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The Ordovician, Silurian and Devonian time scales
Earth and Planetary Science Letters, 51 (1980) 1-8 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
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THE ORDOVICIAN, SILURIAN AND DEVONIAN TIME SCALES W.S. McKERROW 1, R.St.J. LAMBERT 2 and V.E. CHAMBERLAIN 2 1 Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford OX1 3PR (England) 2 Department of Geology, University of Alberta, Edmonton, Alta. T6G 2E3 {Canada)
Received February 16, 1980 Revised version received July 12, 1980
A reassessment of published Ordovician, Silurian and Devonian isotopic age determinations available has necessitated the correction of several calculated dates and revision of several stratigraphic conclusions in the light of later work. The revised dates plot close to a single time scale line, except for Rb-SI dates on acid volcanic rocks. We conclude that the base of the Devonian is at 410 Ma, the base of the Silurian at 437 Ma, and the base of the Ordovician (Tremadoc Series) at 519 Ma. These dates are within the range of previously published dates for the base of the three periods. We place more reliance on our estimates for the Upper Ordovician, Silurian and Devonian than for the Lower Ordovician. A comparison with other time scales shows that our new scale is similar to many previously published, except for those which rely heavily on acid volcanics.
1. Introduction Although there is still a scarcity of accurate isotopic age determinations for Palaeozoic rocks with good stratigraphic control [ 1 ] some recent dates, especially those o f Ross et al. [2], now provide data for a new assessment of the time scale for the Ordovician, Silurian and Devonian periods. In order to compare the stratigraphical ranges of isotopic dates, it is necessary to first plot the ages against a stratigraphic scale which accurately reflects the relative durations o f the time-stratigraphic (chrono-stratigraphic) divisions: periods (systems), epochs (series), ages (stages) and chrono-zones. This is still very uncertain, especially for the Ordovician Period. In Fig. 1, we have used the relative durations of the Devonian stages suggested by Ziegler [3], modified on the advice of A.J. Boucot (personal communication, 1979), and those o f the Silurian stages suggested by Boucot [4]. Our estimates o f the relative durations o f the complete Silurian and Devonian periods also follows Boucot [4]. It has not been easy to estimate the relative dura-
tions of the Ordovician stages either to each other or to the Silurian and Devonian. Boucot [4] suggested that the Caradoc, Ashgill and Llandovery have relative durations in the ratios 2.0 : 1.0 : 1.9. Churkin et al. [5] gave estimates of the relative duration of some Ordovician graptolite zones, based on stratigraphic thicknesses of graptolite shales along the outer margin o f the Cordilleran fold belt of North America, but their table does not include data from the basal Arenig, the early Llanvirn; nor from an unknown part o f file Caradoc, D. clingani Zone. Nevertheless they report very large thicknesses for several Caradoc zones, which suggests to them that the Caradoc has a duration of 17 Ma compared with 2 Ma for the Ashgill. We consider that a Caradoc : Ashgill ratio of 8.5 : 1 is excessive and that the Boucot ratio of 2 : 1 is perhaps too little, so we have selected (rather arbitrarily) an intermediate ratio of 4 : 1. The earlier parts o f the Ordovician Period have been divided (on the basis of the number o f graptolite, trilobite, brachiopod and conodont time-stratigraphic units - e.g., see Bassett [6, pp. 5 7 , 1 3 4 , 2 2 2 ] ) , in the ratios: Caradoc (12), Llandeilo (5), Llanvirn (5), Arenig (8) and
0012-821X/80/0000 0000/$02.50 © 1980 Elsevier Scientific Publishing Company
the Stromlo volcanics [7, p. 86] and the Vilasund Granite [8], were omitted from consideration. Age assignments have also been checked against the original data, and all dates have been corrected to present-day constants [9]. Some determinations were rejected because of excessive error, e.g., Quoddy volcanics, 409 -+ 22 Ma [10], but some others which were rejected by Gale et al. [ 11 ] have been retained.
Tremadoc (8) to give a total time span of the Ordovician (here taken to include the Tremadoc Series) about equal to that of the Silurian and Devonian combined. As will be seen from our discussion below, this calibration of the stratigraphic time scale is in good agreement with the dates which we have selected as giving the best control on the scale.
2. Selection and updating of data 3. The suggested scale We have included all published accurate isotopic age determinations known to us which also have precise enough stratigraphic control to be meaningful in construction of a time scale. Stratigraphic assignments of the data in Fig. 1 were all checked against the original author's statements and modified where necessary in the light of later information. A few supposed marker points were found to be less precise than originally stated, and some previously used for time scale purposes, like
BAS._EE O F
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400 I
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All the data recorded in Fig. 1 are represented by rectangles corresponding to the analytical errors and to the uncertainty in stratigraphical dating. Our time scale represents a best fit line through the following rectangles: (1) A 369 -+ 6 Ma K-Ar date on biotites from the Cerberean volcanics of Victoria, Australia [2, 4], which overlie the fish-bearing Taggerty Group and are thought to have an age "near the top of the Upper
420 I
430 I
440 I
460
450 I
470
480
490
500
510
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K/Ar I 4°Af/39Ar I Fib-St ssochron | ---6 , 7 , 8 , 9 t0.11,12,13 Fission track - - - 15,23 U-Pb on z~rcon - 24,25,26,27,28,29,30,31 Rb-Sf isochrOn on aciG volcanic -- x-- 19a Sm Nd tsochron - 1,4 , 5,17,18.22 ......... 2 , 1 4 , 1 9 , 21
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Fig. 1. Time scale for the Ordovician, Silurian and Devonian. Constructed by choosing best estimates of relative lengths of stages and by analysis of the data for 31 apparent control points. The best line has been subjectively fitted through the data array as described in the text. Key to numbers: 1 = Cerberean volcanics; 2 = Hey basalts; 3 = Shap Granite; 4 = Shiphead Formation; 5 = Gocup Granite; 6 = Bringewood Beds; 7 = Wenlock Limestone; 8 = Buildwas Formation; 9 = Birkhill shales; 10 = Acton Scott Beds; 11 = Gelligrin Ashes; 12 = Bach-y-Graigshales; 13 = Llyfnant Flags; 14 = Descon Formation; 15 = Carters Limestone; 16 = Penobscot Formation; 17 = Bail Hill volcanics; 18 = Chasmops Limestone; 19 = Bay of Islands ophiolite amphiboles; 20 = Benan Conglomerate clasts; 21 = Hare Bay Ophiolite; 22 = Colmonell Gabbro; 23 = Cashel Intrusion; 24 = Hedgehog Formation; 25 = Eastport Formation; 26 = Pembroke Formation; 27 = Dennys Formation; 28 = Stockdale Rhyolite; 29 = Arisaig volcanics; 30 = Borrowdale volcanics; 31 = Krivoklat-Rokycany volcanics.
