U-PB AND LU-HF ISOTOPES FROM DETRITAL ZIRCONS IN THE EAST BRANCH ARKOSE OF NORTHERN MICHIGAN

Studies across Michigan’s upper peninsula have identified Eoarchean aged zircons in metasedimentary and metavolcanic rocks. Additionally, a 3.8 Ga zircon xenocryst has recently been identified in the 3.6 Ga Watersmeet dome. The source of these zircons is unclear. A recent detrital zircon study of the Marquette Range Supergroup found U-Pb ages ranging from 3.4 – 1.8 Ga across the three basal quartzite units found within the supergroup. The majority (>60%) of these zircon ages are ~2.7 Ga and are found in each of the samples collected. Only a few (<1%) of the zircon ages are ~1.8 Ga and their location within the middle quartzite of the supergroup suggests they are not Penokean. Attention now turns to the stratigraphically older Dickinson group where previous work has identified a 3.8 Ga detrital zircon within the East Branch arkose.

Named after the branch of the Sturgeon River along which it crops out, the East Branch arkose marks the lower boundary of the Dickinson group found in Dickinson County, Michigan. It is a meta-conglomerate consisting of stretched quartzite and granitic gneiss pebbles interlayered with a foliated pink arkose. In 1958 the Dickinson group was described by James as “Lower Precambrian” and stratigraphically below the “Middle Precambrian” Chocolay group of the Animikie series (now known as the Marquette Range Supergroup). While this terminology may no longer be in use, detrital zircon studies have determined the basal units of the Chocolay group were deposited in the Paleoproterozoic. Below the East Branch arkose is Archean age granitic gneiss thought to be in erosional contact with the Dickinson group, as no signs of contact metamorphism have been observed in the region.

Continued research on detrital zircons and zircon xenocrysts of Eoarchean age in the upper peninsula is necessary to define better constraints on the enigmatic Eoarchean source in this region. This study uses U-Pb and Lu-Hf isotopes from detrital zircons collected from the East Branch arkose to confirm the presence of more Eoarchean-aged zircon(s) and further define their source. Additionally, this improves upon age constraints of the unit when compared to detrital zircon data from previously conducted research of both the East Branch Arkose and the overlying Sturgeon Quartzite (basal Chocolay unit of the Marquette Range Supergroup).