HEAT SOURCES DURING PALEOPROTEROZOIC MOUNTAIN BUILDING, PENOKEAN OROGEN, MICHIGAN

Igneous and metamorphic features thought to be uniquely associated with plate tectonics, including ophiolites, blueschists and ultrahigh-pressure metamorphism, can only confidently be traced back to the latest 0.8 billion years of Earth history. However, rocks of the Paleoproterozoic Era, especially the interval 2.1–1.7 Ga, host numerous examples of potential proxies for modern-style plate tectonics. Among these proxies for plate tectonics are widespread examples of orogenic regional metamorphism. An exceptionally well preserved example of a chlorite–biotite–garnet–staurolite–sillimanite isograd sequence (the Peavy Node) occurs in the Peavy Pond Area of the Upper Peninsula of Michigan. This regional metamorphism developed during the c. 1.8 Ga Penokean Orogen, and is spatially associated with mafic intrusions (the Peavy Pond Complex). Various thermal models have been proposed for development of orogenic regional metamorphism. Understanding the source of heat for the Peavy Node regional metamorphism—e.g., heat advection associated with a large-scale network of magmatic bodies v. heating as an internal response to crustal thickening—is key to understanding the architecture and dynamics of the Penokean Orogen.

New work on the Peavy Node includes field mapping, microstructural analysis and U/Pb geochronology on zircons from the metasediments and the Peavy Pond Complex. We report new constraints on the temporal and spatial association of the metamorphism and magmatism, with implications for mid-crustal heat sources during the Penokean Orogeny.