Old Red Sandstone of Scotland" [12, p. 1077]. (2) A 4°Ar/39Ar date of 368 + 8 Ma [13,14] on whole-rocks from the Hoy basalts of the Orkney Islands of northern Scotland, indicates a horizon near the base of the Upper Old Red Sandstone. The Late Devonian fish Holoptychius occurs in equivalent beds at Dunnett Head, Caithness. (3) The post-Silurian/pre-Upper Devonian Shap Granite of northern England has an Rb/Sr isochron age of 394 + 3 Ma [15]. (4) Bentonites in the Siegen Shiphead Formation of Quebec [16] give a revised K-At age on sanidine of 395 -+ 16 Ma (H. Baadsgaard, personal communication, 1979). (5) The post-Wenlock and pre-Siegen Gocup Granite of New South Wales has a K-Ar age of 409 +3 Ma [17]. The fission-track dating of zircons in British Lower Palaeozoic bentonites [2] has made a great step forward in establishing a time scale, as they come from horizons accurately dated by fossils. These are: (6) Bringewood Beds (mid-Ludlow): 407 -+ 8 Ma (average of two analyses). (7) Wenlock Limestone (late Wenlock); 416 +- 9 Ma. (8) Buildwas Formation (early Wenlock): 422 +10Ma. (9) Birkhill shales (late Lower Llandovery Monograptus cyphus Zone): 437 +- 11 Ma. (10) Acton Scott Beds (late Caradoc, Actonian Stage, probably late Dicranograptus clingani Zone): 466 -+ 11 Ma (average of two analyses). (11) Gelli-grin Calcareous Ashes (mid-Caradoc, Longvillian Stage): 465 -+ 10 Ma (average of two analyses). (12) Shales at Bach-y-graig, near Llandrindod Wells (topmost Llanvirn or lowermost Llandeilo): 477 +I1 Ma. (13) Llyfnant Flags (high in the Didymograptus extensus Zone): 493 -+ 11 Ma. Most of these are likely to be minimum ages (i.e., they should occur to the left of our time scale line), but the date from the Acton Scott Beds lies to the right. This anomaly may be explained by the assumption that the late Caradoc and Ashgill represent a much longer period of time than that shown on our graph, or by the presence of inherited zircons in the
material analysed, or by the possibility that the large errors present on earlier dates (464 -+ 21 and 468 -+ 12 Ma, [18]) are real. (14) A Lower Llandovery (Monograptus cyphus Zone) volcanic breccia in the Descon Formation of Esquibel Island, Alaska, yields a 4°Ar/39Ar age on hornblende of 433 +--3 Ma [19]. (15) Zircons from a bentonite in the Carters Limestone of the Stones River Group in Tennessee [20, item 156] yield a revised mean age of 443 -+ 10 Ma. These beds are assigned to the Wilderness, which corresponds to the middle Caradoc (Climacograptus wilsoni and C. peltifer Zones) [21 ]. A comparatively high common Pb correction for these zircons makes assessment of this age difficult. (16) The discovery of poorly preserved Upper Ordovician brachiopods from schists of the Penobscot Formation of Maine, together with an Rb-Sr isochron on these schists of 450 -+ 10 Ma [22] suggests that this date "may either reflect a time of metamorp h i s m . . , or a minimum time of deposition for the rocks" [22, p. 1957]. The schists are considered to be overlain by unmetamorphosed Silurian beds. (17) The Bail Hill volcanics of the Southern Uplands of Scotland overlie shales of the l_.landeiloCaradoc Nemagraptus gracilis Zone, and yield a K-Ar age on biotite of 453 -+ 10 Ma [23]. (18) Biotites and sanidines in bentonites in the Chasmops Limestone of Kinnekulle, Sweden, yield a K-Ar age of 457 -+ 4 Ma [20, item 157]. These beds are assigned to the early Caradoc. (19) Hornblendes from the metamorphic aureoles of thrust masses of ophiolites in western Newfoundland, the Bay of Islands ophiolite and the Little Port Complex give a 4°Ar/39Ar.age of 460-+ 5 Ma [24,25]. The stratigraphic time span of these dates is limited by the igneous rocks (probably Middle Arenig) and the age of final emplacement of the thrust sheet (N. gracilis Zone). That this age is likely to refer to metamorphic rather than primary igneous events is confirmed by the Sm-Nd ages of 508 + 6 and 501 -+ 13 Ma on the Bay of Islands gabbros [26] which should refer to Middle Arenig events. These Sm-Nd ages are shown as 507 + 5 in Fig. 1, item 19A, reinforcing our interpretation of the other Lower Ordovician points (12, 13, 20, 21 and 22), though an increase in the age of the Tremadoc-Arenig boundary may be suggested by these latest data.
(20) An Rb-Sr mineral isochron age of 470 Ma (-+5?) for granitic clasts in the Benan Conglomerate, Ayrshire is regarded as a maximum for the Llandeilo and as being inconsistent with published fission-track ages [27]. The four relevant mineral isochrons give 469 _+5,471 -+ 13,466 -+ 14 and 468 -+ 5 Ma. The four corresponding whole-rocks and one other lie on a "tentative" 459 -+ I0 Ma isochron. Longman et al. [27] argue that the base of the Llandeilo must be 460 Ma or younger as it will take "<10 Ma" to unroof the granitic source region and form the Benan Conglomerate, but the Benan Conglomerate probably lies within the N. gracilis zone and near the LlandeiloCaradoc boundary [28, figs. 5.8, 5.9 and 6.2; 29, fig. 3]. Both Walton [28] and Ingham [29] show the Benan Conglomerate forming from an up-faulted block of basement: the time span involved could be short (cf. conglomerates formed against splays of the San Andreas system) and the granitic rocks could have formed during the N. gracilis zone. The lower stratigraphic age limit is therefore given in Fig. 1 as the top of the Llandeilo, a convenient average of the possibilities. There is a problem with the ages which we cannot analyse without more information: it is possible that the mineral isochrons reflect an alteration event and not the age of intrusion, in which case the 470 (-+5?) age is perhaps close to the age of sedimentation. If however, the authors' original interpretation is correct, our scale, based on a best fit through a number of comparatively ill-defined points for the Lower Ordovician, may be a little high at the Llanvirn-Landeilo range (indicated by parentheses in Table 1). (21) A 4°Ar/39Ar date from Hare Bay [30] of 480 -+ 5 Ma, can be established to be post-early Arenig and pre-late Llanvirn. The fact that the time scale line lies near the top of the rectangles on our graph (Fig. 1) suggests that in each case (19 and 21) the metamorphism occurred close to the age of final emplacement of the thrust sheets. This interpretation parallels that proposed for the origin of an amphibolite facies aureole adjacent to serpentinite in the BaUantrae Igneous Complex [31 ]. (22) The Colmonell gabbro of southern Scotland intrudes the Arenig and Llanvirn Ballantrae Volcanic Group, which is overlain by late Llanvirn and younger sediments [29]. It yields a K-Ar age on biotite of 4 8 4 -+ 10Ma [23].
(23) The U-Pb date of 510 _+ 15 Ma for the CashelLough Wheelan basic intrusion [32,33] is contemporary with the Grampian Orogeny in western Ireland. Dalradian rocks, metamorphosed and deformed by this orogeny, occur to the north and to the south of the South Mayo Trough which contains a thick sequence of late Tremadoc to Llanvirn rocks. We now consider that the Grampian Orogeny in western Ireland occurred before the end of the Tremadoc rather than the alternative view [34] that it occurred after the Llanvirn. In Scotland, whole-rock K-Ar ages of 512 -+ 6 Ma on slates of the Highland Border area [35] suggest that the termination of the Grampian Orogeny was synchronous throughout the British Isles (though the stratigraphic age of the Scottish sediments is not well defined). We therefore conclude that 510 Ma represents a point of time during the Tremadoc Series, perhaps in the basal part of the late Tremadoc.
