TY - JOUR
T1 - Petrogenesis and tectonics of the Acasta Gneiss Complex derived from integrated petrology and 142Nd and 182W extinct nuclide-geochemistry
AU - Reimink, Jesse R.
AU - Chacko, Thomas
AU - Carlson, Richard W.
AU - Shirey, Steven B.
AU - Liu, Jingao
AU - Stern, Richard A.
AU - Bauer, Ann M.
AU - Pearson, D. Graham
AU - Heaman, Larry M.
N1 - Funding Information:
We present paired 142 Nd– 182 W from a gabbroic to granitic rocks from the Acasta Gneiss Complex, spanning an age range from 4.02–2.94 Ga. These samples have been characterized with respect to field relationships, geochemistry, geochronology, and petrography and are well-preserved relative to rocks from other parts of the AGC ( Reimink et al., 2014, 2016a ). When interpreted in conjunction with previously published whole-rock geochemistry and zircon Hf- and O-isotope data, the new dataset places constraints on the formation mechanisms and source compositions of the AGC that were previously unavailable: 1. Rocks that comprise the AGC came from distinct sources throughout their >1 Ga emplacement history, exemplified by variations in their μ 142 Nd compositions. 2. AGC rocks ranging in age from 4.02 to 3.4 Ga have homogeneous excesses in 182 W on the order of ∼10 ppm, consistent with data from other locations containing >3.6 Ga terrestrial samples. 3. All sampled Eoarchean tonalites within the AGC were formed by partial melting of hydrated, Hadean mafic crust at depths shallower than the garnet stability field. 4. TTG-composition rocks intruded at ∼3.6 Ga were derived from more juvenile mafic source rocks and subsequently experienced variable degrees of interaction with Hadean rocks, likely by underthrusting of younger crust beneath the Eoarchean nucleus. 5. Some AGC mafic rocks have strongly negative μ 142 Nd, indicating that they were either derived from Hadean mafic crust, or are in fact themselves Hadean. Funding This work was supported by the U.S. National Science Foundation [ OCE-1524384 ] grant to R. Carlson and S. Shirey, National Science and Engineering Research Council Discovery grants to T. Chacko, and Canada Excellence Research Chairs Program funding to D.G. Pearson.
Funding Information:
This work was supported by the U.S. National Science Foundation [OCE-1524384] grant to R. Carlson and S. Shirey, National Science and Engineering Research Council Discovery grants to T. Chacko, and Canada Excellence Research Chairs Program funding to D.G. Pearson.
Publisher Copyright:
© 2018
PY - 2018/7/15
Y1 - 2018/7/15
N2 - The timing and mechanisms of continental crust formation represent major outstanding questions in the Earth sciences. Extinct-nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early Earth, including crust formation. Here, we investigate the whole-rock 182W/184W and 142Nd/144Nd ratios and zircon Δ17O values of a suite of well-studied and lithologically-homogeneous meta-igneous rocks from the Acasta Gneiss Complex, Northwest Territories, Canada, including the oldest-known zircon-bearing rocks on Earth. In the context of previously published geochemical data and petrogenetic models, the new 142Nd/144Nd data indicate that formation of the Hadean–Eoarchean Acasta crust was ultimately derived from variable sources, both in age and composition. Although 4.02 Ga crust was extracted from a nearly bulk-Earth source, heterogeneous μ 142Nd signatures indicate that Eoarchean rocks of the Acasta Gneiss Complex were formed by partial melting of hydrated, Hadean-age mafic crust at depths shallower than the garnet stability field. By ∼3.6 Ga, granodioritic–granitic rocks were formed by partial melting of Archean hydrated mafic crust that was melted at greater depth, well into the garnet stability field. Our 182W results indicate that the sources to the Acasta Gneiss Complex had homogeneous, high-μ182W on the order of +10 ppm—a signature ubiquitous in other Eoarchean terranes. No significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope (16O–17O–18O) compositions of Acasta zircons, confirming homogeneous oxygen isotope compositions in Earth's mantle by 4.02 Ga.
AB - The timing and mechanisms of continental crust formation represent major outstanding questions in the Earth sciences. Extinct-nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early Earth, including crust formation. Here, we investigate the whole-rock 182W/184W and 142Nd/144Nd ratios and zircon Δ17O values of a suite of well-studied and lithologically-homogeneous meta-igneous rocks from the Acasta Gneiss Complex, Northwest Territories, Canada, including the oldest-known zircon-bearing rocks on Earth. In the context of previously published geochemical data and petrogenetic models, the new 142Nd/144Nd data indicate that formation of the Hadean–Eoarchean Acasta crust was ultimately derived from variable sources, both in age and composition. Although 4.02 Ga crust was extracted from a nearly bulk-Earth source, heterogeneous μ 142Nd signatures indicate that Eoarchean rocks of the Acasta Gneiss Complex were formed by partial melting of hydrated, Hadean-age mafic crust at depths shallower than the garnet stability field. By ∼3.6 Ga, granodioritic–granitic rocks were formed by partial melting of Archean hydrated mafic crust that was melted at greater depth, well into the garnet stability field. Our 182W results indicate that the sources to the Acasta Gneiss Complex had homogeneous, high-μ182W on the order of +10 ppm—a signature ubiquitous in other Eoarchean terranes. No significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope (16O–17O–18O) compositions of Acasta zircons, confirming homogeneous oxygen isotope compositions in Earth's mantle by 4.02 Ga.
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U2 - 10.1016/j.epsl.2018.04.047
DO - 10.1016/j.epsl.2018.04.047
M3 - Article
AN - SCOPUS:85046804191
SN - 0012-821X
VL - 494
SP - 12
EP - 22
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -