ERA

Download the full-sized PDF of Petrogenesis of the Acasta Gneiss Complex, Northwest Territories, CanadaDownload the full-sized PDF

Analytics

Share

Permanent link (DOI): https://doi.org/10.7939/R3FB4WV19

Download

Export to: EndNote  |  Zotero  |  Mendeley

Communities

This file is in the following communities:

Graduate Studies and Research, Faculty of

Collections

This file is in the following collections:

Theses and Dissertations

Petrogenesis of the Acasta Gneiss Complex, Northwest Territories, Canada Open Access

Descriptions

Other title
Subject/Keyword
Acasta Gneiss Complex
Hadean Earth
Archean tectonics
Iceland
Zircon
Type of item
Thesis
Degree grantor
University of Alberta
Author or creator
Reimink, Jesse R
Supervisor and department
Chacko, Tom (Earth and Atmospheric Sciences)
Examining committee member and department
Heaman, Larry (Earth and Atmospheric Sciences)
Stern, Richard (Earth and Atmospheric Sciences)
Pearson, D. Graham (Earth and Atmospheric Sciences)
Ketchum, John (Northest Territories Geological Survey)
Kamber, Balz (Trinity College, Dublin)
Department
Department of Earth and Atmospheric Sciences
Specialization

Date accepted
2015-11-17T13:19:03Z
Graduation date
2016-06
Degree
Doctor of Philosophy
Degree level
Doctoral
Abstract
This contribution presents the results of a new mapping and geochemical investigation of well-preserved rock units in the Acasta Gneiss Complex (AGC), Canada, and provides additional support for a broadly mafic Hadean Earth, in which the first evolved crust formation took place in a geodynamic environment unlike modern subduction zones. Rocks in the AGC, with crystallization ages spanning a billion years, document a transition in crust-forming processes starting in a shallow petrogenetic setting with the involvement of plagioclase that eventually evolves to a deep-seated regime dominated by partial melting in the presence of residual garnet. As part of this mapping campaign, we identified and mapped a well-preserved rock unit with an igneous crystallization age of 4.02 Ga, a unit informally termed the Idiwhaa tonalitic gneiss (Idiwhaa meaning “ancient” in the local aboriginal language). The unique geochemical signatures of the Idiwhaa unit are distinct from typical Archean (4.0–2.5 billion year old) evolved crust. Specifically, the strong iron-enrichments and flat rare-earth-element patterns contained in this unit imply a distinct environment of formation; these signatures suggest a petrogenesis involving high degrees of crystal fractionation from a relatively water-poor basaltic magma at shallow levels. Additionally, well-preserved zircons in this unit allow a further investigation of the geochemical evolution as zircon can retain geochemical information through high-temperature metamorphism. In particular, changes in oxygen isotope ratios between two phases of zircon growth, both with ~4.02 Ga crystallization ages, indicate assimilation by the Idiwhaa magma of pre-existing rocks that had been altered by high temperature surface waters. Combined with the whole-rock signatures discussed above, these data imply petrologic processes strikingly similar to those operating on modern Iceland, a setting that has long been proposed as a suitable analog for crust formation on the early Earth. Because of the presence of very well preserved zircons in the Idiwhaa unit, we are also able to evaluate the composition and age of pre-existing crust into which the Idiwhaa magma intruded, as well as the differentiation state of the mantle from which the magma was derived. To this end, Hf isotope data were obtained from two igneous phases of 4.02 Ga zircon material, as well as the highest precision dating yet undertaken on AGC rocks. These isotopic data, combined with the whole-rock data described above, suggest that there was little or no interaction with typical Archean continental crust during the formation of the Idiwhaa unit. This result, along with evolved Hf isotope signatures contained in zircon with mantle-like oxygen isotope ratios, suggests interaction with very ancient mafic crust or an early-enriched mafic reservoir. The combination of early enriched reservoirs and no evidence for interaction with evolved, continental crust suggests that the Hadean Earth may have been dominated by mafic rocks. Whole-rock major- and trace-element data combined with zircon uranium-lead and oxygen isotope data obtained from younger rocks from the AGC, including rocks formed at ~3.95, 3.75, 3.5–3.6 and 3.4–2.9 Ga, document a transition in crust-forming and likely tectonic processes over that time interval. Specifically, the earliest components of the AGC (~4.02 Ga) formed in an oceanic plateau-like environment and produced primarily mafic- to intermediate-composition rocks with flat REE patterns. Episodic melting of this thickened and partially hydrated pile at ~3.95 and 3.75 Ga at moderate depths produced tonalitic magmas with somewhat more fractionated REE patterns. The volumetrically dominant 3.5–3.6 Ga rocks of the AGC have steep REE patterns similar to those found in Archean granitoids worldwide. These patterns require deep-seated melting in the presence of abundant residual garnet in the source rock and possibly occurred in a subduction-like setting. Thus, the AGC as a whole records a gradual transition from an oceanic plateau setting to a subduction-like setting over an ~400 million year time interval. Similar models have been proposed for other ancient gneiss terranes, but this process occurred a few hundred million years earlier in the AGC.
Language
English
DOI
doi:10.7939/R3FB4WV19
Rights
This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for the purpose of private, scholarly or scientific research. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
Citation for previous publication
Reimink, J. R., Chacko, T., Stern, R. A. & Heaman, L. M. Earth’s earliest evolved crust generated in an Iceland-like setting. Nature Geoscience 7 (2014) 529–533

File Details

Date Uploaded
Date Modified
2015-11-17T20:19:14.297+00:00
Audit Status
Audits have not yet been run on this file.
Characterization
File format: pdf (PDF/A)
Mime type: application/pdf
File size: 28717799
Last modified: 2016:11:16 15:01:56-07:00
Filename: Reimink_Jesse_R_201511_PhD.pdf
Original checksum: 48bc77bf0eb83e1eef069bbbcf5d2723
Activity of users you follow
User Activity Date