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Garnet Provenance Studies, Juxtaposition of Laurentian Marginal Terranes and Timing of the Grampian Orogeny in Scotland

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Garnet Provenance Studies, Juxtaposition of Laurentian Marginal Terranes and Timing of the Grampian Orogeny in Scotland Journal of the Geological Society, London, Vol. 155, 1998, pp. 541–550. Printed in Great Britain Garnet provenance studies, juxtaposition of Laurentian marginal terranes and timing of the Grampian Orogeny in Scotland A. R. HUTCHISON* & G. J. H. OLIVER Crustal Geodynamics Group, School of Geography & Geosciences, University of St Andrews, Purdie Building, North Haugh, St Andrews, Fife, KY16 9ST, UK (e-mail: [email protected]) *Present address: BP Exploration Operating Co. Ltd., Fairburn Industrial Estate, Dyce, Aberdeen AB21 7PB, UK Abstract: A study of the composition and zoning patterns of detrital garnets from Ordovician greywackes from the Northern Belt of the Southern Uplands terrane of Scotland reveals characteristics of the metamorphic sources very similar to the Dalradian Supergroup of the Scottish Grampian terrane. The radiometric cooling and uplift ages of the Dalradian metamorphic zones and the depositional ages of the Southern Uplands greywackes support the hypothesis for local provenance. Detrital metamorphic garnet, identical to Dalradian garnet, has also been identified in the Upper Ordovican sandstones of the Highland Border Complex. These observations do not support proposals that the Grampian, Midland Valley and Southern Uplands terranes were exotic to each other in the Late Ordovician time. These new results, together with a review of published age dates, clarify the Late Ordovician palaeogeography for this part of the Laurentian margin. The distances between the Grampian, Midland Valley and Southern Uplands terranes may have been similar to the present day. It is concluded that large rivers flowed out of the uplifting mountainous Grampian terrane and across the Midland Valley into a Southern Uplands trench during the Late Ordovician time. The main orogeny (i.e. mountain building) in the Grampian terrane was therefore post-Cambrian, producing the first high mountains and resultant flysch in the Caradoc. Keywords: Laurentia, Grampian orogeny, provenance, greywackes, palaeogeography. The usefulness of detrital garnet as a provenance indicator is and when the Iapetus Ocean opened and closed, has been well known (Morton 1985; Houghton & Farrow 1989; subject to speculation (e.g. Elders 1987; McKerrow et al. 1991; Tebbens et al. 1995). Garnet is used in such studies because it see discussion below). can withstand long distances of transport in fluvial–deltaic environments; it is resistant to chemical modification during diagenesis and low-grade metamorphism, and its com- Southern Uplands terrane positional variation can make it specific to source area. The Although the origin of the Southern Uplands is still considered present work is the first description of the provenance of UK controversial, this paper follows the fore-arc accretionary Ordovician detrital garnet based on quantitative electron prism model (McKerrow et al. 1977; Bluck 1983; Walton & microprobe analyses. Compositions and zoning patterns of Oliver 1991). Flysch greywacke sedimentation in the Southern garnets from Ordovician greywackes from the Southern Uplands began in earliest Caradoc (N. gracilis) times and Uplands and Midland Valley terranes are compared with continued through to the Upper Wenlock (Peach & Horne published work and new garnet analyses from the Grampian 1899). Lapworth (1889) interpreted the structure of the terrane. The aim of this paper is therefore to present a specific Southern Uplands as a fan of folds in the form of large provenance for the greywackes, namely the metamorphosed anticlinoria and synclinoria. Using the same Ordovician and Dalradian Supergroup, and to propose a palaeogeography. Silurian biostratigraphic evidence, McKerrow et al. (1977) explained the apparent paradox in which fault-bounded tracts become younger towards the southeast, whilst the beds within Geological setting these tracts young in the opposite direction towards the The Southern Uplands terrane (Fig. 1) is defined as those northwest. This is a pattern seen in modern accretionary tectonostratigraphic units that occur between the Southern prisms. The tectonostratigraphic features of the Southern Uplands Fault and the putative Iapetus Suture. The region Uplands of Scotland are therefore interpreted as the result of a between the Southern Uplands Fault and the Highland continuum of deformation and metamorphism in a developing Boundary Fault is termed the Midland Valley terrane whilst accretionary prism terrane (McKerrow et al. 1977; Oliver & the area of rocks between the Highland Boundary Fault and Leggett 1980). The combined results of Leggett et al. (1983), the Great Glen Fault is