4. Acid volcanics The principal group of data shown in Fig. 1 which do not lie close to our time-scale line are all Rb-Sr dates on acid volcanics. (24) Andesites, trachytes and rhyolites from the late Gedinne Hedgehog Formation of Maine yield an Rb-Sr isochron of 405 -+ 10 Ma [36]. (25), (26) and (27) Acid volcanics from the late Silurian and early Devonian of Maine assigned to the Eastport Formation (Geddine), the Pembroke Formation (Pridoli) and the Dennys Formation (Upper Llandovery and Wenlock), give respectively Rb-Sr isochron ages of 399 + 3; 394 -+ 6; and 393 -+ 5 [36,10]. It will be noticed that the oldest formation has the youngest age and vice versa (although there is error overlap). These dates are not consistent unless the time range from the Lower Silurian to the basal Devonian is contracted into less than 3 Ma. (28) The Stockdale Rhyolite (Ashgill) of northern England has been dated by a Rb-Sr isochron of 421 + 3 Ma by Gale et al. [ 11 ], who have used this and other Rb-Sr dates on acid volcanics as the basis of a revised time-scale. It is of note that data from localities within the metamorphic aureole of the Devonian Shap Granite (Fig. 1,3) lie on the same isochron as the remaining data from this rock.
(29) The pre-Lower Llandovery Arisaig volcanics of Nova Scotia include both acid and intermediate volcanics and provide a Rb-Sr isochron age of 419 -+ 10Ma [10]. (30) Rhyolites (and andesites) from the Borrowdale volcanics of northern England have yielded a Rb-Sr isochron date of 442 +- 12 Ma (Chamberlain and Lambert, unpublished data). These lavas are earlier than the Caradoc (17. clingani Zone) and are probably of Llandeilo age [21]. (31) The Krivoklat-Rokycany volcanics of Bohemia have a Rb-Sr isochron age of 490 -+ 9 Ma ([37], revised in Gale et al. [11]) and were extruded before the end of the Tremadoc. All these Rb-Sr dates on volcanic lavas lie well to the left of our line; they appear to be 10-20% lower than the other dates on our graph (Fig. 1). We agree with Van Schmus and Bickford [38] that such dates do not give reliable points on a time scale graph. Gale et al. [39] have discussed a possible difference between rhyolite lavas and pyroclastics, noting that the former may give reliable ages. They cite examples from Portugal analysed by Hamet and Delcey [40] and from Bohemia by Vidal et al. [37] : the former isochron, based on five points, gave 373 -+ 40, scarcely an accurate result, while the latter is one of the examples to which we have referred above (31). Neither set of rocks is adequately described petrographically nor are field relations described in detail. The case for accurate ages from rhyolites remains unproven by these examples. The Stockdale Rhyolite is better described and is analytically secure [11]. We are very surprised that the Wasdale Head samples, which are undoubtedly thermally metamorphosed 50 m from the Shap adamellite contact [41], give the same apparent age as samples further afield. Not only was the Shap pluton very hot when intruded (Harker and Marr [41] mention wollastonite hornfels) but it was also extensively hydrothermally altered by molybdenite-bearing solutions at relatively high temperature, perhaps 500°C. Further, Bott's [42] geophysical studies show the Stockdale area lying within the "roof region" of his inferred largely unexposed, Lake District batholith. The latter must be composite, as exposed portions date at 429 + 4 by Rb-Sr or 427 -+ 8 by K-Ar on biotite (Eskdale granite [43]), 420 -+ 4 (Ennerdale granophyre [43]), 419 -+ 4 (the Harestones felsite, a minor intrusion [43]), 407 +- 10
(Weardale granite [44]), and 394 -+ 3 (Shap [15]). The coincidence of ages at 420 may imply a possible reheating or tectonic event affecting the Stockdale Rhyolite during part of the intrusive phase of the batholith. A further problem with the Stockdale Rhyolite lies in the interpretation of the field and petrographic evidence. In our opinion the Stockdale Rhyolite in the Stockdale section is not "unaffected by the regional cleavage, which is well-developed in the surrounding sediments" [ 11 ]. On the contrary, it is thoroughly cleaved, but being massive, the cleavage planes (the "brittle deformation" of Gale et al.) have a different dip from the cleavage in the sediments and are spaced at 1- to 10-cm intervals throughout the section. Alteration along these cleavage planes is pervasive, the rock being bleached for up to 3 mm from the plane of fracture, while limonite and other weathering products occupy the cleavage cracks. In thin section our Stockdale Rhyolite samples fall into two classes, microspherulitic and microcrystalline rhyolites, both phenocryst-free. The former are flowbanded in hand-specimen, the laminae consisting of rows of incomplete spherules whose dimensions are controlled by original laminae breadth. The spherules are separated by hematitic laminae and may pass into irregular areas of quartz-alkali feldspar aggregate, about 0.1 mm grain size. Some of the microcrystalline, polygonal aggregates of quartz and feldspar lie across spherulite rows, indicating later derivation. The distribution and size of the microcrystalline areas is highly variable. The microcrystalline varieties consist of anhedral quartz and feldspar in an irregular polygonal pattern, the grain size being highly variable, 0.1 mm to less than 0.01 mm. The variations of grain size may reflect original heterogeneities in the lava on the 5-mm scale. The spherulitic and microcrystalline varieties may occur together in the same specimen. Aggregates of chlorite and amorphous opaque material occur rarely and most samples contain abundant very fine-grained sericite. All samples are traversed by thin quartz, quartz-feldspar or quartzmuscovite veins in which the mica grows into the vein from the walls. The quartz-feldspar veins have the same grain size and polygonal texture as the aggregates in the main body of the rock and are probably related to them. The veins and the probably recrystallized areas are indicative of post-eruption alteration. Alteration was also recorded by Harker and Mart
[41, p. 302] who stated that the nodular part of the rhyolite is silicified at Great Yarlside Crag. If the rhyolite has been down-dated, one of two mechanisms is likely: either it suffered internal isotopic re-equilibration at 421 Ma, or complete 88Sr loss. In the first case the original initial 878r/86Sr should not have been lower than 0.7040 (it is not likely to have been derived from depleted mantle) at which value a maximum pre-history of 37 Ma is permissible given an average 8~Rb/a6Sr of 7.5 and STSr/ a6Sr at 421 Ma of 0.70766. Such an increase in the extrusive age is consistent with the Ashgill on our scale: it requires internal isotopic exchange, without intervention of external metasomatism. Alternatively, loss of the 87Sr formed between ~445 and 421 Ma by leaching at the latter date would produce an isochron with the present parameters. The latter seems more likely, given the microcrystalline, cleaved nature of the rhyolite and the alteration along cleavages as described above. However, the very wide range of Rb/Sr within one extrusion may indicate internal exchange as the cause of down-dating. Perhaps there-
fore, the Stockdale Rhyolite has an anomalously low age like all other acid volcanics in Fig. 1. We consider that it may well have been reset by an early Ludlow event, which, so far as evidence is available, would correlate with mild hornfelsing by part of the Lake District batholith.
5. Comparison with other time scales The difference between the proposed scale (Fig. 1, Table 1) and that of Gale et al. [11] is substantial and rests chiefly upon the differences in interpretations of ages obtained from acid volcanics and fission-track data. Much of this has been discussed above. Other scales published over the last few years are not far different from the one we consider best (Table 1). However, some of the similarity, for example with those of Lambert [45] and Armstrong [46], is partly coincidental in that small changes in constants, re-interpretations of assignments and modifications of stratigraphic definitions have tended to cancel each other.
