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

墨西哥西部西马德雷山脉（Sierra Madre Occidental，简称SMO）是地球上变形最小的硅质大火成岩省（LIP），也是最大的新生代火成岩省。SMO火山岩还拥有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%被绘制出来或确定了年代，这给我们对硅质lip的总体认识留下了一个巨大的空白，尤其是盆地和山脉省的演化。SMO可能是地球上研究硅质唇部成因的最佳野外区域，因为它在各种构造水平上都很好地暴露出来，这是由于火山前后的伸展，然后是深部侵蚀解剖。PI和她的合作者在硅质LIP岩石（上火山复合体，UVC）下发现了一个广泛的、以前未绘制和未确定年代的安山岩弧岩（“下火山复合体”，LVC）区域。LVC可能是烟灰爆发的重要前兆，并提供了爆发（UVC）开始时存在的应变状态和条件的记录。新的野外和地质年代学数据表明，扩展开始于爆发之前，而不是像以前的工作者提出的那样在爆发期间或之后。在前安山岩弧中形成的伸展构造在爆发期间部分被重新激活，至少在一定程度上控制了爆发期间裂隙状硅质侵入物和喷口的分布。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峰值岩浆通量有关，例如浪涌状的火成岩，具有大规模的交错层状岩屑角砾岩，以及更广泛的独特矿床，例如巨型浪涌矿床。拟议的工作将解决以下问题：伴随从“正常”、稳态弧系统到“爆发”弧系统过渡的火山和结构事件的性质和时间是什么？爆发开始时的应变状态是怎样的？前兆的延伸是否必要？在稳态弧相（LVC）期间形成的结构如何与UVC中喷口和侵入体的位置、大小和几何形状相关，并可能控制它们？这个非常大的超级火山场的喷发过程、火山成分、面积范围、活动持续时间和岩浆产量与其他较小但研究得更好的超级火山场相比如何？在拟建的油田区域，浅成热液型贵金属矿床的火山和构造背景是什么？由于该研究领域的规模和复杂性，以及实地研究的前沿性质，要求部分支持一名博士后研究员和两名博士生。实地工作将包括绘制地质图，覆盖8000平方公里，具有不同的比例尺，测量剖面和横截面。将进行模态和结构分析，以选择适合定年和地球化学的样品。LA-ICPMS锆石年代学（用于硅质火山岩）将由UNAM的Luigia Solari和Luca Ferrari完成，Ar/Ar年代学（用于基性-中间火山岩，以及广泛的未改变的火成岩的对比）将由伯克利年代学中心的Paul Renne完成。将收集浮石样本（在烟灰岩上）和整块岩石样本（在其他岩石类型上）用于勘查地球化学研究。知识价值-这项工作将有助于：(1)了解SMO的年龄、地层学和结构；(2)认识硅质lip的形成以及伸展构造与岩浆作用的关系；(3)在一个以矿床闻名的地区，了解lip与矿化的关系。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，包括在每个研究所各有一名女教授。PI积极招收女学生，而且很有可能会有一名女博士（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.