Abstract

Fenitization in granitoid rocks is first noticeable in the form of hematite veinlets, accompanied by increased ordering of potash feldspar. With increasing fenitization, plagioclase is converted to albite plus calcite, and K-feldspar progressively converted toward maximum microcline. Acmitic pyroxenes become increasingly abundant. The chemical changes producing these effects consist of the substitution of sodium plus another cafemic ion for silicon in such a way as to maintain charge balance. The fenitizing fluid was essentially an NaCl brine, roughly 1 molal in alkali chlorides, and 0.01 molal total acidity. The content of iron, calcium, and magnesium was high. The oxygen fugacity was near 10−17 bars (hematite–magnetite), and the sulfur fugacity near 10−12 bars. Fenitization may have occurred at temperatures as high as 700°, but the main fenitization took place at temperatures slightly above 500° and fenitization ceased at about 450°. Pressures were low, probably less than 500 bars. The source of the fenitizing solutions was carbonatite, which in turn was derived from nephelinitic magma.

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