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

Tracy9903140在这个合作项目中，我们将重点研究从纽约州韦斯特切斯特县到康涅狄格州布朗森山地体西缘的Taconic(奥陶纪-志留纪)和Acadian(泥盆纪)变质事件。在西部地区(NY)，塔可岩似乎不受阿卡迪期热效应的干扰，而阿卡迪期是东部(CT)唯一的热事件，几乎没有证据表明有任何阿卡迪期前的热史。在这两者之间，高度复杂的结构和组构表明，在较早变质的塔可岩上可能存在不同强度的阿卡迪亚叠加。或者，表观重叠带中的岩石可能不包含早期(塔肯)事件的证据，事实上，当塔肯变质作用发生时，该地点可能根本不存在。该地区的两次造山事件的年龄范围约为100 Ma，从最早的塔肯活动(约475 Ma)到峰值(约450 Ma)，到阿卡迪期的峰值(约390 Ma)，然后是阿卡迪期后的快速减压和去顶板(375~355 Ma)。我们建议利用岩石学、地质年代学和热年代学技术来检验这样的假设，即实际上存在一个与中角闪岩级别的塔肯变质岩重叠的带，并在约60 Ma后叠加了另一类似级别的(阿卡迪期)变质事件。该项目将包括岩石学、温度气压、U-Pb年代学和40Ar/39Ar热年代学分析的全面整合，以推断(1)塔克期和阿卡迪期变质作用的“精细结构”，(2)早期塔克期造山带上阿卡迪期叠置的存在、性质和程度，以及(3)地体的最终冷却和剥落历史。在弗吉尼亚理工学院(Tracy的监督)主要完成的工作中，将从关键样品中提取详细的变质反应历史，并将以尽可能高的精度做出定量的P-T估计。将对带状矿物的主要元素和微量元素进行成分成像，以用于重建变质反应历史，并作为选择适当材料进行放射性测年取样的指南。对纽约州立大学石溪分校精心挑选的变质斑岩矿物的U-Pb测年(汉森的监督)将为这些矿物的生长时间提供关键的制约因素。40Ar/39Ar地质年代学和热年代学将由W.E.Hmes使用各种激光提取Argon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较低品位(低于500摄氏度)部分的原生矿物生长年龄

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