New Isotopic Age Data on Rocks from the Long Mynd, S Hropshire

New isotopic age data on rocks from the Long Mynd, S hropshire

ADRIAN H. BATH

SUMMARY New Rb-Sr isotope data on fine-grained an age of 529 + 6 m.y. with an initial S7Sr/SeSr sediments from the Long Mynd, west of the ratio of o'7o6I + o.oooi, put the depositional Church Stretton Fault in Shropshire, indicate age of the Stretton Series (the stratigraphically a much younger age for these rocks than has lowest part of the sequence) at a maximum of been assumed in recent literature. Wello 600 m.y. defined isochrons, one of which corresponds to

THE SEDIMENTS of the Long Mynd form a succession at least 5000 m thick, the stratigraphy of which has been described in detail by James (1956). The overall structure of the Long Mynd is synclinal. In the west, in an inverted sequence, the Longmyndian is seen to be unconformably overlain by Uriconian volcanics, which have a faulted western contact with Tremadoc shales. To the east, the Long Mynd outcrop is unconformably overlain by Silurian strata, which are separated from an eastern outcrop of Uriconian by the main Church Stretton Fault (CSF). Thus there has been little direct evidence from the type outcrop of Longmyndian to support an accurate assessment of the succession's stratigraphical position. In addition to the main occurrence west of CSF, there are several faulted inliers to the east, which have been inferred, on grounds of lithology and thickness, to belong to the same succession. Prior to the discovery of the Olenellus fauna (Lapworth I888) in the Comley sandstone (overlying the Wrekin quartzite, which is unconformable upon the Uriconian volcanics east of CSF), the Long Mynd succession had been included in the Cambrian system (Cowie et al. I972 ). Subse- quently the Longmyndian, which has been considered to be a little younger than, and partly derived from, the Uriconian (James 1956; Greig et al. 1968) was placed in the Precambrian. Some authors have considered the contact between the inferred Longmyndian and the Uriconian east of CSF to be conformable (James I956; Cobbold & Whittard 1935). K-Ar ages of 677 zk 72 to 632 4- 32 m.y. have been obtained on three samples of Uriconian volcanics (Fitch et al. 1969) but these have been interpreted as post formational (Baker 1971). The correlation of the Wentnor Series (Table i) with the Torridonian in Scotland, suggested on lithological and sedimentological grounds (Lapworth & Watts 19 lo), has received support from palaeomagnetic work, which shows similarity between remanent magnetization axes of Wentnor and Upper Torridonian rocks (Creer 1957). An Rb-Sr isochron age of 805 + 17 m.y. (recalculated to 5.0 × IO 1° yr half-fife for 87Rb) for red shales from the Upper Torridonian (Moorbath 1969) was interpreted as that of diagenesis closely following deposition and compaction. Thus the tentative correlation between Wentnor and Upper Torridonian has lead to an assumed age for the Longmyndian in excess of 600 m.y. (Greig et al. ~968).

Jl geol. Soc. Lond. vol. x3o, I974, pp. 567-574, 2 figs. Printed in Northern Ireland.

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TABLE I" Lithologic succession of the Longmyndian (after Greig et al. 1968)

~-ENTNO R SERIES Bridges group: purple laminated siltstones and sandstones 6OO--1200 m Bayston-Oakswood Group: massive purple and greenish grey sandstones with thick conglomerate bands 1200-2400 m Probable unconformity STRETTON SERIES Portway Group: green and purple siltstones and sandstones with a conglomerate at the base 2OO--1000 m Lightspout Group: greenish grey flaggy silt- stone with sandstone bands, thin tufts near top 500-800 m Synalds Group: purple silty shales with sandstone bands, tuff bands 500--800 m Burway Group: greenish grey sandstones and siltstones with a grit at the top and a silicified tuff at the base 600 m Stretton Shale Group: greenish grey shales with a grit at the base ?IOOO m

Possible correlation of the Mona metamorphic rocks of Anglesey with the Longmyndian has been suggested by Dewey (1969). He has interpreted the two successions as offshore and nearshore facies respectively and considers them to have been formed on the eastern boundary of a Precambrian Proto-Atlantic Ocean. Baker (i973) , on the other hand, has reinterpreted them as western and eastern borders of a marginal Late Proterozoic ocean basin. A detailed petrographic study of lithic fragments in the Longmyndian has lead, however, to the suggestion that they are derived in part from Mona-type schists (Greig et al. 1968) supporting the contention that the Monian predates the Longmyndian. The present study has been undertaken in an attempt to resolve some of these problems of correlation.

