Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift system

Abstract

The earliest Keweenawan plutonism exposed in Minnesota occurs along the northeastern flank of the Duluth Complex as a series of layered gabbros (Nathan's layered series) emplaced at 1106.9±0.6 Ma. This sequence of gabbro sheets shares temporal, spatial, and compositional similarities with the nearby Logan sills in Ontario. Four Duluth Complex anorthositic and troctolitic series samples from widely separated areas have unresolvable ages between 1099.3±0.3 and 1098.6±0.5 Ma, indicating a very short duration for peak intrusive activity (0.5-1 my). The unresolvable ages between anorthositic and troctolitic plutons suggest that these two magma series are more closely related than previously modeled. These dates also imply that the major reverse-to-normal magnetic polarity switch, used throughout the rift system as an important correlation tool, occurred prior to 1099 Ma. Much of the hypabyssal intrusive suite within the volanic pile overlying Duluth Complex plutons may be significantly younger than the main pulse of plutonic activity. -from Authors