Abstract
Meta-igneous lithologies of the Cullivoe inlier in NE Yell, Shetland, have tonalite–trondhjemite–granodiorite (TTG) chemistry and yield U–Pb zircon crystallization ages of c. 2856 – 2699 Ma. Formation was coeval with protoliths of the Lewisian Gneiss Complex and the time of major Neoarchaean crustal growth in the North Atlantic Craton. The adjacent metasedimentary Yell Sound Group accumulated between c. 1019 and c. 941 Ma. The Cullivoe inlier and the Yell Sound Group were metamorphosed at c. 944 – 931 Ma, the former preserving granulite-facies mineral assemblages inferred to be of this age. Similar-aged metamorphic events recorded in other Laurentian metasedimentary successions in the North Atlantic region are attributed to development of the Valhalla orogen along the Rodinia margin. Ordovician (482 ± 30 Ma) and Silurian (428 ± 16 Ma) thermal rejuvenation resulted from successive phases of the Caledonian orogeny during closure of the Iapetus Ocean. The mechanism by which the Cullivoe inlier was emplaced into its current structural setting is uncertain. Either its western or eastern boundary is a major tectonic break, probably an early ductile thrust. However, this is now cryptic as a result of the Caledonian ductile reworking.
Supplementary material: SIMS and LA-ICP-MS analytical data, statistical analyses and major trace element analytical data are available at https://doi.org/10.6084/m9.figshare.c.3575723.
Central problems associated with high-grade, polymetamorphic terrains include the distinction between basement and cover units in situations where ductile deformation has removed all traces of original depositional unconformities, and the identification of early tectonothermal events that may have been masked by subsequent reworking and mineral growth. In such terrains, U–Pb geochronology carried out on accessory minerals such as zircon, monazite and rutile is a key tool for constraining protolith ages and metamorphic events and hence resolving these issues. The Ordovician–Silurian Caledonian orogenic belt (Fig. 1) is classic ground for the investigation of high-grade, polymetamorphic terrains. Caledonian orogenesis resulted from the closure of the Iapetus Ocean and collision of three continental blocks, Laurentia, Baltica and Avalonia (e.g. Pickering et al. 1988; Soper et al. 1992; Leslie et al. 2008). Despite widespread and often intense Caledonian reworking at amphibolite facies, modern geochronological and metamorphic investigations have made significant advances in distinguishing between basement and cover sequences and understanding the complex Precambrian tectonic evolution of the Laurentian-derived Northern Highland Terrane in Scotland (Fig. 1) (e.g. Rogers et al. 1998; Vance et al. 1998; Friend et al. 2008; Storey 2008; Cutts et al. 2009, 2010; Cawood et al. 2010, 2015).
(a) Location of Shetland in the British Isles. Box indicates location of (b). GGF, Great Glen Fault; HBF, Highland Boundary Fault; HT, Hebridean Terrane; IS, Iapetus Suture; MVT, Midland Valley Terrane; GT, Grampian Terrane; MT, Moine Thrust; NHT, Northern Highland Terrane; SUF, Southern Uplands Fault; WBF, Walls Boundary Fault; WKSZ, Wester Keolka Shear Zone. (b) Simplified geological map of the northern part of Shetland (modified after Cutts et al. 2009); the study area in NE Yell (shown in Fig. 2) is enclosed in a box. BMF, Bluemull Sound Fault; BFL, Burra Firth Lineament; H, Hascosay; WKSZ, Wester Keolka Shear Zone.
Shetland (Fig. 1) occupies a key location within the Caledonian orogenic belt because of its pre-Mesozoic proximity to the East Greenland, Scottish and Norwegian sectors of the orogen. The pre-Devonian geology is dominated by orthogneiss complexes and metasedimentary successions, which have been compared on lithological and geochemical grounds with various geological units in the Northern Highland and Grampian terranes of mainland Scotland (Flinn et al. 1972, 1979; Flinn 1985, 1988). However, the validity or otherwise of a number of the proposed correlations, and hence the identification of potential basement–cover relationships, is uncertain because of the lack of modern geochronological studies. In this paper we present U–Pb zircon and rutile data and geochemical analyses obtained from meta-igneous and metasedimentary lithologies that crop out in the NE of the island of Yell in northern Shetland (Fig. 2). The results provide evidence for (1) the existence of an inlier of Archaean basement that was emplaced tectonically into early Neoproterozoic cover successions, (2) early Neoproterozoic (Tonian) high-grade metamorphism of both basement and cover units, and (3) Ordovician and Silurian metamorphism during the Caledonian orogeny. These results provide a firmer basis for regional correlations with other lithotectonic units in the North Atlantic Caledonides.
Geological sketch map of NE Yell, showing the trends of foliation and lineation in the region, and sample locations.
Regional geological framework of Shetland
The Scottish Caledonides is divided into terranes by major faults (Fig. 1). The western margin of the orogen in mainland Scotland is represented by the Moine Thrust, which has traditionally been linked with the Wester Keolka Shear Zone in NW Shetland (Fig. 1; Andrews 1985; Flinn 1985, 1992; Ritchie et al. 1987; McBride & England 1994; see however Walker et al. 2016). To the west is the Laurentian foreland to the Caledonides, the Hebridean Terrane. In mainland Scotland, this is underlain by the Archaean to Palaeoproterozoic Lewisian Gneiss Complex (Friend & Kinny 2001; Love et al. 2004; Kinny et al. 2005; Wheeler et al. 2010). At the supposed equivalent structural position in NW Shetland, the Uyea and Wilgi Geos orthogneisses of the North Roe area are thought to be of Archaean age also (Flinn et al. 1979; Robinson 1983). Structurally above the Moine Thrust in mainland Scotland is the Northern Highland Terrane, which is dominated by the early Neoproterozoic Moine Supergroup (Holdsworth et al. 1994; Strachan et al. 2002). This was deposited between c. 1000 and c. 870 Ma near the margin of the c. 1.0 Ga Rodinia supercontinent, and affected by Knoydartian orogenic events between 840 and 730 Ma (e.g. Rogers et al. 1998; Vance et al. 1998; Cawood et al. 2010, 2015; Cutts et al. 2010). Structurally emplaced inliers of mafic and felsic orthogneisses within the Moine Supergroup have been broadly correlated with the Lewisian Gneiss Complex (Peach et al. 1910; Ramsay 1958, 1963; Winchester & Lambert 1970; Moorhouse & Moorhouse 1988; Strachan & Holdsworth 1988; Holdsworth 1989; Friend et al. 2008). In NW Shetland, the psammites of the Sand Voe Group have been correlated on lithological grounds with the Morar Group of the Moine Supergroup (Flinn 1985, 1988). The Sand Voe Group is interleaved with infolds or tectonic slices of hornblende gneisses (Fig. 1) thought to represent inliers of Lewisianoid basement (Flinn et al. 1979; Robinson 1983).
