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

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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.