Early proterozoic geology of the great lakes region

Abstract

The Belleview metadiabase sill crosscuts the Siamo Slate and the Negaunee Iron Formation of the Marquette Range Supergroup, Michigan. It is one of a series of metamorphosed diabase sills that intrudes the Menominee Group and possibly the Baraga Group. Chloritic sills within the Marquette Range Supergroup have yet to be officially grouped as one or more series or swarms and therefore stratigraphic correlations and temporal relationships between these sills remain speculative. The recovery of the first sensitive high-resolution ion mass spectrometry (ID-TIMS) U–Pb date of 6 baddeleyite grains from the Belleview sill yielding an age of 1891 ± 3 Ma is a positive step to classifying these chloritic metadiabases. This new date has three important implications: (1) it gives a definitive minimum age of Negaunee Iron Formation deposition; (2) it confirms recent stratigraphic correlations that suggest that the Negaunee Iron Formation is older than the Mesabi, Gunflint and Gogebic Iron Ranges and not contemporaneous; and (3) it does not support recent correlations of the Menominee Group chloritic sills with the 1874 Ma Hemlock Formation within the Baraga Group of the Marquette Range Supergroup.This new date does not support a more recently proposed synorogenic foredeep depositional environment for the deposition of the Negaunee Iron Formation. Instead, this age date supports an earlier interpretation that the Negaunee Iron Formation is part of continental shelf deposits laid down prior to onset of 1875–1835 Ma collision. The age of the Belleview sill suggests that it is the result of extension-related magmatism that is prior to the currently defined Penokean orogenic event.