In mainland Scotland, the Northern Highland and Grampian terranes are separated by the Great Glen Fault, the northern extension of which is probably the Walls Boundary Fault of Shetland (Flinn 1961, 1977, 1992). The Grampian Terrane in mainland Scotland incorporates the early Neoproterozoic Badenoch Group (possibly correlative with the Moine Supergroup) (Leslie et al. 2013). Correlative lithological units in Shetland may be the metasedimentary gneisses of the Yell Sound and Westing groups (Fig. 1). Both record evidence for amphibolite-facies metamorphism at c. 930 – 920 Ma (Cutts et al. 2009, 2011) and incorporate strips of mafic and felsic orthogneisses that have been interpreted as inliers of the Lewisian Gneiss Complex (Flinn 1994, 2014). In mainland Scotland and in Shetland, these older metasedimentary successions are overlain by, respectively, the mid-Neoproterozoic to early Cambrian Dalradian Supergroup and the East Mainland Succession (Fig. 1). These were deposited on the margin of Laurentia during supercontinent breakup and development of the Iapetus Ocean (Anderton 1985; Strachan et al. 2002, 2013).
In mainland Scotland, the Highland Boundary Fault is regarded as defining the edge of autochthonous Laurentia. An Ordovician magmatic arc is thought to underlie the Devono-Carboniferous cover of the Midland Valley Terrane (Bluck 1983, 1984, 2001) and its likely northeastern continuation has been identified in the North Sea east of Shetland (Lundmark et al. 2014). The collision of this magmatic arc with the margin of Laurentia at c. 480 – 475 Ma was accompanied by obduction of ophiolites (including the Unst ophiolite of NE Shetland, Fig. 1) and resulted in the Grampian orogenic event (Lambert & McKerrow 1976; Chew et al. 2010). This caused widespread metamorphism and deformation of the Moine and Dalradian Supergroups and equivalent units in Shetland (Dewey & Shackleton 1984; Dewey & Ryan 1990; Kinny et al. 1999; Oliver et al. 2000; Cutts et al. 2011; Tanner 2013; Walker et al. 2016). The subsequent Silurian collision of Laurentia with Baltica resulted in the c. 435 – 425 Ma Scandian orogenic event. In mainland Scotland, regional deformation and metamorphism of the Moine Supergroup was followed by its northwestward translation along the Moine Thrust (Coward 1990; Dallmeyer et al. 2001; Dewey & Strachan 2003). The relative paucity of Silurian mineral ages in Shetland is attributed to a likely location at a high structural level in the regional nappe pile relative to mainland Scotland (Walker et al. 2016).
Geology of NE Yell
The geology of NE Yell is dominated by the metasedimentary rocks of the Yell Sound Group (Fig. 2). These are mainly gneissic, often migmatitic, psammites with subordinate semipelites and quartzites (Flinn 1994). Minor amounts of calc-silicate rock occur locally but no marble. No sedimentary structures have been preserved and hence the order of succession is unknown. Intrusive bodies of granitic orthogneiss and hornblende-schist (amphibolite) appear to record all the main tectonic and metamorphic events within the host metasedimentary rocks (Flinn 1994).
In NE Yell, the Yell Sound Group is underlain structurally by a NNW–SSE-trending strip of hornblendic and felsic orthogneisses that can be traced farther south onto the island of Hascosay, a total distance of 15 km. Flinn (1988, 1994, 2009) interpreted these orthogneisses (referred to here as the ‘Cullivoe orthogneisses’) as an inlier of Lewisian basement on lithological grounds. In eastern Yell and on Hascosay (Fig. 1), the orthogneisses structurally overlie psammites, semipelites and marbles of the Westing Group. Flinn (1994) recognized that if the orthogneisses were of Lewisian affinity, this would require the existence of a tectonic break along the eastern side of Yell, which he termed the ‘Hascosay Slide’, consistent with the often high levels of tectonic strain within all lithological units.
Although tectonic strain within the Cullivoe orthogneisses is often high, it is relatively low on Papil Ness (Fig. 2). Here, banded mafic, intermediate and felsic gneiss are coarsely layered on all scales by variation in the proportions of hornblende, biotite, quartz and feldspar, enhanced by leucocratic segregations and concordant, pink–orange granitic bands (Fig. 3a and b). Pods up to 10 – 20 cm wide of hornblendite are wrapped by the gneissic fabric. To the north, on Migga Ness (Fig. 2), the westernmost orthogneisses are two-pyroxene-bearing with isolated boudins of mafic and ultramafic bodies. The eastern part of the outcrop here mainly comprises a 50 × 150 m zone of metagabbro, locally containing a relic garnet–pyroxene metamorphic assemblage. To the south, on the Ness of Cullivoe (Fig. 2), tectonic strain is relatively high and mafic, intermediate and felsic gneisses with subordinate amphibolite mostly carry a strong, regular planar blastomylonitic fabric. Flinn (1994) reported zoned ultramafic pods from Migga Ness and Papil Ness, as well as 7 m wide clinopyroxene- and orthopyroxene-bearing pods from the south end of the Ness of Cullivoe.
(a, b) Photographs of the basement gneisses south of Bay of Brough (compass for scale). (c) Photograph of sample SH12-04, garnet-bearing metagabbro (d) Photograph from the SH12-09 sample locality at Migga Ness showing the strongly lineated orthogneiss.
Foliation across much of Yell is subvertical, although there is a gradual reduction in dip towards the east such that along the coastal strip foliation is generally inclined gently to the west (Fig. 2). Mineral lineations mainly pitch gently north or south irrespective of dip (Flinn 1994). The contacts of the Cullivoe orthogneisses with the Yell Sound and Westing successions are highly strained and associated with blastomylonites. The contact with the Yell Sound Group on Migga Ness is characterized by an oblique-slip sense of shear: mineral lineations trend at c. 120° and C-type shear bands indicate a top-to-the-ESE sense of displacement broadly parallel to the lineations. In contrast, the lower contact of the Cullivoe orthogneisses farther south in eastern Yell is associated with subhorizontal or gently plunging lineations with no clear sense of shear apparent. There is no discordance in foliation across these contacts that might be consistent with a tectonically modified unconformity between basement and cover, nor is there any trace within the metasedimentary successions of clasts of material that might have been derived from the orthogneisses.
