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

Precise resolution of the timing of igneous activity is crucial to understanding the dynamic processes associated with continental rifting. Mafic intrusive rocks constitute a major portion of the exposed 1100 Ma (Keweenawan) Midcontinent Rift system in northeastern Minnesota; however, prior to this study, geochronological data were insufficient to allow rigorous interpretation of intrusive histories and their relationships to extrusive suites. Eight anorthositic and gabbroic intrusives were chosen to represent both the temporal and spatial ranges of plutonic activity that formed the Duluth Complex and related mafic intrusions. U-Pb isotopic analyses from zircons and baddeleyites result in U-Pb concordant ages with little or no ambiguity introduced by inherited components, Pb loss or common Pb. 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 m.y.). The unresolvable ages between anorthositic and troctolitic plutons suggest that these two magma series are more closely related than previously modeled and argue strongly for the need to reexamine their fundamental petrogenetic relationships. 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. This date is several million years earlier than previously suspected and emphasizes the need for further paleomagnetic and geochronological data from the overlying volcanics. Much of the hypabyssal intrusive suite within the volcanic pile overlying Duluth Complex plutons may be significantly younger than the main pulse of plutonic activity. Two hypabyssal bodies, the Sonju Lake intrusion and gabbro at Silver Bay, were emplaced at 1096. 1 +/- 0.8 Ma and 1095.8 +/- 1.2 Ma, respectively. Dates reported here and in previous studies support the concept of episodic tectonomagmatic rift development where magmatism was apparently concentrated in episodes of short duration (<1-3 m.y.) interspersed with longer hiatuses (2-8 m.y.).