U-Pb 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.

Abstract

Paces and Miller (1993) precisely established the main intrusive periods that created the Duluth Complex and related intrusions associated with the 1.1 Ga Midcontinent Rift in NE Minnesota. They did not, however, resolve differences in emplacement ages within and between major intrusive units due to a small number of dated samples. New high-precision U-Pb baddeleyite and zircon ages from five mafic intrusions related to the Duluth Complex builds on the work of Paces and Miller (1993). Ages from three intrusions that span the entire range of Duluth Complex layered series intrusive activity indicate that the layered series and anorthositic series were synchronously emplaced around 1098 Ma. Moreover, zircon ages obtained from the perceived oldest (Partridge River, 206Pb/238U age 1095.94 ± 0.18 Ma) and youngest (Bald Eagle, 206Pb/238U age 1095.64 ± 0.19 Ma) layered series intrusions suggest the bulk of layered series activity occurred within a period of 670 ka. The Tuscarora intrusion (early layered series) has a distinctly older 207Pb/206Pb baddeleytie age than the Partridge River intrusion (1098.81 ± 0.32 and 1097.98 ± 0.37 Ma, respectively). However, in light of new information regarding the incompatibility between zircon and baddeleyite ages, its temporal relationship with other layered series intrusions remains unclear. Zircon ages obtained from two early Beaver Bay Complex (BBC) intrusions indicate Duluth Complex and BBC magmatism likely overlap. The Houghtaling Creek troctolite is indistinguishable from the other layered series intrusions. Moreover, the Wilson Lake ferrogabbro, long interpreted to be older than the Houghtaling Creek, is distinctly younger (207Pb/206Pb zircon ages 1095.75 ± 0.92 and 1098.62 ± 0.50 Ma, respectively). There still exists an approximate 2 Ma gap between early Beaver Bay and Duluth Complex, and late BBC. A reasonable estimate based on the geometry of layered series intrusions indicate that over 16,000 km3 of mafic magma intruded during layered series emplacement (excluding unknowable amounts of erosion). With over 16,000 km3 of mafic magma being intruded over a period of 670 ka, emplacement rates were at least 0.024 km3/yr, which is similar to estimates for other large continental flood basalt provinces.