J. geol. Soc. London, Vol. US, 1981, pp. 569-572, 1 fig, 2 tables. Printed in Northern Ireland. The significance of isotopic dates from the English for the Ordovician-Saurian time-scale

C. C. Rundle

SUMMARY: The Lower Palaeozoic time-scale is discussed in the light of a new date for the Threlkeld Microgranite and a reassessment of other published dates from the Lake District which suggest ages of c. 410 Ma for the base of the Devonian; c. 421 Ma for the Ashgill; c. 429 Ma for the late Caradoc; c. 439 Ma for the early Llandeilo, and c. 468 Ma for the early Llanvirn.

In a recent publication (McKerrow et al. 1980) con- pre-end-Silurian cleavage. Microgranite clasts, first de- cerning the Lower Palaeozoic time-scale, it is argued scribed by Wadge et al. (1972), from the basal Bor- thatthe time-scale produced by Gale et a1 (1979), rowdaleBampton Conglomerate in theTarn Moor basedlargely on a new Rb-Sr isochronage for the tunnel, to the E of theThrelkeld area, resemble a (Ashgillian) StockdaleRhyolite of the English Lake variety of theThrelkeld Microgranite, both in hand District, is in error due to the possibility that this age, specimen and thin section, and contain small garnets andthe Rb-Sr ages forseveral other acidvolcanic similar to those in the Threlkeld rock. These authors rocks used in their scale, are recording metamorphic also noted clasts of locally derived ‘Borrowdale Group episodes rather than the times of extrusion. lavas and tufi and Groupmudstones’ and It is the purpose of this note to present a new date suggested that ‘The Conglomerate accumulated on the for the Threlkeld Microgranite andto cite other previ- flanks of a tumescent volcanic area as a detrital fan of ously publisheddates from the Lake District which torrent debris’. provideindependent evidence infavour of the In their discussion of 1974, Wadge et al. appeared younger age and longer duration preferred by Gale et to have been more strongly influenced by the coinci- al. (1979) for the Ordovician System. dence of their age for the microgranite with the then All ages quoted in this work have been calculated current estimate of the age of the Caradoc Series, and using the decay and other constants recommended by hence rejected the evidence of the microgranite clasts. Steiger & Jager(1977) and errors on the ages are Despitethe distinctive nature of this rockand the quoted at the 2 sigma level. The Rb-Sr isochron age proximity of the exposed intrusion they suggested that was calculated using a least squares regression based the clastsoriginated from an older, unexposed and on that of York (1969). Where replicate K-Ar deter- previouslyunrecognized intrusion. It is considered minationshave been used todetermine anage, the here that, in view of the uncertainties of both the age standard error of the mean (af A)is quoted. determined by Wadge et al. (1974) and of the age of the Caradoc Series, this special pleading is unjustified and unnecessary alternativethethat and explanation-that themicrogranite clasts areindeed A new date for the derivedfrom the Threlkeld intrusion-is themore early Llandeilo? plausible. Hence, it is suggested thatthe Threlkeld Microgranite is closely constrained stratigraphically to The Threlkeld microgranite was previously dated by the Lower Llandeilo Series. Wadge et al. (1974) at 445 * 15 Ma and was thought The results of new Rb-Sr analyses of 12 fresh by them to have been emplaced in earliest Caradoc samples of the microgranite from the Threlkeld (work- times on the basis of ‘The Caradoc being fairly well ing) andBram Crag (disused)quarries are givenin dated at 447 Ma’. However, it was noted by Rundle Table 1. All twelve samples define a line on an iso- (1979) that recalculation of their data to conform with chron diagram with an MSWD of only 1.7, giving an current practice yields an age of 459*25 Ma, and it is age of 439+8Ma and intercept of 0.7056*6. evidentfrom the conflictingtime-scales mentioned Thisresult is not significantlydifferent from that above that there is still considerable dispute and un- quoted by Wadge et al. but, despite the new samples certainty about the age of the Caradoc. Hence, there havinga smaller range of Rb-Srratios, is far more was a need for a new assessment of the age of this precise, reflecting the improved analytical techniques intrusion. and more localized sampling. As a check on the earlier According to Wadge et al. (op. cit.) the microgranite work,and alsoin an attemptto improve the new is youngerthan the lowest part of theBorrowdale result,sample WG404 wasre-analysed during this Volcanic Group sequence (which is post late Llanvirn study. This sample was chosen because it was the only and pre-early Caradoc in age (Wadge1978)) and is one collected by Wadge et al. from the working quarry 0016-7649/81/0900-0569$02.00 @ 1981 The Geological Society

