Late-Proterozoic tectonics in northwest Scotland: one contractional orogeny or several?

Abstract

The Glenelg-Attadale Inlier of the northwest Highlands of Scotland comprises a Western Unit (WU) containing Trondhjemite-Tonalite-Granodiorite (TTG) gneisses and subordinate mafic bodies and an Eastern Unit (EU) containing abundant mafic rocks (eclogite), minor ultrabasic bodies and manganiferous metasedimentary rocks and marbles intercalated with intermediate to acid gneiss. The boundary between these two units is an amphibolite facies ductile shear zone, which contains kinematic criteria indicating a top-to-the-west (contractional) sense of shear. The EU was exhumed from eclogite facies to amphibolite facies conditions of the mid-crust at ca. 1000 Ma via amphibolite facies ductile shearing. A series of shear zones with identical metamorphic grade and kinematics occur at the contacts of the EU with the WU and the surrounding Moine Supergroup. In this study, an attempt is made to directly date the shear zone between the EU and WU, which also contains Moine Supergroup metasediments. A U–Pb titanite age of 669 ± 31 Ma, from syn-kinematic titanite within a mylonite, indicates a minimum age for movement on this shear zone, demonstrating that it must be Late-Proterozoic in age, and not Caledonian as previously supposed. If the shear zone is Grenvillian (ca. 1000 Ma) in age, this would then require that the Moine be deposited before ca. 1000 Ma in order to undergo burial to the mid-crust at this time. However, the youngest detrital zircons in the Moine indicate a maximum deposition age of 1000 Ma and, therefore, preclude this possibility. An alternative possibility is that movement along this shear zone was punctuated, with an initial phase involving uplift of eclogites to the mid-crust at ca. 1000 Ma, followed by a further period of top-to-the-west shearing, coupling of the Moine Supergroup and EU in the mid-crust during a putative Neoproterozoic “Knoydartian” orogenic event. However, since there are now three contractional “orogenic” events affecting the Moine, ca. 820, 740 and 670 Ma, it is suggested here that the term “Knoydartian” is no longer well-defined and requires re-appraisal. A later phase of extensional ductile shearing and large-scale folding, during a period of lower amphibolite facies metamorphism, is shown to be broadly Caledonian, based on a 437 ± 6 Ma U–Pb titanite age from a pegmatite, that is deformed during this event, and prior to movement along the Moine Thrust Zone. This further suggests an Ordovician “Grampian” orogenic event within the Northwest Highlands of Scotland.