Sample and zircon characteristics
Samples of orthogneiss and metagabbro were collected from the Cullivoe orthogneisses for U–Pb zircon geochronology and chemistry to establish (1) their protolith age(s) and tectonic affinities, and hence test the hypothesis that these represent a Lewisian-type (i.e. Archaean) basement inlier, and (2) their subsequent metamorphic history. U–Pb zircon dating was undertaken by either secondary ionization mass spectrometry (SIMS) or by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A sample of Yell Sound Group paragneiss was also collected for U–Pb zircon and rutile geochronology by SIMS to constrain (1) its provenance and depositional age by dating detrital zircon grains, and (2) its regional metamorphic history.
SH12-04: metagabbro [HP 53974 03230]
Sample SH12-04 is from the medium-grained garnet-bearing metagabbro exposed on the eastern margin of Migga Ness (Figs 2 and 3c). The sample contains the mineral assemblage garnet–orthopyroxene–plagioclase–hornblende–muscovite with accessory ilmenite–rutile–zircon–monazite. Garnet up to 2 mm in size is commonly replaced by chlorite. The sample contains a weak west-dipping foliation defined by coarse-grained orthopyroxene. Plagioclase commonly shows extensive sericite alteration and defines a NW–SE-trending lineation. Zircons are typically anhedral, and range between 50 and 300 µm in size. All zircons display cathodoluminescence (CL)-intermediate brightness. Some grains display regular internal boundaries, both narrow and broadly spaced, with thin CL-bright rims that are often discontinuous. Some grains have few or no visible growth zones; many show zones of recrystallization (Fig. 4a).
(a) CL images of representative zircons from sample SH12-04 (metagabbro). (b) CL images of representative zircons from sample SH12-14 (paragneiss, YSG). (c) CL images of representative zircons from sample SH12-16 (orthogneiss). SHRIMP analytical sites are shown as circles together with site number and the indicated U–Pb age in Ma (207Pb/206Pb ages are quoted if older than 1.2 Ga, and 206Pb/238U ages if younger, with 1σ errors).
SH12-14: paragneiss (Yell Sound Group) [HP 53736 05217]
The sample is from a 10 m wide shear zone at the boundary between the orthogneisses and the Yell Sound Group to the west (Fig. 2). The mineral assemblage is plagioclase–quartz–biotite–chlorite–muscovite–epidote–chloritoid–garnet with accessory zircon, ilmenite, rutile and monazite, and is characterized by a mylonitic fabric. Interleaved plates of muscovite and chlorite define the foliation on a millimetre scale. Rutile was observed in both random orientations as well as aligned with the foliation. Sample SH12-14 yielded abundant rounded to subrounded zircon ranging between 70 and 200 µm in size. Zircon grains comprise two types based on CL response. Type 1 grains display elongate subrounded morphology with cores of intermediate CL brightness surrounded by CL-darker rims. Some cores display oscillatory zoning. Type 2 grains are generally ‘soccer-ball’ grain shapes, some with weak sector zoning (Fig. 4b).
SH12-16: orthogneiss [HP 53780 05241]
The sample is from a narrow (c. 4 m) west-dipping shear zone located in the orthogneiss (SH12-09) (Fig. 2). The mineral assemblage consists of clinopyroxene–plagioclase–quartz–biotite–chlorite–epidote–chloritoid with accessory zircon and monazite. Coarse elongate clinopyroxene and orthopyroxene replaced by chlorite, biotite and epidote define the strong gneissic foliation. Ductile deformation in the shear zone has resulted in the development of a weak C-type shear band cleavage. Zircon occurs as inclusions in plagioclase and quartz, ranging up to 400 µm in size, and is subrounded prismatic to subhedral in shape. Zircon grains comprise two types based on CL response (Fig. 4c). Type 1 grains display elongate subrounded morphology with CL-dark cores often with oscillatory zoning, surrounded by unzoned CL-bright rims. Rims are typically discontinuous and of varying thickness. Type 2 zircons are typically rounded to equant, with zones of recrystallization and some sector zoning.
SH12-09: orthogneiss [HP 53982 05217]
Sample SH12-09 is felsic gneiss from the eastern margin of Migga Ness (Fig. 2). The mineral assemblage is quartz–plagioclase–biotite–chlorite–orthopyroxene with accessory zircon, ilmenite and rutile. Gneissic banding is developed on a centimetre scale by alternation of felsic layers and streaks with biotite- and hornblende-rich bands with some orthopyroxene, and dips moderately to the west. Felsic minerals define a lineation that plunges shallowly to the NW (Fig. 3d). Zircon grains comprise two types based on CL response, equivalent to the types observed in sample SH12-16.
11CL-04: orthogneiss [HP 54921 02599]
The sample is a medium- to coarse-grained felsic gneiss collected near Papil Ness (Fig. 2). Gneissic banding is developed on a centimetre scale by alternation of felsic layers and streaks with biotite-rich bands, and dips moderately westwards. The mineral assemblage consists of plagioclase–quartz–biotite with accessory zircon and ilmenite. In thin section, a weak foliation is defined by aligned biotite grains, which are commonly replaced by secondary chlorite. Zircons are mostly subhedral and rounded, rarely euhedral. They range from 100 to 300 µm in size and are often made up of a homogeneous dark core in CL, sometimes partially resorbed, surrounded by rims of bright to moderate CL response (Fig. 5e–h).
(a–d) CL images of representative zircons from orthogneiss sample 11CL-02. (e–h) CL images of representative zircons from orthogneiss sample 11CL-04. LA-ICP-MS analytical sites are shown as raster lines together with site number and the indicated U–Pb age in Ma (207Pb/206Pb ages are quoted if older than 1.2 Ga, and 206Pb/238U ages if younger, with 1σ errors).
11CL-02: orthogneiss [HP 54054 04479]
The sample is a banded, intermediate orthogneiss collected on the eastern side of the Ness of Cullivoe (Fig. 2). The mineral assemblage consists of feldspar–quartz–hornblende–biotite with accessory ilmenite, zircon and rutile. A weak foliation is defined by aligned hornblende and biotite grains, with the latter occasionally replaced by chlorite. Zircons are typically subhedral or rounded and are 100 – 300 µm in size. In CL, zircons have either a dark core often with oscillatory zoning surrounded by brighter rims, or a moderate to bright homogeneous core with rare thin dark rims (<20 µm) (Fig. 5a–d).
Analytical methods
U–Pb zircon (SIMS)
Zircons from samples SH12-04, SH12-16, SH12-09 and SH12-14 were separated from crushed rock using traditional magnetic and heavy liquid separation techniques. Selected grains chosen to be as representative as possible of the whole population were mounted in epoxy resin with standard zircons (BR266, NBS610 glass, OGC-1 and Temora-2). The grains were imaged using a CL detector fitted to a Phillips XL30 scanning electron microscope (SEM) using a 12 kV beam current. CL images were used to select analysis locations (e.g. Hanchar & Miller 1993; Nasdala et al. 2003).
