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大火成岩省：西方马德雷山脉(墨西哥)(由EAR/Petroology&Amp；地球化学和OISE/America联合支持的项目)墨西哥西部的马德雷山脉(SMO)是地球上变形最小的硅质大火成岩省(LIP)，也是最大的新生代大火成岩省(LIP)。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，只有不到10%的硅质嘴唇被绘制或定年，这给我们对硅质嘴唇，特别是盆地和山脉省的演化的了解留下了一个很大的漏洞。SMO可能是地球上研究硅质嘴唇起源的最佳野外区域，因为它在不同的构造水平上暴露得很好，这是由于同生到后火山伸展，随后是深刻的侵蚀解剖。Pi和她的合作者在硅质唇岩(上火山杂岩，UVC)下面发现了一片先前未绘制地图和未确定日期的安山岩弧岩区域(“下火山杂岩”，LVC)。LVC可能代表了褐沸石爆发的重要前兆，并提供了在爆发开始时存在的应变状态和条件的记录(UVC)。提供了新的现场和地质年代学数据，以表明伸展是在地震爆发之前立即开始的，而不是像以前的工作人员所说的那样在爆发期间或之后开始。前驱安山岩弧中形成的伸展构造在耀斑过程中被部分重新激活，至少部分似乎控制了耀斑过程中裂隙状硅质侵入体和喷口的分布。UVC似乎包含了一些不同寻常的喷口和接近喷口的特征，这些特征可能与非常高的峰值岩浆流量有关，例如具有大规模交错层状岩屑滞后角砾岩的类似涌浪的原始岩，以及更广泛的独特矿床，如巨型涌浪沉积。拟议的工作将解决以下问题：伴随着从“正常”、稳定状态的弧系向“爆发”弧系的转变，火山和构造事件的性质和时间是什么？暴发开始时的应变状态是什么，前兆延伸是必要的吗？在稳态弧段(LVC)期间形成的构造如何与UVC中喷口和侵入的位置、大小和几何形状相关，并可能控制这些位置、尺寸和几何形状？与其他规模较小但研究得更好的超级火山场相比，这个超大型火山场的喷发过程、火山成分、面积范围、活动持续时间和岩浆产出率如何？拟建矿区浅成热液贵金属矿床的火山和构造环境是什么？由于该地区的规模和复杂性，以及野外研究的前沿性，需要为一名博士后研究人员和两名博士生提供部分支持。实地工作将包括绘制覆盖8000平方公里的各种比例尺的地质图，其中包括测量的剖面和横断面。将进行模式和纹理分析，以选择适合测年和地球化学的样品。La-ICPMS锆石年代学(硅质火山岩年代学)将由UNAM的Luigia Solari和Luca Ferrari完成，Ar/Ar年代学(镁铁质至中间火山岩，以及广泛的未改变的原生橄榄岩对比)将由伯克利地质年代学中心的Paul Renne完成。我们将收集浮石样品(在褐沸石上)和全岩样品(在其他岩石类型上)，用于侦察地球化学研究。智力价值-这项工作将显著有助于：(1)了解SMO的年龄、地层和结构；(2)了解硅质嘴唇的形成，以及伸展构造和岩浆作用之间的关系；以及(3)了解嘴唇与矿化之间的关系，在一个以矿藏闻名的地区。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，包括两所大学的一名女教授。PI积极招收女学生，其中一名博士生很可能是女性(待定)。这项研究还将使墨西哥的贵金属勘探行业受益。

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.