Grenvillian age decompression of eclogites in the Glenelg–Attadale Inlier, NW Scotland

Abstract

The Glenelg-Attadale Inlier is the largest basement inlier within the Caledonian Moine nappe of NW Scotland. In the eastern part of the inlier amphibolite-facies retrogression of the eclogites is associated with tectonic fabrics, and P-T estimates indicate significant decompression (c. 20 km). Previous Sm-Nd mineral-whole-rock dates indicated that peak eclogite-facies metamorphism occurred around c. 1.08 Ga, which was correlated with the Grenvillian orogeny. However, the middle REE enrichment of the analysed garnets suggests the influence of apatite inclusions. It is therefore likely that the interpretation of the c. 1.08 Ga age is complex, possibly reflecting re-equilibration at lower temperatures. Sampled eclogites contain zircon in a number of distinct textural forms that are mainly associated with pargasite and plagioclase, and are part of the retrograde amphibolite-facies assemblages. Titanite extensively replaces rutile, and is clearly associated with the retrograde amphibolite-facies event. A second textural type of titanite forms anhedral grains with plagioclase and pargasite, which is developed where the retrograde amphibolite-facies assemblage overprints the eclogite mineralogy. U-Pb dating has yielded the following ages: zircon age of 995 AE 8 Ma, and variably discordant rutile ages between 416 and 480 Ma. U-Pb and Pb-Pb isochrons on titanite and plagioclase/quartz separates yielded ages of 971 AE 65 Ma and 945 AE 57 Ma, respectively, in agreement with the zircon age. Analysed zircons and titanites are texturally part of the amphibolite-facies assemblage. The new zircon age demonstrates that amphibolite-facies metamorphism during exhumation occurred at 995 AE 8 Ma; the titanites could have closed with respect to Pb at this time or alternatively at some time between c. 1000 and 900 Ma. These data clearly demonstrate that parts of the Scottish basement underwent major thick-skinned tectonics during the Grenvillian orogeny. Rutile is part of the eclogite-facies paragenesis, and yet has young ages; these data are best explained by reheating producing near-total Pb loss related to emplacement of the late-to post-tectonic Ratagain Granite Complex at c. 425 Ma, at the end of the Caledonian orogeny.

P–T conditions of Grenville-age eclogite facies metamorphism and amphibolite facies retrogression of the Glenelg–Attadale Inlier, NW Scotland

“…The P-T estimates from the post-peak high-pressure granulite and upper amphibolite facies phases are the first attempt to establish the retrograde exhumation history of the eclogites and co-facial lithologies. Insights into the amphibolite facies retrograde conditions are particularly important given that Brewer et al (2003) now date this at c. 995 Ma. It is most likely that the P-T conditions following thermal re-equilibration of the Eastern Unit at amphibolite facies were c. 13 kbar and 650-700 • C. In terms of the decompression history, either partial re-equilibration of the Eastern Unit rocks occurred during decompression, or the rocks cooled under isobaric conditions following initial isothermal decompression into the high-pressure granulite facies stability field.…”

“…Traditionally, rocks of the Morar Group are interpreted as being deposited unconformably upon basement represented by the Glenelg-Attadale Inlier (Ramsay, 1958). Brewer et al (2003) note that the exhumation of the Eastern Unit eclogites was achieved partly by ductile shearing, with the shear zone along the contact of the Eastern Unit and Western Unit representing a structure during this process. However, this shear zone contains discontinuous slivers of Morar Group metasediments that appear to have undergone kinematically identical shearing to that seen in the eclogite at amphibolite facies (Storey, Brewer & Parrish, 2004) and, hence, most likely during the same event as the retrogression of the Eastern Unit.…”

“…Ph.D. thesis, Univ. Leicester, 2002;Brewer et al 2003;Oberti et al 1991). Garnet neoblasts most likely grew during eclogite facies metamorphism in equilibrium with omphacite and plagioclase as the textures and assemblages are identical to those described by Sanders (1988), whereas kyanite in the leucocratic areas was originally part of the eclogite facies paragenesis and reacted to form plagioclase and garnet during decompression within the eclogite facies.…”

“…They interpreted the older age to be close to the date of the peak of eclogite facies metamorphism. Brewer et al (2003) concluded that upper amphibolite facies retrogression of the Eastern Unit occurred at 995 ± 8 Ma by studying the U-Pb systematics of zircon and titanite. They therefore showed that retrograde metamorphism (c. 1000 Ma) occurred within the Grenville orogenic cycle.…”

Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions

“…Eclogite-facies metamorphism and subsequent retrogression and exhumation occurred between 1080 and 995 Ma (Sanders et al 1984;Brewer et al 2003) during the Grenville Orogeny. No isotopic evidence of Grenvillian metamorphism has been found in the Western Glenelg gneiss.…”