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Journal of the Geological Sociery, London, Vol. 151, 1994, pp. 1017-1021, 4 figs, 2 tables. Printed in Northern Ireland

Sm-Nd isotopic characteristics of sedimentary provenance: the of NW England

W. D. McCAFFREY Department of Earth Sciences, University of Leeak, Leeds LS2 9JT, U.K

Abstract: Nd model ages of Windermere Supergroup sediments increase from the upper Ashgill to theupper Ludlow. Below thelower Llandovery, increasing t,, reflects mixing betweenthe underlying Borrowdale Volcanic Group and hemipelagic detritus, possibly sourced from the Lauren- tian margin. The ongoing rise of tDM into the upper Ludlow probably reflects the growing influenceof sediment sourced from the Scandian Orogen. A Piidol5 drop in t,, is interpreted as recording the re-establishment of supply fromthe local Laurentian margin: northto south sediment dispersal indicating the effective demise of the as a barrier to sediment dispersal.

It hasgenerally been accepted that the Sm/Ndratio of siltstones, deposited in a deep shelf environment. A period crustal rocks is not significantly altered by chemical of terrigenous sand-turbidite sedimentation (Birk Riggs and weathering, transport, deposition, diagenesis or all but the Austwick Formations;Kneller et al. in press) precedesa highestgrades of metamorphism(Taylor & McLennan returnto laminatedsiltstone deposition. The Coniston 1985). The Sm-Nd characteristics of sediments are therefore Group and the Bannisdale Formation record a resurgenceof taken to reflect accurately those of their protoliths, and to deep waterturbidite sedimentation. Thereafter, a aid in the characterization of source areas. Nd model ages diachronousreturn to shallow watersedimentation (the (‘provenance ages’ sensu Mearns et al. 1989) providea UnderbarrowFormation, Shaw1971) culminates in the weighted average of the amount of time that the sample Nd storm influenced Kirkby Moor Formation. has resided in the crust. Much use has been made of the Sm-Nd system to aidprovenance studies on Caledonian The Windermere Supergroup data set rocks (e.g. O’Nions et al. 1983; Miller & O’Nions 1984; Davies et al. 1985; Haughton 1988; Thorogood 1990; Evans The Windermere Supergroup analyses were performed on et al. 1991). Inthis study Windermere Supergroup clay/silt-gradesamples when these were available at the sedimentshave been characterized in terms of Sm-Nd desiredstratigraphic level. Otherwise,medium-grained isotopiccomposition and temporal trends compared with sandstonesamples were used (seeTable 2). Thedata set those in related basins. comprises 19 analyses. Of these,three have been taken from the literature (Miller & O’Nions, 1984). The rest were produced by the author. Model ages were calculated using The Windermere Supergroup the depleted mantle model of DePaolo (1981). TheWindermere Supergroup of northwestEngland TheWindermere Supergroup data exhibita distinct (Moseley 1984) comprisespredominantlya sedimentary trend in which tDMincreases smoothly from c. 1.1 Ga in the Ashgill toPiidoli marine sequence whichis almost Ashgill to c. 1.75 Ga in the upper Ludlow, then falls sharply complete. It was deposited on thenorthern margin of to c. 1.45 Ga in the Pfidoli (Fig. 1). ,and was thereforeideally positioned to record Mixing lines are approximately linear on a tDM-Sm/Nd changes in provenanceassociated with theclosure of the plot. On this plot the Windermere Supergroup data exhibit Iapetus Ocean. The Windermere Supergroup succession is a distinct distribution (Fig. 2). The oldest sediments clearly interpreted as the fill of a post-arc thermal subsidence basin, exhibit derivation from the underlying Borrowdale Volcanic overlain by that of aperipheral (Kneller Group. The diminishing influence of the volcanic rocks is 1991). It unconformably overlies a mid- volcanic seen in the temporal trend towards the higher values of t,, complex and its early Ordovician basement, the Borrowdale and lower Sm/Ndcharacteristic of the Llandovery Volcanic Group(Branney 1988) and hemipelagites.In asecond. linear trend, both tDM and (Moore 1992), respectively,and is in turn unconformably Sm/Nd increase in progressively younger sediments from overlain by essentiallyundeformed Dinantian rocks. In the late Ashgill until the late Ludlow (413Ma), when the beingdeposited after late Ordovician volcanic shut-down trendreverses; the Piidoli (412Ma) fall in tDM being butbefore Acadian deformation, it is analogous tothe accompanied by a reduction in Sm/Nd. Powys Supergroup of Wales (Woodcock 1990). A revised WindermereSupergroup stratigraphy is Trends in related basins presented by Kneller et al. (in press)(see Table 1). The sediments record a transition from shore-face, through storm-dominated mixed carbonate/clastic shelf to Northern England deep shelf; the succeeding Skelgill and Browgill Formations of the Stockdale Group comprise graptolitic mudstones and (i) The Skiddaw Group. The Skiddaw Group comprises an siltstonesdeposited in relatively deep shelf marine early Ordovician clastic marine succession, deposited on the environments(Scott & Kneller 1990). The Brathay northern Gondwanan margin (Jackson 1978; Moore 19% Formationcomprises laminated hemipelagic carbonaceous which crops out principally in the Skiddaw, Ullswater and 1017

