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
SUMMARYThe complex can be divided into three units, here named the Lower, Middle and Upper Layered Series respectively, on the basis of petrochemical data and correction of the dips of the layering for the effects of post-consolidational flexuring. There are no chilled margins, and the contact with the country rock consists of granular basic and ultrabasic rocks, older than the cumulates of the layered series. The intrusive mechanism is considered in the light of field and chemical data.
A sheet-like appinite from Breaghy Head, North Donegal, is closely associated with a spessartite intrusion. Field, mineralogical, and geochemical similarities indicate that the parent magmas were very similar, and the appinite magma may have been derived from the spessartite by small degrees of crystal fractionation. The appinite is divided into an arnphibole-rich lower part and a felsic upper part; the mineralogical contrast between the two is best explained by tao-pulse intrusion of a batch of spessartitic magma within which amphibole and pyroxene had already segregated due to crystal settling or flow sorting. Unlike other Caledonian appinites, the Breaghy Head intrusion is not one of a cluster, and appears to be an isolated development. Possible reasons for its location are discussed.K E Y WORDS Donegal Spessartite Appinite Lamprophyrc Caledonian Differentiation
Major, trace and isotopic element data are reviewed, in the light of current views on the petrogenesis of Caledonian granite magmas. The information at present available tends to confirm a crustal anatexis model rather than a mantle origin, but more work is needed to confirm this.
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