282-6 Barrovian metamorphic episodes in the Lake Superior Area, Michigan (USA): Insights into mid-crustal thermal evolution during Paleoproterozoic orogenesis

Session: Crustal Petrology, Part II

Presenting Author:

Supratik Roy

Authors:

Roy, Supratik¹, Holder, Robert M.², Ramezani, Jahandar³, Hames, Willis E. ⁴, Webb, Laura E. ⁵, Guice, George L. ⁶, George, Freya R. ⁷, Stokes, Martha Rebecca⁸, Brenner, Dana C.⁹, Nelson, Lyle L.¹⁰, Viete, Daniel R.¹¹

(1) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (2) Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, USA, (3) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MI, USA, (4) Department of Geosciences, Auburn University, Auburn, AL, USA, (5) Department of Geography and Geosciences, University of Vermont, Burlington, VT, USA, (6) Department of Physics, Astronomy and Geosciences, Towson University, Towson, MD, USA; Department of Mineral Sciences, Smithsonian National Museum of Natural History, Washington, D.C., USA, (7) School of Earth Sciences, University of Bristol, Bristol, United Kingdom; Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (8) Geology, Energy & Minerals Science Center, U.S. Geological Survey, Reston, VA, USA, (9) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (10) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA, (11) Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA,

Abstract:

Barrovian-type regional metamorphism—characterized by ordered appearance of chlorite, biotite, garnet, staurolite, kyanite, and sillimanite isograds—provides an excellent archive of past mid-crustal tectonothermal conditions. Although ubiquitous in Phanerozoic orogenic belts, Barrovian-type metamorphism is rarely documented from the Paleoproterozoic and older rock records. The Lake Superior area in Michigan, USA, preserves several distinct, concentrically arranged Paleoproterozoic Barrovian-type isograd sequences (‘nodes’) cored by sillimanite-grade rocks.

In this study, we investigate the timing and tempo of Barrovian-type metamorphism in the Peavy and Republic nodes of the Lake Superior area to better understand the mid-crustal tectonothermal histories during Paleoproterozoic orogenesis. Textural and petrologic relationships, along with zircon and monazite U–Pb geochronology, suggest that Barrovian-type metamorphism of the Peavy node was a short-lived event that occurred at c. 1830 Ma, with metasediments experiencing deposition, burial and peak metamorphic temperatures in < 12 Myr. In contrast, monazite U–Pb and whole-rock ⁴⁰Ar/³⁹Ar geochronology suggest that Barrovian-type metamorphism of the Republic node occurred significantly later, at c. 1745 Ma. Timescales of Mn diffusion in garnet and retention of detrital white mica radiogenic Ar components in biotite grade rocks that experienced peak temperatures of ~475 °C suggest a similarly short overall duration for the Republic node thermal event, but temporally distinct from that recorded at the Peavy node. 

This study identifies one of the oldest known examples of episodic Barrovian regional metamorphism, providing insights into tectonothermal processes active during the Paleoproterozoic. We compare these data with the archetypal example of Barrovian metamorphism in Scotland to assess similarities/differences in the drivers of orogenic heating in the Paleoproterozoic and Phanerozoic.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8607

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