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/5/569/4886625/gsjgs.138.5.0569.pdf by guest on 28 September 2021 570 C. C. Rundle TABLE1. Rb-Sr analytical results for Threlkeld Micro- because of the increased spread in Rb-Sr ratios. granite The age produced by all 13 samples is 438+ 6 Ma with an intercept of 0.7057 f4, MSWD = 1.74 (Fig. 1). Sample ppm Rb* ppm Sr* 87Rb/86Sr s7Srls6Sr It is proposed that this date represents a better esti- mate of the age of theearly Llandeilo than any TH1 126 83 4.423 0.73312 previously published data. m2 158 126 3.638 0.72868 TH 3 153 TH3 67 6.678 0.74730 TH 4 141 TH4 86 4.791 0.73549 A new date for the Caradoc? TH5 134 86 4.561 0.73416 TH6 121 71 4.951 0.73713 The age of the Eskdale Graniteis now well established both by Rb-Sr and K-Ar dates (429f4 Ma, 427 f BM C l 121 BMCl 91 3.856 0.72964 8Ma (Rundle1979)) and by U-Pb on zircons (c. BMC2 128 55 6.700 0.74726 425Ma Pidgeon & Aftalion1978). It is widelyac- BMC3 141 66 6.172 0.74433 cepted that the granite is a syntectonic intrusion (e.g. BMC4 126 82 4.453 0.73365 Dwerryhouse1908; Simpson 1934;Trotter et al. 87 4.060 0.73052 BMC5 122 1937; Firman 1960) but the age is considerably older BMC6 122 106 3.330 0.72639 than the main Caledonian deformation (c. 400 Ma, see W G404 129WG404 36 10.510 0.77123 later). Hence, since the granite intrudes the Borrow- daleVolcanic Group of Llandeiloage and since no * Approximate values. Analytical and calculation detailsas major intra-Silurian folding has been recorded in the given in Rundle(1979) except that all strontiumisotopic area, it is concluded that intrusion occurred during the analyseswere made with a VG-Micromass“30, fully widely recognised pre-Ashgillian deformation (e.g. see automatic spectrometer, 6 samples being analysed overnight Moseley 1972; Ingham & McNamara 1978; Soper & without operator intervention. 1 sigma errors are *0.5% on Rb/Sr ratios and +0.02% on Sr87’86 Moseley 1978;Wadge 1978). Consequently, it is TH = Threlkeld Quarry; BMC = Bram Crag Quarry. suggested that the best date for the Eskdale granite (429+4 Ma) may also be taken as a good estimate of atThrelkeld and also because of itsrelatively high the age of the late Caradoc Series. Rb-Sr ratio. The new value for the 87Rb/86Sr ratio (10.5)is 3.4% A new date for the early Llanvirn? higher than that obtained by Wadge et al. (1974) but, because of the high errors associated with the earlier In the northern Lake District, the Great Cockup Pic- analyses, is not significantly different at the 95%confi- rite has given a K-Ar hornblende date of 458+ 9 Ma dence level. Similarly the new value for the s7Sr/86Sr (Rundle1979). The stratigraphic age of thisrock is ratio (0.77123) is also just within error of the earlier somewhat uncertain. Eastwood et al. (1968) suggested result (0.21% lower). that it is consanguineous with the Embleton quartz- These new values bothtend to move the point diorite and Firman (1978) presented geochemical data closer to the best fit line on the isochron diagram for from which he concluded that the diorite is related to the new samples and if this point is includedin the the Eycott Volcanic Group; but Rundle (1979) dis- regression there is asignificant increase in precision puted Firman’sconclusion onthe grounds thatthe diorite was ‘emplaced afterthe development of a I I I I I l I major fold in the Skiddaw Slates, whereas the Eycott 87Sr/86Sr lavas are conformably interbedded with the Skiddaw 8.788 - Threlkeld Slates sequence’. However, the recent division of the 8.778 - SkiddawSlates intothe ‘Skiddaw Group’and the - overlying ‘Eycott Group’ (Wadge 1978) and the rec- 8.760 ognition of an early pre-Eycott Group phaseof nappe

