Iron—titanium oxide minerals in the Series, Kap Edvard Holm, East Upper Layered Greenland

Abstract

S U M M A R Y .Electron-probe data are presented for coexisting magnetite and ilmenite from seven gabbro cumulates and a gabbro pegmatite, and bulk FeO-F%O3-TiO2 analyses for ten magnetiteilmenite assemblages. The oxides equilibrated at 58o to 72o ~ and oxygen fugacities of IO -1~'0 to to -20.5 bar. Late-stage alteration, associated with growth of hydrothermal silicates, resulted in the breakdown of ilmenite to rutile and the dissolution of magnetite. Higher levels in the intrusion equilibrated at lower temperatures than deeper levels. All features described are consistent with the retention of the initial water content of the magma within the walls of the intrusion.THE Upper Layered Series of the Kap Edvard Holm complex consists of plagioclaseaugite-magnetite and plagioclase-augite-olivine-magnetite orthocumulates. The presence, throughout the exposed succession, of cumulus magnetite indicates a high Fe3+: Fe 2+ ratio in the crystallizing magma and this is considered to be the result of a high water content.Other features associated with the high water content are, firstly, the ubiquitous occurrence of pargasitic amphibole as a primary crystallizing phase; secondly, extensive hydrothermal alteration of the primary minerals over a wide range of temperature during cooling, including metasomatic formation of quartz-albite-epidote-ilmenite rock (Elsdon, in preparation); thirdly, variations in the sites of initial primocryst nucleation (Elsdon, 5970, I97Ia).The petrographic and chemical features shown by the Fe-Ti oxides of the Upper Layered Series afford information on the physico-chemical conditions prevailing during crystallization of the magma and on the cooling history of the rocks down to temperatures at which plagioclase was replaced by albite+epidote and albite § nite. The clinopyroxenes similarly yield information regarding the cooling history of the rocks over an extensive temperature range (Elsdon, I97Ib).General petrography. All the Upper Layered Series cumulates contain cumulus magnetite, which forms subhedral to euhedral crystals up to I m m in size, with cumulus plagioclase, augite, and occasional olivine. Typical textures are shown in fig. I. With increasing height in the stratigraphic succession the modal proportion of Fe-Ti oxides increases, partly because of more extensive cumulus crystallization but mainly through an increasing quantity of intercumulus material, which often forms extensive clots up to 5 mm in size, enclosing silicate phases ( fig. IA). Near the top of the succession the cryptic layering is obscured by irregular variations in the iron ratios O

Autometasomatic alteration of gabbro, Kap Edvard Holm intrusive complex, East Greenland

“…In the Upper Layered Series (Elsdon, 1969a) there is evidence that the magma had a high water content, based mainly on observations on the Fe-Ti oxide assemblage (Elsdon, 1972). A notable feature of the Upper Layered Series is the widespread hydrothermal alteration of the primary minerals, and evidence to be presented in this paper suggests that the source of the alteration was a hydrous phase emanating from the crystallizing water-rich magma itself.…”

“…Chemical data presented elsewhere (Elsdon, 1972) show that these trellis-like networks are the remains of oxidation-exsolution intergrowths of magnetite and ilmenite, where the former phase has subsequently been removed. The volume formerly occupied by the magnetite is now largely occupied by epidote, which is sometimes rendered semi-opaque by the presence of an extremely fine-grained brown substance, probably a hydrated Fe-oxide, which also occurs elsewhere in the rock in small quantities.…”

“…Near the southern end of the Upper Layered Series the highest part of the sequence is exposed, as deduced from the geometry of the rhythmic layering (Abbott, 1962;Elsdon, 1969b). In one locality here ( fig.…”

“…1). It contains three units of layered gabbros (Elsdon, 1969a), mainly plagioclaseaugite-olivine-magnetite orthocumulates. Cutting all the gabbros, but themselves cut by later syenites (Lwin, 1960), are numerous dolerite dykes of the East Greenland coastal dyke swarm (Wager and Deer, 1939).…”

Geochemistry of ilmenite from the Asama ultramafic-mafic layered igneous complex, Mikabu greenstone belt, Sambagawa metamorphic terrane, central Japan.

“…The composition of ilmenite changes with magmatic differentiation (e.g., Snyder et al, 1993); it also varies according to type of parental magma (e.g., Cawthorn et al, 1988). The MgO content of ilmenite reaches 20 wt.% in kimberlites (Shee, 1984;Haggerty and Tompkins, 1984), but is usually less than 3 wt.% in basaltic rocks (Duchesne, 1972;Elsdon, 1972;Haggerty, 1976); ilmenite in granitic rocks is de pleted in MgO and tends to be enriched in MnO (Tsusue, 1973;Neumann, 1974;Agata and Suwa, 1983;Frost and Linsley, 1991). Although some authors (Lightfoot et al, 1987;Agata, 1995) have suggested that subsolidus compositional changes of ilmenite occur as the result of reactions with silicate minerals, the composition of ilmenite has been often used as an indicator of chemical con ditions of parental magma (Cawthorn et al, 1985;Cawthorn and Biggar, 1993).…”