Geochemical evidence for loss of Na and K from Moinian calc-silicate pods during prograde metamorphism

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Abstract
Summary: A total of 111 major element analyses have been made of impure calcareous rocks from the Late Precambrian Moinian sequence in NW Scotland. Samples were collected along a 36 km traverse from three stratigraphic horizons which are repeated by folding and vary in metamorphic grade from lower to upper amphibolite facies. Anorthite content of the plagioclase, which increases progressively from albite to anorthite, was assessed by X-ray diffraction measurement of 2θ (131–11); this value is used as a ‘metamorphic index’ for rocks with a CaO/Al2O3 (wt. %) ration of 0.3–0.7 and Na2O/Al2O3, < 0.35. Na2O/Al2O3 and K2O/Al2O3 ratios plotted against this index, together with the spatial relationship between metamorphic/chemical isograds and stratigraphic boundaries show that both Na (0.24–4.82% Na2O) and K (0.01–3.28% K2O) are lost from the calc-silicate rocks during prograde metamorphism. These chemical changes result in a variation of the stable mineralogy with increasing metamorphic grade and take place through the medium of the intergranular fluid.