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Table 1. Lithostratigraphicaldivision of theWindermere Super- Caradoc Ash 'I1 Llandovery Wenlock Ludlow Ridoli group in the southern 2.0 I I I I 1 1.8

1.6

~DM,lGa 1.4

1.2

440 430 420 410 420 450 430 440 Stratigraphic Age l Ma

Fig. 1. Graph of Windermere Supergroup f,, versus stratigraphic age. Numbers correspond to WDM sample numbers (Table 2); asterisks indicate data fromMiller & ONions (1984).

AfterKneller efal. (inpress) Lsd Mbr, Longsleddale Member. Ages in Ma after McKerrow et al. (1985).

Table 2. New Borrowdale Volcanic Group, WindermereSupergroup and Southern Uplands Nd analyses

Code Grid Host Age Host Grid Code Nd Sm Sm/Nd I4'Sm/ 143Nd/1"Nd t,, reference stratigraphyreference (PP4 (PP4 (M4 '"Nd (G4 (f24

Borrowdale Volcanic Gro 'UP WDMl* NY49650670 Garburn Formation 460 8.00 34.97 0.229 0.1383 0.512403(39) 1.29 WDM2* NY49230678 Garburn Formation 460 6.70 29.98 0.223 0.1447 0.512282(37) 1.67 WDM3* NY49000640 Garburn Formation 460 8.29 39.40 0.210 0.1262 0.512548(30) 0.87 WDM4* NY48890606 Garburn Formation 460 4.17 17.31 0.241 0.1456 0.512341(55) 1S6 WDMS* NY48810576 Garburn Formation 460 6.25 29.59 0.211 0.1277 0.512371(22) 1.19 Windermere Supergroup WDM6*t NY48880573 Longsleddale Member 446 13.13 62.84 0.209 0.1263 0.512395(20) 1.13 WDM8 NY48910572 Stile End Formation 445 7.07 34.73 0.204 0.1231 0.512372(25) 1.12 WDM8t NY48910572 Stile End Formation 445 6.40 30.27 0.211 0.1278 0.512339(20) 1.24 WDM17-FSD24239230 Skelgill Formation 430 8.18 43.76 0.187 0.1130 0.512019(20) 1.54 WDM18 NY49630583 Skelgill Formation 430 8.68 44.42 0.195 0.1188 0.512165(19) 1.40 WDM20 NY497.50585 Browgill Formation 427 8.76 46.60 0.189 0.1137 0.512050(21) 1.50 WDM22t SD70659723 Brathay Formation 424 4.42 23.64 0.187 0.1130 0.512042(20) 1.51 WDM28* SD802 705 Austwick Formation 422 1.60 8.20 0.195 0.1182 0.512062(32) 1.56 WDM29SD802 705 Austwick Formation 422 6.76 37.15 0.182 0.1100 0.512014(27) 1.50 WDM34*SD265 940 Birk Riggs Formation 422 8.37 44.17 0.189 0.1147 0.512043(20) 1.53 WDM46 SD256 794 Birk Riggs Formation 422 12.30 65.15 0.189 0.1142 0.512048(20) 1.51 WDM62 SD61499878 Poolscar Formation 418 1.30 6.43 0.202 0.1226 0.512015(23) 1.71 WDM8l-F NY55530643 Yewbank Formation 418 6.27 32.76 0.191 0.1157 0.512010(20) 1.60 WDM90 SD54639480 Kirkby Moor Formation 413 5.81 30.00 0.194 0.1170 0.511925(20) 1.75 WDM93 SD54689412 Kirkby Moor Formation 412 12.91 71.94 0.179 0.1085 0.512055(18) 1.42 WDM94t SD54689412 Kirkby Moor Formation 412 6.46 35.44 0.182 0.1101 0.512029(20) 1.48 Southern Uplands SY49*t NX69884373 Raeberry Castle Fmn. 422 2.22 10.84 0.205 0.1240 0.512106(20) 1S8 SYS1.tNX69204365 Raeberry Castle Fmn. 422 4.45 21.06 0.211 0.1276 0.512187(20) 1.50 SY53't NX68704360 Raeberry Castle Fmn. 422 4.31 22.16 0.194 0.1177 0.512043(20) 1.58 SY54*t Raeberry Castle Fmn. 422 5.10 26.72 0.191 0.1153 0.512013(20) 1S8

