METAMORPHIC SCALES ILLUMINATE PAST TECTONIC PROCESSES (Invited Presentation)

By utilizing a well-maintained compilation of the nature of metamorphism throughout Earth history, Mike Brown and colleagues have made key contributions to our understanding of the emergence and evolution of plate tectonics. Though the primary focus of these papers has been metamorphic pressure–time (P–T) conditions (and thermobaric ratio) through time, considerable attention has also been given to the shape of associated P–T paths (clockwise v. counterclockwise), the nature of metamorphic heating, and implications for tectonism. Two more parameters that are beginning to be compiled for rocks from throughout geological time are the time scales and length scales of regional metamorphism, which provide additional information on the nature of past tectonic processes.

Sub-crustal-scale (5–15 km-thick) chlorite–biotite–garnet(–chloritoid–staurolite–kyanite)–sillimanite isograd sequences are a near-ubiquitous feature of Phanerozoic collisional orogeny (often loosely called “Barrovian” metamorphism). In rocks that predate the widespread emergence of low T/P (e.g., blueschist-facies) metamorphism in the rock record at c. 850 Ma, such isograd sequences are almost exclusively found in 2.2–1.7 Ga terranes. This part of the Paleoproterozoic is also known for preserving anomalously low T/P metamorphic rocks. Might these decidedly modern-like metamorphic records signify an isolated period of modern-like tectonism at 2.2–1.7 Ga?

One such Paleoproterozoic isograd sequence, with similar dimensions to Phanerozoic examples, is found in the Penokean Orogeny of the Lake Superior area. We outline similarities among the Penokean metamorphism and Phanerozoic examples of orogenic metamorphism, assess potential tectonothermal scenarios (metamorphic drivers), and discuss some directions of future research in the Lake Superior area.