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美国新墨西哥州杰米兹山火山田火山口岩浆成岩作用的矿物学证据

摘要

杰米兹山脉火山场（JMVF）是班德利尔凝灰岩的两个巨大的火山口形成成员的地点，在长期和连续的火山口前火山史（〜10 Myr）后于1·60和1·25 Ma爆发。 ）。先前利用全岩石地球化学进行的研究确定了破火山口前岩浆作用的两个主要脉冲中的复杂岩浆过程，包括同化-分形结晶（AFC）和岩浆混合。在这里，我们将火山口前火山岩的岩石学研究扩展到微观领域，并利用矿物化学和纹理信息完善岩浆演化模型。结果表明，随着火山田的发展，矿物种群的多样性不断增加。一系列斜长石质地（例如 筛分的核和轮缘表明几乎所有火山灰前岩浆中的不平衡状况，从安山岩到流纹岩，反映斜长石溶解和再生长。粗筛或溶解的斜长石核心可以通过从深度融化，同化，储藏和均质化（MASH）区向上迁移过程中通过水不饱和减压进行吸收来解释。筛分后缘的斜长石晶体在以达铁矿为主的岩浆作用中几乎无处不在（La Grulla高原安山岩和辉绿岩在〜8-7 Ma处喷发，以及Tschicoma形成安山岩，安辉石和流纹岩在〜5-2 Ma处），反映了由岩浆混合。这些斜长石晶体通常具有贫贫核，其化学性质不同于富贫边缘。f O ₂基团和高Al，高温，低f O ₂组。由斜长石核心和边缘组成计算熔体中Sr，Ba，La和Ce的浓度表明，这些化学变化主要是由岩浆混合产生的。在中下部地壳的MASH区域中，发现了多个可能由AFC过程连接的镁铁质端构件。早期安山岩为主的岩浆系统（Paliza峡谷安山岩，钠铁矿和流纹岩，10-7 Ma）中的硅质成分为同期的早期流纹岩（Canovas Canyon Rhyolite）。推测浅层地壳中的硅质糊状带既是硅铁矿占主导地位的岩浆系统中的硅质端粒，又是晚期低温流纹岩的来源（Bearhead Rhyolite，7-6 Ma）。镁铁质熔体充填硅质糊状物可以解释观察到的镁铁质岩浆中矿物不平衡质地和成分的多样性。总体而言，火山口前的JMVF岩浆系统随着时间的推移逐渐趋向凉爽和氧化程度更高，表明局部地壳逐渐热成熟，形成了跨壳岩浆系统，最终形成了“超大规模”硅质火山作用。早期岩浆作用对地壳进行热量和质量调节可能在其他长寿命火山岩田中很常见。