The Role of Partial Melting and Fractional Crystallization in Determining Discordant Migmatite Leucosome Compositions

Abstract

Anatectic migmatite leucosomes in the Quetico Metasedimentary Belt (Superior Province) are discordant to the host rock layering. Two morphological varieties within the anatectic leucosome suite are distinguished. The first type show little compositional or textural variation either across, or along, the leucosomes. In contrast, the second variety exhibits both compositional and textural variations in a single leucosome, typically with internal cross-cutting relationships. Major-oxide contents vary comparatively little in the Quetico anatectic leucosome suite, but there is a considerable range in the incompatible element (REE, Hf, Zr, Y and Th) concentrations. In particular La contents range from 1.8 to 78.1 p.p.m. and the La/Yb ratios from 9.1 to 101.9. Samples with high REE contents have negative Eu anomalies, whereas those with low total REE abundances have positive Eu anomalies, which indicate that feldspar fractionation was important in their petrogenesis. Three samples which have no Eu anomalies, and which are taken not to have experienced significant feldspar fractionation, are regarded as the closest approximation to a primary melt composition. Petrographic evidence indicates that only the most aluminous bulk compositions in the host rocks have melted, with cordierite and biotite as the principal residual phases. Batch partial melting models indicate that the three leucosomes without Eu anomalies could have been derived from 40–80 per cent partial melting of the aluminous metasediments, but garnet must have been a residual phase. Since the residuum from 40 per cent partial melting is more mafic than any of the rocks currently exposed in the area, it is concluded that the melting which gave rise to the leucosomes occurred at greater depth. Crystallization models indicate that the observed range of leucosome compositions can be derived by crystal fractionation of melt compositions similar to the three leucosomes lacking Eu anomalies (i.e. the assumed primary melts). Samples with high abundances of incompatible elements and negative Eu anomalies represent fractionated melts, whereas those with low levels of REE and positive Eu anomalies represent cumulates. Leucosome composition, morphology and texture can be related to crystallization history, notably the timing of crystallization with respect to leucosome intrusion. In particular, those leucosomes that exhibit compositional and textural zoning are interpreted to have undergone crystal fractionation during intrusion. Although a suite of migmatite leucosomes may be derived by partial melting, it is concluded that the trace-element composition of any particular leucosome depends, to a great extent, upon its segregation and crystallization history. Indeed, the primary melt composition may not be preserved.