U-PB ZIRCON GEOCHRONOLOGY OF THE DULUTH COMPLEX AND RELATED HYPABYSSAL INTRUSIONS: INVESTIGATING THE EMPLACEMENT HISTORY OF A LARGE MULTIPHASE INTRUSIVE COMPLEX RELATED TO THE 1.1 GA MIDCONTINENT RIFT

Previous geochronology of mafic intrusive magmatism associated with the 1.1 Ga Midcontinent Rift in NE Minnesota by Paces and Miller (1993) established, with high precision, the main intrusive periods that created the Duluth Complex and related intrusions. This study did not, however, resolve differences in emplacement ages within and between major intrusive units due to a small number of dated samples. We present new high-precision isotope dilution U-Pb baddeleyite and CA-TIMS zircon ages from four mafic intrusions that build on the work of Paces and Miller (1993). Three of these are from troctolitic intrusions (Partridge River, Bald Eagle, Tuscarora) belonging to the layered series of the Duluth Complex (DC) that, based on field relationships, appear to represent the entire range of layered series intrusive activity. They yielded irresolvable weighted mean 207Pb/206Pb ages between 1098.8 ± 0.3 and 1098.0 ± 0.7 Ma. These ages fall in line with the 1098.6 ± 0.5 Ma and 1099.3 ± 0.3Ma ages reported by Paces and Miller for two layered series intrusions. Moreover, these ages are irresolvable from the ages of two DC anorthositic series samples analyzed by Paces and Miller (1099.0± 0.6 Ma and 1099.1± 0.5 Ma) and recently confirmed by unpublished data from the Boise State University isotope laboratory.

An early intrusion from the Beaver Bay Complex (BBC), the Houghtaling Creek troctolite, was also dated in this study to test the implication from Paces and Miller’s study that a 3 Ma magmatic hiatus exists between emplacement of the deeper seated DC and the more hypabyssal BBC. The Houghtaling Creek sample yielded an age, which is irresolvable from the Duluth Complex ages and thus implies that BBC and DC magmatism likely overlaps.

The evidently synchronous emplacement of layered series and anorthositic series intrusions indicate that voluminous main stage magmatism occurred during an extremely short interval of within 1 Ma. Reasonable estimates of the geometry of the dozen or so layered series intrusions that comprise the DC indicate that over 15,000 km³ of mafic magma intruded during emplacement of the layered series alone. The total volume of intruded magma is likely much higher when erosion is factored in and rocks of the anorthositic series and early BBC are included.