TABLE 1 Comparison of scales (age of base of divisions; ages in million years) This paper Carboniferous Famenne Frasne Givet Eifet Ems Siegen Gedinne Pridoli Ludlow Wenlock Llandovery Ashgill Caradoc Llandeilo Llanvirn Arenig Tremadoe * See item (20). ** See item (19).
360 369 377 383 390 398 403 411 413.5 420 425.5 438 445 467 (479) * (489) (504) *-~ " ; 519
Gale et al. [11]
Ross et al. [21
354
Fitch et al. [47]
368
394 405 413 423 435 460 474
418 434 (453) (464) ' ' (482) (490)
Armstrong [46]
.....
(487) 494
Lambert [45] 375
417
410
420
446 455 465 477 492 500 510
450
450
480 515 520
A g r e e m e n t w i t h t h e scale o f F i t c h et al. [47] is m o r e a p p a r e n t t h a n real, for these a u t h o r s used o n l y a small n u m b e r o f d a t a p o i n t s , some o f w h i c h we also use. Ross et al. [2] used some o f t h e d a t a w h i c h we rely o n , b u t assigned a m u c h y o u n g e r age t o t h e U p p e r Silurian ( b a s e d d i r e c t l y o n t h e i r fission-track ages) a n d used a very d i f f e r e n t set o f O r d o v i c i a n stage l e n g t h s , as we have discussed earlier. T h e m u c h l o n g e r scale o f L a m b e r t a n d M c K e r r o w [34] was p a r t l y based on assumptions about correlations of tectonic a n d s t r a t i g r a p h i c e v e n t s in t h e G r a m p i a n s a n d is n o t considered further here, T h e r e f o r e , in c o n c l u s i o n , we observe t h a t the fund a m e n t a l division o f o p i n i o n is b e t w e e n t h e considerable revision o f Gale, Beckinsale a n d Wadge, a n d o u r r e f i n e m e n t o f the s t a n d a r d 1 9 7 0 ' s scale for the O r d o vician, Silurian a n d D e v o n i a n .
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Acknowledgements
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This research forms part of a program financed by t h e N a t u r a l Sciences a n d E n g i n e e r i n g R e s e a r c h C o u n cil o f C a n a d a in a g r a n t to R . S t . J . L a m b e r t , w h i c h is g r a t e f u l l y a c k n o w l e d g e d . Dr. N.H. Gale a n d R.D. Beckinsale are t h a n k e d for h e l p f u l c o m m e n t s .
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References 17 1 W. Compston, The place of isotope age determinations in stratigraphy, Episodes 1 (1979) Ottawa, I.U.G.S. 2 R.J. Ross, C.W. Naeser, G.A. Izett, H.B. Whittington, C.P. Hughes, R.B. Rickards, J. Zalasiewicz, P.R. Sheldon, C.J. Jenkyns, L.R.M. Cocks, M.G. Bassett, P. Toghill, W.T. Dean and J.K. Ingham, Fission-track dating of Lower Palaeozoic bentonites in British stratotypes, in: Short Papers of the 4th International Conference, Geochronology, Cosmochronology and Isotope Geology, R.E. Zartman, ed., U.S. Geol. Surv. Open-file Rep. 78-701 (1978) 363-365. 3 W. Ziegler, Devonian, in: Contributions to the Geologic Time Scale, G.V. Cohee, M.F. Glaessner and H.D. Hedberg, eds., Am. Assoc. Pet. Geol., Stud. Geol. 6 (1978) 337-339. 4 A.J. Boucot, Evolution and Extinction Rate Controls (Elsevier, Amsterdam, 1975). 5 M. Churkin, C. Carter and B.R. Johnson, Subdivisions of of Ordovician and Silurian time scale using accumulation rates of graptolite shales, Geology 5 (1977) 4 5 2 - 4 5 6 . 6 M.G. Bassett, ed., The Ordovician System (University of
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Wales Press and National Museum of Wales, Cardiff, 1976) 696 pp. J.A. Talent, W.B.N. Berry and A.J. Boucot, Correlation of the Silurian rocks of Australia, New Zealand and New Guinea, Geol. Soc. Am. Spec. Paper 150 (1975) 108 pp. D.G. Gee and M.R. Wilson, The age of orogenic deformation in the Swedish Caledonides: reply, Am. J. Sci. 277 (1977) 1353-1354. R.H. Steiger and E. J~iger, Subcommision on Geochronology: convention on the use of decay-constants in geoand cosmochronology, Earth Planet. Sci. Lett. 36 (1977) 359-362, P.D. Fullagar and M.L. Bottino, Rb-Sr whole rock ages of Silurian-Devonian volcanics from eastern Maine, Maine Geol. Surv. Bull. 23 (1970) 4 9 - 5 2 . N.H. Gale, R.D. Beckinsale and A.J. Wadge, A Rb-Sr whole rock isochron for the Stockdale Rhyolite of the English Lake District and a revised mid-Palaeozoic timescale, J. Geol. Soc. London 136 (1979) 235-242. I. McDougall, W. Compston and V.M. Bofinger, Isotopic age determinations on Upper Devonian rocks from Victoria, Australia, Geol. Soc. Am. Bull. 77 (1966) 1 0 7 5 1088. A.N. Halliday, A. McAlpine and J.G. Mitchell, The age of the Hoy Lavas, Orkney, Scott. J. Geol. 13 (1977) 4 3 - 5 2 . A.N, Halliday, A. McAlpine and J.G. Mitchell, Erratum: the age of the Hoy Lavas, Orkney, Scott. I. Geol. 15 (1979) 79. A.J. Wadge, N.H. Gale, R.D. Beckinsale and C.C. Rundle, A Rb-Sr isochron for the Shap Granite, Proc. Yorks. Geol. Soc. 42 (1978) 297-305. D.G.W. Smith, H. Baadsgaard, R.E. Folinsbee and J. Lipson, K/Ar age of Lower Devonian bentonites of Gaspe, Quebec, Canada, Geol. Soc. Am. Bull. 72 (1961) 171 174. J.R. Richards, J.P. Barkas and T.G. Vallance, A Lower Devonian point in the geological time scale, Geochem. J. (Japan) l l (1977) 147-153. R.J. Ross, C.W. Naeser and G.A. Izett, Fisslon-track dating of Lower Paleozoic bentonites in British stratotypes, U.S. Geol. Surv. Open-file Rep. 77-348 (1977) 12 PP. M.A. Lanphere, M. Churkin and G.D. Eberlein, Radiometric age of the Monograptus cyphus zone in southeastern Alaska - an estimate of the Ordovician-Silurian boundary, Geol. Mag. 114 (1977) 15-24. W.B. Harland, A.G. Smith and B. Wilcock, eds., The Phanerozoic time scale, Q. J. Geol. Soc. London 120S (1964). A. Williams, I. Strachan, D.A. Bassett, W.T. Dean, J.K. Ingham, A.D. Wright and H.B. Whittington, A correlation of Ordovician rocks in the British Isles, Geol. Soc. London, Spec. Rep. 3 (1972) 74 pp. A.J. Boucot, D. Brookins, W. Forbes and C.V. Guidotti, Staurolite zone Caradoc (Middle-Late Ordovician) age, Old World province brachiopods from Penoscot Bay, Maine, Geol. Soc. Am. Bull. 83 (1972) 1953-1860.
