P – T constraints and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides: implications for the structural setting of the Unst ophiolite

Abstract

An integrated in situ monazite laser-ablation inductively coupled plasma mass spectrometry and metamorphic equilibria study is used to establish the P – T conditions and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides. The results have implications for the structural setting of the Unst ophiolite, which was obducted onto metasedimentary rocks of the Dalradian Supergroup. Metapelites in the footwall of the ophiolite yield U–Pb ages between 462 and 451 Ma with P – T conditions varying from c . 7.5 kbar and 550 °C directly below the ophiolite to c . 10 kbar and 775 °C at structurally deeper levels. The timing of peak metamorphism corresponds closely to that of Grampian ( c . 450–470 Ma) metamorphism in mainland Scotland and Ireland, and Taconic ( c . 450–460 Ma) metamorphism in the Appalachians, thus confirming the near-synchroneity of this important arc accretion event along the Laurentian margin. There is a significant metamorphic contrast between the low-grade rocks associated with the Unst ophiolite and the P – T conditions recorded in its footwall. If published K–Ar ages of c. 470 Ma broadly record obduction of the ophiolite, its present basal contact is probably a younger tectonic break that was associated with the excision of at least c . 10 km of crustal section. Supplementary material: Electron microporobe chemical analyses of garnet grains are available at www.geolsoc.org.uk/SUP18494

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

“…Crustal thickening and metamorphism of the footwall successions followed obduction. Pelites on NW Unst and Yell yield U-Pb monazite ages of c. 451-462 Ma with P-T conditions varying from c. 7.5 kbar and 550°C directly below the ophiolite to c. 10 kbar and 775°C at structurally deeper levels (Cutts et al 2011). The gently dipping foliations, east-west-trending mineral lineations and prograde kyanite-staurolite assemblages preserved in NW Unst are plausibly related to obduction and nappe stacking (Cannat 1989).…”

“…The gently dipping foliations, east-west-trending mineral lineations and prograde kyanite-staurolite assemblages preserved in NW Unst are plausibly related to obduction and nappe stacking (Cannat 1989). However, the metamorphic contrast between the low-grade rocks associated with the ophiolite and regional P-T conditions in its footwall indicate that its present basal contact is a younger tectonic break (Cutts et al 2011). A gently dipping, retrogressive greenschistfacies shear zone underlies the lower ophiolite sheet (Read 1934;Cannat 1989;Flinn 2014), but whether final ophiolite emplacement resulted from orogen-parallel shear (Cannat 1989) or orthogonal, NW-directed thrusting (Flinn & Oglethorpe 2005) is uncertain.…”

“…3). These kyanite-garnet-staurolite-bearing lithologies have yielded peak P-T estimates of 7.5 kbar and 630°C (Cutts et al 2011). The ages of these samples are therefore interpreted to represent the timing of formation of the mica-bearing schistosity.…”

“…AB08-05, a foliated syntectonic pegmatite from northern Yell, has yielded a similar age of 459.4 ± 1.4 Ma. Cutts et al (2011) reported peak P-T conditions of 9 kbar and 650°C and a U-Pb monazite age of 451 ± 4 Ma from the Yell Sound Group on Yell. …”

“…3) may form part of essentially the same sedimentary package. Both successions record evidence for amphibolite-facies metamorphism at c. 930-920 Ma (Cutts et al 2009(Cutts et al , 2011 and incorporate strips of mafic and felsic orthogneisses that have been interpreted as inliers of the Lewisian Gneiss Complex (Flinn & Roddam 1994;Flinn 2014). The Yell Sound Group and the Westing Group are succeeded eastwards by the c. 14 km thick East Mainland Succession consisting of psammites, pelites and marbles with minor volcanic horizons (Flinn 1967), although the presumed intervening unconformity is obscured by high tectonic strain (Flinn 1988).…”