GROWTH AND MODIFICATION OF NEOARCHEAN-PALEOPROTEROZOIC CRUST: A CASE STUDY OF THE NORTHWEST WYOMING PROVINCE

Tsai-Wei Chen

doi:10.7273/000007339

The temporal distribution of Earth’s crustal rocks exhibits distinctive age peaks, representing periods of significant crustal growth. Among these, the Neoarchean (2.7-2.5 Ga) and Paleoproterozoic (2.0-1.8 Ga) stand out as major periods of crustal formation in Earth’s early history. Investigating these geological records is crucial for understanding the evolution of continental crust and the underlying tectonic processes that shaped our planet. The northwest Wyoming Province in Montana, USA, serves as an ideal natural laboratory to examine the ancient crustal processes. This study employs an integrated approach utilizing multiple geochronometers and isotope systems (U-Pb, Lu-Hf, and Sm-Nd) to investigate a suite of metaigneous rocks from this region. Zircon U-Pb geochronology identifies three discrete crust-forming periods at ~2.7, 2.5- 2.4, and 1.8-1.7 Ga, revealing episodic growth of continental crust in the northwest Wyoming Province. Hafnium isotope compositions in zircon indicate that Neoarchean magmatism involved significant juvenile additions from the mantle, while Paleoproterozoic crust formation was characterized by extensive reworking of existing crust with complex mixing of juvenile input. Garnet Lu-Hf and Sm-Nd geochronology identifies three distinct Paleoproterozoic tectonothermal events: the Tendoy orogeny (2.5-2.4 Ga), a rifting phase (2.2-2.0 Ga), and the Big Sky orogeny (1.8-1.7 Ga). Analysis of fourteen garnetiferous samples demonstrates a spatial and temporal trend of decreasing metamorphic grade from the northwestern boundary toward southeast from 1.8 to 1.7 Ga, indicating southeastward propagation of Big Sky orogenic activity. Additional isotopic analyses of apatite and titanite Sm-Nd isotopes, together with monazite and titanite U-Pb ages, reveal Sm-Nd isotopic re-equilibration between 1.82 and 1.68 Ga, coinciding with the Big Sky orogeny. Notably, accessory minerals demonstrate evidence of widespread Sm-Nd re-equilibration, while zircon in the same samples preserves its primary Hf isotopic signatures, indicating the differential response of isotope systems to high-grade metamorphism. This highlights the importance of using multiple chronometers to unravel complex geological histories. The integration of multiple geochronometers and isotope systems provides a comprehensive picture of crustal formation and modification in the northwest Wyoming Province. Results in this study reveal a complex history of magmatism, metamorphism, and subsequent cooling spanning hundreds of millions of years, offering new insights into the geological evolution of this region.

GROWTH AND MODIFICATION OF NEOARCHEAN-PALEOPROTEROZOIC CRUST: A CASE STUDY OF THE NORTHWEST WYOMING PROVINCE

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