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大火成岩省的稳定状态到爆发弧岩浆活动：西马德雷西方（墨西哥）（项目由地球物理/岩石地球化学和OISE/美洲联合支持）西墨西哥的西马德雷西方（SMO）是地球上变形最小的大火成岩省（LIP），也是最大的新生代火成岩省。SMO火山岩也是世界上最大的浅成热液贵金属矿床之一。然而，只有不到10%的地区被绘制成地图或确定了年代，这在我们对大裂谷的总体认识上留下了一个大洞，特别是盆地和山脉省的演变。SMO可能是地球上最好的领域领域研究的起源，因为它是非常好的暴露在各种结构水平，由于同后火山扩展，其次是深侵蚀解剖。PI和她的合作者发现了一个广泛的，以前未绘制地图和未注明日期的安山岩弧岩（“下火山杂岩”，LVC）下的PALIP岩石（上火山杂岩”，UVC）。LVC可能代表了熔结凝灰岩耀斑的基本前兆，并提供了应变机制和耀斑（UVC）开始时现存条件的记录。新的领域和地质年代学数据表明，扩展开始前立即耀斑，而不是在它之前的工人提出的。在前体安山岩弧中形成的伸展结构在耀斑期间部分地重新激活，并且至少部分地似乎控制了耀斑期间裂隙状侵入体和喷口的分布。UVC似乎包含一些不寻常的喷口和近喷口特征，这些特征可能与非常高的高峰岩浆通量有关，例如浪涌状熔结凝灰岩，具有大规模的交叉层岩屑滞后角砾岩，以及更广泛的独特矿床，例如巨型浪涌矿床。伴随着从“正常”、稳定状态的弧系统过渡到“突然爆发”的弧系统的火山和结构事件的性质和时间是什么？耀斑开始时的应变状态是什么？偶然的扩张是必要的吗？在稳定状态电弧阶段（LVC）期间形成的结构如何与UVC中的喷口和侵入体的位置、大小和几何形状相关并可能对其进行控制？如何喷发过程，火山成分，面积范围，活动持续时间和岩浆生产率在这个非常大的超级火山领域与其他相比，较小的，但更好地研究超级火山领域？在拟议的作业区，浅成低温热液贵金属矿床的火山和构造背景是什么？由于该地区的规模和复杂性，以及该领域研究的前沿性质，部分支持要求博士后研究人员和两名博士生。实地工作将包括绘制各种比例尺的8 000平方公里的地质图，并附有测量的剖面和横截面。 将进行模态和结构分析，以选择适合测年和地球化学的样品。LA-ICPMS锆石地质年代学（用于灰岩火山岩）将由UNAM的Luigia Solari和Luca Ferrari完成，Ar/Ar地质年代学（用于镁铁质至中质火山岩，以及用于广泛分布的未蚀变熔结凝灰岩的相关性）将由伯克利地质年代学中心的Paul Renne完成。浮石样品（关于熔结凝灰岩）和全岩样品学术价值-这项工作将有助于：（1）了解SMO的年龄、地层和结构;（2）了解伸展构造和岩浆作用之间的关系;（3）了解SMO-LIPs的形成;（4）了解SMO-LIPs的形成。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，包括在每个学院都有一名女教授。PI积极招募女学生，其中一名博士生可能是女性（待定）。这项研究也将有利于墨西哥的贵金属勘探行业。

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.