Deciphering Polymetamorphic History of the Archean Wyoming Province Margins: An Application of New Mineral-Growth Geochronometers

该项目提出了一种独特的地质年代学方法，用于揭示太古代怀俄明州省西北和东南边缘复杂前寒武纪地形中多变质作用的细节。 研究将集中在黑山（南达科他州）和狄龙地块（蒙大拿州西南部）的结晶岩上，在那里，不确定的前寒武纪年龄的叠印变形组构和矿物组合广泛存在。 将在这些地区进行为期三年的野外工作、岩石学、温压学和地质年代学的综合计划，以确定：（1）变质作用的时间、持续时间、速率、强度、频率和热源;（2）变形和流体在空间和时间上的相互作用。 将使用1990年代后期开发和验证的新的单矿物地质年代计确定组构和组合的时间。 一种新的方法是对成岩变质矿物进行Pb分步淋滤（PbSL），然后进行207 Pb-206 Pb同位素分析。 PbSL为石榴石等低铀矿物提供了可靠的生长年龄，即使存在锆石和/或独居石等高铀夹杂物。 另一种新方法涉及独居石（一种铅保持剂）的电子探针（EMP）分析-Th，U，Pb和其他主要元素。电磁脉冲分析提供了一个可行的化学手段侦察测年独居石生长代-原位和高空间分辨率。 在区域上，在这个项目中获得的结果将提供新的见解的性质和时间的前寒武纪大陆碰撞（S），涉及怀俄明州边缘超大陆劳伦提斯组装。 在全球范围内，这些结果将展示一种新的方法来研究前寒武纪（板块）构造过程一般。

DahlEAR -9909433 This project advances a unique geochronometric approach for unravelling the details of polymetamorphism in complex Precambrian terrains along the northwestern and southeastern margins of the Archean Wyoming Province. The research will be focused on crystalline rocks magnificently exposed in the Black Hills (South Dakota) and the Dillon Block (southwestern Montana), where overprinted deformational fabrics and mineral assemblages of uncertain Precambrian age are widespread. A comprehensive three-year program of field work, petrology, thermobarometry, and geochronology will be undertaken in these terrains to establish: (1) the timing, duration, rate, intensity, frequency, and heat source(s) of metamorphism; and, (2) the interplay of deformation and fluids in space and time. Timing of fabrics and assemblages will be determined using new single-mineral geochronometers developed and validated during the late 1990's. One new method involves Pb stepwise leaching (PbSL) of rock-forming metamorphic minerals, followed by 207Pb-206Pb isotopic analysis. PbSL offers reliable growth ages for low-U minerals like garnet, even where high-U inclusions like zircon and/or monazite are present. The other new method involves electron microprobe (EMP) analysis of monazite (a Pb-retentive accessory) - for Th, U, Pb, and other major elements. EMP analysis offers a viable chemical means for reconnaissance dating of monazite growth generations - in situ and with high spatial resolution. Regionally, the results obtained in this project will provide new insights into the nature and timing of Precambrian continental collision(s) involving the Wyoming Province margins during the assembly of supercontinent Laurentia. Globally, these results will demonstrate a new approach for investigating Precambrian (plate-)tectonic processes in general.