282 Crustal Petrology, Part II
This session explores various igneous and metamorphic processes that have shaped Earth’s crust. We welcome contributions from the micro- to macroscale, from the deep to shallow crust, and from field- to laboratory- to modeling-based.
Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB
Barrovian metamorphic episodes in the Lake Superior Area, Michigan (USA): Insights into mid-crustal thermal evolution during Paleoproterozoic orogenesis
Paper number: 282-7
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM
Authors
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Robert HolderUniversity of Michigan, Ann Arbor, MI
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Daniel VieteJohns Hopkins University, Baltimore, MD
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Jahandar RamezaniMassachusetts Institute of Technology, Cambridge, MI
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Willis HamesAuburn University, Auburn, AL
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Laura WebbUniversity of Vermont, Burlington, VT
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George GuiceTowson University, Towson, MD
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Freya GeorgeUniversity of Bristol, Bristol
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Martha StokesU.S. Geological Survey, Reston, VA
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Dana BrennerJohns Hopkins University, Baltimore, MD
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Lyle NelsonMassachusetts Institute of Technology, Cambridge, MA
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 Programs. Vol. 57, No. 6
doi: 10.1130/abs/2025AM-413305
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