Authors / CoAuthors
Smithies, R.H. | Champion, D.C.
Abstract
Modern adakite forms in subduction zones with unusually high geotherms that result in high-pressure melting of subducted basaltic crust. Close geochemical similarities between adakite and Archaean TTG, which forms the major component of Archaean crust, have supported a view that Archaean TTG is likewise subduction related. However, recent studies have shown that conditions of adakite formation are not unique to a subducting slab and can be attained in basaltic lower crust in both subduction and non-subduction environments. Non-subduction environments may be equally relevant to the genesis of Archaean TTG. Heat flow calculations suggest that Archaean subduction (if it occurred), must have been at a low angle ? the same style of subduction that produces most modern adakites. However, if Archaean TTG was derived from a subducting slab, then like most modern adakites, it should show 1) evidence for interaction with a mantle wedge, and 2) an association with diagnostic subduction influenced magmas such as high-Mg andesite (sanukitoid), Nb-enriched basalts and boninites. Whereas this does appear to be the case for some Late Archaean terrains (e.g. Superior Province), such is not unequivocally the case for older terrains. This suggests either that Early Archaean TTG is not subduction related, or that the style of Early Archaean subduction was significantly different from modern subduction, including low-angle or flat subduction. The volume of preserved felsic crust in the Early Archaean Pilbara Craton, Western Australia, requires a complimentary volume of dense mafic material (residual after basalt melting) equaling a combined thickness of ~170 km, which is difficult to conceptualize through solely magmatic processes (e.g. underplating, mantle plume). Subduction provides a means whereby large volumes of mafic crust can be progressively cycled through a melting zone, but if applicable to the Early Archaean, features such as typically low Mg#, Cr, and Ni indicate that TTG melts of that crust must have avoided interaction with the mantle. We suggest that Early Archaean slabs were pushed or thrust beneath overriding basaltic crust, without the development of a mantle wedge. Early Archaean TTGs are melts of the underthrusted slab and their petrogenesis combines components of the subduction model for modern adakite and models for lower crustal sodic melts (e.g. Cordillera Blanca ? Peru; Ningzhen ? China).
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nonGeographicDataset
eCat Id
47636
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
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Keywords
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- External PublicationAbstract
- ( Theme )
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- geochemistry
- ( Theme )
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- plate tectonics
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- AU-WA
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
Publication Date
2003-01-01T00:00:00
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geoscientificInformation
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[-23.0, -20.0, 116.0, 121.5]
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