i. Sample description and isotopic data Fine grained rock samples were collected from single sections (max. IOO m stratigraphical height) of the Burway, Synalds and Lightspout Groups, which lie within the older part of the Longmyndian succession (Fig. I; Table I), west of CSF. These shales consist of quartz and plagioclase as the major coarse grained phases, with the clay fraction consisting solely of illite and chlorite. Potassium felspar (generally representative of a detrital component in such low grade sediments) and calcite were not detected in any of the samples. The illite crys- taUinity index (Kubler i968), the half-height peak width for the Io A reflection measured under standardized conditions, was found to be remarkably high (in the range 6 to IO on the Kubler scale), i.e. representative of very low grade illite.

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Furthermore, the illite crystallinity index increases with stratigraphical height, suggesting that the iUite crystallinity is a remnant of burial diagenesis, and has not been subsequently recrystallized under metamorphic conditions. The Rb/Sr and 87Sr/seSr values for rock samples are presented in Table 2 and Fig. 2. Using a 8~Rb half-life of 5"o × IO~° yr, the data from the Burway Group (Fig. 2a) define an isochron (mean of the squared weighted deviates 1.2) corresponding to an age of 529 4-6 m.y. (2a), with an initial 87Sr/s6Sr ratio of o'7o61 4- o.oooi (2a). Those from the Synalds Group (Fig. 2b) define an isochron corresponding to an age of 452 4- 31 m.y. (M.S.W.D.o.9) , with (87Sr/SeSr)0 of o-71oo 4-o.ooI2. The large error is due to the poor spread in Rb/Sr ratios, however the age given does appear to be significantly younger than that from the Burway Group. The stratigraphically highest group sampled, the Lightspout Group, yielded data (Fig. 2c) giving an age of 529 + 23 m.y. (M.S.W.D. 4"4), with an (87Sr/86Sr)0 of o'7o75 4-o'ooo5. The latter age is statistically indis- tinguishable from that from the Burway Group.

Silurian I I u. i

FIG ! Sketch map of part of Longmyndian outcrop, with sample IocMities. (Adapted from Greig et al. 1968). B-O. G. Bayston-Oakswood Group P.G. Portway Group WALES L.G. Lightspout Group S.G. Synalds Group t B.G. Burway Group S.S.G. Stretton Shale Group

.F U. Uriconian Volcanics.

sit. \ oI ~t CSF Church Stretton Fault LPF Linley-Pontesford Fault.

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2. Interpretation Unpublished work by the author on Silurian shales from Pembrokeshire and the Welsh Borderlands has shown that such rocks can define isochrons giving ages significantly younger than the time of deposition adduced from faunal evidence. It is considered that such ages represent the advancement of dewatering such that ionic transport in solution (the process by which large scale isotopic homogenization takes place) ceases to operate over the scale of sample collection or over a scale delimited by the minimum volume representative of the overall chemical

0.740

~Sr ~ J

0-730'

a) o.T~o.

0-710 •706121 ~R:,

0.700

O-'J~ A I %, jjl 0-~0 ~'"'~'Wr~ 462+_31m.y.

b) ~

0-710" IrtRb e6$r 0,700 0

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composition of the section. Since it has been shown that complete Sr-isotope homogenization does not occur under normal marine depositional conditions (Dasch I969) , it is necessary that burial occurs before homogenization may commence. The threshold depth necessary for homogenization will depend on the geothermal gradient and on the chemistry of the connate fluids. These conditions appear to have been satisfied, at least in the groups studied, within the great thickness of the Long Mynd succession. The dewatering episode may be associated purely with burial phenomena, or may have been induced by folding; therefore it is not possible, using these considerations, to give a definite time interval between deposition and the dewatering. However, it is possible to define within reasonable certainty a maximum age of deposition, by consideration of the initial sTSr/S6Sr ratios for each isochron~a method which has been used elsewhere (O'Nions et al. 1973). If the (87Sr/S6Sr)0 ratios for each group are extrapolated backwards in time, using average Rb/Sr ratios representative of each group (close to unity for each group), it is found that at c.6oo m.y. each of the groups would correspond to an overall 8VSr/S6Sr ratio of c.o.7o 4. This latter ratio is a reasonable minimum value for the source materials of the Longmyndian sediments, particularly in view of the evidence for these sources being the Uriconian calc-alkaline suite and, perhaps, Mona-type metamorphics (Greig et al. 1968 ). An age greater than c.6oo m.y. can be derived only by Rb-metasomatism subse- quent to deposition, or by reaction with a fluid with an 87Sr/S6Sr ratio less than the initial ratios on the isochrons, i.e. less than c.o'7o6. The former possibility is not suggested by the Rb contents of the shales, which are compatible with values from unaltered shales, and furthermore it is unlikely that shales could yield an isochron after such a process. The latter possibility can be ruled out because, although

0-740 A eTSr ~Sr

0.730 629+_ 23m.y.

