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

在这个合作项目中，我们将把研究重点放在从纽约州威彻斯特县到康涅狄格州布朗森山地体西部边缘的一个样带上的塔克尼克（奥陶-志留纪）和阿卡迪亚（泥盆纪）变质期。在西部地区（NY）， Taconic岩石似乎没有受到任何阿卡迪亚热作用的干扰，而阿卡迪亚热作用是东部地区（CT）唯一的热事件，很少或没有任何前阿卡迪亚热历史的证据。在这两者之间，高度复杂的结构和构造表明，在早变质的塔克onic岩石上可能有不同强度的阿卡迪亚套印。另一种可能是，表面重叠带的岩石可能不包含更早的（塔尼克）事件的证据，事实上，当塔尼克变质作用发生时，可能根本不存在于这个位置。这一地区的两次造山活动覆盖了大约100 Ma的时间范围，从最早的Taconic活动（约475 Ma）到高峰（约450 Ma），到阿卡迪亚高峰（390 Ma），然后是快速的后阿卡迪亚减压和拆顶（375 - 355 Ma）。我们建议使用岩石学、地质年代学和热年代学技术来验证这样的假设，即在60 Ma之后，存在一个由中等角闪岩品位的Taconic变质岩叠加而成的重叠带，并被另一个类似品位的阿卡迪亚变质事件所覆盖。该项目将涉及岩石学、热气压学、U-Pb地质年代学和40Ar/39Ar热年代学分析的全面整合，以推断(1)塔科尼克和阿卡迪亚变质作用的“精细结构”，(2)阿卡迪亚对早期塔科尼克造山带的叠印存在、性质和程度，以及(3)地体的最终冷却和拆顶历史。工作主要在弗吉尼亚理工大学完成（Tracy的监督），详细的变质反应历史将从关键样品中提取出来，定量的P-T估计将尽可能准确地进行。将对分带矿物进行主要元素和微量元素组成成像，用于重建变质反应历史，并作为选择适当材料样品进行放射性测年的指南。对纽约州立大学石溪分校精心挑选的变质卟啉母岩矿物进行U-Pb定年（汉森的监督），将为这些矿物的生长时间提供关键的约束条件。40Ar/39Ar地质和热年代学将由W.E. Hames利用各种激光提取氩气实验室（主要是麻省理工学院的实验室）来完成，以限制整个造山带的冷却历史和较低质量（低于500摄氏度）部分的主要矿物生长年龄

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