Detrital Zircon Records of Cordilleran Mountain Building

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This lecture applies detrital zircon geochronology to track the complicated erosion, transport, deposition, and recycling history of zircons in Cordilleran and Tertiary basins. Much Paleozoic and Mesozoic passive margin strata came from Laurentia’s E and SW margin. The sediments initially traveled westwards along the continental shelf in Mississippian time, reaching west to California and north to Montana. Following Late Jurassic initiation of Cordilleran mountain building, Mesozoic, Paleozoic, and pre-Cambrian (meta)-sedimentary rocks were recycled into the Cordilleran foreland basin unroofing sequence. Young detrital zircons deposited alongside recycled grains record periods of magmatism, including Late Cretaceous intrusion of the Boulder, Idaho, and Pioneer Batholiths. Four samples from Pioneer District placer deposits have ages consistent with derivation from the footwall of a low-angle extensional detachment in the Flint Creek Range. Models show that the placers came from a source area containing Pioneer Batholith and older sedimentary strata, including foreland basin rocks. Our interpretation is that the placer gold originated in a skarn, which is consistent with Gold fineness measurements. We present a tectonic model that links exhumation and concentration of the gold to extension of the Anaconda metamorphic core complex.

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Dr. Laskowski grew up in Clancy, Montana and involuntarily began his geology career near Butte at the age of 8 as a field assistant for his father, an exploration geologist for Newmont. He earned a BS in geology, then both MS and PhD from the University of Arizona. He is an assistant professor of Earth Sciences at MSU, and uses structural geology, geo-, and thermo-chronology to study continental tectonism. He currently works in Tibet and on core-complex extension in western Montana.