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.
University of Minnesota M.S. thesis. July 2010. Major: Geological Sciences. Advisor: James D. Miller Jr. 1 computer file (PDF); vii, 103 pages. Ill. (some col).
Hoaglund, Steven Arthur.
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..
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