termed the Grampian terrane (Stone Freeman et al. (1988), Oliver & McKerrow (1984) and & Kimble 1995). On most continent reconstructions (using Anderson & Oliver (1996) suggest that the Southern Uplands palaeogeography, faunas and palaeomagnetism) for latest crystalline basement is composed of high-grade metamorphic Precambrian and Early Palaeozoic times, all three terranes are equivalents of the prism in thrust contact with mylonitonized juxtaposed against the margin of a Laurentian Supercontinent Lake District-type Borrowdale Volcanics. (Scotese & McKerrow 1990; McKerrow et al. 1991; Soper Alternative models for the Southern Uplands involve 1994; Dalziel et al. 1994; Dalziel 1997). However, the amount Ordovician back-arc or marginal basins and island arcs, with of lateral separation between these terranes, particularly where Silurian successor basins and advancing foreland propagating 541 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/3/541/4886850/gsjgs.155.3.0541.pdf by guest on 01 October 2021 542 A. R. HUTCHISON & G. J. H. OLIVER thrust stacks (Morris 1987; Stone et al. 1987; Hutton & Midland Valley terrane Murphy 1987; Kelling et al. 1987; Armstrong et al. 1996) with an island arc separating the Northern and Central Belts being According to Bluck (1984), during the Ordovician, the subsequently excised by the Orlock Bridge Fault (Anderson & Midland Valley terrane was composed of a central Midland Oliver 1986). Valley Block of batholithic and volcanic arc with interarc basins, a southern Ballantrae Igneous Complex with proximal fore-arc basin (coeval with a Northern Belt accretionary prism but now missing due to overthrusting by the Southern Uplands) and a northern, marginal Highland Border basin containing the Highland Border Complex. The latter contains probable pre-Arenig to Arenig ophiolite plus associated Arenig to Late Llanvirn aged deep-sea sediments and lavas unconformably overlain by Caradoc sandstones and lime- stones. These Caradoc sediments were supposedly derived from the Midland Valley Block since the sandy detitus (for example the Bofrishlie Burn and Achray Sandstone For- mations) is granitic in provenance and apparently unlike the Dalradian metamorphics (Curry et al. 1984; Bluck 1984). On the contrary, Robertson & Henderson (1984) argued on the basis of geochemistry that the sediments were derived from a terrigenous terrane similar to that which supplied the Dalradian Supergroup and therefore did not completely rule out the Dalradian as a source. According to Bluck (1984) the Cambrian–Arenig Ballantrae Igneous Complex has a marginal basin-arc setting; con- glomerates of Early Llanvirn, Late Llanvirn and Caradoc ages with fluviatile and marine components transgress northwards over the Complex (see summary in Walton & Oliver 1991). MacNiociall & Smethurst (1994) and Dalziel (1997) include the Ballantrae Igneous Complex as part of a Peri-Laurentian intraoceanic arc. Grampian Terrane This terrane is made up of the Dalradian Supergroup of late Proterozoic and early Early Palaeozoic age. These are mostly marine sediments, laid down on a rifted continental margin, and regionally metamorphosed under conditions varying from Barrovian kyanite–sillimanite high-temperature and high- pressure to Buchan cordierite–andalusite low-pressure and high-temperature conditions during the late Proterozoic–early Ordovician (summary in Johnson 1991; Dempster 1985; Dempster et al. 1995). The bulk of the Dalradian has been metamorphosed to garnet grade and above (see Fig. 1). Dempster et al. (1995) place great emphasis on the oldest Rb–Sr mineral ages, 515 Ma from Angus and 505 Ma from the Buchan coast, as the minimum ‘cooling’ ages after a Late Cambrian (or end-Proterozoic) Grampian orogeny. The long delay before cooling in the mid-Ordovician is a problem that this paper throws light on. Fig. 1. Maps of sample localities 1–21. Upper map shows the Grampian, Midland Valley and Southern Uplands terranes. Note the large extent of garnet-bearing subcrop (i.e. garnet+kyanite+ sillimanite Barrovian zones). BB, Bofrishlie Burn; LCQ, Lime Craig Quarry; GE, Glen Esk; GGF, Great Glen Fault; HBF, Highland Boundary Fault; SUF, Southern Uplands Fault; NB, Northern Belt; CB, Central Belt; SB, Southern Belt; IS, Iapetus Suture; BV, Borrowdale Volcanics; LD, Lake District. Lower map shows the geology of the Ballantrae–Barrhill–Cairnryan area. Map references to sample localities given in text. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/3/541/4886850/gsjgs.155.3.0541.pdf by guest on 01 October 2021 GARNET PROVENANCE IN THE SOUTHERN UPLANDS 543 Previous work on Southern Uplands greywacke One clast from the Craigputtock conglomerate was subse- provenance quently reported as having a muscovite Rb–Sr age of 463&5 Ma (van Breemen pers. comm. to Kelley & Bluck In the early work on the Southern Uplands of Scotland, Peach 1989). & Horne (1899) used rock fragments
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