Title

Sr-Nd-Hf Isotope Geochemistry of 3.5 Ga Gneisses of the Minnesota River Valley and U-Pb Geochronology of Detrital Zircon from Cambrian Sedimentary Rocks of the Laurentian Rifted Margin

Date of Award

5-2013

Degree Type

Dissertation

Embargo Date

5-23-2013

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Scott D. Samson

Keywords

Archean, geochronology, Hf, Nd, Sr

Subject Categories

Earth Sciences

Abstract

Zircon and apatite are highly refractory minerals; this suggests they will retain their primary isotopic composition during high temperature deformational events or low temperature fluvial transport over long distances. The focus of this dissertation research is to exploit the refractory nature of zircon and apatite to understand the formation and evolution of highly deformed rocks in the Minnesota River Valley and the provenance of Neoproterozoic-Cambrian sedimentary rocks deposited during the continental rifting of Laurentia.

The Archean rocks of the Minnesota River Valley consist of ~3.50-3.10 Ga highly deformed orthogneisses, a ~2.6 Ga non-deformed granite and garnet-biotite paragneiss. Initial Hf isotopic values in zircon and whole-rock trace element data from the 3.5 Ga gneisses suggest that these rocks were not formed in a modern plate tectonic regime, consistent with other datasets that go back as far as ~4.4 Ga. Not until post 3.14 Ga are the data consistent with those that would be observed in rocks that formed by more modern day plate tectonic processes. This suggests that plate tectonics as it is observed today did not operate for the first 1.42 b.y. of Earth history.

To further investigate whether modern-style plate tectonic processes operated in the early Earth, Sr isotopes were collected from single apatite crystals in a 3.5 Ga gneiss from the Minnesota River Valley where Hf isotopic data already exist. The high initial 87Sr/86Sr ratio (average = 0.7049) of the apatite, compared to the bulk earth or depleted mantle suggest that the source had a long crustal residence time before melting, consistent with the Hf isotopic data in chapter 1 and also suggesting that modern-style plate tectonic processes were not responsible for the formation of 3.5 Ga crust in the Minnesota River Valley.

Finally, U-Pb geochronologic data were collected from detrital zircon from Neoproterozoic-Cambrian sedimentary rocks along the rifted margin of eastern Laurentia from New York to Georgia. These data confirm the presence of a large-scale unconformity at the base of most Cambrian sequences and are consistent with detachment faulting as the mechanism for continental rifting beginning in the latest Neoproterozoic.

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