23 P.M. Harris, E. Farrar, R.M. Maclntyre and D. York, Potassium-argon age measurements on two igneous rocks from the Ordovician system of Scotland, Nature 205 (1965) 3 5 2 - 3 5 3 . 24 R.D. Dallmeyer and H. Williams, 4°Ar/agAr ages from the Bay of Islands metamorphic aureole: their bearing on the timing of the Ordovician ophiolite subduction, Can. J. Earth Sci. 12 (1975) 1685-1690. 25 D.A. Archibald and E. Farrar, K-At ages of amphiboles from the Bay of Islands ophiolite and the Little Port Complex, western Newfoundland, and their geological implications, Can. J. Earth Sci. 13 (1976) 5 2 0 - 5 2 9 . 26 S.B. Jacobsen and G.J. Wasserburg, Nd and Sr isotopic composition of oceanic crust and mantle and the evolution of the source of mid-ocean ridge basalts, J. Geophys. Res. 84 (1979) 7429-7445. 27 C.D. Longman, B.J. Bluck and O. Van Breeman, Ordovician conglomerates and the evolution of the Midland Valley, Nature 280 (1979) 5 7 8 - 5 8 1 . 28 E.K. Walton, Lower Paleozoic rocks, in: The Geology of Scotland, G.Y. Craig, ed. (Oliver and Boyd, Edinburgh, 1965) 161-227. 29 J.K. Ingham, Geology of a continental margin, 2. Middle and late Ordovician transgression, Girvan, in: Crustal Evolution in Northwestern Britain and Adjacent Regions, D.R. Bowes and B.E. Leake, eds. (Seel House Press, Liverpool, 1978) 163-176. 30 R.D. Dallmeyer, Diachronous ophiolite obduction in western Newfoundland evidence from 4°Ar/39Ar ages of the Hare Bay metamorphic aureole, Am. J. Sci. 277 (1977) 6 1 - 7 2 . 31 J.G. Spray and G.D. Williams, The sub-ophiolite metamorphic rocks of the BaUantrae igneous complex, S.W. Scotland, Geol. Soc. London, Newslett. 8, No 5 (1979) 13. 32 R.T. Pidgeon, Zircon U-Pb ages from the Galway granite and the Dalradian, Connemara, Ireland, Scott. J. Geol. 5 (1969) 3 7 5 - 3 9 2 . 33 P.J. Leggo and R.T. Pidgeon, Geochronological investigations of Caledonian history in western Ireland, Eclogae Geol. Helv. 63 (1970) 2 0 7 - 2 1 2 . 34 R.St.J. Lambert and W.S. McKerrow, The Grampian Orogeny, Scott. J. Geol. 12 (1976) 271-292. 35 C.T. Harper, The geological interpretation of K-Ar ages of metamorphic rocks from the Scottish Caledonides, Scott. J. Geol. 3 (1967) 4 6 - 6 6 .
36 M.L. Bottino and P.D. Fullagar, Whole rock rubidiumstrontium age of the Silurian Devonian boundary in northeastern North America, Geol. Soc. Am. Bull. 77 (1966) 1167-1176. 37 P. Vidal, B. Auvray, R. Charlot, F. Fediuk, J. Hameurt and J. Waldhansrova, Radiometric age of volcanics of the Cambrian "Krivoklat-Rokycany" complex (Bohemian Massif), Geol. Rundsch. 64 (1975) 5 6 3 - 5 7 1 . 38 W.R. Van Schmus and M.E. Bickford, Rotation of Rb-Sr isochrons during low-grade events, Abstracts for European Colloquium of Geochronology, Amsterdam (1976). 39 N.H. Gale, R.D. Beckinsale and A.J. Wadge, Rb-Sr whole rock dating of acid rocks, Geochem. J. (1979) 2 7 - 2 9 . 40 J. Hamet and R. Delcey, Age, synchronisme et affiliation des roches rhyolitiques de la province pyrito-carbonif~re de Baixo-Alentejo (Portugal) mesures isotopiques par la m~thode 87Rb/aTSr, C.R. Acad. Sci. Paris 272 (1971) 2134-2146. 41 A. Harker and J.E. Marr, The Shap granite and associated rocks, Q. J. Geol. Soc. London 47 (1891) 266-328. 42 M.H.P. Bott, The geological interpretation of a gravity survey of the English Lake District and the Vale of Eden, J. Geol. Soc. London 130 (1974) 309-331. 43 C.C. Rundle, Ord0vician intrusions in the English Lake District, J. Geol. Soc. London 136 (1979) 2 9 - 3 8 . 44 J.G. Holland and R.St.J. Lambert, Weardate granite, in: Geology of Durham County, G. Hickling, ed., Trans. Nat. Hist. Soc. Northumberl., Durham Newcastle upon Tyne 41 (1970) 103-118. 45 R.St.J. Lambert, The pre-Pleistocene Phanerozoic timescale: a review, Geol. Soc. London Spec. Publ. 5 (1971) 9-31. 46 R.L. Armstrong, Pre-Cenozoic Phanerozoic time scale computer file of critical dates of new and in-progress decay constant revisions, in: Contributions to the Geological Time Scale, G.V. Cohee, M.F. Glaessner and H.D. Hedberg, eds., Am. Assoc. Pet. Geol. Stud. Geol. 6 (1978) 7 3 - 9 1 . 47 F.J. Fitch, S.C. Forster and J.A. Miller, The dating of the Ordovician, in: The Ordovician System, M.G. Bassett, ed., (University of Wales Press and National Museum of Wales, Cardiff, 1976) 1 5 - 2 7 .
1
[5]
THE ORDOVICIAN, SILURIAN AND DEVONIAN TIME SCALES W.S. McKERROW 1, R.St.J. LAMBERT 2 and V.E. CHAMBERLAIN 2 1 Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford OX1 3PR (England) 2 Department of Geology, University of Alberta, Edmonton, Alta. T6G 2E3 {Canada)
Received February 16, 1980 Revised version received July 12, 1980
A reassessment of published Ordovician, Silurian and Devonian isotopic age determinations available has necessitated the correction of several calculated dates and revision of several stratigraphic conclusions in the light of later work. The revised dates plot close to a single time scale line, except for Rb-SI dates on acid volcanic rocks. We conclude that the base of the Devonian is at 410 Ma, the base of the Silurian at 437 Ma, and the base of the Ordovician (Tremadoc Series) at 519 Ma. These dates are within the range of previously published dates for the base of the three periods. We place more reliance on our estimates for the Upper Ordovician, Silurian and Devonian than for the Lower Ordovician. A comparison with other time scales shows that our new scale is similar to many previously published, except for those which rely heavily on acid volcanics.