Zircon U–Th–Pb isotopic compositions were measured using the sensitive high-resolution ion microprobe (SHRIMP II) instrument at the John de Laeter Centre, Curtin University, Perth, Western Australia. Analytical procedures for the instrument have been described by Kennedy & de Laeter (1994) and de Laeter & Kennedy (1998), and are similar to those described by Compston et al. (1984) and Williams (1998). SHRIMP was operated at a mass resolution of 5000, with a primary beam current of 2 nA and beam diameter of 20 – 25 µm. Data from the sample were collected successively cyclic field-stepping through the mass range of 196Zr2O+, 204Pb+, background, 206Pb+, 207Pb+, 208Pb+, 238U+, 248ThO+ and 254UO+. Data were combined after six cycles.
The zircon standards BR266 (Stern & Amelin 2003) and Temora-2 (Black et al. 2004) were used for 206Pb/238U age corrections. Zircon reference material OG1 (Stern et al. 2009) was used to correct for mass fractionation. Common-Pb corrections were made using the measured 204Pb in each sample. The common-Pb component, being largely surface contaminant, was modelled on the composition of Broken Hill ore Pb. The observed covariance between Pb+/U+ and UO+/U+ (Compston et al. 1984) obtained from analyses of the standard BR266 (599 ± 0.3 Ma; 206Pb*/238U = 0.09059) was used to correct instrumental inter-element discrimination of Pb/U ratios (Stern & Amelin 2003). Data were processed using SQUID II software and Isoplot/Ex (Ludwig 2003, 2009).
External spot-to-spot errors on zircon U–Pb calibration sessions were <1%, and a minimum error of 1% was applied, which reflects the long-term performance of the SHRIMP II facility. Uncertainties assigned to all isotopic ratios and dates for single analyses in data tables are at the 1σ level. Uncertainties of weighted mean values for pooled analyses in the figures are at the 95% confidence level. Error ellipses in concordia diagrams are at the 2σ level. 207Pb/206Pb ages are quoted if older than 1.2 Ga, and 206Pb/238U ages if younger. Percentage age discordance is defined as the percentage difference between 206Pb/238U and 207Pb/206Pb ages.
U–Pb rutile (SIMS)
Rutile grains were separated in conjunction with zircon from rock samples by conventional magnetic and heavy liquid separation. Grains were mounted in random orientation in epoxy resin with rutile standard WHQ (Taylor et al. 2012). Backscattered electron (BSE) imaging was undertaken to choose spot locations. Imaging was conducted on a tungsten-sourced Zeiss EVO SEM at Curtin University, Perth, Western Australia using a 20 kV beam current.
U–Pb isotopic measurements were collected on the SHRIMP II, and analysed using a primary beam current of c. 2.5 nA, with a beam diameter of c. 25 µm and an impact energy of 10 keV. Corrections for common Pb were made using the measured 208Pb isotope (Hinthorne et al. 1979; Compston et al. 1984). An assessment of potential radiogenic 208Pb from Th was made by measuring ThO+ at mass 264 u.
Pb/U ratios were determined relative to the WHQ (Windmill Hill Quartzite) standard, which has a measured age of 2625 Ma, 206Pb*/238U ratio of 0.5025 and U content of 164 ppm (Clark et al. 2000). A fixed slope of 1.12 was used to represent the observed covariance between Pb+/UO+ and UO2+/UO+ for the standard to calibrate unknown analyses (Taylor et al. 2012).
U–Pb zircon (LA-ICP-MS)
Samples 11CL-04 and 11CL-02 were crushed using a jaw crusher and disc mill and then sieved to <500 µm. Heavy mineral cuts were separated using a Wilfley table, then zircons were handpicked, mounted in epoxy resin and polished to half height. Grains were imaged using a Phillips XL30CP SEM and a CL detector at the University of Portsmouth.
Zircon U–Pb ages were determined by LA-ICP-MS at the University of Portsmouth using an Agilent 7500cs (quadrupole) ICP-MS system and a New-Wave UP213 (λ = 213 nm) solid-state Nd:YAG laser, after Jeffries et al. (2003). Each analysis consisted of c. 30 s background acquisition and 60 s sample acquisition. A 30 – 40 µm spot was rastered along a 60 µm line, with typical laser conditions of a 10 Hz repetition rate and a fluence of c. 4 J cm−2. Ratios were calculated using standard-sample bracketing and an in-house spreadsheet based on LamTool (Košler et al. 2008), measuring GJ-1 (Jackson et al. 2004) as the primary standard and Plešovice and Temora-2 as secondary standards. All uncertainties were propagated in quadrature. GJ-1 yielded a mean 206Pb/238U ratio of 0.09758 ± 00251 (MSWD = 0.36, n = 58, with 95% confidence limit) and 207Pb/206Pb ratio of 0.06013 ± 00018 (MSWD = 0.3, n = 58, with 95% confidence limit), whereas Plešovice and Temora-2 zircon standards yielded mean 206Pb/238U ages of 340.7 ± 3 Ma (2SD; 337.1 ± 0.37 Ma, Slama et al. 2008) and 419.1 ± 6.3 Ma (95%; 416.78 ± 0.33 Ma, Black et al. 2004) respectively. The amount of 204Pb in these analyses was below the detection limit, and no common-Pb correction was undertaken. All zircons of sufficient size were analysed, but only ages from a single growth zone and avoiding irregular features such as cracks and inclusions were used.
Major and trace element chemistry
Samples were split, passed through a jaw-crusher and powdered in a TEMATM disc mill. Major elements and Sc, Cr, V, Cu, Zn, Ni, Rb, Sr, Y, Zr, Nb, Ba, Pb and Sn were analysed by X-ray fluorescence spectrometry at the University of Portsmouth, against calibrations defined with international certified reference materials (CRMs). Fusion discs were used for the major elements and pressed powder pellets for trace elements. REE, Hf, Ta, Th and U were analysed by ICP-MS at the University of Portsmouth following fusion dissolutions with Li metaborate (rock:flux ratio 1:3) into 10% HNO3, also against calibrations defined with international CRMs. Accuracy and precision were monitored with independent CRMs and are both estimated to be better than 1% for major elements and 5% for trace elements.
