Journal of Petrology | Volume 32 | Number 4 | Pages 701-738 | 1991
© Oxford University Press 1991
research-article |
Disequilibrium Melting and the Rate of Melt–Residuum Separation During Migmatization of Mafic Rocks from the Grenville Front, Quebec
Sciences de la Terre, Université du Québec à Chicoutimi Chicoutimi G7H 2B1, Québec, Canada
Received March 2, 1990; Revised typescript accepted November 28, 1990
ABSTRACT
Migmatites are developed in Archaean metabasites south of the Grenville Front. Relative to equivalent greenschist facies metabasites, those hosting the migmatites have undergone some mobilization of CaO, Na2O, and Sr, and, in the case of sheared metabasites, the introduction of K2O, Ba, Cs, and Rb, before migmatization. Three types of anatectic migmatite are recognized, based on their leucosome-melanosome relationships: (1) non-segregated migmatites in which new leucocratic and magic phases are intimately mixed in patches up to 15 cm across, (2) segregated migmatites in which the leucosomes are located in boudin necks and shear bands, and are separated from their associated mafic selvedges by 5–100 cm, and (3) vein-type migmatites where discordant leucosomes lack mafic selvedges. The non-segregated and segregated migmatites have a local and essentially isochemical origin, whereas the vein-type represent injected melt. Leucosomes from the segregated and vein-type migmatites have similar tonalitic major oxide compositions, but they differ greatly in their trace-element characteristics.
The vein-type leucosomes are enriched in K2O, Ba, Cs, Rb, LREE, Th, Hf, Zr, and P2O5 relative to their metabasite hosts, and have greater La/YbN ratios (27 compared with 0·6–17). These veins may have formed by between 5 and 25%equilibrium batch partial melting of Archaean metabasalt, leaving garnet + hornblende in the residuum.
In contrast, leucosomes from the segregated migmatites are depleted in REE, Sc, V, Cr, Ni, Co, Ti, Th, Hf, Zr, Nb, and P2O5 relative to their source rocks; the associated mafic selvedges are enriched in these elements. The leucosomes and mafic selvedges both have La/YbN ratios that are similar to those of the source metabasites irrespective of whether the source is LREE depleted or LREE enriched. The abundances of many trace elements in the leucosomes appear to be controlled by the degree of contamination with residuum material. Zr concentrations in the leucosomes are between 10 and 52% of the estimated equilibrium concentrations in felsic melts at the temperature (750–775 °C) of migmatization. A numerical simulation of disequilibrium melting using both LREE-depleted and LREE-enriched sources yields model melts with trace element abundances that match those of the natural leucosomes. Mafic selvedge compositions indicate that the segregated migmatites represent a range of between 12 and 36% partial melting of their host metamatization.
Based upon calculated dissolution times for zircon in wet melts, the melt and residuum were separated in less than 23a, otherwise melts would have become saturated in Zr. Rapid melt extraction is thought to be driven by pressure gradients developed during non-coaxial deformation of the anisotropic palaeosome during migmatization.
The common occurrence, based on published work, of disequilibrium compositions in migmatite leucosomes implies that during mid-crustal melting the melt-segregation rates are greater than the rate of chemical equilibration between melt and the residual solid. In contrast, at the higher temperatures of granite formation, the rate of chemical equilibration exceeds that of melt-segregation and equilibrium melt compositions are reached before segregation can occur. On the basis of their trace element characteristics, the melt which forms segregated migmatites cannot be the same as that which forms the vein-like migmatites, or granitoid plutons.
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