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，用于揭示沿太古代怀俄明州西北部和东南部边缘复杂的前寒武纪地形的多变质作用细节。研究将集中在布拉克山（南达科他州）和狄龙地块（蒙大拿州西南部）壮观的结晶岩石上，在那里，不确定的前寒武纪时代的叠印变形结构和矿物组合广泛存在。一项为期三年的野外工作、岩石学、热气压学和地质年代学的综合计划将在这些地区进行，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)变形与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的单矿物地球时计来确定。一种新的方法是将Pb逐步浸出（PbSL）用于造岩变质矿物，然后进行207Pb-206Pb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/或独居石等高铀包裹体。另一种新方法涉及电子探针（EMP）分析独居石（一种保持铅的附件）- Th， U， Pb和其他主要元素。电磁脉冲分析提供了一种可行的化学手段，用于原位和高空间分辨率的独居石生长世代的侦察测年。在区域上，本项目获得的结果将为在超大陆Laurentia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，这些结果将为研究前寒武纪（板块）构造过程提供一种新的途径。

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.