Abstract
The Ikátoq zoned ultramafic lens occurs as an elliptical body in Archaean gneisses and amphibolites to the south of Færingehavn, southern West Greenland. From the unaltered serpentinite core to the contact with the surrounding gneisses five zones were distinguished: serpentinite (5), talc-carbonate rock (4), tale (3), amphibole (2) and chlorite (1): their interrelationships are discussed in the light of the theory of zoning. It is shown that these five main lithological types developed from a common parent rock under the inf1uence of low temperature metamorphism as a process of steatisation – the outer chlorite zone representing altered host rocks, whilst the inner tale, tale-carbonate and serpentinite shells represent changed serpentine core. The original junction between the two parent rocks lies in the amphibole zone. The growth process of the zones is discussed and evidence concerning these mechanisms is presented. Estimates of bulk transport and ion migration during the alteration are developed from the chemical analyses, based on whole-rock formulas and equivalent volumes, using ideal-mineral reaction stoichiometry. The system was open to migrating components, and it is shown that the body lost Mg to the gneisses, which in turn supplied some Ca and K to the reacting system. It is likely that CO2 and H2O were permanently in excess. A volume contraction of approximately 6 per cent during the steatisation process is postulated.
Highlights
The growth process of the zones is discussed and evidence concerning these mechanisms is presented
The validity of the mass-balanced manipulations in the preceding section is supported by Fick's First Law of Diffusion, and Korzhinskii (1970, p. 111) indicates that the conditions to be satisfied for the application of this law are as follows: first, that the components must be transported through pore-solution diffusion, and secondly that the reaction must proceed with no substantial change of volume
In the case of the Ikåtoq model, the mechanics of grain-boundary diffusion are not completely applicable, because the pattern was disturbed by the formation of 'migration channeis', suggesting that fluid flow in certain parts of the lens may have predominated over cation diffusion in this case
Summary
The sequential occurrence of gneiss, chlorite, amphibole and tale in reaction between ultramafic bodies and more siliceous host is similar to that discussed in the literature (cf Read, 1934; Chidester, 1962; Jahns, 1967; Mathews, 1967; Curtis & Brown, 1969, 1971) These mineralogical zones are formed by the selective movements of ionic components migrating at different rates by a combination of pore-solution transport and ion diffusion, infiuenced by activity gradients set up at the contact of the two compositionally distinct materiais. The increase in Mg concentration towards the outer portions of the lens is indicative of the developing tale and amphibole zones It one uses ideal mineral formulas instead of rock formulas (Curtis & Brown, 1969), a volume contraction of 9.35 per cent can be caleulated. 'Flux' estimates for zone boundaries with the slope value to be equated with component activity gradients in a qualitative sense
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