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首页> 外文期刊>Contributions to Mineralogy and Petrology >Building zoned ignimbrites by recycling silicic cumulates: insight from the 1,000 km~3 Carpenter Ridge Tuff, CO
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Building zoned ignimbrites by recycling silicic cumulates: insight from the 1,000 km~3 Carpenter Ridge Tuff, CO

机译：通过回收硅堆积物来构造带区的火成岩：科罗拉多州1000 km〜3 Carpenter Ridge Tuff的见识

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摘要

The ~ 1,000 km~3 Carpenter Ridge Tuff (CRT), erupted at 27.55 Ma during the mid-tertiary ignimbrite flare-up in the western USA, is among the largest known strongly zoned ash-flow tuffs. It consists primarily of densely welded crystal-poor rhyolite with a pronounced, highly evolved chemical signature (high Rb/Sr, low Ba, Zr, Eu), but thickly ponded intracaldera CRT is capped by a more crystal-rich, less silicic facies. In the outflow ignimbrite, this upper zone is denned mainly by densely welded crystal-rich juvenile clasts of trachydacite composition, with higher Fe-Ti oxide temperatures, and is characterized by extremely high Ba (to 7,500 ppm), Zr, Sr, and positive Eu anomalies. Rare mafic clasts (51-53 wt% SiO_2) with Ba contents to 4,000-5,000 ppm and positive Eu anomalies are also present. Much of the major and trace-element variations in the CRT juvenile clasts can be reproduced via in situ differentiation by interstitial melt extraction from a crystal-rich, upper-crustal mush zone, with the trachydacite, crystal-rich clasts representing the remobilized crystal cumulate left behind by the melt extraction process. Late recharge events, represented by the rare mafic clasts and high-Al amphiboles in some samples, mixed in with parts of the crystal cumulate and generated additional scatter in the whole-rock data. Recharge was important in thermally remobilizing the silicic crystal cumulate by partially melting the near-solidus phases, as supported by: (1) ubiquitous wormy/sieve textures and reverse zoning patterns in feldspars and biotites, (2) absence of quartz in this very silicic unit stored at depths of >4-5 km, and (3) heterogeneous melt compositions in the trachydacite fiamme and mafic clasts, particularly in Ba, indicating local enrichment of this element due mostly to sanidine and biotite melting. The injection of hot, juvenile magma into the upper-crustal cumulate also imparted the observed thermal gradient to the deposits and the mixing overprint that partly masks the in situ differentiation process. The CRT provides a particularly clear perspective on processes of in situ crystal-liquid separation into a lower crystal-rich zone and an upper eruptible cap, which appears common in incrementally built upper-crustal magma reservoirs of high-flux magmatic provinces.
机译：在美国西部的第三纪中层火成岩爆发期间，在27.55 Ma处爆发的〜1,000 km〜3 Carpenter Ridge凝灰岩（CRT）是已知的最大的强区灰流动凝灰岩。它主要由具有显着，高度演化的化学特征（高Rb / Sr，低Ba，Zr，Eu）的密集焊接的晶体差流纹岩组成，但浓密的火山口内CRT被晶体含量更高，硅含量较低的岩层所覆盖。在流出的火成岩中，该上部区域主要是由密晶的富含长晶石成分的富含晶体的少年熔结所界定的，具有较高的Fe-Ti氧化物温度，并且具有极高的Ba（至7,500 ppm），Zr，Sr和正异常。也存在钡含量为4,000-5,000 ppm的稀有镁铁矿屑（51-53 wt％SiO_2），并且存在正Eu异常。 CRT少年岩屑的大部分主要和微量元素变化可通过从富含晶体的上地壳糊状区的间隙熔体抽提来原位分化来再现，其中的菱镁矿，富含晶体的岩屑代表重新固定的晶体累积熔体提取过程留下的痕迹。后期补给事件，以某些样品中稀有的镁铁质碎屑和高铝闪石为代表，与部分晶体混合在一起，在整个岩石数据中产生了额外的散射。补给在通过部分融化近固相而热迁移硅质晶体方面很重要，这受以下因素支持：（1）长石和黑云母中普遍存在的蠕虫/筛子质地和反向分区模式，（2）在这种硅质含量很高的硅质中不存在石英单位储存在> 4-5 km的深度，以及（3）菱锰矿辉石和镁铁矿碎屑，特别是Ba中的异质熔体成分，表明该元素的局部富集主要是由于山梨酸和黑云母的熔化。将热的，青少年的岩浆注入到上地壳堆积物中，也使沉积物和混合的叠印物观测到了热梯度，混合的叠印物部分地掩盖了原位分化过程。 CRT提供了一个清晰的观点，将原位的液-液分离成富含下部晶体的区域和上部可喷出的顶盖，这在高通量岩浆省的渐增上地壳岩浆储层中很常见。

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