Refertilisation of the Almklovdalen dunite body at the north of Gusdal Quarry locality, Western Gneiss Region, Norway.

Abstract

One of the most intriguing characteristics of the Western Gneiss Complex (Norway) are the commonly occuring ultramafic bodies that have been emplaced during continent subduction from the northwest into the surrounding quartzofeldspathic gneisses during the Scandian. The Almklovdalen garnet peridotite (AGP) is one of such enclosures and forms the topic of the current study. The AGP was originally formed in Archean times as a dunite/ harzburgite. During Proterozoic and Scandian times it was refertilised. Four refertilised dunite locations are recognized within the AGP: Lien, Rødhaugen, Raudkleivane, and north of Gusdal Quarry. The first three locations are thoroughly studied (respectively by Medaris Jr., L.G. 1980, Carswell, D.A. 1981, and Lappin M.A. 1974). From the last locality garnet websterite samples were taken in order to do mineral chemical analyses and geothermobarometry. The philosophy used was that when a positive correlation can be made between the mineral chemistry and calculated PT conditions of the various refertilised locations, the age of refertilisation in the north of Gusdal quarry location can be inferred as the other three locations have already been dated. Results of geothermobarometric calculations indicate PT conditions for the stage 1 assemblage (porphyroclasts) of 32 ± 3 kbar and 740 ± 20 °C for the combination of TEG79 and PNimisTaylor00. For the combination of T¬Powell85 and PNimisTaylor00 the estimated PT conditions for the stage 1 assemblage are 31 ± 2 kbar and 712 ± 17 °C. For the stage 2 assemblage (matrix minerals) the calculated PT conditions are 31 ± 3 kbar and 725 ± 25 °C for the combination of TOpxBK90 and PNimisTaylor00. The equilibrium conditions for both assemblages are thus the same, most likely implying a reset of the stage 1 assemblage during Caledonian times by solid state diffusion. The age of refertilisation in the north of Gusdal quarry location is inferred to have happened during the Proterozoic, either at the Gothian event (1.8 – 1.65 Ga) or at the Sveconorwegian event (1.3 – 0.8 Ga).