Steady State to Flare-Up ARC Magmatism in the Largest Cenozoic Silicic Igneous Province on Earth: The Sierra Madre Occidental (MEXICO)

地球上最大的新生代硅质火成岩省的 ARC 岩浆活动从稳定状态到爆发：西马德雷山脉（墨西哥）

• 批准号: 1019559
• 负责人: Cathy Busby
• 金额: $ 22.6万
• 依托单位: University of California-Santa Barbara
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-06-01 至 2014-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1019559&HistoricalAwards=false
• 关键词: Steady; State; Flare; Up; ARC

Steady state to flare-up ARC Magmatism in the largest Cenozoic silicic iIgneous province on Earth: The Sierra Madre Occidental (MEXICO)(project jointly supported by EAR/Petrology & Geochemistry and OISE/Americas)The Sierra Madre Occidental (SMO) of western Mexico is the least deformed silicic large igneous province (LIP) on Earth, as well as the largest Cenozoic one. The SMO volcanic rocks also host one of the largest epithermal precious-metal deposits in the world. Yet less than 10% of it has been mapped or dated, leaving a big hole in our knowledge about silicic LIPs in general, and the evolution of the Basin and Range province in particular. The SMO may be the best field area on Earth for studying the origin of silicic LIPs, because it is very well exposed at a variety of structural levels, due to syn- to post-volcanic extension, followed by deep erosional dissection. The PI and her collaborators have discovered an extensive, previously unmapped and undated area of andesite arc rocks ("Lower Volcanic Complex", LVC) beneath the silicic LIP rocks (Upper Volcanic Complex", UVC). The LVC may represent an essential precursor to the ignimbrite flareup, and provides a record of the strain regime and conditions extant at the beginning of the flareup (UVC). New field and geochronological data are presented to show that extension began immediately before the flareup, rather than during or after it as proposed by previous workers. The extensional structures that formed in the precursor andesite arc were partially reactivated during the flareup, and at least in part appear to have controlled the distributions of fissure-like silicic intrusions and vents during the flareup. The UVC appears to contain some unusual vent and near-vent features that may be related to very high peak magma fluxes, such as surge-like ignimbrites, with large-scale cross-bedded lithic lag breccias, as well as more widespread distinctive deposits, such as mega-surge deposits.The proposed work will address the following questions: What is the nature and timing of volcanological and structural events that accompany the transition from a "normal", steady-state arc system to a "flare-up" arc system? What is the strain regime present at the start of flareup, and is precursory extension essential? How do structures that developed during the steady-state arc phase (LVC) correlate with, and possibly control, the location, size, and geometry of vents and intrusions in the UVC? How do eruptive processes, volcanic compositions, areal extent, duration of activity, and magmatic production rates in this very large supervolcano field compare with with other, smaller but much better-studied supervolcano fields? What are the volcanic and structural settings of epithermal precious metal deposits in the proposed field area?Due to the great size and complexity of the area, as well as the frontier nature of the field research, partial support is requested for a postdoctoral researcher and for two PhD students. Fieldwork will include construction of geologic maps that cover 8,000 km2, at a variety of scales, with measured sections and cross sections. Modal and textural analysis will be done to select samples suitable for dating and geochemistry. LA-ICPMS zircon geochronology (for silicic volcanic rocks) will be done by Luigia Solari and Luca Ferrari at UNAM, and Ar/Ar geochronology (on mafic- to intermediate volcanic rocks, and for correlation of widespread unaltered ignimbrites) will be done by Paul Renne at the Berkeley Geochronology Center. Pumice samples (on ignimbrites) and whole rock samples (on other rock types) will be collected for reconnaisance geochemical studies.Intellectual merit - This work will contribute significantly to: (1) understanding the age, stratigraphy and structure of the SMO; (2) understanding the formation of silicic LIPs, as well as the relationships between extensional tectonics and magmatism; and (3) understanding the relationship between LIPs and mineralization, in a region famous for its mineral deposits.Broader impact - The work will strengthen collaborations between the University of California and the National Autonomous University of Mexico, including one female professor at each insitute. The PI has aggressively recruited female students, and one of the PhD students will likely be female (TBD). The research will also benefit the precious metal exploration industry in Mexico.