Results and interpretation: U–Pb geochronology
Zircon
SH12-04: metagabbro
Eighteen analyses from this sample plot in a discordant array with upper and lower intercepts at 2687 ± 64 Ma and 977 ± 32 Ma respectively (MSWD = 4.8; Fig. 6a). Taking into account the potential for multi-stage Pb loss, the best estimate of the magmatic zircon age is given by the mean 207Pb/206Pb age of those analyses that are within 10% of the upper intercept with concordia, representing the least isotopically disturbed domains. Additional core analyses together with analyses of rims and recrystallized zones plot further along the array between those ages and the c. 946 Ma lower intercept. A group of four analyses consisting of three regularly internally zoned grains and one rim yields a weighted mean 207Pb/206Pb age of 2636 ± 150 Ma (2σ, MSWD = 36). The high MSWD, however, suggests that even this subgroup has been disturbed significantly. Excluding analysis 16.1.1 from the group yields a refined weighted mean 207Pb/206Pb age of 2699 ± 48 Ma with acceptable MSWD (2σ, MSWD = 1.2). This value is our best estimate of the protolith age of this sample. At the lower end of the discordance array, three analyses consisting of the recrystallized zones of two zircons and one oscillatory zoned zircon, and that are concordant within error have a weighted mean 206Pb/238U age of 944 ± 52 Ma (2σ, MSWD = 2). This value is interpreted as dating metamorphism. The spread in discordant apparent ages is interpreted as variable Pb loss during this event and subsequent metamorphic event(s).
Error ellipses shown at 2σ level. (a) Concordia diagram showing the results of all zircon analyses for sample SH12-04. (b) Concordia diagram showing the results of all zircon analyses for sample SH12-09. (c) Concordia diagram showing the results of zircon analyses for sample SH12-16; three analyses with large errors have been omitted. (d) Concordia diagram showing the subset of results of younger zircon with <10% discordance for sample SH12-16. (e) Concordia diagram showing the subset of results of zircon with <10% discordance for sample SH12-14. (f) Concordia diagram showing the subset of younger zircon with <10% discordance for sample SH12-14 colour coded according to Th/U ratio.
SH12-16: orthogneiss
Twenty-two spots were analysed. Analyses targeted the cores and rims of type 1 and type 2 grains. Single 207Pb/206Pb ages range from c. 2818 to c. 722 Ma. Four analyses were >50% discordant with high common-Pb content and large uncertainties; these have been excluded from the plot and age calculations. The remaining data plot in two groups defining a discordia with approximate upper and lower intercepts of 2821 ± 60 Ma and 917 ± 39 Ma respectively (2σ, MSWD = 10.8; Fig. 6c). Given the extent of isotopic disturbance and the lack of near-concordant analyses, this upper intercept age is considered to be the best estimate of the protolith age of this sample. All analyses of rims and recrystallized zones of type 1 grains, one type 1 grain and all type 2 grains define a younger population with 206Pb/238U ages ranging from c. 1006 to 835 Ma. Analyses that are <10% discordant at the lower end of the discordance array yielded a weighted mean 206Pb/238U age of 931 ± 29 Ma (2σ, MSWD = 7.1; Fig. 6d). Given that these analyses have distinctly low Th/U with respect to typical magmatically crystallized zircon (e.g. Hanchar & Miller 1993; Corfu et al. 2003) and that they are from unzoned rims of predominantly CL-bright responses or unzoned recrystallized grains, this age is considered a best estimate of the time of new zircon growth during metamorphism. The high MSWD of this grouping, however, together with attendant discordant analyses, indicates that additional, later isotopic disturbance has occurred.
SH12-09: orthogneiss
Seventeen spots were analysed. Both cores and rims of grains were analysed. All analyses were between 14 and 39% discordant (Fig. 6b). Although the plotted data points do not define a meaningful discordance line, the majority of analyses have apparent 207Pb/206Pb ages ranging from c. 2674 to c. 2250 Ma, consistent with a late Archaean magmatic crystallization age and later disturbance. Two analyses record distinctly older Archaean 207Pb/206Pb ages of c. 3079 and c. 3030 Ma; these may represent pre-magmatic inherited zircon.
SH12-14: paragneiss
Twenty-three analyses were obtained from 21 zircons. Analyses targeted the cores and rims of type 1 and type 2 grains. Cores of type 1 grains are interpreted to be detrital based on the presence of concentric zoning and typical magmatic Th/U ratios. Rims of type 1 zircon and grains of type 2 are interpreted to be metamorphic based on their morphology. The majority of analyses plot close to concordia (Fig. 6e). Three analyses with large uncertainties have been excluded from the plot; analyses that were >10% discordant have been excluded from age calculations. The inferred detrital grains have apparent 207Pb/206Pb ages ranging from c. 3113 to c. 1019 Ma with Th/U ratios between 0.08 and 0.71 (Fig. 6e and f). Analyses of type 1 rims and type 2 grains yielded a weighted mean 206Pb/238U age of 906 ± 33 Ma (2σ, MSWD = 13). The high MSWD indicates a significant spread in ages, and probably reflects the additional Caledonian disturbance of the original metamorphic grains. Three analyses with 206Pb/238U ages ranging from 859 to 865 Ma with lower Th/U ratios (0.2 – 0.3; Fig. 6f) are excluded and are interpreted to reflect minor loss of radiogenic Pb, resulting in partially reset grains. The remaining analyses form a single population with lower Th/U ratios (<0.03) than the inferred detrital grains, with a weighted mean 206Pb/238U age of 941 ± 16 Ma (2σ, MSWD = 1.4; Fig. 6f). This age is considered to be the best estimate of the timing of new zircon growth during metamorphism.
11CL-04: orthogneiss
Thirty-seven spots were analysed, of which 14 plot in a broad discordant array with upper and lower intercepts crossing concordia at 2702 ± 71 Ma and 1229 ± 100 Ma respectively (MSWD = 8.9; Fig. 7c). Eight analyses of zircon cores that are either homogeneous or show oscillatory zoning plot close to concordia (<5.3% discordant) at the upper end of the array. These analyses range in 207Pb/206Pb age from 2814 to 2630 Ma and are interpreted as magmatic zircons that have experienced some early isotopic disturbance to produce the observed spread in apparent ages. Additional core analyses together with analyses of bright and intermediate rims plot further along the array between those ages and the c. 1229 Ma lower intercept. The analyses defining the array plot along most of its length, and are interpreted to record variable resetting by the regional Neoproterozoic high-temperature metamorphic event. However, no concordant analyses of that age were obtained. Instead, three analyses of dark zircon rims with low Th/U define a younger population with a weighted mean 206Pb/238U age of 482 ± 30 Ma (2σ, MSWD = 2.5; Fig. 7d). This distinctly younger age records an episode of further metamorphism that resulted in the growth of new zircon rims, some grains showing distinct morphological overgrowths (grain 38, Fig. 5f).
(a) Concordia diagram showing the results of all zircon analyses for sample 11CL-02. (b) Concordia diagram showing the younger population of zircon analyses for sample 11CL-02. (c) Concordia diagram showing the results of all zircon analyses for sample 11CL-04. (d) Concordia diagram showing the younger population of zircon analyses for sample 11CL-04.
