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The geochemistry of the Engebøfjell eclogite is a reflection of its complex geologic evolution, and mirrors both primary magmatic chemical variation in the Proterozoic gabbroic protolith and probably also the influence of Caledonian metamorphism.
Based on a large number of Fe2O3 and TiO2 analyses of cores by a portable XRF spectrometer and analyses of surface samples, some general features can be observed. Firstly, the Fe2O3 -TiO2 chemical variation is continuous from leuco eclogite and through transitional eclogite into ferroeclogite. Secondly, the ferroeclogite (>14% Fe2O3 and >3% TiO2) can be split into two groups, a "normal ferroeclogite" and a "Ti-enriched ferroeclogite". The metamorphic influence is considered minor, although evidence for this interpretation is not documented. Thirdly, some ferroeclogite samples at approx. 18% Fe2O3 are distinctly enriched in P2O5 (0.5-2% P2O5 vs. the normal values of < 0.2% P2O5); see Korneliussen et al. (1998) for further information.
Rutile is determined by the so-called rutile analytical procedure (% rutile = %TiO2 (analysed by XRF) - %TiO2 (dissolved in HCl; analysed by ICP)). In this procedure rutile is not dissolved while ilmenite is dissolved by HCl. Silicates are not dissolved in HCl, i.e. TiO2 in silicates will come out as rutile by this method. The method is only to be used on rocks, such as eclogites, where TiO2 in silicates are low enough to be neglected for practical purposes. In the typical Engebøfjell eclogite more than 90% of the titanium in the rock occurs as rutile. Those samples that plot well away from the line rutile/TiO2 = 1 have a distinct portion of retrograde ilmenite formed during the deformational episodes D3, D4 and D5. The effects of retrograde overprinting are shown in a variety of illustration in Alteration effects on rutile ore.
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Alteration effects
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