formation(Banham et al., inpress) invalidates this

argument. Thefold cut by the intrusion at Embleton is - /+’+

8.758 - 8.748 - inSkiddaw Groupslates and there isnow no good ++ reason why the Eycott lavas should not be related to 8.738 /+: p++‘ the Great Cockup rock. - M= 438.36 t 6.8 k (23igmd - Intercept= 8.78566 8.W2 - It may be noted that the picrite date is the mean of W 1.74 three separate determinations on three different sam- - ples and the oldest of these, on the hornblende with 87WAKr 1 I I l I the highest K content, is 468f 15Ma (Rundle 1979); 2.8 4.8 6.8 0.8 18.8 identical to that determined for the gab- FIG. 1. Isochron diagram for new data fromthe bro (468 f 10 Ma,Ibid) which has also been related to Threlkeld Microgranite. the Eycott Group lavas (Firman 1978).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/5/569/4886625/gsjgs.138.5.0569.pdf by guest on 28 September 2021 Isotopic dates, English Lake District 571 It is concluded here that the best estimateof the age TABLE2. Comparison of time-scales (ages in Ma) of the Eycott Group, and hence of the D. bifidus zone of the Llanvirn (Downie & Soper 1972) is currently Gale McKerrow 468 f 9 Ma (the meanof the determinations from Car- et al* et al* rock Fell and Great Cockup). This work 197919801980 DEVONIAN The Lower Devonian Gedinne 399*8 394 400 411 (Skiddaw) According to Wadge et al. (1978) the main Caledonian SILURIAN cleavage in the Lake District affected strata up to the Pridoli 415 Scout Hill Flags of Downtonian age (=early Gedin- Ludlow 422 nian). Theemplacement of the Skiddawgranite is Wenlock 428 thought to have overlapped this Caledonian deforma- Llandovery 418 425 440 tion (Soper & Roberts 1971) and hence the K-Ar age ORDOVICIAN of 399*9 Ma determined by Shepherd et al. (1976) Ashgill 421zt9 447 for this intrusion can be taken as a minimum estimate (Stockdale) of the age of the early Gedinnian andsuggests that the Late Caradoc 429 f 9 base of theDevonian is somewhat inexcess of (Eskdale) E arly Caradoc Early 434 469 400 Ma. Llandeilo453 439*9 478 (Threlkeld) The Lower Palaeozoic time-scale Llanvirn464 10 468* 488 (Eycotts) In recent publications (Gale et al. 1979,1980; Arenig 482 504 McKerrow et al. 1980)the authors considered it Tremadoc 4905 10 490 500 5 19 necessary toerect some independent scale propor- (Bohemia) tional tothe absolute duration of thestratigraphic * Ages to base of divisions. divisionsin orderto make a statisticalcorrelation FollowingRundle (1979) and Gale et al. (1979) a between these divisions and the radiometric dates. In minimum absolute error of *2% (2 sigma) has been assigned view of the uncertainty of the stage lengths used by to all ages in order to allow for possible errors in decay and Gale et al. (based on rates of animal evolution) and of other constants and inter-laboratory differences. The resolu- the discussion by McKerrow et al. of what ratio to use tion between particular data points determined by the same forthe relative lengths of theCaradoc and Ashgill method in the same laboratory maybe considerably less than Series (2: 1 or 8.5 : 1) it is considered here that this this. attempted correlation is unjustified and gives a false impression of the accuracy of the interpolated points. any disagreement between the new data and the scale This is demonstrated admirably by the latter authors of Gale et al. (1979). The Skiddaw granite age clearly who,after noting the uncertainty of establishinga indicates that the early Gedinnian must be older than relativestratigraphic time-scale ‘especially for the about 400 Ma and hence, in this case, the estimate of Ordovician’, then present a table which assigns an age 411 Ma by McKerrow et al. appears to be the more (with no error estimates) to the base of every stage of reasonable. the Lower Palaeozoic. In the author’s opinionit would The time-scalepreferred here suggests thata be far more useful and less misleading if geochono- reasonableestimate of theduration of the Silurian logists applied themselves to producing a list of reli- period is only some 10-15 Ma compared with perhaps able data points for the stratigraphic column and left 70Ma for theOrdovician. (However, it shouldbe the interpolations to the imagination and predilection noted that if one accepts the interpolated values for of those who wish to use the data. the boundaries andassumes a 2% error on them at the 95% confidence level, the Silurian could be anywhere Conclusion between minus 9 and plus 23 Ma in length; beware stratigraphic interpolations and statistics!). That this is The new ages presented here (see Table 2) are more in not unreasonable can be demonstrated by the geologi- accordwith the time-scale produced by Gale et al. cal history of the Lake District during these periods. (1979) than any other version and support the lower- TheOrdovician is characterized by at leastthree ing of the Ordovician-Silurian boundary to something periods of deformation, four periods of magmatic ac- less than about 420 Ma. If the date for the Stockdale tivity anda widevariation in depositional environ- Rhyolite is accepted then there is no justification for ments producingin excess of 11 000 m (Moseley 1978) placing the boundary older than this(as Gale et al. of deep sea turbidites, sub-aereal lavas, volcaniclastics 1980, have done in their revised scale). and limestones. In contrast the Silurian was a remark- Only for the Silurian-Devonian boundary is there ably quiescent periodof tranquil marine sedimentation

