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

DahlEAR -9909433 该项目提出了一种独特的地质测时方法，用于揭示太古代怀俄明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。 该研究将集中于布莱克山（南达科他州）和狄龙地块（蒙大拿州西南部）裸露的结晶岩，那里广泛分布着叠印的变形结构和不确定前寒武纪年龄的矿物组合。 将在这些地形中开展为期三年的综合性实地工作、岩石学、温压测量和地质年代学计划，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。 织物和组合的时间将使用 1990 年代后期开发和验证的新型单矿物地质计时仪来确定。 一种新方法涉及对造岩变质矿物进行 Pb 逐步浸出 (PbSL)，然后进行 207Pb-206Pb 同位素分析。 PbSL 为石榴石等低 U 矿物提供可靠的生长年龄，即使存在锆石和/或独居石等高 U 包裹体。 另一种新方法涉及对独居石（一种铅保留附件）进行电子微探针 (EMP) 分析，以分析 Th、U、Pb 和其他主要元素。 EMP 分析为独居石生长世代的勘察测年提供了一种可行的化学方法 - 原位且具有高空间分辨率。 从区域来看，该项目获得的结果将为了解劳伦大陆超大陆组装期间涉及怀俄明州边缘的前寒武纪大陆碰撞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.