1. Introduction Although there is still a scarcity of accurate isotopic age determinations for Palaeozoic rocks with good stratigraphic control [ 1 ] some recent dates, especially those o f Ross et al. [2], now provide data for a new assessment of the time scale for the Ordovician, Silurian and Devonian periods. In order to compare the stratigraphical ranges of isotopic dates, it is necessary to first plot the ages against a stratigraphic scale which accurately reflects the relative durations o f the time-stratigraphic (chrono-stratigraphic) divisions: periods (systems), epochs (series), ages (stages) and chrono-zones. This is still very uncertain, especially for the Ordovician Period. In Fig. 1, we have used the relative durations of the Devonian stages suggested by Ziegler [3], modified on the advice of A.J. Boucot (personal communication, 1979), and those o f the Silurian stages suggested by Boucot [4]. Our estimates o f the relative durations o f the complete Silurian and Devonian periods also follows Boucot [4]. It has not been easy to estimate the relative dura-
tions of the Ordovician stages either to each other or to the Silurian and Devonian. Boucot [4] suggested that the Caradoc, Ashgill and Llandovery have relative durations in the ratios 2.0 : 1.0 : 1.9. Churkin et al. [5] gave estimates of the relative duration of some Ordovician graptolite zones, based on stratigraphic thicknesses of graptolite shales along the outer margin o f the Cordilleran fold belt of North America, but their table does not include data from the basal Arenig, the early Llanvirn; nor from an unknown part o f file Caradoc, D. clingani Zone. Nevertheless they report very large thicknesses for several Caradoc zones, which suggests to them that the Caradoc has a duration of 17 Ma compared with 2 Ma for the Ashgill. We consider that a Caradoc : Ashgill ratio of 8.5 : 1 is excessive and that the Boucot ratio of 2 : 1 is perhaps too little, so we have selected (rather arbitrarily) an intermediate ratio of 4 : 1. The earlier parts o f the Ordovician Period have been divided (on the basis of the number o f graptolite, trilobite, brachiopod and conodont time-stratigraphic units - e.g., see Bassett [6, pp. 5 7 , 1 3 4 , 2 2 2 ] ) , in the ratios: Caradoc (12), Llandeilo (5), Llanvirn (5), Arenig (8) and
0012-821X/80/0000 0000/$02.50 © 1980 Elsevier Scientific Publishing Company
the Stromlo volcanics [7, p. 86] and the Vilasund Granite [8], were omitted from consideration. Age assignments have also been checked against the original data, and all dates have been corrected to present-day constants [9]. Some determinations were rejected because of excessive error, e.g., Quoddy volcanics, 409 -+ 22 Ma [10], but some others which were rejected by Gale et al. [ 11 ] have been retained.
Tremadoc (8) to give a total time span of the Ordovician (here taken to include the Tremadoc Series) about equal to that of the Silurian and Devonian combined. As will be seen from our discussion below, this calibration of the stratigraphic time scale is in good agreement with the dates which we have selected as giving the best control on the scale.
2. Selection and updating of data 3. The suggested scale We have included all published accurate isotopic age determinations known to us which also have precise enough stratigraphic control to be meaningful in construction of a time scale. Stratigraphic assignments of the data in Fig. 1 were all checked against the original author's statements and modified where necessary in the light of later information. A few supposed marker points were found to be less precise than originally stated, and some previously used for time scale purposes, like
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All the data recorded in Fig. 1 are represented by rectangles corresponding to the analytical errors and to the uncertainty in stratigraphical dating. Our time scale represents a best fit line through the following rectangles: (1) A 369 -+ 6 Ma K-Ar date on biotites from the Cerberean volcanics of Victoria, Australia [2, 4], which overlie the fish-bearing Taggerty Group and are thought to have an age "near the top of the Upper
420 I
430 I
440 I
460
450 I
470
480
490
500
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Fig. 1. Time scale for the Ordovician, Silurian and Devonian. Constructed by choosing best estimates of relative lengths of stages and by analysis of the data for 31 apparent control points. The best line has been subjectively fitted through the data array as described in the text. Key to numbers: 1 = Cerberean volcanics; 2 = Hey basalts; 3 = Shap Granite; 4 = Shiphead Formation; 5 = Gocup Granite; 6 = Bringewood Beds; 7 = Wenlock Limestone; 8 = Buildwas Formation; 9 = Birkhill shales; 10 = Acton Scott Beds; 11 = Gelligrin Ashes; 12 = Bach-y-Graigshales; 13 = Llyfnant Flags; 14 = Descon Formation; 15 = Carters Limestone; 16 = Penobscot Formation; 17 = Bail Hill volcanics; 18 = Chasmops Limestone; 19 = Bay of Islands ophiolite amphiboles; 20 = Benan Conglomerate clasts; 21 = Hare Bay Ophiolite; 22 = Colmonell Gabbro; 23 = Cashel Intrusion; 24 = Hedgehog Formation; 25 = Eastport Formation; 26 = Pembroke Formation; 27 = Dennys Formation; 28 = Stockdale Rhyolite; 29 = Arisaig volcanics; 30 = Borrowdale volcanics; 31 = Krivoklat-Rokycany volcanics.
Old Red Sandstone of Scotland" [12, p. 1077]. (2) A 4°Ar/39Ar date of 368 + 8 Ma [13,14] on whole-rocks from the Hoy basalts of the Orkney Islands of northern Scotland, indicates a horizon near the base of the Upper Old Red Sandstone. The Late Devonian fish Holoptychius occurs in equivalent beds at Dunnett Head, Caithness. (3) The post-Silurian/pre-Upper Devonian Shap Granite of northern England has an Rb/Sr isochron age of 394 + 3 Ma [15]. (4) Bentonites in the Siegen Shiphead Formation of Quebec [16] give a revised K-At age on sanidine of 395 -+ 16 Ma (H. Baadsgaard, personal communication, 1979). (5) The post-Wenlock and pre-Siegen Gocup Granite of New South Wales has a K-Ar age of 409 +3 Ma [17]. The fission-track dating of zircons in British Lower Palaeozoic bentonites [2] has made a great step forward in establishing a time scale, as they come from horizons accurately dated by fossils. These are: (6) Bringewood Beds (mid-Ludlow): 407 -+ 8 Ma (average of two analyses). (7) Wenlock Limestone (late Wenlock); 416 +- 9 Ma. (8) Buildwas Formation (early Wenlock): 422 +10Ma. (9) Birkhill shales (late Lower Llandovery Monograptus cyphus Zone): 437 +- 11 Ma. (10) Acton Scott Beds (late Caradoc, Actonian Stage, probably late Dicranograptus clingani Zone): 466 -+ 11 Ma (average of two analyses). (11) Gelli-grin Calcareous Ashes (mid-Caradoc, Longvillian Stage): 465 -+ 10 Ma (average of two analyses). (12) Shales at Bach-y-graig, near Llandrindod Wells (topmost Llanvirn or lowermost Llandeilo): 477 +I1 Ma. (13) Llyfnant Flags (high in the Didymograptus extensus Zone): 493 -+ 11 Ma. Most of these are likely to be minimum ages (i.e., they should occur to the left of our time scale line), but the date from the Acton Scott Beds lies to the right. This anomaly may be explained by the assumption that the late Caradoc and Ashgill represent a much longer period of time than that shown on our graph, or by the presence of inherited zircons in the
material analysed, or by the possibility that the large errors present on earlier dates (464 -+ 21 and 468 -+ 12 Ma, [18]) are real. (14) A Lower Llandovery (Monograptus cyphus Zone) volcanic breccia in the Descon Formation of Esquibel Island, Alaska, yields a 4°Ar/39Ar age on hornblende of 433 +--3 Ma [19]. (15) Zircons from a bentonite in the Carters Limestone of the Stones River Group in Tennessee [20, item 156] yield a revised mean age of 443 -+ 10 Ma. These beds are assigned to the Wilderness, which corresponds to the middle Caradoc (Climacograptus wilsoni and C. peltifer Zones) [21 ]. A comparatively high common Pb correction for these zircons makes assessment of this age difficult. (16) The discovery of poorly preserved Upper Ordovician brachiopods from schists of the Penobscot Formation of Maine, together with an Rb-Sr isochron on these schists of 450 -+ 10 Ma [22] suggests that this date "may either reflect a time of metamorp h i s m . . , or a minimum time of deposition for the rocks" [22, p. 1957]. The schists are considered to be overlain by unmetamorphosed Silurian beds. (17) The Bail Hill volcanics of the Southern Uplands of Scotland overlie shales of the l_.landeiloCaradoc Nemagraptus gracilis Zone, and yield a K-Ar age on biotite of 453 -+ 10 Ma [23]. (18) Biotites and sanidines in bentonites in the Chasmops Limestone of Kinnekulle, Sweden, yield a K-Ar age of 457 -+ 4 Ma [20, item 157]. These beds are assigned to the early Caradoc. (19) Hornblendes from the metamorphic aureoles of thrust masses of ophiolites in western Newfoundland, the Bay of Islands ophiolite and the Little Port Complex give a 4°Ar/39Ar.age of 460-+ 5 Ma [24,25]. The stratigraphic time span of these dates is limited by the igneous rocks (probably Middle Arenig) and the age of final emplacement of the thrust sheet (N. gracilis Zone). That this age is likely to refer to metamorphic rather than primary igneous events is confirmed by the Sm-Nd ages of 508 + 6 and 501 -+ 13 Ma on the Bay of Islands gabbros [26] which should refer to Middle Arenig events. These Sm-Nd ages are shown as 507 + 5 in Fig. 1, item 19A, reinforcing our interpretation of the other Lower Ordovician points (12, 13, 20, 21 and 22), though an increase in the age of the Tremadoc-Arenig boundary may be suggested by these latest data.
