Paleomagnetism of the ~1.1 Ga Baraga-Marquette dykes (Michigan, USA)

Abstract:

We present new detailed paleomagnetic and rock magnetic results from ~50 dykes exposed in the Baraga-Marquette (BM) area of the Upper Peninsula of Michigan (USA). The dyke intrusion was associated with the development of the ~1.1 Ga North American Mid-Continent Rift (MCR) system. Most rocks representing the early stage of MCR are characterized by steep reversed directions of the characteristic remanent magnetization (ChRM), whereas the rocks from the main stage of MCR generally yield shallower normal polarity ChRM directions. Rock magnetic experiments show that the principal magnetic carrier in the BM dykes is single-domain to pseudosingle-domain magnetite or low-Ti titanomagnetite with minor occurrences of hematite, maghemite, or pyrrhotite in some dykes. The majority of BM dykes yielded steep reversed-polarity directions of ChRM indicating that they belong to the early stage of MCR development. The corresponding paleomagnetic pole plots close to the apex of the so called “Logan Loop”, a segment of the Apparent Polar Wander Path (APWP) for the North American continent for ~1000-1200 Ma. Several BM dykes yielded steep normal-polarity ChRM directions, significantly different from the direction exhibited by the normally magnetized MCR sequences. This may indicate that these dykes are older than the reversed polarity dykes. In addition, a single mafic dyke from the Baraga basin with a recently published U/Pb age of 1120±4 Ma resulted in a shallow normal ChRM direction. The implications of our new paleomagnetic results for the North American APWP and the evolution of MCR will be discussed.