Engebøfjellet
Geologic overview
RutilNett
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| Geologic map, Førdefjord |
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| Geologic map, Engebøfjellet |
The grey gneisses which are intimately associated with the mafic rocks, are of two types: cm-dm thick leucocratic bands (quartz + feldspar + white-mica) alternating with mafic bands of garnet amphibolites and eclogites, and granodioritic to tonalitic gneisses within units several meters to several tens of meters thick. The Engebøfjellet eclogite and the surrounding undifferentiated mafic and felsic rocks belong to the Hegreneset complex.
Larger units of granitoid gneisses belong to the Helle complex.
The Engebøfjell eclogite forms a 2.5 km long E-W-trending lens with a distinctly massive character compared to the surrounding rocks. It is believed to represent a Proterozoic gabbroic intrusion that experienced crystal fractionation processes leading to the enrichment of Fe and Ti, and transformed into eclogite during Caledonian high-pressure metamorphism at approx. 400 Ma. In this process ilmenite in the protolith has been replaced by rutile, and the Ti-enriched parts of the body is now rutile ore.
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| Fe2O3 -TiO2 scattergram |
Relict magmatic zircons from the leuco-eclogite variety at Engebøfjell has been dated at 1500 Ma (unpubl. data from Thomas Krogh, Royal Ontario Museum, Canada).
In the Engebøfjell geologic map, the body is subdivided into two major eclogite types based on their varying iron and titanium contents:
- The ferro eclogite is Fe2O3-, TiO2- and garnet-rich (>14% Fe2O3, >3% TiO2 and > 25% garnet). In general, it has a more massive character than the other eclogite varieties found at Engebøfjellet, although in parts it is significantly banded and folded. This eclogite is the rutile ore-type eclogite, the major volume of which is found in the central and western parts of the deposit. This eclogite occasionally shows fairly sharp contacts to the leuco eclogite (see below), but usually the contacts are gradational over several decimeters to several meters via a "transitional" eclogite variety.
- The leuco- and transitional eclogite is TiO2 - and Fe2O3-poor (<14% Fe2O3 and < 3% TiO2), locally with preserved gabbroic protolith textures.
Major lithologic units
| The rocks enclosing the Engebøfjell eclogite body | |||
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Granitoid gneisses (undifferentiated) |
The granitoid gneisses represent Proterozoic granitic and tonalitic plutonic rocks belonging to the Helle complex. They intruded into the mafic and felsic rocks of the Hegreneset complex and were extensively deformed by the Caledonian orogeny. |
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Amphibolite unit (undifferentiated) |
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| The Engebøfjell eclogite rocks | |||
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Leuco eclogite |
Fine-grained eclogite with a fairly low garnet content. In varieties with minor deformation a gabbroic texture is preserved. Leuco- and transitional eclogite is distinguished from ferroeclogite by its leucocratic character incl. lower garnet content. The increase in iron and titanium values is gradual. |
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Transitional eclogite |
Eclogite intermediate between the leuco- and the ferro eclogite. |
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Ferro eclogite |
The ferro eclogite (ore type) is a relatively massive, dark, garnet-rich eclogite with a distinct rutile-content. A characteristic feature that is commonly seen is a series of thin cracks (D5) in which the eclogite has been retrograded (amphibolitised). Occasionally 1-2 mm large holes after a leached-out mineral can be seen along certain zones in this eclogite. |
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