HEAT SOURCES FOR BARROVIAN-TYPE METAMORPHISM: WHAT COOKED THE ROCKS OF THE PALEOPROTEROZOIC PENOKEAN OROGEN, MICHIGAN (USA)?

The Michigamme Formation, a foredeep turbidite sequence deposited, deformed, and metamorphosed during the 1.9–1.8 Ga Penokean Orogeny, records one of the oldest known examples of Barrovian-type regional metamorphism. The regional metamorphism is manifested in the form of isograd sequences involving independent ‘nodes’ cored by sillimanite-grade rocks. A mafic intrusive complex known as the Peavy Pond Complex (PPC) is located at the center of one of the nodes (the Peavy Node), emphasizing a strong spatial association between magmatism and metamorphism.

Correlations among deformation, metamorphism and magmatism suggest a pre- to syn-metamorphic and broadly syn-tectonic timing of magmatism. Structural timing of inclusion trails in cores and rims of porphyroblasts suggests episodic growth of the minerals, reflecting punctuated metamorphism. New U-Pb geochronology data suggest: 1) the Michigamme Formation was deposited after c. 1850 Ma, and 2) the PPC was emplaced at c. 1830 Ma, synchronous to the timing of peak metamorphism from published monazites ages (c. 1828 Ma; Holm et al., 2007, Precambrian Res., 157, 106-126).

The Michigamme Formation was deposited, lithified, intruded by the PPC and regionally metamorphosed within a time window of less than 20 million years (1850–1830 Ma). The rapid, episodic regional metamorphism recorded in the Peavy Node cannot be explained by passive thermal incubation and heat accumulation during a protracted collisional history; instead, it may record transient heating associated with intrusion of the PPC. Such examples of short-duration regional metamorphism are common for the Phanerozoic. The similarities among the Penokean Orogen and Phanerozoic orogens suggest that modern style tectonics may have been operational during the Paleoproterozoic.