Petrochemistry of opaque minerals in eclogites from the Western Gneiss Region, Norway: 1. Petrology of the oxide microassemblages

Abstract

Oxide minerals in various rock types from ten different eclogite pods in the Western Gneiss Region of southern Norway have been examined by reflected light microscopy. Thirteen main types of oxide micro-assemblage have been distinguished. The results include: 1. (a) the establishment of magnetite and hematite, as well as of ilmenite and rutile, as oxide phases stable in eclogite-facies parageneses in appropriate local bulk-rock chemical compositions; 2. (b) the recognition of relict magnetite derived from the protoliths of certain eclogites; 3. (c) evidence for the following petrological processes which correlate with, and add to, the available data and interpretations from the accompanying silicates: exsolution of rutile from garnet, of hematite from rutile and of ilmenite and hematite from ilmenite—hematite ss; carbonation of magnetite and of ilmenite; hydration and oxidation of orthopyroxene; oxidation of clinopyroxene, garnet, ilmenite and magnetite; reduction of magnetite and of ilmenite—hematite ss, and migrations of Fe, Mg and Mn; 4. (d) recognition of true exsolution rather than of oxidation as the process which generated many preamphibolitisation oxide microassemblages; 5. (e) evidence for the synchronous development of exsolution textures in the oxides and in the silicates prior to the initial eclogite-facies equilibration; 6. (f) further petrological evidence to distinguish the eclogite pods and their associated rocks, and to support the geodynamic model of lithospheric interdigitation of rock types derived from diverse foreign sources. Oxide minerals can evidently be valuable petrogenetic markers in complex polymetamorphic rocks such as eclogites and are well worthy of systematic investigation.