Degree of Oxidation of Adirondack Iron Oxide and Iron-Titanium Oxide Minerals in Relation to Petrogeny

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.     

Iron-Titanium Oxide Minerals and Synthetic Equivalents

Phase-equilibrium studies in the system FeO-Fe₂O₃—TiO₂permit determination of the temperature and oxygen fugacityof formation of coexisting pairs of titaniferous magnetite andilmenite in many rocks. Temperatures thus obtained are probablyaccurate to ?50?C. Temperatures indicated for most igneous andmetamorphic rocks for which data are available are generallyconsistent with temperatures inferred by other methods. Temperaturesfor certain gabbroic rocks are too low for magmatic crystallizationand probably reflect the migration of ilmenite from titaniferousmagnetite to form separate granules upon cooling. The experimentally determined solubility of ilmenite in magnetiteis much too small to account for most ilmeno-magnetites by simpleexsolution. Subsolidus oxidation of magnetite-ulv?spinelg_SSto yield ilmenite-magnetite intergrowths has been experimentallyverified and probably takes place during cooling of many igneousand perhaps some metamorphic rocks. Oxidation at surface orhypabyssal conditions may produce metastable titanomaghemites. In order of increasing intensity of oxidation, the followingFe—Ti oxide pairs occur in plutonic rocks: ulv?spinel-richmagnetite_SS+ilmenite_SS, ulv?spinel-poor magnetite_SS+ilmenite_SS,ulv?spinel-poor magnetite_SS+hematite_SS, hematite_SS+rutile.