No 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.

Peak and retrograde P-T conditions of Grenville-age eclogites from the Glenelg-Attadale Inlier of the northwest Highlands of Scotland are presented. Peak conditions are estimated as c. 20 kbar and 750-780 • C, in broad agreement with previous work. The eclogites subsequently followed a steep decompression path to c. 13 kbar and 650-700 • C during amphibolite facies retrogression. Peak eclogite facies metamorphism occurred > 1080 Ma and retrogression at c. 995 Ma, suggesting fairly sluggish uplift rates of < 0.3 km/Ma and cooling rates of < 1.25 • C/Ma, when compared with other parts of the Grenville orogeny and/or modern orogens. However, current poor constraints on the timing of peak metamorphism mean that these rates cannot be used to interpret the geodynamic evolution of this part of the orogen. The P-T-t data, together with petrology and the field relationships between the basement rocks of the Glenelg-Attadale Inlier and the overlying Moine Supergroup, mean that it is difficult to support the currently held view that an unconformable relationship exists between the two. It is suggested that more data are required in order to re-interpret the Neoproterozic tectonic evolution of the northwest Highlands of Scotland.

[1] Volcanic systems in Iceland comprise a central volcano linked to magmatic fissures, whose subsurface architecture remains enigmatic. The surface dimensions and trends of active fissures show their spatial relation with fault systems. Structure of the Fremri-Namur and Hengill volcanic systems was analyzed. The linear vent arrays marking recent fissures are tightly bound by vertical segmented faults, while normal faults accommodating extension are common on the flanks of the fissure swarm. Evidence of normal frictional slip on the fissure-bounded faults suggests that failure occurred in response to subsidence with subsequent dilation. This is supported by inward tilting of strata adjacent to faults compatible with a downsag phase. We propose that magmatic fissures have vertical feeders with lateral offshoots extending along the rift zone. Their inflation/deflation during an eruptive cycle causes subsidence. Such magmatically generated faults can be subsequently modified by tectonic extension.