地球上最大的新生代硅质火成岩省的 ARC 岩浆活动处于稳定状态：西马德雷山脉（墨西哥）（由 EAR/岩石学与地球化学和 OISE/美洲联合支持的项目）墨西哥西部的西马德雷山脉 (SMO) 是地球上变形最小的硅质大型火成岩省 (LIP)，也是地球上最大的硅质大型火成岩省 (LIP)。 新生代之一。 SMO 火山岩还蕴藏着世界上最大的浅成热液贵金属矿床之一。然而，其中只有不到 10% 被绘制或测年，这给我们关于硅质 LIP 的总体知识，特别是盆地和山脉省的演化的知识留下了一个大漏洞。 SMO 可能是地球上研究硅质 LIP 起源的最佳野外区域，因为由于同火山后延伸以及随后的深层侵蚀剥离，它在各种结构水平上都很好地暴露。 PI 和她的合作者在硅质 LIP 岩石（上火山杂岩”，UVC）下方发现了一个广泛的、以前未绘制地图且未标明日期的安山岩弧岩（“下火山杂岩”，LVC）区域。LVC 可能代表了火凝灰岩爆发的重要前兆，并提供了爆发 (UVC) 开始时现存的应变状态和条件的记录。新领域和 所提供的地质年代学数据表明，延伸是在爆发之前立即开始的，而不是像以前的工作人员所建议的那样，在爆发期间或之后开始。在前体安山岩弧中形成的伸展结构在爆发期间部分重新激活，并且至少部分地在爆发期间控制了裂隙状硅质侵入体和喷口的分布。 UVC 似乎包含一些不寻常的通风口和近通风口特征 这可能与非常高的峰值岩浆通量有关，例如具有大规模交错层状岩屑滞后角砾岩的涌状凝灰岩，以及更广泛的独特沉积物，例如巨型涌动沉积物。拟议的工作将解决以下问题：伴随从“正常”稳态弧系统向“爆发”过渡的火山和结构事件的性质和时间是什么？ 电弧系统？爆发开始时存在什么应变状态，前期延伸是否重要？稳态电弧阶段 (LVC) 期间形成的结构如何与 UVC 中的通风口和侵入物的位置、尺寸和几何形状相关联，并可能控制这些位置、尺寸和几何形状？这个非常大的超级火山区的喷发过程、火山成分、面积范围、活动持续时间和岩浆生产率如何 与其他较小但研究更深入的超级火山场相比？拟建矿区的浅成热液贵金属矿床的火山和构造环境如何？由于该地区规模庞大、复杂，以及该矿区研究的前沿性，需要为一名博士后研究员和两名博士生提供部分支持。实地工作将包括绘制覆盖 8,000 平方公里、不同地点的地质图 刻度，带有测量的截面和横截面。 将进行模态和结构分析，以选择适合年代测定和地球化学的样品。 LA-ICPMS 锆石地质年代学（针对硅质火山岩）将由墨西哥国立自治大学的 Luigia Solari 和 Luca Ferrari 完成，Ar/Ar 地质年代学（针对镁铁质到中质火山岩，并用于关联广泛分布的火山岩）。 未改变的熔凝灰岩）将由伯克利地质年代学中心的 Paul Renne 完成。将收集浮石样品（关于火凝结岩）和整个岩石样品（关于其他岩石类型）用于勘探地球化学研究。智力价值 - 这项工作将显着贡献于：（1）了解 SMO 的年龄、地层学和结构； (2)了解硅质LIPs的形成及其相互关系 伸展构造与岩浆作用之间； (3) 了解 LIP 与矿化之间的关系，该地区以矿藏闻名。 更广泛的影响 - 这项工作将加强加州大学和墨西哥国立自治大学之间的合作，包括每个研究所各一名女教授。 PI 积极招收女学生，其中一名博士生很可能是女性（待定）。该研究也将有利于贵金属 墨西哥的勘探业。