Parental magmas of Grenville Province massif-type anorthosites, and conjectures about why massif anorthosites are restricted to the Proterozoic

Abstract

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.