Evidence for 930 Ma metamorphism in the Shetland Islands, Scottish Caledonides: implications for Neoproterozoic tectonics in the Laurentia–Baltica sector of Rodinia

Abstract

Zircon and monazite laser-ablation inductively coupled plasma mass spectrometry U–Pb geochronological data for two metasediment samples from the Westing Group, northern Shetland Islands, Scottish Caledonides yield ages between 938 ± 8 and 925 ± 10 Ma (Tonian) for upper amphibolites-facies metamorphism. Texturally early metamorphism is recorded by a migmatitic garnet + sillimanite + plagioclase + muscovite + biotite assemblage, which formed at c . 650–700 °C and 7 kbar. Subsequent reworking resulted in the growth of a secondary garnet + kyanite + plagioclase + muscovite + biotite assemblage at c . 650 °C and 8–9 kbar. In situ electron probe microanalysis (EPMA) U–Th–Pb chemical dating of monazite hosted within garnet grains and the matrix of one sample also give Tonian ages, apparently indicating that all the metamorphism occurred during the Neoproterozoic. However, the dominant structural fabrics appear to have formed during the Ordovician–Silurian Caledonian orogeny, suggesting that the reworking was substantially younger despite the apparent absence of Caledonian monazite or zircon ages. Detrital zircons are consistent with Laurentia–Baltica provenance. Deposition of the Westing Group is constrained to between c . 1030 and 930 Ma. The timing of Tonian metamorphism suggests possible correlations with sequences elsewhere in the northern Caledonides, including the Krummedal Succession of East Greenland and Laurentian-derived successions in Svalbard and northern Norway. Supplementary material: U–Pb LA-ICPMS and EPMA data are available at http://www.geolsoc.org/SUP18379 .

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

“…Also shown in Figure 8 is an additional data point of previously published conventional P-T data (Cutts et al 2009b). …”

“…This minor age difference could be the result of partial resetting of older grains, perhaps sourced from older metamorphism in Shetland (e.g. Cutts et al 2009b) or detrital grains. For sample FRQ-20, the older population (c. 474 Ma) was found in cores of large grains and could plausibly have grown at an early stage of Grampian metamorphism in response to a prograde silicate (garnet-bearing) reaction that produced monazite (Hermann & Rubatto 2003;Spear & Pyle 2010; see also Kelsey et al 2008).…”

Laurentia-Baltica-Amazonia relations during Rodinia assembly

“…2) and in correlative successions along northern Laurentia and possibly extending as far as Siberia (Cawood et al, 2016;Likhanov et al, 2015;Malone et al, 2014 (Cutts et al, 2009;Friend et al, 2003;Kirkland et al, 2006;Kirkland et al, 2007;Kirkland et al, 2008;Watt et al, 2000). The units were Balashov et al (1996) reported U-Pb zircon evaporation ages of 1160 ± 40 Ma for mafic and felsic igneous rocks of the Eimfjellet Group, which separates metasedimentary rocks of the underlying Isbjörnhamma and overlying Deilegga groups.…”

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

“…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).…”

“…The overall tectonic significance of these orogenic events has been much debated, the most recent syntheses suggesting that they correspond to accretionary events along a proximal active margin of the Rodinia supercontinent (Cawood et al 2010(Cawood et al , 2015Kirkland et al 2011). Although there is evidence of Neoproterozoic metamorphism within the Westing Group in western Unst at c. 930 Ma (Cutts et al 2009), younger Cryogenian Knoydartian orogenesis has hitherto been unreported in Shetland. This is important for regional tectonic models, as Knoydartian events are now shown to extend significantly northwards from northern mainland Scotland to Shetland.…”