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A trace element perspective on Archean crust formation and on the presence or absence of Archean subduction

Stephen Foley
A trace element perspective on Archean crust formation and on the presence or absence of Archean subduction (in When did plate tectonics begin on planet Earth?, Kent C. Condie (editor) and Victoria Pease (editor))
Special Paper - Geological Society of America (2008) 440: 31-50


The early continental crust is dominated by high-grade gneisses with the composition of sodic granites (the tonalite trondhjemite-granodiorite or TTC suite) that date as for back as >3800 Ma. These are considered by many to be formed in subduction zones, and so have a critical role in the discussion about when plate tectonics may have begun. Trace elements can be used to learn about the Identity of minerals in the source rocks during melting, but only indirectly to infer tectonic environments. The integrated results from experimental petrology and major and trace element geochemistry of the TTG suite indicate that most of them formed by melting of garnet amphibolites of broadly basaltic composition. This can explain low Nb/Ta coupled with high Zr/Sm, as well as low concentrations of HREEs, Melting of eclogite probably increased in importance in the Late Archean, as shown by an increase in Nb/Ta. Melting of garnet amphibolite can be achieved either in subduction zones at appropriate geotherms or in the lower reaches of thick basaltic crust. Water contents must be much higher than in the original basalts, indicating hydrothermal alteration at near-surface conditions. Thus, the thick crust scenario requires volcanic piling to deeply bury hydrothermally altered basalts, and also delamination of underlying thick cumulates. If subduction occurred in the Archean (implying the operation of plate tectonics), than with a slightly higher average mantle temperature, subduction geotherms would have been disproportionately hotter than today. However, there is no evidence for large volumes of TTG gneisses formed by melting of garnet-free amphibolites in the Early Archean, which constrains the average mangle temperature to be only marginally hotter than on modern Earth. In the Early Archean, melting of more magnesian volcanics and cumulates produced low SiO (sub 2) melts and prevented the production of voluminous continental crust with >55 wt% SiO (sub 2) . Given current trace element evidence, the most likely scenario for Archean tectonics is a slightly modified plate tectonics with only marginally hotter average geotherms, but substantially hotter subduction geotherms, enabling melting of garnet amphibolites in the Late Archean.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 440
Title: A trace element perspective on Archean crust formation and on the presence or absence of Archean subduction
Title: When did plate tectonics begin on planet Earth?
Author(s): Foley, Stephen
Author(s): Condie, Kent C.editor
Author(s): Pease, Victoriaeditor
Affiliation: University of Mainz, Institue of Geosciences, Mainz, Federal Republic of Germany
Affiliation: New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science, Socorro, NM, United States
Pages: 31-50
Published: 2008
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Meeting name: Penrose Conference
Meeting location: Lander, WY, USA, United States
Meeting date: 20060613June 14-18, 2006
References: 162
Accession Number: 2008-124313
Categories: General geochemistrySolid-earth geophysics
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: Stockholm University, SWE, Sweden
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200848
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