(20) An Rb-Sr mineral isochron age of 470 Ma (-+5?) for granitic clasts in the Benan Conglomerate, Ayrshire is regarded as a maximum for the Llandeilo and as being inconsistent with published fission-track ages [27]. The four relevant mineral isochrons give 469 _+5,471 -+ 13,466 -+ 14 and 468 -+ 5 Ma. The four corresponding whole-rocks and one other lie on a "tentative" 459 -+ I0 Ma isochron. Longman et al. [27] argue that the base of the Llandeilo must be 460 Ma or younger as it will take "<10 Ma" to unroof the granitic source region and form the Benan Conglomerate, but the Benan Conglomerate probably lies within the N. gracilis zone and near the LlandeiloCaradoc boundary [28, figs. 5.8, 5.9 and 6.2; 29, fig. 3]. Both Walton [28] and Ingham [29] show the Benan Conglomerate forming from an up-faulted block of basement: the time span involved could be short (cf. conglomerates formed against splays of the San Andreas system) and the granitic rocks could have formed during the N. gracilis zone. The lower stratigraphic age limit is therefore given in Fig. 1 as the top of the Llandeilo, a convenient average of the possibilities. There is a problem with the ages which we cannot analyse without more information: it is possible that the mineral isochrons reflect an alteration event and not the age of intrusion, in which case the 470 (-+5?) age is perhaps close to the age of sedimentation. If however, the authors' original interpretation is correct, our scale, based on a best fit through a number of comparatively ill-defined points for the Lower Ordovician, may be a little high at the Llanvirn-Landeilo range (indicated by parentheses in Table 1). (21) A 4°Ar/39Ar date from Hare Bay [30] of 480 -+ 5 Ma, can be established to be post-early Arenig and pre-late Llanvirn. The fact that the time scale line lies near the top of the rectangles on our graph (Fig. 1) suggests that in each case (19 and 21) the metamorphism occurred close to the age of final emplacement of the thrust sheets. This interpretation parallels that proposed for the origin of an amphibolite facies aureole adjacent to serpentinite in the BaUantrae Igneous Complex [31 ]. (22) The Colmonell gabbro of southern Scotland intrudes the Arenig and Llanvirn Ballantrae Volcanic Group, which is overlain by late Llanvirn and younger sediments [29]. It yields a K-Ar age on biotite of 4 8 4 -+ 10Ma [23].
(23) The U-Pb date of 510 _+ 15 Ma for the CashelLough Wheelan basic intrusion [32,33] is contemporary with the Grampian Orogeny in western Ireland. Dalradian rocks, metamorphosed and deformed by this orogeny, occur to the north and to the south of the South Mayo Trough which contains a thick sequence of late Tremadoc to Llanvirn rocks. We now consider that the Grampian Orogeny in western Ireland occurred before the end of the Tremadoc rather than the alternative view [34] that it occurred after the Llanvirn. In Scotland, whole-rock K-Ar ages of 512 -+ 6 Ma on slates of the Highland Border area [35] suggest that the termination of the Grampian Orogeny was synchronous throughout the British Isles (though the stratigraphic age of the Scottish sediments is not well defined). We therefore conclude that 510 Ma represents a point of time during the Tremadoc Series, perhaps in the basal part of the late Tremadoc.
4. Acid volcanics The principal group of data shown in Fig. 1 which do not lie close to our time-scale line are all Rb-Sr dates on acid volcanics. (24) Andesites, trachytes and rhyolites from the late Gedinne Hedgehog Formation of Maine yield an Rb-Sr isochron of 405 -+ 10 Ma [36]. (25), (26) and (27) Acid volcanics from the late Silurian and early Devonian of Maine assigned to the Eastport Formation (Geddine), the Pembroke Formation (Pridoli) and the Dennys Formation (Upper Llandovery and Wenlock), give respectively Rb-Sr isochron ages of 399 + 3; 394 -+ 6; and 393 -+ 5 [36,10]. It will be noticed that the oldest formation has the youngest age and vice versa (although there is error overlap). These dates are not consistent unless the time range from the Lower Silurian to the basal Devonian is contracted into less than 3 Ma. (28) The Stockdale Rhyolite (Ashgill) of northern England has been dated by a Rb-Sr isochron of 421 + 3 Ma by Gale et al. [ 11 ], who have used this and other Rb-Sr dates on acid volcanics as the basis of a revised time-scale. It is of note that data from localities within the metamorphic aureole of the Devonian Shap Granite (Fig. 1,3) lie on the same isochron as the remaining data from this rock.
