DECIPHERING THE ARCHEAN CARNEY LAKE COMPLEX AND ADJOINING ROCK UNITS IN MICHIGAN’S UPPER PENINSULA BY REMAPPING 60 YEARS LATER: APPROACH, EARLY IMPRESSIONS, AND NEXT STEPS

Much of the Precambrian terrane in Michigan’s Upper Peninsula was last mapped in the period 1950-1965 prior to acceptance of plate tectonics and subsequent advances in understanding related tectonic, magmatic, and metamorphic processes. A decline in base and ferrous metal mining in the region during the same period reduced interest in commercial or scientific investigations, but recent concern about critical mineral supply has rejuvenated such interest. The Archean Carney Lake Complex and Paleoproterozoic cover in Dickinson County, Michigan are similar to nearby granite-gneiss complexes surrounded by continental margin deposits in part older than and coeval with Penokean orogenesis (~1.85 Ga) along the margin of Laurentia. Publications up to the mid-1960s depict the Carney Lake Complex as mostly felsic gneiss, whose areal extent includes ~10% mafic inclusions and ~5% granodiorite and syenite dikes. A leading idea of that time was that outcrop relationships were explained by invasion of a mostly basaltic protolith by granitic magma resulting from crustal anatexis or differentiation of basaltic magma.

Ongoing mapping in this densely forested area, funded by the USGS Earth MRI program, is revealing a wider variety of rocks than previously reported, differences in metamorphic grade, and new structural relationships. Whereas the Carney Lake Complex had been interpreted to be 85% gneissic rocks by area, new mapping indicates that granitoid intrusions with little to no foliation are far more abundant than previously thought. An earlier classification of gneissic rocks is being revised to reflect textural and compositional attributes that better reflect their protoliths. In this regard, we suggest that many “gneissic” rocks in the area are lower grade meta-sedimentary rocks whose banding reflects original bedding and whose texture retains some sedimentary characteristics. Similarly, many previously designated meta-gabbro intrusions have little to no metamorphic character and instead are likely Mesoproterozoic (Keweenawan) in age. Other such intrusions are instead ultramafic rocks likely related to the Penokean Orogeny. Our early results suggest a more complex history with widespread late Archean to Mesoproterozoic magmatism and derivation of a significant volume of gneiss from a sedimentary protolith.