Paper No. 5
Presentation Time: 2:05 PM


MUELLER, Paul A., Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, MOGK, D.W., Dept. Earth Sciences, Montana State Univ, Bozeman, MT 59717, HENRY, D.J., Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, WOODEN, J.L., Stanford University, Stanford, CA 94305, STAFFENBERG, Jennifer, School of Earth and Environmental Sciences, Washington State University, Box 642812, Pullman, WA 99164 and VYVERBERG, Karen L., Department of Geological Sciences, University of Florida, Gainesville, FL 32606,

U-Pb ages, Lu-Hf systematics, and trace element contents of zircons from Archean metasedimentary rocks in the northern Wyoming Province (NWP) show three distinct episodes of crustal growth superimposed on a record of continuous magmatism and crustal maturation over a period of >1 Ga. This record shows variable extents of crustal recycling and juvenile (depleted mantle) input over this period. The record begins with extraction of material from a juvenile or primitive mantle source ~4.0 Ga and suggests an evolving crust with recycling dominating over juvenile additions for ~ 600 Ma. The interval from 3.2-3.4 Ga is one of prolific crustal growth that produced sufficient numbers of zircons to dominate the record in Archean metasedimentary rocks. Rocks of this age (3.2-3.4 Ga) are dominantly of the TTG suite. They exhibit geochemical traits (e.g. HFSE depletion, enriched LREE, etc.) indicative of generation by subduction. Isotopic compositions (e.g., Sm-Nd model ages 3.8-4.1 Ga) in these rocks show evidence of interaction with an even older crust, likely represented in the 3.5 to 4.0 Ga detrital zircons preserved in the intercalated, quartz-rich metasedimentary rocks. After a period of ~500 Ma, a third episode of subduction-driven magmatism and crustal growth began ~2.8 Ga, which was short lived (<100 Ma), but extensive (>400,000 sq. km). Prior to, or coeval with, this third crust-forming event, metaplutonic and supracrustal rocks were tectonically mixed with the 3.2-3.4 TTG rocks, and were metamorphosed at 6-8 Kbar and 750-800oC (kyanite is locally present); thick, stable continental crust was clearly established by this time. TTG rocks produced during this third generation of crustal growth (2.8-2.9 Ga) now comprise the dominant lithologies in the Beartooth-Bighorn magmatic zone of the northern Wyoming Province. These TTG rocks have enriched initial isotopic signatures in the U-Pb, Rb-Sr, Sm-Nd, and Lu-Hf systems and non-radiogenic trace element signatures that suggest they also were produced in an environment similar to a modern continental margin arc. The enriched initial isotopic compositions in these systems argue strongly that the late Mesoarchean (~2.8 Ga) arc was built on pre-existing, thick, continental crust.