0-720. c)

0-710- 0.7O75 + 5 ".b ' II~

0.70C o i ~ ~ i FIo. 2. Rb-Sr whole rock plots for (a) the Burway group, (b) the Synalds group, and (c) the Lightspout group from the Longmyndian. Errors shown are 217.

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TAB L E 2 " Analytical data Sample No. Rb/Sr* S7Sr/S6Sr*

Burway Group BI 0"997 4- 0.028 0"72733 4- 4 B2 i.o33 4- 0-028 0"72793 4- 6 B 3 o.9oi 4- o-o26 o.72516 4- Io B4 o'94I -t- o.o26 o.72634 4- I8 B5 o.975 4- o.o28 o-72648 4- I4 B6 o.954 4- o.o26 o-7262o 4- I2 B7 o.I I9 4- 0"004 0"70857 4- 4 B9 o'8x4 4- o'o24 o'72349 4- 6 Bio 1.3i I 4- 0"036 0"73353 4- 8 BI2 I'46I 4- 0-040 0"73708 4- 6 BI 3 I.I62 4- 0"032 0"73083 4- 4 BI5 o'898 4- o-o26 o'72598 4- 6 Synalds Group SI 0.856 4- 0-022 0"72576 4- 9 $2 0"877 4- 0.022 0"72584 4- 5 $3 o-9I 3 4- 0"024 0"72669 4- IO $4 0"896 4- 0"024 0"72639 4- x8 $5 0.765 4- 0.020 o'7239I 4- 5 $6 I-O69 4- 0.028 0"72999 4- 6 $7 I-IO6 4- 0.030 0.73000 + xo $8 0.940 4- 0-026 o'727 I2 4- 9 SIo I.x I I 4- 0-030 0"72993 4- 6 Six i.o97 4- 0.03 ° 0.72984 4- I2 Lightspout Group LI o'712 4- o.o2o o.72314 4- 6 L2 0.868 4- 0"024 0"72633 4- 7 L 3 0.326 4- o-oIo o'71478 4- 3 L4 x-oo5 4- 0-028 0-72866 4- x6 L 5 o'483 4- o-oi4 o'71767 4- 4 L6 o'422 4- o-ox2 o'7x597 4- 4 L7 1.622 4- o'o48 o'7413 ° 4- 4 L8 o'7I I 4- o-o2o o.72266 4- 5 L 9 o.593 4- o.oi6 o'72o59 4- 7 Lxo o.918 4- o.o26 o'72723 4- 6 LI x o'96I 4- 0-028 0"72776 4- 4 LI2 0.346 4- o.oIo o.7148o 4- xo

* All errors quoted are 2a.

connate fluids clearly contribute some Sr during equilibration, present knowledge of early marine Sr isotopic composition for which the earliest figure is o.7o7 8 for Palaeozoic sea-water (Peterman et al. I97o ) indicates that such fluids would not have a ratio falling below o.7o6. No evidence, therefore, suggests any model for the systematics other than the simple one described above. The agreement of Rb-Sr isochron ages obtained from the oldest and youngest groups, whilst the intermediate Synalds Group yields a younger age, presents a problem of interpretation. It is questionable whether this apparent difference is,

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in fact, significant, there being a poor spread of STRb/seSr ratios from the Synalds Group samples. If it is accepted as significant, further work would be required to elucidate the cause of such a discrepancy.

