Testing Models for Magma Transfer and Emplacement In 4-Dimensions:The Bergell Intrusion, N. Italy

知识价值。岩浆产生、搬运和侵位的时间尺度和体积有助于确定岩石圈的热、构造和地球化学结构。构造和岩石学数据，结合高精度地质年代学的最新进展，得出了不同的模型，这些模型要么涉及在短时间尺度上大量的岩浆运输和就位，要么涉及在数百万年里逐渐就位的小批岩浆。岩浆侵位的模拟受到以下因素的阻碍：a)完整的深部系统暴露有限；b)对岩浆侵位的时间尺度认识不完全。该提案概述了一个为期三年的项目，针对位于意大利北部阿尔卑斯山中部的约30 Ma Bergell入侵的两个挑战。选择该岩体的原因如下：[1]高阿尔卑斯地区的特殊暴露和起伏，[2]其年轻的年代允许非常高精度的地质年代学（单个锆石碎片约±10-20 ka），[3]其岩性范围从辉长岩到花岗闪长岩，具有典型的正分带，[4]和~12-15垂直km的地壳截面通过侵位后阿尔卑斯隆升和倾斜暴露出来。主要根据构造和野外资料，推断Bergell岩体侵位时间延长（约5 Ma），中地壳岩浆停留，然后是底液作用/膨胀进入上地壳。该模型与最近的中上部地壳岩体增量侵入模型形成鲜明对比，需要对具有良好场关系特征的样品进行年代学确证。本研究的目标是在详细的野外工作和高精度的地质年代学的基础上，建立一个强大的四维岩体建设模型，该模型涵盖了从馈线带到顶板的精心挑选的样本。本文所述的初步工作总结了Bergell的野外工作，并提出了一种新的分析方法，将锆石年代学与锆石碎屑的原位和散装矿物微量元素分析相结合。辅助矿物地球化学将有助于通过识别异种和反晶体来解释高精度的地质年代学，并用于详细说明潜在的岩浆分异趋势，从而确定AFC过程的时间尺度，并评估岩浆侵入保持封闭系统的程度。全岩主微量元素数据的整合将用于破译岩浆脉冲的大小和频率。最后，这些信息将用于建立岩浆侵位和演化的数值模型，及其对中上部地壳流变特征的影响。更广泛的影响。这项工作将培养一名博士生（Kyle Samperton）和至少两名大四研究生(Christine Chen ？在不同的工具，包括野外地质学，构造地质学，火成岩岩石学，并补充这些技能与经验，使用尖端的地球化学和同位素分析方法。这项工作需要在另一个实验室进行培训(约翰·科特尔教授？在加州大学圣巴巴拉分校的LA-ICPMS实验室)，并通过与凯瑟琳·安博士（布里斯托尔大学）合作，在数值技术方面取得了进展。该项目将为普林斯顿大学新建立的分析设施以及PI？美国国家科学基金会支持的地球时间计划的参与。该项目与瑞士多所大学联合研究Adamello岩基，该岩基位于拟建现场东南约100公里处。阿达梅洛岩基是研究岩浆过程的一个经典地点，但值得注意的是，它缺乏贝吉尔侵入所提供的穿过地壳的垂直分辨率。PI担任该项目的外部专家，并将于2013年夏季组织一次由瑞士博士生和合作者参加的联合实地专题讨论会，以分享两个项目的成果。

Intellectual Merit. The timescales and volumes of magma generation, transport, and emplacement help set the thermal, structural, and geochemical architecture of the lithosphere. Structural and petrologic data, in conjunction with recent advances in high-precision geochronology have led to contrasting models that involve either large volumes of magma transported and emplaced over short timescales or small batches of magma that are emplaced incrementally over many millions of years. Modelling of magma emplacement is hampered by : a) limited exposure of complete plutonic systems, and b) incomplete understanding of timescales timescales for magma emplacement. This proposal outlines a three year project targeting both challenges in the ca. 30 Ma Bergell intrusion, located in the Central Alps in N. Italy. This pluton was chosen for the following reasons: [1] extraordinary exposure and relief in the high Alps, [2] its young age permits very high-precision geochronology (ca. ±10-20 ka on single zircon fragments), [3] it exhibits a range in lithologies from gabbroic to granodioritic with archetypal normal zoning, and [4] an ~12-15 vertical km crustal cross section has been exposed through post-emplacement Alpine uplift and tilting. Based primarily on structural and field data, emplacement of the Bergell pluton is inferred to have been prolonged (ca. 5 Ma) with mid-crustal magma residence followed by diapirism/ballooning into the upper crust. This model is in stark contrast to recent incremental intrusion models for middle and upper crustal plutons and demands corroborating geochronology of samples with well-characterized field relations. The goal of this study is to construct a robust 4D model for pluton construction based on detailed field work and high-precision geochronology of carefully selected samples spanning the feeder zone to the roof. Initial work described in this proposal summarizes field work in the Bergell and presents a novel analytical approach combining zircon geochronology with in situ and bulk mineral trace element analyses on dated zircon fragments. Accessory mineral geochemistry will aid interpretations of high-precision geochronology by identifying xeno- and antecrysts and be used to detail potential magmatic differentiation trends, which place timescales on AFC processes and evaluate the degree to which magmatic intrusions remained closed systems. Integration of whole-rock major/trace element data will be used to decipher the size and frequency of magma pulses. Finally, this information will be used to develop numerical models of magma emplacement and evolution, and its implications for rheological characteristics of middle and upper crust. Broader Impacts. This work will train one PhD student (Kyle Samperton) and at least two senior Under graduates (Christine Chen ?13 in year 1) in a diverse set of tools including field geology, structural geology, and igneous petrology, and to supplement these skills with experience using cutting edge geochemical and isotopic analytical methods. This work involves training in one other laboratory (Prof. John Cottle?s LA-ICPMS lab at Univ. California Santa Barbara) and also in numerical techniques through collaboration with Dr. Catherine Annen (Univ. Bristol). The project will provide support for newly established analytical facilities at Princeton as well as the PI?s participation in the NSF-supported EARTHTIME initiative. The project links with a multi-university Swiss initiative to study the Adamello batholith, located ~100 km SE of the proposed field locale. The Adamello batholith is a classic locality for studies of magmatic processes but, significantly, lacks the vertical resolution through the crust provided by the Bergell intrusion. The PI has served as an external expert for that project and will organize a joint field symposia involving swiss PhD students and collaborators in summer 2013 in order to share the results of both projects.