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Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust

Thorsten J. Nagel, J. Elis Hoffmann and Carsten Muenker
Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust
Geology (Boulder) (February 2012) 40 (4): 375-378


The earliest compounds forming Earth's first continental crust were magmatic rocks with tonalitic-trondhjemitic-granodioritic composition (TTGs). TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones. Alternative models argue that they may have formed by melting within thickened mafic oceanic protocrust. To simulate formation of Eoarchean TTGs in different tectonic regimes, we combine for the first time the thermodynamic calculation of residual assemblages with subsequent modeling of trace element contents in TTGs. We compare water-absent partial melting of two hydrated starting compositions, a modern mid-oceanic-ridge basalt (MORB) and a typical Eoarchean arc tholeiite from the Isua Supracrustal Belt that represents the country rock of Earth's oldest TTGs in southern West Greenland. At 10 kbar, partial melting of MORB-like residues results in modeled TTG compositions that are very different from natural ones. Melting at higher pressures (14 and 18 kbar) leads to a better match, but several key trace element parameters in TTGs are still amiss. A perfect fit for trace element compositions is achieved by melting of Eoarchean arc tholeiites at 10 and 14 kbar. These protoliths contain less Al and Na and more Fe and Mg as compared to present-day MORB and form amphibole-rich and plagioclase-free residues even at low pressures. Formation of Earth's oldest continental crust is therefore best explained by melting within tectonically thickened mafic island-arc crust.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 40
Serial Issue: 4
Title: Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust
Affiliation: Universitaet Bonn, Steinmann-Institute, Bonn, Germany
Pages: 375-378
Published: 20120228
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 31
Accession Number: 2012-037635
Categories: Igneous and metamorphic petrologyGeochemistry of rocks, soils, and sedimentsSolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2012101; accessed on March 22, 2012. IGCP Project No. 599
Illustration Description: illus. incl. sects.
Source Medium: WWW
N60°00'00" - N64°00'00", W60°00'00" - W50°00'00"
Secondary Affiliation: Universitaet zu Koeln, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201220
Program Name: IGCPInternational Geological Correlation Programme
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