Direct dating of mylonite evolution: a multi-disciplinary geochronological study from the Moine Thrust Zone, NW Scotland

Abstract

Rb-Sr dating of sub-closure temperature, syn-deformationly crystallized white micas from the Moine mylonites (Knockan and Dundonnell), has yielded ages which vary from 437 Ma to 408 Ma. Morphological and major element analyses of the micas indicate that all the micas within the analysed samples were (re)crystallized during the Moine Thrust shearing. The ages yielded are therefore interpreted as marking the end of crystal plastic deformation associated with shearing on the Moine Thrust. The variation in ages between samples is significantly greater than the individual analytical errors and may be a product of strain localization within the shear zone. Alternatively it may indicate flow of strontium-rich fluids derived from outside the shear zone implying that feldspars may act as potentially unreliable initial isotopic reservoirs even in very high strain samples within the greenschist-facies shear zones. However, the Rb-Sr age are internally consistent and compatible broadly with existing geological and geochronological data for the region suggesting that even if the feldspars have not acted as perfect initial isotopic reservoirs the resulting errors were minimal. The general cessation of ductile deformation at c. 430 Ma probably dates the general initiation of imbrication within footwall to the Moine Thrust. The age of 430 Ma is therefore likely to be a time of significant brittle displacement on the underlying Ben More Thrust.K-Ar dating of the (re)crystallized Moine mylonite micas yielded anomalously old ages in comparison to Rb-Sr values as a result of incorporation of excess Ar during shearing. The degree of incorporated excess Ar decreases away from the base of the shear zone into the hanging wall, becoming indiscernible at c. 1.5 km structurally above the base of the shear zone. The distance (c. 1.5 km) may represent the distance that fluid can have flowed out of the shear zone during deformation.

Dating deformation in the Gran Paradiso Massif (NW Italian Alps): Implications for the exhumation of high-pressure rocks in a collisional belt

“…This is because of the contrasts between the Sr-isotopic signatures of different white mica grain size fractions, with a higher apparent age for the larger mica crystals. This can be interpreted as evidence for isotopic inheritance and incomplete recrystallisation of the white mica during the last ductile deformation episode (Freeman et al, 1998;Glodny et al, 1998;Müller et al, 1999;Glodny et al, 2008). With presence of augen-like feldspar, incomplete deformation-induced recrystallisation in this sample is also evident from the microtexture.…”

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

“…This is entirely feasible in the context of published data that indicate diachronous, northward-younging evolution of the marginal thrust belt that defines the western margin of the Laurentian Caledonides. In Scotland, development of the Moine Thrust Zone is known to have been essentially complete by c. 430 Ma (Freeman et al 1998;Dallmeyer et al 2001;Goodenough et al 2011). In contrast, marginal thrusting in East Greenland continued until at 400-380 Ma (Dallmeyer et al 1994), a necessary requirement to accommodate early Devonian (c. 400 Ma) high-pressure metamorphism within the internal sector of the orogen (Gilotti et al 2004).…”

Mylonites: lessons from Eriboll

“…But often these are closure dates, that is they represent the time that mineral passed through its blocking temperature. Rb -Sr dating of white mica which had syntectonically crystallized in the Moine mylonites gave a sub-closure age of c. 430 Ma (Freeman et al 1998) with a span from 437 to 408 Ma. This is within the span from c. 440 to 410 Ma for movement on the Moine Thrust Zone (Johnson et al 1985;Kelley 1988;Dallmeyer et al 2001).…”