36 Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories

This session welcomes contributions  applying modern approaches in metamorphic petrology, geochronology, and structural geology to unravel the dynamics of convergent margin processes from the Appalachians and beyond. We encourage submission from Early Career participants.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

Timing and Rates of Orogenic Regional Metamorphism and Post-metamorphic Cooling in the Paleoproterozoic Lake Superior Region (USA)

Paper number: 36-11
Program: Technical Sessions
Time: 11:20 AM – 11:40 AM

Authors

Abstract

Regional metamorphism provides a fundamental record of orogenic processes, including of microcontinental and/or arc collisions in accretionary settings. Regional metamorphism is recorded in field gradients typified by sequential appearance of metamorphic minerals, or “isograds.” Rates of orogenic regional metamorphism determined for individual isograd sets provide key insights into the geodynamics of past tectonism. Recently, studies have suggested a secular increase in these rates from the Precambrian to the Phanerozoic. However, the temporal resolution of radiometric dating techniques commonly used to constrain metamorphic rates becomes progressively coarser with increasing age, limiting our ability to accurately resolve Precambrian values.

One of the oldest examples of regional metamorphism associated with accretionary orogenesis is centered around Republic, in the Upper Peninsula of Michigan, Lake Superior region (USA). The Republic regional metamorphic node preserves a concentrically arranged isograd sequence cored by sillimanite-grade rocks. Here, we present results on the timing and rate of regional metamorphism and post-metamorphic cooling in the Republic node. U–Pb monazite geochronology indicates that regional metamorphism occurred at ca. 1750 Ma. 40Ar/39Ar white mica geochronology from peak metamorphic grades, as well as 40Ar/39Ar whole-rock geochronology across the isograd sequence, suggests cooling through temperatures < 200–400 °C by ca. 1745 Ma. Additionally, diffusion length scales of Ar in white mica and Mn in garnet indicate that the metamorphic event centered at Republic was short-lived (<10 Myr). Together, these results document one of the oldest records of short-duration regional metamorphism.

We compare these data with Phanerozoic examples of regional metamorphism during accretionary orogenesis from the Appalachian–Caledonian belt to assess similarities in tectonic settings and drivers between the Paleoproterozoic and the Phanerozoic. Our findings suggest that crustal heating dynamics during accretionary orogenesis in the Paleoproterozoic may not have differed substantially from those in the Phanerozoic. We emphasize that more comprehensive studies of metamorphic heating and cooling rates from the Precambrian are essential for improving our understanding of the evolution of tectonic processes through time.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411277

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