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TitleConstraints from experimental melting of amphibolite on the depth of formation of garnet-rich restites, and implications for models of Early Archean crustal growth
AuthorZhang, C; Holtz, F; Koepke, J; Wolff, P E; Ma, C; Bédard, J H
SourcePrecambrian Research vol. 231, 2013 p. 206-217,
Alt SeriesEarth Sciences Sector, Contribution Series 20130001
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf; html
Subjectstectonics; igneous and metamorphic petrology; Archean; continental crust; partial melting; subduction; oceanic crust; oceanic lithosphere; amphibolites; bulk composition; garnet; Precambrian
Illustrationstables; plots
ProgramTargeted Geoscience Initiative (TGI-4) TGI-4 Program Coordination
Released2013 07 01
AbstractThe felsic continental crust formed in the early Earth most likely resulted from melting of basaltic pro-toliths, but the geodynamic processes leading to partial melting are still debated. Attempts to reconcilegeochronological data, thermal modeling and experimental results have led to two major alternativescenarios: (1) partial melting of subducted oceanic slabs and (2) dehydration melting at the base of thick(or thickened) oceanic/simatic protocrusts. Existing experimental data on melting of metabasalt suggestthat garnet only becomes an important residual phase (>10 wt%) at depths >50-60 km, which has beenseen as difficulty for model 2. We present results of amphibolite dehydration-melting experiments atpressures of 5-15 kbar and provide constraints on melting reactions of a hydrated metabasalt with SiO2of 47.5 wt% and Al2O3of 16.4 wt%. Our experiments demonstrate that felsic melts and complementaryrestites with ~20 wt% garnet can form at ca. 900°C and 12 kbar, conditions corresponding to the base of a40-km thick basaltic protocrust that might be prevalent in the Early Archean. Based on phase proportionsdetermined experimentally and trace element partitioning data, our modeling suggests that such partialmelts resemble the Early Archean tonalite-trondhjemite-granodiorite (TTG) suites, including high Al2O3and low MgO contents, and modestly high La/Yb and Sr/Y ratios. The garnet-rich restite is calculated tobe denser than the underlying Early Archean lherzolitic upper mantle and would have the potential todelaminate. Our experimental results and combined geochemical modeling are consistent with modelswhere the initial growth of continental crust on the Archean Earth occurred in non-subduction settingsby anatexis of the base of basaltic plateaus.
Summary(Plain Language Summary, not published)
Melting experiments on metabasaltic compositions were performed to simulate generation of Archaean felsic crust. Results show that felsic melts similar to Archaean crust can be generated at temperatures of about 900C and pressures near 12 kb. These pressures and temperatures are not consistent with subduction zones, but could correspond to the base of a 40km thick basaltic plateau crust. Trace element models based on the experimentally determined phase proportions can reproduce Archaean felsic magmas. The density of the residues exceeds that of the mantle, suggesting that delamination of residues is plausible.

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