Tectono-metamorphic evolution of the Glenelg-Attadale Inlier, northwest Scotland

The Glenelg-Attadale Inlier (GAI) is the largest tract of exposed basement rocks underlying the Moine Supergroup of northwest Scotland. The GAI is divided into two distinct crustal units, the Western Unit (WU) and the Eastern Unit (EU). The WU comprises dominantly Late Archaean acid gneisses of Tonalite-Trondhjemite-Granodiorite (TTG) affinity, cut by basic sheets. The WU underwent high-pressure granulite facies metamorphism and concomitant partial melting producing trondhjemites in the period 2600-2800 Ma ago. The WU underwent pervasive reworking at c.1750 Ma, possibly under eclogite facie conditions. A third event at c.1000 Ma affected the WU and involved amphibolite facies metamorphism and possibly eclogite facies metamorphism. The EU comprises mixed orthogneiss and paragneiss. Late Archaean trondhjemitic gneisses are intercalated with a Proterozoic volcano-sedimentary succession comprising marble, calc-pelite, pelite, manganiferous sediments and basic rocks. The EU underwent reworking at c.1440 to 1490 Ma, concomitant with the Pinwarian orogeny in North America and the Hallandian event in Scandinavia, suggesting a link. Eclogite facies metamorphism at c.1080 Ma occurred with the EU, at conditions of c.20kbar and 750°C. The EU was subsequently retrogressed and exhumed by 20-25km into the mid-crust at amphibolite facies by c.1000 Ma. The exhumation process was facilitated by ductile shearing at amphibolite facies. This major orogenic event in the period c.1080 to 1000 Ma is correlated with the Grenvillian-Sveconorwegian orogeny in north America and Scandinavia. The Moine Supergroup was deposited between c.1000 Ma and 870 Ma and was subsequently interleaved with the GAI by a process combining folding and ductile shearing at amphibolite facies. This interleaving process occurred in the period c.1000 to 670 Ma. The area underwent Caledonian folding and ductile shearing in the period c.440 to 425 Ma, prior to intrusion of the syn- to post-tectonic Ratagain Granite Complex.