~ ~~ ~ ~ ~ ~ ~ ~~~~~ ~ ~ ~~ * Samples medium sand (otherwise mud/silt-grade). t Analysis undertaken at the British Geological Survey, Grays Inn Road (GIR), otherwise at the Isotope Laboratory, Departmentof Earth Sciences, Leeds University. Stratigraphic ages interpolated from the timescale of McKerrow et al. (1985). All analyses corrected to '46Nd/'"Nd = 0.7219. GIR data corrected to 143Nd/1"Nd = 0.512657 f12 (10) for BCR-1 (contemporary GIR BCR-1analysis gave '43Nd/'"Nd=0.512619). Analyses ofLa Jollagave '43Nd/'"Nd as follows: Leeds0.511897 fll (labaverage of 23 analyses; GIR 0.511820 f12 (labaverage of eight analyses. Note: sample WDM8 was analysed in both the Leeds and GIR laboratories. The analytical difference is significantly less than the range in Nd values encompassed by the Windermere Supergroup trend.

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succession. They range in tD, from 1.39 to 1.49 Ga and in Sm/Ndfrom 0.19 to 0.20. Thesedata would appearto indicate that only the younger volcanic rocks contributed to the basal Windermere Supergroup clastic rocks. 1.6 Wales t,, l Ga Alarge-scale study of the Nd characteristics of the 1.4 pre- sediments of Wales and the Welsh Borders hasbeen published (Thorogood 1990). The post-Ashgill 1.2 secular trend of tDM shows a close similarity to that of the WindermereSupergroup (Fig. 3). On this basis it canbe argued that the Welsh and Windermere Supergroup basins 1.0 probably shared broadly similar sediment sources in terms 0.15 0.17 0.19 0.21 0.23 0.25 of Ndisotopic composition during this period. The wide Sm/Nd geographicalextent of thistrend emphasizes its likely geotectonic significance. Fig. 2. Windermere Supergroup Sm/Ndversus t,,. Numbers give stratigraphic age in Ma. BVG av, equal volume averageof the Borrowdale Volcanic Group samples (Table2). Hatched area The Southern Uplanak represents data field of Thirlwall & Fitton (1983). Average upper Thedata set for the Southern Uplands comprises30 continental Sm/Nd ratio isc. 0.173 (Taylor & McLennan 1985). analyses. Of these 26 have been taken from the literature (ONions et al. 1983; Miller & ONions 1984; Davies et al. Black Combe inliers in the north and southwestof Cumbria, 1985; Evanset al. 1991) andfour were produced by the northwestEngland. Three published Nd analyses exist, author(Table 2).The youngest SouthernUplands ranging in tDM from 1.44 to 1.52 Ga (O'Nions et al. 1983; sediments persist into the lundgreni Biozone of the upper Davies et al. 1985). The range in the published data does Wenlock and are thus coeval with the oldest Windermere not extend to the oldest Windermere Supergroup values. Supergroup turbidites. On a t,,-time plot (Fig. 4) the data exhibit awedge-shaped distribution. The stratigraphically oldest samples range in tDM from 1.09 to 1.85 Ga, whereas (ii) The Borrowdale Volcanic Group. The Borrowdale Vol- canic Group comprises a mid-Ordovician sub-aerial volcanic the youngest range only from 1.50 to 1.58 Ga.This distribution contrasts markedly with the Eastern Avalonian arc (Branney 1988; Branney & Soper 1988), and is uncon- formably overlain by the Windermere Supergroup. Analysis trend in tDM, and serves to emphasize the separation of the was undertaken by the author on five samples from Long- two areas,at least before the mid-Wenlock. Two sleddale, immediately beneath the Windermere Supergroup end-member explanations could account for the progressive basalunconformity at [NY 490 0601. Thesedata indicate diminution in isotopicheterogeneity. The Wenlock sediments might either be composed of material reworked that even after an averaging affect, derivation of the basal WindermereSupergroup sediments in Longsleddale from from within theSouthern Uplandsterrane (e.g. Walton the underlyingBorrowdale Volcanic Group is plausible 1983), or might comprise material from a third source which (Table 2; Fig. 2); an equal volume mix of the five Borrow- swamped local sedimentary input. The Nd evidence permits no discrimination between these alternatives. dale Volcanic Group samples analysed would have tDM1.21 Gaand Sm/Nd 0.222. Petrographic analysis of the sand gradeLongsleddale Member confirms thisderivation. Baltica Thirlwall & Fitton (1983) presentedfour Ndanalyses on Unfortunately the Baltican data set is poor. However, tDM samples from deeper within the Borrowdale Volcanic Group in modem Scandinavian river sediments ranges from 1.47 to

Unndeilo Caradcc Ashgill Llyldovay Wdcck MowRidoli I 2.0 I I I I I 0 1.8 6 00 00

A 1.6 0 e toMI Ga 00 0 0 0 1.4 0 $1 I I I 1.2 m l.O!'I'I'!'i.I

460 450 440 430 420 410420 430 440 450 460 1 .o ~ Stratigraphic Age l Ma 410 420 430 440 450 460 Stratigraphic Age l Ma Fig. 3. Graph of Windermere Supergroupand Welsh Basin tDM versus stratigraphic age. Welsh Basindata from Thorogood (1990). Fig. 4. Graph of Southern Uplands t,, versus stratigraphic age.

