DETRITAL ZIRCON AGES AND HF ISOTOPE CONCENTRATIONS FROM THE MARQUETTE RANGE SUPERGROUP, NORTHERN MICHIGAN

The study of Eoarchean age zircons is fundamental to our understanding of the early history of the Earth and the formation of continental crust. Zircons of such an age have been identified as xenocrysts found in gneisses and some volcanic rocks throughout the upper peninsula of Michigan, however, the source of these zircons is enigmatic and outcrops of the host rocks in this region are limited. Detrital zircons in surrounding Paleoproterozoic metasedimentary rocks may provide more insight into an early Archean provenance. The Marquette Range Supergroup (MRS), spanning the length of the upper peninsula, consists of metasedimentary rocks deposited uncomformably over Archean basement. The MRS is divided into three major groups; the Chocolay, Menominee, and Baragra groups, each containing a basal quartzite. It is thought to be one of the Paleoproterozoic basins correlative to the Animikie Group rocks in Canada and Minnesota, and the Huronian Supergroup rocks in Canada. U-Pb and Lu-Hf data from the detrital zircons from the basal quartzites in each group within the MRS are key to improving our understanding of the evolution of Eoarchean (and older?) crust of the upper peninsula in addition to providing a better understanding of the interaction between the Archean and Paleoproterozoic terranes (Penokean) along the southern margin of the Superior Province. Samples of each of the three basal quartzites from across the upper peninsula and zircons were analyzed for U-Pb and Lu-Hf isotopic compositions by laser ablation multi-collector ICP-MS. Detrital zircon U-Pb ages from the middle quartzite in the MRS range from 2.5 – 3.1 Ga with the majority of zircons being around 2.7 Ga. Lu-Hf model ages from these same samples range from 2.7 – 3.4 Ga. Zircons analyzed from samples in the upper quartzite have U-Pb ages ranging from 1.8 – 2.8 Ga with the majority of ages also being ~2.7 Ga. The upper quartzite also yielded Lu-Hf model ages ranging from 2.7 – 3.1 Ga. The ages and Hf isotopic compositions calculated from the isotope ratios measured in these zircons are used to determine the age and constrain the source(s) of these zircons which, in turn, shed light on their provenances and potential early Archean sources.