Collaborative Research: Documenting the Fine-Structure of Metamorphic Events and Ages in a Complex Polymetamorphic Orogen

合作研究：记录复杂多变质造山带的变质事件和时代的精细结构

• 批准号: 9903140
• 负责人: Robert Tracy
• 金额: $ 5.17万
• 依托单位: Virginia Polytechnic Institute and State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-09-01 至 2001-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9903140&HistoricalAwards=false
• 关键词: Collaborative; Research; Documenting; Fine; Structure

Tracy9903140In this collaborative project we will focus our research effort on the Taconic (Ordovician-Silurian) and Acadian (Devonian) metamorphic episodes in a transect from Westchester County, New York, to the western margin of the Bronson Hill terrane in Connecticut. In the western area (NY), the Taconic rocks appear to be undisturbed by any Acadian thermal effects, whereas the Acadian is the only thermal event in the east (CT), with little or no evidence of any pre-Acadian thermal history. In between, highly complex textures and fabrics suggest there may be a variably intense Acadian overprint on earlier-metamorphosed Taconic rocks. Alternatively, the rocks in the apparent overlap zone may contain no evidence for an earlier (Taconic) event, and in fact may not have been present at all in this location when the Taconic metamorphism occurred. The two orogenic events in this region cover a range in age of about 100 Ma, from earliest documented Taconic activity at about 475 Ma to a peak at about 450 Ma, to an Acadian peak at 390 Ma followed by rapid post-Acadian decompression and unroofing at 375 - 355 Ma. We propose to use petrologic, geochronologic and thermochronometric techniques to test the hypothesis that there is in fact an overlap zone with Taconic metamorphic rocks of middle amphibolite grade overprinted by another (Acadian) metamorphic event of similar grade about 60 Ma later. This project will involve thorough integration of petrologic, thermobarometric, U-Pb geochronologic and 40Ar/39Ar thermochronometric analyses to deduce (1) the "fine structure" of the Taconic and Acadian metamorphisms, (2) the presence, nature and extent of Acadian overprint on the earlier Taconic orogen and (3) the ultimate cooling and unroofing history of the terrane. In work primarily done at Virginia Tech (supervision of Tracy), detailed metamorphic reaction histories will be extracted from key samples and quantitative P-T estimates will be made with the maximum accuracy possible. Zoned minerals will be compositionally imaged for major and trace elements for use both in reconstruction of the metamorphic reaction history and also as a guide to selection of appropriate material to sample for radiometric dating. U-Pb dating of carefully chosen metamorphic porphyroblast minerals at SUNY-Stony Brook (supervision of Hanson) will provide key constraints on the timing of growth of these minerals. 40Ar/39Ar geo- and thermochronology will be done by W.E. Hames using various laser-extraction Argon laboratories (primarily that at MIT) to constrain the cooling histories for the entire orogenic belt and primary mineral growth ages for lower-grade (sub-500 degrees C) portions

Tracy 9903140在这个合作项目中，我们将集中研究从纽约韦斯特切斯特县到康涅狄格州布朗森山西缘的塔康尼克（奥陶纪-志留纪）和奥陶纪（泥盆纪）变质事件。 在西部地区（纽约州），塔康尼克岩石似乎没有受到任何西印度热效应的影响，而西印度热事件是东部地区（CT）唯一的热事件，几乎没有或没有任何前西印度热历史的证据。在这两者之间，高度复杂的纹理和组构表明，在早期变质的塔康尼克岩石上可能有一个强烈的黑云母叠印。 或者，在明显的重叠带的岩石可能不包含早期（塔康尼克）事件的证据，事实上可能根本没有在塔康尼克变质作用发生时，在这个位置。 该地区的两个造山事件的年龄范围约为100 Ma，从最早记录的约475 Ma的Taconic活动到约450 Ma的高峰，再到390 Ma的Escherichian高峰，随后是375 - 355 Ma的快速后Escherichian减压和去顶。 我们建议使用岩石学，地质年代学和热年代学技术来测试的假设，事实上有一个超覆带与塔康次变质岩石的中角闪岩级叠加的另一个类似的等级（Pastodian）变质事件约60 Ma后。 该项目将涉及岩石学、热压、U-Pb地质年代学和40 Ar/39 Ar热年代学分析的全面综合，以推断（1）塔康和奥陶系变质作用的“精细结构”，（2）奥陶系在早期塔康造山带上的存在、性质和范围，以及（3）奥陶系的最终冷却和去顶历史。 这项工作主要在弗吉尼亚理工大学完成（特雷西的监督），将从关键样品中提取详细的变质反应历史，并以最大的准确度进行定量的P-T估计。 带状矿物的主要和微量元素的成分将被成像，用于重建变质反应历史，并作为指导选择适当的材料进行放射性测年。 在纽约州立大学石溪分校（汉森的监督）精心挑选的变质斑晶矿物的U-Pb测年将提供这些矿物生长时间的关键限制。 40 Ar/39 Ar地质年代学和热年代学将由W.E. Hames使用各种激光提取氩实验室（主要是麻省理工学院的实验室）来限制整个造山带的冷却历史和低品位（低于500摄氏度）部分的原始矿物生长年龄