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1.95 Ga (E.W. Mearns pers. comm.). This material, largely constraintsupon interpretation provided by the study of derived from the Scandian Orogen, is thus characterised by derived microfossils prove particularly useful. On provenanceages generally older than those of the palynological grounds, McCaffrey et al. (1992) arguethat Windermere Supergroup. the bulk of the sediment entering the Iapetus Ocean after the lateLlandovery wasunlikely to have originated in , but much more likely to' have been of Baltican or Scotland north of the Highland Boundary Fault Avalonianprovenance, unless the Laurentiansource area The Dalradian Supergroup, an upper Proterozoic to lower was comprised of predominantlypre- (i.e. Cambrian succession of marine clastic rocks and unfossiliferous)rocks. The Ndevidence cited above deposited onthe southern Laurentian margin hasold, mitigatesagainst significant recycling of pre-Cambrian inheritedprovenance ages. tDM rangesfrom 1.64 to Laurentian rocks. 2.69 Ga, reflecting derivation from older Moine or Lewisian In order to accountfor the Proterozoic values of tDM rocks(O'Nions et al. 1983;Daly & Menuge 1989). These measured in Caradocand AshgillKirkholm Formation data indicatethat any sedimenteroded off thesouthern sediments of theSouthern Uplands, Evans et al. (1991) Laurentian margin and derived from rocks of Cambrian age support the contention of Elders (1987) that the Southern orolder, would probablyhave exhibited old provenance Uplandsfault was the locus formajor sinistral strike-slip ages. IfLaurentia did contributetothe Windermere movements, and infer that a Proterozoic terrane formerly Supergroup or SouthernUplands sediment budgets, its exposed on the southern Laurentian margin has been strike contribution was either volumetrically small or was derived slipped away and is now hidden. Were a similar terrane to from Phanerozoic successions of younger provenance age. have sourced the Windermere Supergroup it would have to havebeen barren of diagnostic Laurentian microflora; to Discussion account for evenpart of the post-Ashgill trend in tDM it would have to have been extensive. The likelihood that a Possible source areas now-hiddensource terrane movedalong theLaurentian margin and supplied Eastern Avalonia must, therefore, be It is difficult to envisage erosion of the Southern Uplands slim. producing the observedsecular increase in Windermere An alternative source exists, however, in the Scandian Supergroup tDM: Southern Uplands formations characterized Orogen, which began to develop in the Llandovery by relatively old values of tDM and those by relatively young (Harland & Gayer 1972; Stephens & Gee 1985; Dallmeyer values interdigitate in thecurrent outcrop (Evans et al. 1988). The explanationthat the bulk of the Windermere 1991). Erosion of average Avalonian crustwould produce Supergroup turbidite clastics (and probably those of Wales detritus with average tDM 1.8 Ga (Thorogood 1990). Even if too) were ultimately sourcedfrom Balticais therefore an Avaloniansource exhibited the necessary isotopic favoured; the secular increase in tDM reflecting