Degree of Oxidation of Adirondack Iron Oxide and Iron-Titanium Oxide Minerals in Relation to Petrogeny |
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| Authors: | BUDDINGTON A F; FAHEY JOSEPH; VLISIDIS ANGELINA |
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| Institution: | U.S. Geological Survey, Princeton University Princeton, N.J., U.S.A.
U.S. Geological Survey Washington, D.C., U.S.A. |
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| Abstract: | Partial chemical analyses of the accessory iron oxide and iron-titaniumoxide minerals from more than one hundred samples of Adirondackigneous, metamorphic, and metasomatic rocks portray the degreeof oxidation of the minerals and afford a basis for a discussionof their relation to the accompanying silicate assemblage andpetrogeny. Among the granitic igneous rocks, the green pyroxenicfacies have the least oxidized iron-titanium oxide mineralsand a pink potassium-rich microperthitic microcline facies themost oxidized. Granites and syenites may crystallize with eitheran iron-rich variety of hornblende (or pyroxene) plus ilmenite,or with an iron-magnesium variety of hornblende (or of pyroxene)plus magnetite plus ilmenite, depending on the degree of oxidation.The granite facies of the orthogneisses of the Diana Complex(Precambrian) have a more oxidized mineral assemblage than thesyenitic and quartz syenitic facies. The anorthosites and gabbroanorthosites have relatively oxygen-rich oxide minerals. Orthogneissesin the granulite facies have a lower ratio of Fe2O3/FeO thansimilar gneisses in the amphibolite facies. At least part ofthis lower ratio is due to metamorphism at higher temperaturesand pressures. A regional belt of granitic orthogneiss metamorphosedin the amphibolite facies has a magnetite-sphene instead ofa magnetite-ilmenite assemblage. Metasomatism of biotite-quartz-plagioclasegneiss to sillimanitic quartz-microcline gneisses is accompaniedby decrease in mafic silicates and a series of changes of mineralsin intermediate stages such that, in general, biotite yieldswith increasing degrees of oxidation successively such assemblagesas (1) biotite, garnet, magnetite, hemo-ilmenite, and ilmeno-hematite;(2) pale brown mica, sillimanite, magnetite, and ilmeno-hematite;and (3) rutilo-hematite, rutile, and meagre silicates includingsillimanite, muscovite, and chlorite. Diabase altered to monzodioriteby potassium-bearing solutions has ilmeno-magnetite partly alteredto ilmeno-maghemite. |
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