The structural setting and U–Pb geochronology of Knoydartian pegmatites in W Inverness-shire: evidence for Neoproterozoic tectonothermal events in the Moineof NW Scotland

Abstract

U-Pb monazite data for the Ardnish and Sgurr Breac pegmatites in the SW Moine block give Knoydartian ages of 827 2 and 784 1 Ma respectively. Structural and metamorphic studies of the pegmatites and the local Moinian metasediments suggest that pegmatite generation was a consequence of localized high-temperature shearing and metamorphism within the pelitic horizons. Petrographic evidence from the sheared pelites is interpreted as indicating that the local garnet-grade metamorphism was contemporaneous with the shearing and pegmatite generation, and was, therefore, Knoydartian in age. Chemical zoning profiles in garnets are consistent with their growth during prograde regional metamorphic increases in P and T. The c. 45 Ma difference between the pegmatite ages implies that the Knoydartian tectonothermal event was diachronous, being either a prolonged or episodic event. The metamorphic and geochronological data are consistent with the presence of Neoproterozoic orogenic events in the SW Moine.

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

“…Morar Group rocks and we concur with the concept that D1 isoclinal folds were formed during a Knoydartian orogeny (see also Vance et al 1998;Rogers et al 1998). However, the exact timing and nature of events remains problematic as the Knoydartian ages appear to cluster in (at least) two phases (see Cutts et al 2010).…”

“…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.…”

“…It has been proposed that D2 in the southern Moine Nappe is also of Knoydartian age, but unequivocal evidence for this is lacking (see discussion in Rogers et al 1998). The structural relationships of individual dated pegmatites have been disputed: e.g.…”

“…The structural relationships of individual dated pegmatites have been disputed: e.g. for the c. 827 Ma pegmatite at Ardnish, Powell et al (1983) suggested a post-D2/pre-D3 age whereas Rogers et al (1998) assigned a post-D1/pre-D2 age, which is more likely. Storey et al (2004) reported a 437 ± 6 Ma U-Pb titanite age from a 'post-D2' pegmatite within the Eastern Glenelg gneiss.…”

“…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).…”