Late Precambrian U–Pb titanite age for peak regional metamorphism and deformation (Knoydartian orogeny) in the western Moine, Scotland

Abstract

There has been controversy over the number and timing of orogenies in the Precambrian Moine block in the Scottish Caledonides since the earliest radiometric dating in the 1960s. This work challenges a recent hypothesis, that this sector of the Laurentian margin was subjected to continuous crustal extension between .900 and 470 Ma. U-Pb dating (thermal ionization mass spectrometry) of titanite from a calcsilicate pod in the Moine (Morar Group) of the western Highlands gives an age of 737 AE 5 Ma. The titanite grew from Fe-Ti-bearing detrital minerals during the main progressive, syn-D 2 , amphibolite-facies (sillimanite zone) regional metamorphism, thus demonstrating that a Neoproterozoic contractional tectonothermal event (Knoydartian orogeny) affected the Moine block following the rift-related emplacement of the West Highland granite gneiss at 873 Ma. We conclude that the Sgurr Beag Thrust, a major tectonic break separating the Morar and Glenfinnan groups of the Moine, is mainly of Neoproterozoic, not Caledonian, age. The early tectonothermal event was succeeded by the Grampian Phase (Caledonian orogeny) at 460-470 Ma.

87 Sr/ 86 Sr chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and Ireland: constraints on depositional ages and time scales

“…2). This is consistent with the emerging evidence for early and mid-Neoproterozoic tectonothermal events between c. 800 and 730 Ma in the Moine Supergroup of the Northern Highlands and the Dava and Glen Banchor successions in the Central Highlands (Strachan et al 2002;Tanner & Evans 2003), as these events represent significant crustal thickening and so are likely to be orogenic in origin. Given the evidence for regional unconformity of Grampian Group rocks on the gneissose rocks of the Dava and Glen Banchor successions , the 'Moinian' metamorphic edifice must have been uplifted and significantly eroded before the Grampian Group was deposited, implying a significant time gap following the tectonothermal events.…”

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

“…van Breemen et al 1974;Powell et al 1983;Rogers et al 1998), U-Pb titanite ages from below the Sgurr Beag Thrust (Tanner & Evans 2003), and Sm-Nd ages from garnet (Vance et al 1998) -see Figure 1 for some localities. These ages fall into two clusters: c. 830-800 Ma and 780-740 Ma.…”

“…Giletti, 1961;van Breemen et al 1974;Rogers et al 1998;Vance et al 1998;Tanner & Bluck 1999;Oliver 2002;Tanner & Evans 2003;Cutts et al 2010;Cawood et al in press). deformation and magmatism appears to be mainly restricted to the higher Sgurr Beag and Naver nappes (Kinny et al 1999;Cutts et al 2010).…”

The internal structure of the Moine Nappe Complex and the stratigraphy of the Morar Group in the Fannichs–Beinn Dearg area, NW Highlands

“…Vance et al 1998;Rogers et al 1998;Tanner & Evans 2003;Cutts et al 2010). However, it remains unclear how the different mineral ages relate to the regional tectonic evolution because the extent and limits of Grampian and Knoydartian deformation and metamorphism remain uncertain (e.g.…”

“…This implies that the concept of a regional, Moine-wide D2 or D3 deformation phases (e.g. Rogers et al 1998;Tanner & Evans 2003;Kocks et al 2006), is not valid. We have shown here that a D1-Dn sequence is only valid within a single thrust sheet and that it is necessary (and possible) to analyze the relationship between D1-Dn sequences in different thrust sheets.…”