FROM DIABASE DIKES AND RB-SR TO ION PROBES AND HF ISOTOPES: A TRIBUTE TO P.A. MUELLER AND 50 YEARS OF COLLABORATIVE RESEARCH IN THE WYOMING PROVINCE (Invited Presentation)

A hallmark of Paul Mueller’s long career in geology are the contributions he and collaborators have made to the Precambrian crustal evolution of the Wyoming Province. These contributions include over 40 papers and numerous abstracts and involve dozens of colleagues and students. This research effort has spanned a dynamic period in the earth sciences marked by dramatic changes in how the geologic history of the earth was viewed and major advances in the techniques of analytical geochemistry. This research effort began in the late 1960’s with examination of the geochemistry and geochronology (K-Ar and Rb-Sr) of the Precambrian diabase dikes of the Beartooth Mtns. and southwestern Montana as a monitor of mantle evolution. A long term effort to understand the Archean geology of southwestern Montana was started in 1977 stimulated by a request to host a workshop for the U.S. Archean Geochemistry Working Group in the Beartooth Mtns. in 1979. Numerous papers published in the 1980’s and early 1990’s established that the northern Wyoming province was divisible into the Beartooth-Bighorn Magmatic (BBMT) and the Montana Metasedimentary (MMT) terranes which are separated by a major late Archean tectonic boundary. Both terranes contain 3.2-3.5 Ga intrusive rocks and metasediments dominated by abundant 3.2-3-5 Ga detrital zircons and sparse detrital zircons as old as 4.0 Ga. The BBMT is characterized by a wide variety of 2.8 Ga intrusive rocks that represent a major period of subduction related magmatism and the establishment of an enriched mantle keel. Hf isotopic data indicate that the magmatic-tectonic environment switched from plume driven to subduction as early as 3.5-3.6 Ga. Crust older than 2.8 Ga represents a high U/Pb reservoir, but the 2.8 Ga magmatism established a major low U/Pb reservoir. Additional research on Mesozoic plutons and crustal xenoliths, the Paleoproterozoic reworking of the Great Fall tectonic zone and the Mesoproterozoic depositional history of the Belt Basin has expanded the regional crustal history. In summary this persistent research effort established the Archean crustal evolution of the northern Wyoming Province, made a significant contribution to the crustal evolution of North America and provided stimulating data and models about the processes involved in early crustal evolution.