11CL-02: orthogneiss
Nineteen analyses from this sample plot in a discordant array with upper and lower intercepts at 2856 ± 44 Ma and 946 ± 23 Ma respectively (MSWD = 6.7; Fig. 7a). The analysed magmatic cores generally have a higher U content (78 – 341 ppm) than other parts of the grains, accounting for their darker appearance in CL. Those core analyses that are <10% discordant record 207Pb/206Pb ages ranging between 2983 and 2747 Ma. A distinct younger zircon population consisting of five cores of rounded zircons and four CL-intermediate rims, with the low Th/U signature that is typical of metamorphic zircon, record a weighted mean 206Pb/238U age of 940 ± 14 Ma (2σ, MSWD = 1.4; Fig. 7b). This younger concordant population is interpreted to again represent the high-temperature metamorphic event that also affected some of the magmatic Archaean zircons. Unlike sample 11-CL04, a younger Ordovician metamorphic age has not been identified in this sample but could be represented in narrow dark rims observed on some zircons that were too small to be analysed.
Rutile
SH12-14: paragneiss
Rutile from sample SH12-14 showed no signs of alteration, nor systematic zonation in BSE images, therefore only one analysis per grain was made. Thirty-five grains were analysed. The data define a close population (Fig. 8) with apparent 206Pb/238U ages ranging from c. 484 to c. 345 Ma. Nine analyses with large uncertainties have been excluded from the plot and age calculations. Twenty-six analyses were combined to yield a weighted mean 206Pb/238U age of 428 ± 16 Ma (2σ, MSWD = 0.62; Fig. 8). This age is interpreted to represent post-metamorphic cooling.
Concordia diagram showing the results of rutile analyses for sample SH12-14 (analyses with large errors have been omitted).
Results and interpretation: major and trace element chemistry
A suite of 10 samples of the Cullivoe gneisses was collected for the purposes of whole-rock chemistry. Two of these (11CL-02 and 11CL-04) provided zircons for dating, described above. Silica ranges from 45.22 to 51.21 wt% in the mafic gneisses, associated with the following oxide ranges: TiO2 from 1.32 to 3.00 wt%, Al2O3 from 12.51 to 20.14 wt%, Fe2O3(T) from 9.08 to 23.65 wt%, MnO from 0.12 to 0.32 wt%, MgO from 2.46 to 5.27 wt%, CaO from 8.21 to 10.78 wt%, Na2O from 1.90 to 4.54 wt%, K2O from 0.17 to 0.29 wt%, P2O5 from 0.09 to 0.18 wt%. For the felsic gneisses the following ranges apply: SiO2 from 63.66 to 70.27 wt%, TiO2 from 0.26 to 0.88 wt%, Al2O3 from 14.28 to 17.05 wt%, Fe2O3(T) from 3.97 to 7.58 wt%, MnO from 0.02 to 0.09 wt%, MgO from 0.64 to 3.35 wt%, CaO from 2.64 to 4.43 wt%, Na2O from 2.24 to 4.69 wt%, K2O from 0.61 to 2.72 wt%, P2O5 from 0.05 to 0.12 wt%. Harker diagrams (Fig. 9) display decreasing Fe2O3, MgO, TiO2 and P2O5 amongst the mafic gneisses, with increasing Al2O3, CaO and Na2O. K2O is uniformly low. For the felsic gneisses, there are strongly decreasing trends with SiO2 for MgO, Fe2O3, TiO2, K2O and P2O5, and broadly increasing trends for Na2O and Al2O3. On a plutonic total alkalis v. silica plot (Fig. 10) the data are bimodal, with mafic gneisses falling in the gabbro field and the felsic gneisses in the granodiorite–granite fields. However, on a normative Ab–An–Or diagram (Fig. 11a; Barker 1979), the felsic gneisses plot as tonalites and granodiorites, according rather better with their mineralogy, which often lacks abundant K-feldspar. On an AFM diagram (Fig. 11b) the mafic gneisses plot in the tholeiitic field whereas the felsic gneisses plot as calc-alkaline.
Harker variation diagrams for major and trace elements in mafic (filled symbols) and felsic (open symbols) samples of the Cullivoe gneisses.
Total alkalis v. silica diagram showing data from the mafic (filled symbols) and felsic (open symbols) samples of the Cullivoe gneisses.
(a) Ab–An–Or diagram showing data from the felsic Cullivoe gneisses. (b) AFM diagram showing data from the mafic (filled symbols) and felsic (open symbols) samples of the Cullivoe gneisses.
Trace element abundances are summarized with selected Harker-style diagrams in Figure 9. Alkali and alkaline earth metals are higher in the felsic rocks (e.g. Rb a few tens of ppm, Sr and Ba several hundreds of ppm). Rb is closely correlated with K2O, with high K/Rb ratios, consistent with granulite-facies metamorphism of the protoliths. Of the high field strength elements, Zr is more abundant in the felsic rocks (hundreds of ppm v. single-figure ppm), but Y is considerably lower (single-figure ppm; but compare with a few tens of ppm in the mafic rocks). Th (not shown) is very low (<1 ppm) in relation to Archaean crustal averages of 5.7 and 3.8 ppm (upper crust and bulk crust values: Taylor & McLennan 1985), with the exception of sample 11CL-03 at a more normal 4.7 ppm, again possibly indicative of granulite-facies metamorphism. Predictably, transition metals are higher in the mafic rocks (Sc, Cr and Ni a few tens of ppm; V, Cu and Zn (not shown) a few hundred ppm) than the felsic ones and generally their abundances fall with silica abundance.
In rocks such as these with a protracted metamorphic history, petrogenetic constraints are best derived from relatively immobile trace elements such as the REE, whose abundances are displayed as chondrite-normalized plots in Figure 12. Even so, given the presence of garnet in the mafic rocks, these should be treated with some caution. The majority of the mafic gneisses show flat heavy REE (HREE) at 20 – 40 times chondrites, insignificant Eu anomalies and mild light REE (LREE) depletion. One sample (08MN-02) has a significant positive Eu anomaly and mild LREE enrichment. On the other hand, the felsic gneisses show low HREE, large positive Eu anomalies in the more silicic examples and strong LREE enrichment. These features are sufficiently distinct to preclude a direct genetic relationship between the gneiss types. The mafic gneisses are likely to have been derived as tholeiitic magmas from a depleted mantle source, but must have undergone significant fractionation (olivine ± pyroxene) to reduce Cr and Ni abundances. La- and Eu-rich sample 08MN-02 has c. 20 wt% Al2O3 and the highest Sr of the mafic gneisses sampled (264 ppm); it may therefore have been enriched in cumulus plagioclase. The range of REE patterns generally falling with increasing SiO2, plus the large positive Eu anomalies and concave-upward HREE patterns in the more silicic rocks, strongly suggest amphibole involvement either as a residual phase during melting or during fractional crystallization.