(29) The pre-Lower Llandovery Arisaig volcanics of Nova Scotia include both acid and intermediate volcanics and provide a Rb-Sr isochron age of 419 -+ 10Ma [10]. (30) Rhyolites (and andesites) from the Borrowdale volcanics of northern England have yielded a Rb-Sr isochron date of 442 +- 12 Ma (Chamberlain and Lambert, unpublished data). These lavas are earlier than the Caradoc (17. clingani Zone) and are probably of Llandeilo age [21]. (31) The Krivoklat-Rokycany volcanics of Bohemia have a Rb-Sr isochron age of 490 -+ 9 Ma ([37], revised in Gale et al. [11]) and were extruded before the end of the Tremadoc. All these Rb-Sr dates on volcanic lavas lie well to the left of our line; they appear to be 10-20% lower than the other dates on our graph (Fig. 1). We agree with Van Schmus and Bickford [38] that such dates do not give reliable points on a time scale graph. Gale et al. [39] have discussed a possible difference between rhyolite lavas and pyroclastics, noting that the former may give reliable ages. They cite examples from Portugal analysed by Hamet and Delcey [40] and from Bohemia by Vidal et al. [37] : the former isochron, based on five points, gave 373 -+ 40, scarcely an accurate result, while the latter is one of the examples to which we have referred above (31). Neither set of rocks is adequately described petrographically nor are field relations described in detail. The case for accurate ages from rhyolites remains unproven by these examples. The Stockdale Rhyolite is better described and is analytically secure [11]. We are very surprised that the Wasdale Head samples, which are undoubtedly thermally metamorphosed 50 m from the Shap adamellite contact [41], give the same apparent age as samples further afield. Not only was the Shap pluton very hot when intruded (Harker and Marr [41] mention wollastonite hornfels) but it was also extensively hydrothermally altered by molybdenite-bearing solutions at relatively high temperature, perhaps 500°C. Further, Bott's [42] geophysical studies show the Stockdale area lying within the "roof region" of his inferred largely unexposed, Lake District batholith. The latter must be composite, as exposed portions date at 429 + 4 by Rb-Sr or 427 -+ 8 by K-Ar on biotite (Eskdale granite [43]), 420 -+ 4 (Ennerdale granophyre [43]), 419 -+ 4 (the Harestones felsite, a minor intrusion [43]), 407 +- 10
(Weardale granite [44]), and 394 -+ 3 (Shap [15]). The coincidence of ages at 420 may imply a possible reheating or tectonic event affecting the Stockdale Rhyolite during part of the intrusive phase of the batholith. A further problem with the Stockdale Rhyolite lies in the interpretation of the field and petrographic evidence. In our opinion the Stockdale Rhyolite in the Stockdale section is not "unaffected by the regional cleavage, which is well-developed in the surrounding sediments" [ 11 ]. On the contrary, it is thoroughly cleaved, but being massive, the cleavage planes (the "brittle deformation" of Gale et al.) have a different dip from the cleavage in the sediments and are spaced at 1- to 10-cm intervals throughout the section. Alteration along these cleavage planes is pervasive, the rock being bleached for up to 3 mm from the plane of fracture, while limonite and other weathering products occupy the cleavage cracks. In thin section our Stockdale Rhyolite samples fall into two classes, microspherulitic and microcrystalline rhyolites, both phenocryst-free. The former are flowbanded in hand-specimen, the laminae consisting of rows of incomplete spherules whose dimensions are controlled by original laminae breadth. The spherules are separated by hematitic laminae and may pass into irregular areas of quartz-alkali feldspar aggregate, about 0.1 mm grain size. Some of the microcrystalline, polygonal aggregates of quartz and feldspar lie across spherulite rows, indicating later derivation. The distribution and size of the microcrystalline areas is highly variable. The microcrystalline varieties consist of anhedral quartz and feldspar in an irregular polygonal pattern, the grain size being highly variable, 0.1 mm to less than 0.01 mm. The variations of grain size may reflect original heterogeneities in the lava on the 5-mm scale. The spherulitic and microcrystalline varieties may occur together in the same specimen. Aggregates of chlorite and amorphous opaque material occur rarely and most samples contain abundant very fine-grained sericite. All samples are traversed by thin quartz, quartz-feldspar or quartzmuscovite veins in which the mica grows into the vein from the walls. The quartz-feldspar veins have the same grain size and polygonal texture as the aggregates in the main body of the rock and are probably related to them. The veins and the probably recrystallized areas are indicative of post-eruption alteration. Alteration was also recorded by Harker and Mart
[41, p. 302] who stated that the nodular part of the rhyolite is silicified at Great Yarlside Crag. If the rhyolite has been down-dated, one of two mechanisms is likely: either it suffered internal isotopic re-equilibration at 421 Ma, or complete 88Sr loss. In the first case the original initial 878r/86Sr should not have been lower than 0.7040 (it is not likely to have been derived from depleted mantle) at which value a maximum pre-history of 37 Ma is permissible given an average 8~Rb/a6Sr of 7.5 and STSr/ a6Sr at 421 Ma of 0.70766. Such an increase in the extrusive age is consistent with the Ashgill on our scale: it requires internal isotopic exchange, without intervention of external metasomatism. Alternatively, loss of the 87Sr formed between ~445 and 421 Ma by leaching at the latter date would produce an isochron with the present parameters. The latter seems more likely, given the microcrystalline, cleaved nature of the rhyolite and the alteration along cleavages as described above. However, the very wide range of Rb/Sr within one extrusion may indicate internal exchange as the cause of down-dating. Perhaps there-
fore, the Stockdale Rhyolite has an anomalously low age like all other acid volcanics in Fig. 1. We consider that it may well have been reset by an early Ludlow event, which, so far as evidence is available, would correlate with mild hornfelsing by part of the Lake District batholith.
5. Comparison with other time scales The difference between the proposed scale (Fig. 1, Table 1) and that of Gale et al. [11] is substantial and rests chiefly upon the differences in interpretations of ages obtained from acid volcanics and fission-track data. Much of this has been discussed above. Other scales published over the last few years are not far different from the one we consider best (Table 1). However, some of the similarity, for example with those of Lambert [45] and Armstrong [46], is partly coincidental in that small changes in constants, re-interpretations of assignments and modifications of stratigraphic definitions have tended to cancel each other.
TABLE 1 Comparison of scales (age of base of divisions; ages in million years) This paper Carboniferous Famenne Frasne Givet Eifet Ems Siegen Gedinne Pridoli Ludlow Wenlock Llandovery Ashgill Caradoc Llandeilo Llanvirn Arenig Tremadoe * See item (20). ** See item (19).
360 369 377 383 390 398 403 411 413.5 420 425.5 438 445 467 (479) * (489) (504) *-~ " ; 519
Gale et al. [11]
Ross et al. [21
354
Fitch et al. [47]
368
394 405 413 423 435 460 474
418 434 (453) (464) ' ' (482) (490)
Armstrong [46]
.....
(487) 494
Lambert [45] 375
417
410
420
446 455 465 477 492 500 510
450
450
480 515 520
A g r e e m e n t w i t h t h e scale o f F i t c h et al. [47] is m o r e a p p a r e n t t h a n real, for these a u t h o r s used o n l y a small n u m b e r o f d a t a p o i n t s , some o f w h i c h we also use. Ross et al. [2] used some o f t h e d a t a w h i c h we rely o n , b u t assigned a m u c h y o u n g e r age t o t h e U p p e r Silurian ( b a s e d d i r e c t l y o n t h e i r fission-track ages) a n d used a very d i f f e r e n t set o f O r d o v i c i a n stage l e n g t h s , as we have discussed earlier. T h e m u c h l o n g e r scale o f L a m b e r t a n d M c K e r r o w [34] was p a r t l y based on assumptions about correlations of tectonic a n d s t r a t i g r a p h i c e v e n t s in t h e G r a m p i a n s a n d is n o t considered further here, T h e r e f o r e , in c o n c l u s i o n , we observe t h a t the fund a m e n t a l division o f o p i n i o n is b e t w e e n t h e considerable revision o f Gale, Beckinsale a n d Wadge, a n d o u r r e f i n e m e n t o f the s t a n d a r d 1 9 7 0 ' s scale for the O r d o vician, Silurian a n d D e v o n i a n .
7
8
9
10
11
12
Acknowledgements
13
This research forms part of a program financed by t h e N a t u r a l Sciences a n d E n g i n e e r i n g R e s e a r c h C o u n cil o f C a n a d a in a g r a n t to R . S t . J . L a m b e r t , w h i c h is g r a t e f u l l y a c k n o w l e d g e d . Dr. N.H. Gale a n d R.D. Beckinsale are t h a n k e d for h e l p f u l c o m m e n t s .
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15
16
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