progressive heterogeneity to encompass the post-Ashgill range in Nd unroofing of, and increasingsupply fromthe Scandian compositionseen in Windermere Supergroup and Welsh Orogen. Basin sediments, it is difficult to conceive how such a source might havebeen eroded toproduce the observedsecular End- drop in tDM increasein tDM. Nevertheless, the possibility thatthe Southern Uplands or Avalonia sourced these basins for a The Pfidoli drop in tDM, reflecting increased juvenile input, limited part of their history cannot be ruled out on isotopic alsois presentin Wales (Thorogood 1990) and is grounds. undoubtedly significant. Piidoli volcanicity in the Midland At least four remaining explanations could account for Valley and Glen Coe produced a sediment source with a the progressive Ashgill tolate Ludlow increasein tDM. relatively high juvenilecontent (Thirwall1988), and early Firstly, the sediments could all be sourced from Laurentia, Devoniansediment distribution waspredominantly to the or Laurentian terranes. In this case it could be argued that south (Anderton et al. 1979). The petrography, whole rock thetrend reflects the increasinginfluence of Lewisian geochemistry and REE distributions of upper Silurian basement and/orother Archaean and earlyProterozoic WindermereSupergroup sediments, however, do not rocks. Davies et al. (1985) favour a comparable mechanism indicate major volcanic input (McCaffrey 1991). The drop to explain the Silurian to increase in tDM seen in in tDM might, however, reflect erosionand southward southernBritain. Secondly, unroofing of the Scandian dispersal of Ordovician and Silurian material from the orogenand distribution of thedetritus through Eastern Southern Uplands. In either case, by this time the remnant Avalonia could account for the secular increase in tDM. A Iapetus seaway would have ceased to be either a barrier to Baltican provenance for the Welsh sediments is favoured by or conduit for sediment distribution. Thorogood (1990) onthe basis of the post-Ordovician proximity of Eastern Avalonia to Baltica inferred by Cocks Conclusions & Fortey (1982). Thirdly, nowa absent or hidden The post-Ashgill WindermereSupergroup trend in tDM terrane(s) formerly exposed along the southern Laurentian extends into Wales. Although the possibility of widespread margin could have sourced the sediment (e.g. Evans et al. recycling within the Southern Uplands cannotbe eliminated, 1991). Finally, a combination of these mechanismscould it is unlikely that this terrane constituted a unique source for have operated. theWindermere Supergroup sediments. It is more likely that Wenlock sediments in both areas were supplied from a Other provenance techniques thirdsource. Of potentialsource areas, Avalonia, the The Eastern Avalonian trend in tDMcannot be interpretedin Laurentian hinterland and a hidden terrane can be ruled out isolation from other provenance techniques. In this case the on isotopic orother grounds. The Ndisotopic evidence

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Received 23 November 1993; revised typescript accepted 4 January 1994.

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