U-Pb, TRACE ELEMENTS, AND O-Hf ISOTOPES IN ZIRCON FROM THE SUPERIOR CRATON, UPPER PENINSULA MICHIGAN: IMPLICATIONS FOR ARCHEAN CRUSTAL DEVELOPMENT

The temporal and spatial formation of continental crust is fundamental to our understanding of Earth evolution; however, the paucity of exposed Archean rocks globally presents a challenge for developing accurate continental growth models. We report the first comprehensive analysis of zircon U-Pb-Hf-O-trace element data from igneous and metamorphic rocks from the southeast Superior Craton, Upper Peninsula, Michigan. The Archean rocks are exposed as a complex assemblage of granitoids (tonalite-trondhjemite-granodiorite), gneisses, and migmatites intruded by numerous mafic dikes and/or sills. We focused our study on the batholith- sized body of high-K megacrystic granitoid rocks informally called the Bell Creek batholith located south of the Great Lakes Tectonic Zone (GLTZ). U-Pb zircon dating suggests a ca. 2.5-2.6 Ga emplacement age for the Bell Creek granitoids, gneisses and migmatites. Inherited zircons have ages ranging from ca. 2.7-3.7 Ga, suggesting that a range of supracrustal material of various ages was involved in the formation of this felsic batholith. The δ¹⁸O (VSMOW) values of the zircons ranges from +5.0 to +13.0‰, from typical asthenospheric mantle (+5.5‰±0.3) towards metasedimentary crustal compositions enriched in ¹⁸O (δ¹⁸O of 5.8 to 13.0‰). The high-K nature of the granitoids and gneisses in general, and trace element characteristics of the high δ¹⁸O crustal zircons (high U, P, and Al) specifically, are consistent with at least one of the upper crustal assimilants being pelitic in composition. Remnants of this pelitic component can be observed in numerous outcrops as cm-sized biotite-chlorite-garnet clots. However, the abundance of zircon with a wide range in negative εHf(t) values (-5 to -20) and mantle-like δ¹⁸O indicates the batholith was produced by melting of preexisting juvenile supracrustal basement lithologies dating back to the Eoarchean. These findings indicate that recycling of a variety of both high δ¹⁸O metasedimentary crust and low δ¹⁸O mantle sources of basement material was an important process involved in the formation of felsic magmas in this portion of the Superior Province during the Neoarchean.