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

DahlEAR-9909433这个项目提出了一种独特的地质计时方法来揭示怀俄明省太古宙西北和东南边缘复杂的前寒武纪地形中的多变质作用细节。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)宏伟暴露的结晶岩石上，那里普遍存在不确定前寒武纪时代的叠印变形组构和矿物组合。将在这些地区进行为期三年的野外工作、岩石学、温压测量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(S)；(2)变形和流体在空间和时间上的相互作用。组构和组合的时间将用上世纪90年代末S开发和验证的新的单矿物地质年代学来确定。一种新的方法是用成岩变质矿物的铅逐步浸出(PbSL)，然后进行207Pb206Pb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，即使在存在锆石和/或独居石等高U包裹体的地方也是如此。另一种新方法包括独居石(一种铅保持附件)的电子探针(EMP)分析-Th、U、Pb和其他常量元素。EMP分析为独居石生长世代的现场和高空间分辨率的勘测提供了一种可行的化学手段。在区域上，该项目所获得的结果将为在超大陆劳伦西亚组装期间涉及怀俄明省边缘的前寒武纪大陆碰撞(S)的性质和时间提供新的见解。在全球范围内，这些结果将展示一种研究前寒武纪(板块)构造过程的新方法。

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.