Chondrite-normalized REE plots showing data from the mafic (filled symbols) and felsic (open symbols) samples of the Cullivoe gneisses.
Discussion
Ages of the igneous protoliths of the Cullivoe orthogneisses
U–Th–Pb SIMS analyses of zircon from samples SH12-04 and SH12-16 have yielded data that provide estimates of original igneous protolith ages. These estimates are from the upper intercepts of the discordant array from sample SH12-16 (2821 ± 60 Ma) and discrete, near-concordant populations of zircon core and recrystallized growth domains from sample SH12-04 (2699 ± 48 Ma). U–Pb LA-ICP-MS analyses of zircon from samples 11CL-04 and 11CL-02 have also yielded discordant arrays, with upper intercepts of 2702 ± 71 Ma and 2856 ± 44 Ma, respectively. These upper intercept ages are similarly interpreted as providing estimates of protolith ages. The orthogneiss protolith ages indicate Neoarchean crystallization ages, which range between c. 2850 to c. 2700 Ma. These ages provide the first constraints on the age of emplacement of the basement gneisses in this area, and are broadly in agreement with K–Ar ages of c. 2873 Ma (Flinn et al. 1979) and U–Pb zircon ages of c. 2870 – 2730 Ma (Davis 2012; Kinny & Strachan, unpublished data) obtained from gneisses of the Uyea Group, North Roe, which lie west of the Walls Boundary Fault on mainland Shetland (Fig. 1).
Provenance and age of the Yell Sound Group
Sample SH12-14 yielded insufficient detrital zircons for statistical analysis. However, concordant analyses of interpreted detrital grains of igneous origin have ages ranging between 3113 and 1019 Ma. In combination with the interpreted metamorphic zircon ages of c. 941 Ma (see below), the data indicate deposition of the sedimentary protolith between c. 1019 and c. 941 Ma. The ages of the youngest detrital grains in SH12-14 are broadly consistent with the U–Pb zircon age of the main detrital peak reported by Cutts et al. (2009) from metasedimentary sample SH11 of the Westing Group on the west coast of Unst (Fig. 1).
Neoproterozoic metamorphism
Three samples have yielded U–Pb SIMS data that provide estimates for the age of high-temperature metamorphism in the Neoproterozoic. These data are from concordant populations of zircon rims and metamorphic growth domains characterized by distinctly low Th/U ratios from samples SH12-04, SH12-14 and SH12-16. These returned weighted mean 206Pb/238U ages of 944 ± 52 Ma, 941 ± 16 Ma and 931 ± 29 Ma, respectively. This Neoproterozoic (Tonian) metamorphic event represents the principal time of isotopic disturbance of the protolith zircons as well as a time of growth of new zircon. These findings are consistent with the U–Pb LA-ICP-MS data obtained from samples 11CL-04 and 11CL-02, the discordant arrays recording lower intercept ages of 984 ± 120 Ma and 933 ± 51 Ma respectively, and a distinct population of low Th/U rims and rounded grains within 11CL-02 yielding weighted mean 206Pb/238U age of 940 ± 14 Ma. In summary, the data indicate that the Cullivoe orthogneisses and the Yell Sound Group both record a high-temperature metamorphic event at c. 940 – 930 Ma, similar to the c. 938 – 925 Ma age deduced by Cutts et al. (2009) for early Neoproterozoic metamorphism in the nearby Westings Group.
Caledonian metamorphism
The age of 482 ± 30 Ma derived from three concordant analyses from sample 11CL-04 is most easily interpreted as corresponding to the time of high-grade metamorphism during the Ordovician Grampian orogenic event (Oliver et al. 2000; Chew et al. 2010; Cutts et al. 2011; Crowley & Strachan 2015). The U–Pb rutile age of 428 ± 16 Ma obtained from sample SH12-14 is interpreted as representing the timing of the youngest metamorphic event the Yell Sound Group experienced during the Scandian orogenic event in the Silurian. As the mineral assemblage in the dated sample is mylonitic, consisting of low- to moderate-temperature metamorphic minerals muscovite, chlorite and epidote, this might suggest that Silurian metamorphism was associated with shearing at greenschist-facies conditions. However, because rutile grains locally overprint the mylonite fabric it is possible that this foliation formed during an older event.
Implications for regional correlations
The new data reported here confirm the hypothesis of Flinn (1994, 2009) that the Cullivoe orthogneisses probably correlate in a broad way with the Lewisian Gneiss Complex of the Hebridean Terrane in mainland Scotland and the Hebrides. The protolith ages of the Cullivoe orthogneisses are similar to those recorded by many of the older components of the Lewisian Gneiss Complex (e.g. Kinny et al. 2005; Wheeler et al. 2010), representing a period of significant Neoarchaean crustal growth in the North Atlantic Craton (e.g. Garde et al. 2000; Nutman et al. 2010; Tappe et al. 2011; Dyck et al. 2015). The geochemical characteristics of the felsic components of the Cullivoe orthogneisses are typical of tonalite–trondhjemite–granodiorite (TTG) suite rocks of the kind that dominate the Lewisian Gneiss Complex (Fowler & Plant 1987; Rollinson & Fowler 1987) and Archaean cratons globally (Moyen & Martin 2012). There is sufficient spread amongst the less-disturbed Archaean zircons (especially in sample 11CL-04) to suggest that isotopic disturbance had occurred prior to the 940 – 930 Ma event. We have no precise constraints on the timing of this early isotopic disturbance, but suggest that it most probably occurred during the c. 1.8 – 1.7 Ga Laxfordian event, which was a period of major reworking at high metamorphic grade of significant tracts of the Lewisian Gneiss Complex (Kinny et al. 2005; Wheeler et al. 2010, and references therein).
The inferred detrital zircon data from SH12-14, in combination with the metamorphic zircon ages, indicate that deposition of the Yell Sound Group occurred between c. 1019 and 940 Ma, during or shortly after the assembly of Rodinia at c. 1.2 – 1.0 Ga (Cawood et al. 2007). Broad correlation with the Torridon Group of the Hebridean Terrane and the Morar Group of the Moine Supergroup would be feasible if deposition occurred after c. 990 – 980 Ma. The Yell Sound Group is not lithologically similar to either succession, but could have been deposited in a contemporaneous but entirely separate and distal basin. Correlation of the Yell Sound Group with the Glenfinnan Group was proposed on lithological grounds by Flinn (1988) but this is now ruled out as the latter is thought to have been deposited after c. 920 Ma (Cutts et al. 2011; Cawood et al. 2015).
