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TitleParental magmas of Grenville Province massif-type anorthosites, and conjectures about why massif anorthosites are restricted to the Proterozoic
AuthorBédard, J H
SourceSixth Hutton Symposium on The Origin of Granites and Related Rocks: Proceedings; Geological Society of America, Special Paper vol. 472, 2010.,
Alt SeriesNatural Resources Canada, Contribution Series 20180252
PublisherGeological Society of America
MeetingHutton Symposium; Stellenbosch; ZA; July 2-6, 2007
Mediapaper; on-line; digital
File formatpdf
NTS12J; 12K; 12L; 12M; 12N; 12O; 12P; 13A; 13B; 13C; 13D; 13F; 13G; 13H; 21M; 22; 23A; 31I; 31J; 31K; 31L; 31M; 31N; 31O; 31P; 32A
Lat/Long WENS -78.0000 -58.0000 57.0000 46.0000
Subjectsigneous and metamorphic petrology; tectonics; igneous rocks; gabbros; ilmenite; apatite; continental crust; crustal studies; crustal evolution; plate tectonics; tectonic evolution; tectonic setting; tectonic interpretations; Archean; Grenville Province; Precambrian
Illustrationslocation maps; tables; plots
AbstractTrace element inversion modelling of Grenvillean anorthosite massifs and associated rocks yield NMORB-normalised trace element profiles enriched in highly incompatible elements; commonly with negative Nb and Th anomalies. Model melts can be divided into subtypes that cannot be linked through fractional crystallisation processes. Most model melts are depleted in the heavy rare-earth elements and can be explained by partial melting of arc basaltic sources (5–60 melting %) with garnet-bearing residues. Some of the model melts have flat NMORB-normalised profiles (for rare-earth elements), have high compatible element contents, and might have been derived from mantle fertilised by arc magmatism, followed by low-pressure fractional crystallisation. Intermediate Ce/Yb types may represent mixtures of these end-members, or less probably, variations in the crustal source composition and residual assemblage. The active tectonic context now favoured for the Grenville Province appears to be inconsistent with plume or thermal insulation models. The heat source for crustal and mantle melting could record either post-orogenic thermal relaxation of a tectonically-thickened arc crust, or basaltic underplating caused by delamination of a mantle root or subduction slab beneath this arc crust. In this context, pre-Proterozoic anorthosites may be lacking, because prior to ca. 2·5 Ga, the crust may have been too weak to be thickened tectonically. The absence of post-Proterozoic anorthosites may be due to the secular decrease in radiogenic heating and cooling of the mantle and crust.

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