PRELIMINARY GEOCHEMISTRY ON THE MARSHFIELD TERRANE, WEST-CENTRAL WISCONSIN

The assembly of Laurentia has long been studied by geologists; however, much of its evolution is still in question due to its vast complexity that resulted from numerous tectonic events coalescing over time. One of the many pieces involved in the ancestral North America assemblage is the Marshfield terrane (MT), which is associated with Paleoproterozoic Penokean Orogeny (PPO). The PPO is responsible for the amalgamation of the Superior Craton (SC), the Pembine-Wausau terrane (PWT) and the Marshfield terrane (MT). While the PWT and the SC have well understood origins, the origins of the MT are poorly constrained. Both the MT and the SC contain Archean gneisses that suggest a possible relationship; however, studies based on Pb isotopic data indicate that the MT is not related to the SC based on Pb isotope differences. These data however, are not strong enough to bring conclusions to establish if the MT is a rift fragment of the SC, a fragment from another continental body, or its own entity.

From this investigation, preliminary whole-rock geochemical data on meta-igneous rock collected from the western MT (west-central Wisconsin) illustrate the role of subduction in the terrane origin and tectonic evolution. Most felsic samples yielded a geochemical signature typical of a volcanic arc or syn-collisional system when plotted on tectonic discrimination diagrams. Likewise, the mafic samples of the MT were found to be plate margin basalts, and a majority appears to come from a continental arc system. One mafic sample yielded data suggestive of an oceanic arc system. These data support previous studies indicating that the MT is a displaced continental fragment, and that the SC, MT, and PWT converged together through subduction of oceanic crust during the PPO.

To further explore the origins of the MT, future work will include U-Pb zircon geochronology and trace element geochemistry; including utilizing the Lu-Hf system.