REVISITING THE WESTERN EXPOSURES OF THE PENOKEAN MARSHFIELD TERRANE: PRELIMINARY ZIRCON LA ICPMS U-PB AND LU-HF DATA REVEAL NEW COMPLEXITY IN AN OLD TERRANE

The Marshfield terrane is a sparsely studied crustal block in the North American midcontinent, consisting of a Paleoproterozoic volcanic arc complex constructed above Archean gneissic basement. This unit collided with the southern margin of the Superior craton during the c. 1.88 – 1.83 Ga Penokean orogeny. Much of the terrane is concealed beneath Phanerozoic sedimentary rocks, with exposure limited to areas where rivers cut through the cover into crystalline basement. Geochronology and geochemistry of the Marshfield terrane is limited due to this poor exposure, leading to uncertainties in the extent of the Archean basement and the role it played in the subsequent evolution of the Penokean orogeny. This study presents new zircon U-Pb geochronology and Lu-Hf isotopic data, as well as whole-rock geochemistry from a five samples of inferred meta-igneous rocks collected from presumed Archean units exposed in the valleys of the Chippewa and Yellow Rivers (Chippewa County, WI). Three samples were collected from amphibole and /or biotite bearing schists, one from a mylonitic quartzofeldspathic gneiss, and one from a weakly deformed granodiorite. Whole-rock geochemistry focused on immobile trace elements, and reveal a consistent, volcanic arc origin for the samples. Two schist samples yielded zircon LA-ICPMS U-Pb ages of c. 1,858 and c. 1,860 Ma, surprising from units previously described as Archean. A third sample, collected from the mylonitic gneiss located at Jim Falls, previously yielded a U-Pb age of c. 2.5 Ga. This study revealed a range of near-concordant zircon ages from c. 1,780 Ma to 2,840 Ma, possibly reflecting detrital zircon in a metasedimentary protolith, and metamorphic zircon growth in later tectonothermal events. Lu-Hf isotopic data from the two schist samples yielded T_DM model ages of c. 2.2 Ga, and average ɛHf_{(crystallization)} values for the samples were 3.6 and 1.7 respectively. These data are suggestive of modest Archean (?) basement input to the original magma. This study highlights the poorly documented complexity of the Marshfield terrane, and the need for further investigation into its pre-Penokean origin and evolution. These data also help improve tests for finding a possible parental craton to the Marshfield terrane.