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/5/569/4886625/gsjgs.138.5.0569.pdf by guest on 28 September 2021 572 C.C. Rundle

producing only c. 3-4000 m of beds, with little evi- byshire, Miss J. Evans and D. Bradley for assistance with the dence of majormagmatic or tectonic activity. analyticalwork, and to R. J. Pankhurstand N. J. Snellingfor helpful comments. This paper is published with the permis- sion of the Director of the Institute of Geological Sciences, ACKNOWLEDGEMENTS.Thanks are due to Mrs D. P. F. Dar- London (N.E.R.C.).

References

BANHAMP. H., HOPPER,F. M. W. & JACKSON,J. B., in press. granites of Scotland and . In: Bows, D. R. & The Gilbrea Nappe inthe Skiddaw Slates, Cockermouth, LEAKE, B. E. (eds). Geol. J. Spec. Iss. No 10, 183-220. . Geol. Mag. RUNDLE,C. C. 1979. Ordovician intrusion in the English DOWNIE, C.& SOPER, N.J. 1972. Age of the Eycott Volcanic Lake District. J. geol. Soc. London, 136, 29-38. Group and its conformable relationship to the Skiddaw SHEPHERD,T. J., BECKINSALE,R. D., RUNDLE,C. C. & Slatesin the EnglishLake District. Geol. Mag. 109, DURHAM,J. 1976.Genesis of CarrockFell tungsten 259-68. deposits,Cumbria: fluid inclusion and isotopic study. DWERRYHOUSE,A. R. 1908. On some intrusive rocks in the Trans. B. Znst. Min. Met. 85, 63-74. neighbourhood of Eskdale (Cumberland). Q. J. geol. Soc. SIMFSON, B. 1934. The petrology of the Eskdale (Cumber- London, 65, 55-80. land) granite. Proc. Geol. Assoc. London, 45, 17-34. EASTWOOD, T., HOLLINGWORTH,S. E., ROSE, W. C. C. and SOPER, N.J. & MOSELEY,F. 1978. Structure. In: MOSELEY, TROTTER,F. M. 1968. Geology of the country around F. (ed.). The Geology of the Lake District. Yorks. Geol. Cockermouth and Caldbeck. Mem. Geol.Sum. Sheet 23. SOC. 45-67. RRMAN,R. J. 1960.The relationship between joints and - & ROBERTS,D. E. 1971.Age of cleavagein the fault patterns in the Eskdale granite (Cumberland) and Skiddaw Slates in relation to the Skiddaw aureole. Geol. the adjacent Borrowdale Volcanic Series.Q. J. geol. Soc. Mag. 108, 293-302. London 116, 317-47. STEIGER,R. H. & JAGER, E. 1977.Subcommission on -1978. Intrusions. In: MOSELEY,F. (ed.). The Geology of