3. Conclusion Therefore it is proposed that the Longmyndian sediments were deposited no earlier than c.6oo m.y., i.e. in the Cambrian or the very late Precambrian. Correlation with the Upper Torridonian (Lapworth & Watts 191o ) is thus ruled out completely. K-Ar dates of 58o--6oo m.y. on muscovites and Rb-Sr ages of 6o 9 -¢- 13, 613 4- 18 and 614 4- 14 m.y. (recalculated to *TRb half-life of 5"o × I o 1° yr) on whole rock--muscovite pairs were obtained from the Coedana granite and its hornfels in the Mona complex (Moorbath & Shacldeton I966), and were interpreted as representative of some late event in the metamorphic history of the complex (Cobbold & Whittard 1935). On the basis of this interpretation and the present data, the Monian clearly pre-dates the Longmyndian, and direct correlation between the two is not possible. It is of interest to note that the Ingletonian succession of Northern England, previously attributed to the Precambrian, has been shown (by Rb-Sr methods) to have an early Palaeozoic time of deposition (O'Nions et al. 1973). Analytical methods Clay fractions ( <2/~m) of all samples were separated by a centrifugal sedimentation technique. Oriented specimens for X-ray diffraction analysis were prepared by a suction-onto-ceramic disc method described by Shaw (1972). Illite crystallinity index measurements were standardized to the Kubler (1968) scale by running concurrently three standards of known index. Rb/Sr ratios were determined in duplicate on pressed pellets of rock powders by X-ray fluorescence techniques, employing a single standard (Pankhurst & O'Niom I973). Strontium was extracted by conventional dissolution and ion-exchange techniques. The isotopic composition of the strontium was determined using a 3o cm solid source mass-spectrometer (designed by N. H. Gale) in the Oxford Isotope Geology Laboratory. STSr/SeSr for the Eimer and Amend SrCO 3 standard measured under the same conditions was o.7o83 ° 4- o.oooo4.

A~OWLEDGEMENTS. I thank Dr R. K. O'Nions for help in sample collection, many useful discussions, and critically reading the manuscript; also Dr W. S. McKerrow for helpful discussion. This work was carried out during the tenure of a B.P. Research Fellowship.

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FITCH, F. J., MILLER, J. A., EVANS, A. L., GRASTY, R. L. & MENEISY, M. Y. I969. Isotope age determinations on rocks from Wales and the Welsh Borders. In 'The Pro-Cambrian and Lower Palaeozoic Rocks of Wales,' ed. Wood, A., University of Wales Press. GREIG, D. C., WRIGHT, J. E., HAINS, B. A. & MITCHELL, G. H. i968. Geology of the country around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee. Mere. geol. Surv. G.B. 379PP. JAMES, J. H. 1956. The structure and stratigraphy of part of the pro-Cambrian outcrop between Church Stretton and Linley, Shropshire. Q. J. geol. Soc. Ix2, 3 I5-337. KUBLER, B. 1968. ]~valuation quantitative du mfitamorphisme par la cristallinit6 de l'illite. Bull. Centre Rech. Pau-SNPA, 2, 385-397. LAPWORTH, C. • WATTS, W. W. I9IO. Shropshire. Geol. Ass. Lond. Jubilee Vol., 739-769 . LAPWORTH, C. I888. On the discovery of the Olenellus fauna in the Lower Cambrian rocks of Britain. Geol. Mag. 5, 484-487 • MOORBATH, S. & SHACKLETON, R. M. 1966. Isotopic ages from the Precambrian Mona complex of Anglesey, North Wales (Great Britain). Earth planet. Sci. Lett. x, x 13-I 17. MOORBATH, S. x969 . Evidence for the age of deposition of the Torridonian sediments of N.W. Scotland. Scot. J. Geol. 5, x54-I 7o. O'NIONS, R. K., OXBURGH, E. R., HAWKESWORTH, C. J. & MACINTYRE, R. M. I973. New isotopic and stratigraphical evidence on the age of the Ingletonian: probable Cambrian of Northern England. Jl. geol. Soc. Lond. x29, 445-452- PANKHtJRST, R. J. & O'NioNs, R. K. t973. Determination of Rb/Sr and aTSr/S6Sr ratios of some standard rocks and evaluation of X-ray fluorescence spectrometry in Rb-Sr geochemistry. Chem. Geol. x2, I27-I36. PETERMAN, Z. E., HEDGE, C. E. & TOURTELOT, H. A. I97o. Isotopic composition of strontium in sea-water throughout Phanerozoic time. Geochim. Cosmochim. Acta. 34~ IO5-12o. SHAW, H. F. I972. The preparation of oriented clay mineral specimens for XRD analysis by a suction-onto-ceramic tile method. Clay 3¢inerals, 9, 349-35 °.

Received 6 March I974. Adrian Hubert Bath, Department of Geology and Mineralogy, Oxford University, Parks Road, Oxford, OXI 3PR.

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