Eklogitiska bergarter i Roan, Vestranden, Norge : en mineralinventering och texturstudie

Sammanfattning: During reconnaissance field work in the westernmost part of Vestranden, Nord-Trøndelag, Norway, some eclogitic rocks were found (by Leif Johansson and Charlotte Möller). They are of interest, because until now, eclogites have not been found in this northern part of the Western Gneiss Region. It is important to consider these rocks in the evolutionary context of the entire Western Gneiss Region. This B.Sc. examination paper includes an inventory of minerals and textures, and brief comments on observed reactions. The eclogitic bodies occur as lenses of varying size in a gneissic and migmatized countryrock, and consist of garnet, clinopyroxene and/or amphibole, and often kyanite. Clinopyroxenes are jadeite poor omphacites (<7,5 mole-% Jd) with a high molar proportion of diopside. In most of the samples, the clinopyroxenes are not stable, but are rimmed by symplectitic orthopyroxene and plagioclase. The garnets fall into two groups: one with pyrope contents of 29-37 mole-%, the other with 40-58 mole-% pyrope. In some samples, garnets are zoned with an increasing pyrope content towards the rim. They are rimmed by very fine-grained symplectitic orthopyroxene, spinel and plagioclase. In the kyanite-bearing samples, the kyanite crystals always have a reaction rim of corundum and sapphirine, with an outer rim of granoblastic, strongly zoned plagioclase. Sapphirine forms beautiful rims around kyanite. Observed reactions are typical of granulite facies. Are these reactions due to a temperature increase, or are they the products of an isothermal pressure drop during uplift of the Western Gneiss Region?