Geochronology:Convention on the use of decaycon- the Lake District. Yorks. Geol. Soc. 146-63. stants in geo- and cosmochronology. Earth planet. Sci. GALE,N. H., BECKINSALE,R. D. & WADGE,A. J. 1979. A Lett. 36, 359-62. Rb-Sr whole rock isochron for the Stockdale Rhyolite of TROTTER,F. M., HOLLINGWORTH,S. E., EASTWOOD,T. & the English Lake District and a revised mid-Palaeozoic ROSE,W. C. C. 1937.Gosforth District. Mem.Geol. time-scale. J. geol. Soc. London 136, 235-42. Sum., Sheet 37. __-, & -1980. Discussion of a paper by McKerrow, WADGE,A. J. 1978. Classification and stratigraphical rela- Lambertand Chamberlain on the Ordovician, Silurian tionships of the Lower Ordovician Rocks. In: MOSELEY, andDevonian time-scales. Earth. planet. Sci.Lett. 51, F. (ed.). The Geology of the Lake District. Yorks. Geol. 9-17. SOC. 68-78. INGHAM,J. K. & MCNAMARA,K. 1978. Upper Ordovician - GALE,N. H., BECKINSALE,R. D. & RUNDLE,C. C. rocks. In: MOSELEY,F. (ed.). The Geology of the Lake 1978. A Rb-Sr isochron age for the Shap Granite. Proc. District. Yorks. geol. Soc. 121-9. Yorks. Geol. Soc. 42, 297-305. MCKERROW, W.S., LAMBERT, R. ST. J. & CHAMBERLAIN, V.- HARDING, R. R. & DAREIYSHIRE, D.P. F. 1964. The E. 1980. The Ordovician, Silurian and Devonian time- rubidium-strontium age and field relations of the Threl- scales. Earth. planet. Sci. Lett. 51, 1-8. keld Microgranite. Proc. Yorks. Geol. Soc. 40, 211-22. MOSELEY,F. 1972. A tectonic history of north-west England. - NUT, M. J. C., & SKEVINGTON, D. 1972. Geologyof J. geol. Soc. London, 128, 561-98. the Tarn Moor Tunnel in the Lake District. Bull. Geol. - 1978. The Geobgy of the LakeDistrict. Introductory Sum. G.B. 41, 55-73. Review. Yorks. Geol. Soc. 284p. YORK,D. 1969. Least squares fitting of a straight line with PIDGEON,R. T. & AFTALION,M. 1978.CO-genetic and correlated errors. Earth. planet. Sci. Lett. 5, 320-4. inheritedzircon U-Pb systems in granites. Palaeozoic

Received 22 February 1981. CHRISC. RUNDLE, Isotope Geology Unit, Institute of GeologicalSciences, 64 Gray’s Inn Road, London WClX 8NG.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/5/569/4886625/gsjgs.138.5.0569.pdf by guest on 28 September 2021