143-9 Detrital Zircon Geochronology of Midcontinent Rift-Fill Strata in Iowa, USA

Abstract:

Sandstone units penetrated by deep drill cores within the Iowa segment of the ~1100 Ma Midcontinent Rift provide a rare opportunity to investigate sediment provenance and stratigraphic relationships in a poorly characterized portion of the Rift. Although correlative sedimentary rocks exposed in the Lake Superior region have been studied, the origin and depositional history of Iowa Rift-fill strata remain uncertain. Detrital zircon U-Pb geochronology was conducted by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) on three core splits: one from the Nonesuch (Eischeid well; (15,101-15,102 ft) and two from the Freda (Eischeid and Lehman wells; 11,383-11,386 ft and 2,953.6-2,954.6 ft).  A total of 457 zircons were analyzed.  The Nonesuch sample had two principal age peaks: 1640 and 1750 Ma, which are consisted with the ages of local basement rocks.  The two Freda samples are statistically indistinguishable from Freda data sets in Michigan and Wisconsin, with most zircons ranging from 1000-1450 Ma in age.  The origin of these zircons is less well constrained.  The geon 14 subset can be derived from local Quimby Granite, the geon 13 zircons could be derived from the Spencer intrusive complex along the Western Spirit Lake Tectonic zone.  Each of these plutons are proximal to the Eischeid well.  The small number of geon 11 grains can be derived from Midcontinent Rift volcanic rocks.  Alternatively, these zircons can be derived from distal Grenville source areas.  We do not favor this possibility because of the paucity of the ubiquitous Archean zircons available.  The absence of Midcontinent Rift derived zircons indicates that rift rocks were either zircon infertile or buried by the time these strata were deposited.  We favor a model where local source areas from the rift shoulder provided the bulk of the sediment to this depositional system.

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