~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex
The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded...
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2015
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Online Access: | http://hdl.handle.net/1885/34307 |
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ftanucanberra:oai:digitalcollections.anu.edu.au:1885/34307 |
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openpolar |
institution |
Open Polar |
collection |
Australian National University: ANU Digital Collections |
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ftanucanberra |
language |
unknown |
topic |
Keywords: Archean crustal evolution igneous geochemistry isotopic composition magma migmatite neodymium isotope orthogneiss SHRIMP dating tonalite zircon Arctic Greenland Nuuk Crustal evolution Eoarchaean Itsaq Gneiss Complex Nd isotopes Tonalites Zircons |
spellingShingle |
Keywords: Archean crustal evolution igneous geochemistry isotopic composition magma migmatite neodymium isotope orthogneiss SHRIMP dating tonalite zircon Arctic Greenland Nuuk Crustal evolution Eoarchaean Itsaq Gneiss Complex Nd isotopes Tonalites Zircons Nutman, Allen Bennett, Victoria Friend, Clark R L Horie, Kenji Hidaka, Hiroshi ~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex |
topic_facet |
Keywords: Archean crustal evolution igneous geochemistry isotopic composition magma migmatite neodymium isotope orthogneiss SHRIMP dating tonalite zircon Arctic Greenland Nuuk Crustal evolution Eoarchaean Itsaq Gneiss Complex Nd isotopes Tonalites Zircons |
description |
The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 μm) younger (3,660-3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110-140 ppm Zr in the tonalites and likely magmatic temperatures of >850°C, zircon solubility - melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo - feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial εNd (3,85 0Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800-3,600 Ma) rocks of the Itsaq Gneiss Complex. |
format |
Article in Journal/Newspaper |
author |
Nutman, Allen Bennett, Victoria Friend, Clark R L Horie, Kenji Hidaka, Hiroshi |
author_facet |
Nutman, Allen Bennett, Victoria Friend, Clark R L Horie, Kenji Hidaka, Hiroshi |
author_sort |
Nutman, Allen |
title |
~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex |
title_short |
~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex |
title_full |
~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex |
title_fullStr |
~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex |
title_full_unstemmed |
~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex |
title_sort |
~3,850 ma tonalites in the nuuk region, greenland: geochemistry and their reworking within an eoarchaean gneiss complex |
publisher |
Springer |
publishDate |
2015 |
url |
http://hdl.handle.net/1885/34307 |
long_lat |
ENVELOPE(-52.150,-52.150,68.717,68.717) |
geographic |
Arctic Greenland Nuuk |
geographic_facet |
Arctic Greenland Nuuk |
genre |
Arctic Greenland Itilleq Nuuk |
genre_facet |
Arctic Greenland Itilleq Nuuk |
op_source |
Contributions to Mineralogy and Petrology (Electronic) |
op_relation |
1432-0967 http://hdl.handle.net/1885/34307 |
_version_ |
1766350214440943616 |
spelling |
ftanucanberra:oai:digitalcollections.anu.edu.au:1885/34307 2023-05-15T15:20:00+02:00 ~3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex Nutman, Allen Bennett, Victoria Friend, Clark R L Horie, Kenji Hidaka, Hiroshi 2015-12-08T22:29:58Z http://hdl.handle.net/1885/34307 unknown Springer 1432-0967 http://hdl.handle.net/1885/34307 Contributions to Mineralogy and Petrology (Electronic) Keywords: Archean crustal evolution igneous geochemistry isotopic composition magma migmatite neodymium isotope orthogneiss SHRIMP dating tonalite zircon Arctic Greenland Nuuk Crustal evolution Eoarchaean Itsaq Gneiss Complex Nd isotopes Tonalites Zircons Journal article 2015 ftanucanberra 2015-12-28T23:24:33Z The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 μm) younger (3,660-3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110-140 ppm Zr in the tonalites and likely magmatic temperatures of >850°C, zircon solubility - melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo - feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial εNd (3,85 0Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800-3,600 Ma) rocks of the Itsaq Gneiss Complex. Article in Journal/Newspaper Arctic Greenland Itilleq Nuuk Australian National University: ANU Digital Collections Arctic Greenland Nuuk ENVELOPE(-52.150,-52.150,68.717,68.717) |