U–Pb zircon evidence for a high-temperature metamorphic event within the Cullivoe orthogneisses and the Yell Sound Group at c. 940 Ma is consistent with emerging datasets from peri-Laurentian terranes around the North Atlantic region. Recent geochronological investigations synthesized by Cawood et al. (2010) have revealed two major cycles of sedimentation and orogenesis along the northern Laurentian margin in the North Atlantic region between 1030 and 710 Ma (see also Kirkland et al. 2011; Malone et al. 2014). These events are recorded in regions currently exposed in Scotland, Shetland, East Greenland, Svalbard, Pearya and Norway, and are currently interpreted in the context of the development of an exterior, accretionary ‘Valhalla’ orogen along the margin of Rodinia (Cawood et al. 2010). An early ‘Renlandian’ phase of deformation, metamorphism and granite intrusions occurred between 980 and 920 Ma. It is this event that resulted in the high-temperature reworking of the Cullivoe orthogneisses and early metamorphism of the Yell Sound Group as well as the Westing Group (Cutts et al. 2009).
Thermal rejuvenation during the Caledonian orogeny is represented by the U–Pb zircon age of 482 ± 30 Ma from the Cullivoe orthogneisses and the U–Pb rutile age of 428 ± 16 Ma from the Yell Sound Group. Both ages are consistent with the c. 467 to c. 451 Ma monazite ages from northeasten Shetland reported by Cutts et al. (2011) and the currently understood tectonic framework for the Caledonian orogeny, involving Ordovician arc–continent collision and final continental amalgamation in the Silurian (e.g. Dewey & Ryan 1990; Dallmeyer et al. 2001; Chew et al. 2010). Cutts et al. (2011) did not find isotopic evidence of the c. 430 Ma Silurian metamorphic event recorded in the Yell Sound Group. However, this sample was obtained from a shear zone, and the 428 ± 16 Ma rutile age obtained from sample SH12-14 could reflect reactivation of the shear zone during the Scandian event. It is also possibly attributed to the lower closure temperature of rutile compared with monazite, c. 600°C (Cherniak 2000), and could represent cooling from the peak temperatures (c. 650°C) defined by Cutts et al. (2011) for meta-igneous lithologies on Yell and Unst. This age is reflected in c. 420 Ma K–Ar and 40Ar/39Ar mineral ages from NE Unst (Flinn & Oglethorpe 2005).
Structural setting of the Cullivoe orthogneisses
The isotopic data reported here have established that the Cullivoe orthogneisses constitute an inlier of Neoarchaean basement; outstanding issues relate to how and when it was interleaved tectonically with the Yell Sound and Westing successions. Consideration of these issues is made problematic by the generally high tectonic strains within, and along the margins of, the inlier. Because the metasedimentary successions west and east of the inlier differ lithologically (Flinn 1994), this means that the inlier cannot simply occupy the core of a major isoclinal fold. Either its eastern or western boundary must represent a significant tectonic break, and it is therefore possible that the other boundary could be a tectonically modified unconformity with either the Yell Sound Group or the Westing Group. However, the regionally gently plunging mineral lineations are not commonly associated with kinematic indicators, suggesting a strain regime close to overall bulk pure shear. The deformation event that produced these structures is thus unlikely to have been responsible for the primary interleaving of basement and cover, but instead reworked a pre-existing basement–cover relationship. We therefore suggest that the ‘Hascosay Slide’ of Flinn (1994) is an early tectonic break, most probably a thrust, located along either the western or eastern edge of the Cullivoe orthogneisses, but entirely cryptic as a result of the intensity of Caledonian reworking.
Conclusions
The geochemical characteristics of the felsic components of the Cullivoe orthogneisses are typical of TTG suite rocks. The igneous protoliths of the orthogneisses and metagabbro of the Cullivoe inlier crystallized at c. 2856 – 2699 Ma. Their evolution is broadly coeval with development of the protoliths of the Lewisian Gneiss Complex of mainland Scotland and the Outer Hebrides, and a major period of Neoarchaean crustal growth in the North Atlantic Craton.
Deposition of the sedimentary precursors of the Yell Sound Group occurred between c. 1019 Ma (the age of the youngest detrital zircon grain) and c. 941 Ma (the age of the oldest metamorphic event to affect these rocks). If sedimentation occurred at <1000 Ma, the Yell Sound Group might be coeval with the Torridon Group and Morar Group of mainland Scotland, but correlation with the younger (<920 Ma) parts of the Moine Supergroup (Glenfinnan and Loch Eil groups) is ruled out.
Both the Cullivoe inlier and the Yell Sound Group were metamorphosed at c. 944 – 931 Ma, with the former containing granulite-facies mineral assemblages inferred to be of this age. Similar-aged Tonian metamorphic events have been recorded in various Laurentian sedimentary successions in the North Atlantic region and attributed to development of an accretionary Valhalla orogeny along the margin of Rodinia (Cawood et al. 2010).
Thermal rejuvenation during the Ordovician (482 ± 30 Ma) and the Silurian (428 ± 16 Ma) resulted from successive phases of the Caledonian orogeny during closure of the Iapetus Ocean.
The mechanism by which the Cullivoe inlier was emplaced into its current structural setting in relation to the adjacent Yell Sound and Westing groups is uncertain. Either its western or eastern boundary must represent a major tectonic break, most probably an early ductile thrust of Tonian or Ordovician age. However, this structure is now entirely cryptic as a result of intense ductile reworking during the Caledonian orogeny.
Acknowledgements and Funding
Funding for fieldwork and analytical work was provided through Australian Research Council (ARC) project DE120103067 to C.C. SIMS zircon and rutile U–Pb analyses were carried out using the SHRIMP-II ion microprobe at the John de Laeter Centre, Perth, managed by A. Kennedy on behalf of a consortium consisting of Curtin University, the Geological Survey of Western Australia and the University of Western Australia with the support of the Australian Research Council. The geochemistry and LA-ICP-MS zircon analyses were carried out at the University of Portsmouth. We also acknowledge the facilities, and scientific and technical assistance of the Centre for Microscopy, Characterisation and Analysis. S.J. Daly and D. Chew are thanked for constructive reviews and A. Bird for her editorial handling. We thank C. Storey and S.P. Davis for discussions in the field.
- © 2017 The Author(s)
References
Evidence from U–Pb zircon geochronology for early Neoproterozoic (Tonian) reworking of an Archaean inlier in northeastern Shetland, Scottish Caledonides
- Article
- Abstract
- Regional geological framework of Shetland
- Geology of NE Yell
- Sample and zircon characteristics
- Analytical methods
- Major and trace element chemistry
- Results and interpretation: U–Pb geochronology
- Results and interpretation: major and trace element chemistry
- Discussion
- Conclusions
- Acknowledgements and Funding
- References
- Figures & Data
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