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Rapid cooling of the Neoarchean Quetico metamorphic belt challenges the paradigm of long-lived, slow-cooling Archean metamorphism

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part I

Presenting Author:

Dr. Robert M. Holder

Authors:

Holder, Robert M.¹, Xu, Yiruo², Hames, Willis³

(1) Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA, (2) Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA, (3) Geosciences, Auburn University, Auburn, AL, USA,

Abstract:

Higher and more variable metamorphic cooling rates among Phanerozoic rocks, compared to Archean, have been interpreted to indicate secular change in the global system of (plate) tectonics. We present metamorphic cooling rates from the c. 2.7 Ga Quetico Subprovince, Superior Province, Canada, using both garnet diffusion chronometry and 40 Ar/ 39 Ar biotite thermochronology. In contrast to the paradigm of low cooling rates for Archean metamorphic rocks (1’s of °C/Ma), our results require cooling rates ≥ 20–50 °C/Ma, comparable to many Phanerozoic examples. We argue that apparent homogeneity of low cooling rates for Archean rocks reflects, in part, the limited range of metamorphic environments from which data have been compiled. This itself might be an indirect indication of secular change in tectonic processes, insofar as there is greater variety of metamorphic and structural environments preserved from Phanerozoic orogenic belts. However, more comprehensive studies of cooling rates from a larger and more representative variety of Archean metamorphic environments and structural settings—comparable to the diversity of available Phanerozoic data—are needed to rigorously test hypotheses of secular change in global tectonic processes.