TECTONOTHERMAL EVOLUTION OF THE SOUTHERN LAURENTIAN MARGIN FROM THE NEOARCHEAN TO THE PALEOPROTEROZOIC RECORDED IN THE WATERSMEET GNEISS DOME, MI, USA

The Watersmeet terrane, located in northern Michigan, USA, represents a critical record of Precambrian growth of the southern Laurentian margin. Existing tectonothermal models (Peterman et al., 1986) suggest suturing of the Archean Watersmeet terrane to the southern margin of the Superior Province at ~2.75–2.6 Ga, and polyphase metamorphism and deformation in the Paleoproterozoic beginning with the ~1.89–1.83 Ga Penokean Orogeny. This study integrates textures and petrologic context with in situ U-Th-Pb isotopic and trace-element LASS-ICPMS data of titanite and apatite to test and enhance existing P-T-t constraints on Neoarchean and Paleoproterozoic metamorphic events in the Watersmeet area.

Our samples come from an interlayered amphibolite and orthogneiss unit within the Watersmeet Gneiss Dome core. Three titanite sub-populations discriminated by lithology, composition, and texture record dates of ~2.64 Ga, ~1.84 Ga, ~1.78 Ga, while apatite records a date of ~1.80 Ga. The ~2.64 Ga titanite dates from the amphibolite unit appear to be a metamorphic counterpart to ~2.65 Ga U-Pb zircon dates that are interpreted to record granitoid magmatism. Neoarchean titanite dates are interpreted to record metamorphism associated with suturing to the Superior Province. These Neoarchean metamorphic titanite grains record higher apparent Zr-in-titanite temperatures than Paleoproterozoic titanite, suggesting peak temperatures of the Penokean Orogeny were not high enough to fully reset the U-Pb isotopic system in Archean titanite. Paleoproterozoic neocrystallized titanite and apatite dates expand on existing thermochronology for the Penokean Orogeny, supporting tectonic models of peak amphibolite-facies P-T conditions at ~1.83 Ga, orogenic cooling to below 550 ± 50°C by ~1.82 Ga, and subsequent pulses of arc magmatism at ~1.80 and ~1.78 Ga associated with northward subduction of Yavapai-age oceanic crust (Holm et al., 2005). This is the first application of titanite and apatite petrochronology to the Archean gneiss domes in northern Michigan, revealing the utility of these phases towards dating and interpreting Precambrian tectonothermal events.