Magmatic and metamorphic controls on chemical variations within the Eiksunddal eclogite complex, Sunnmøre, western Norway

Abstract

The Eiksunddal eclogite complex is one of the larger group B (“country-rock”) eclogites within the Western Gneiss Region (WGR) of Norway. Whole-rock chemical data suggest a magmatic origin for the rocks within the eclogite complex. The chemical variations among the eclogitic rocks were controlled by accumulation of olivine (Fo 80) and plagioclase (An 70) and probably magnetite. The composition of the interstitial (“trapped”) liquid is derived by linear regression methods and is a high-Al orthopyroxene normative basalt. The most likely conditions of crystallization are: T ≈ 1150°C, P < 10 kbar. The Al-rich nature of the interstitial liquid may have implications for the formation of anorthosites and leucogabbros associated with similar mafic rocks elsewhere in the WGR. Trace-element characteristics are compatible with a continental origin of these rocks. During the subsequent metamorphic history, the marginal parts of the complex have suffered variable degrees of contamination. Abundant amphibole inclusions in large garnets in garnet websterites suggest local hydration at pre-eclogitic stages. This may be associated with introduction of radiogenic Sr as previously reported by W.L. Griffin and H.K. Brueckner. During prograde metamorphism, alkali exchange occurred during growth of garnet, clinopyroxene and biotite at the expense of pargasitic amphibole and orthopyroxene. The most extensive metasomatism however, was probably associated with the introduction of fluid during amphibolitization (retrogression) of the eclogitic rocks. Three model reactions — eqs. 5–7 — qualitatively explain the relation between degree of retrogression and the observed metasomatic effects. Whether Si and Ca are gained or lost during retrogression seems to be controlled by the extent of amphibole growth in diopside + plagioclase symplectites initially formed by breakdown of omphacite. Alumina has behaved in a more or less immobile way during metasomatism. Low mobility of Al is also reflected by the formation of subsilic (Al-rich) amphiboles and Al-rich plagioclase during garnet breakdown. This demonstrates the importance of kinetic factors in controlling mineral chemistry.