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Wealden Detrital Tourmaline: Implications for Northwestern Europe

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Wealden Detrital Tourmaline: Implications for Northwestern Europe Wealden detrital tourmaline: implications for northwestern Europe PERCIVAL ALLEN CONTENTS Introduction 273 Detrital petrology 274 (A) Weald 274 (B) Wessex 275 Isotopic ages 276 (A) Weald 276 (B) Wessex 279 Source area 28x Wider questions 283 References 284 Appendix A on jaros, te by A. Parker • 289 Appendix B on ~Ar/S9Ar dating by F. J. Fitch and J. A. Miller • 291 SUMMARY Wealden detritus in S England and NW Hercynian-Galician-Caledonian convergence. France suggests ultimate derivation from an Eastwards the hills diverged to encompass a ancient complex. This had affinities with lowland depression, divided into northern and present-day Cornubia, Armorica and NW southern basins by the Cornubian island. After Iberia, which apparently represent its relict the first sediment arrived in the ?Permian there flanks. Westwards, lay a Cadomian-Caledonian were important fluctuations of supply and com- core. Repeated tourmalinizations occurred position during the late Trias/early Jurassic, from pre-Cambrian times onward. These late Jurassic/early Cretaceous and Hastings were postdeformational and may have followed Beds/Weald Clay. These may have been separate orogenies. Geographically, the massif related to movements of the massif generated probably focussed on what is now the con- by changes in North Atlantic spreading rates tinental margin, over the Southwestern Ap- and opening of Biscay. proaches. Structurally, it was the Cadomian- I. Introduction A WELTER of tourmaline grains and pebbles of tourmalinized rocks poured into southern England after the Hercynian orogeny. There were two main pulses, early ?Permian and early Cretaceous, separated by a Jurassic 'low'. Comparable detritus is originating today from heavily tourmalinized and tourmaliniferous Upper Palaeozoic rocks in Cornwall and Devon. For this and other reasons the Jl geol. Soc. vol. x28, I972 , pp. 273-294 , I fig. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/3/273/4884473/gsjgs.128.3.0273.pdf by guest on 26 September 2021 274 P. Allen Cornubian* Hercynides are widely regarded as a major source (or the only source) of nearby tourmaline-rich formations. Late Hercynian ages for the clasts seem confirmed by their associates: alkali-granite, topaz, andalusite, kaolinite, etc. (from in and around the intrusions ?), biotite with strong optical dispersion (from the ?Permian lavas?) and radiolarian chert (from the Carboniferous country rocks ?). Broadly, and as expected, the tourmaline contents of the older formations increase westwards towards the assumed source-area (e.g. Hancock 1969 and refs.), especially in non-marine facies. Most striking of these is the Wealden. Sands and pebble beds, tourmaline-poor in Kent and East Sussex, are thickly speckled with the mineral in Dorset. Concomitantly their contents of topaz, andalusite, trans- ported kaolin, ?Permian-type biotite, Culm-facies chert, etc. become appreciable as they are traced through the western Weald into Wessex (e.g. Groves 193I , Oakley 1947, Allen 1959, Cosgrove & Salter 1966 ). Micropalaeontology (Anderson et al. 1968 ) and detrital petrography of deep boreholes in the middle ground of Hampshire (Chilcombe Down (Winchester) and Kingsclere) seem to confirm that the Weald and Wessex basins (Allen 1959, fig. 1 ; Dingwall 197 I) were connected during upper Wealden (Wealden Shales/Weald Clay) times. At first sight, therefore, the picture is simple: southern England was dominated by Cornubian detritus of late Carboniferous to early Permian origin, progressively diluted eastwards by sediment from elsewhere (Relf 1916, Allen 1969). But difficulties are revealed by a closer look at the materials. 2. Detrital petrology (A) WEALD In the Hastings Beds, tourmaline and feldspar grains are nowhere abundant (Allen 1949, 1959). Pebbles of tourmalinized rocks are exceedingly rare, com- prising tourmaline-quartz-schists and foliated or disrupted feldspar-free quartzites and ?veinstones riddled with postdeformational tourmaline (Allen 1967 a, p. 255 & pl. 7G'H). Tourmalinized shale or slate resembling killas or culm is not known. Unstained (but not bleached) and unzoned, the pebbles do not 'look' Cornubian. Rather, their metamorphic textures suggest more ancient tourmalinized rocks. In the western Weald Clay (but not the eastern) several sandstones reveal major incursions of fresh Cornubian-type detritus. Orthoclase comprises up to 12 % of the whole rock, and is correlated with total-tourmaline which reaches 64 % of the heavy fraction. Overall geometries of Beds I, 3 a, 5 c, 5 f, 7 b, 7c and I I C (nomen- clature of Thurrell et al. I968), and sparse observations of their cross-lamination, cross-bedding and plant-lineations, hint at easterly to southerly currents. More of the grains in these beds are highly rounded (maximum = 7 % among quartz) than in the associated sandstones lacking Cornubian-type materials, suggesting aeolian * Districts needing definition here are: 'Cornubia' = general region of Cornwall-Devon-Scilly Isles- Haig Fras and 'Armorica' ---- Brittany-Lower Normandy-Channel Islands (used for pre;Permian massifs) ; 'Wessex'= Hampshire-I.o.W.-Dorset-contiguous English Channel (Dingwall 1971) and 'North Celtic Sea' = northern part of Southwestern Approaches between Cornubia and Ireland (used for Wealden basins). • Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/3/273/4884473/gsjgs.128.3.0273.pdf by guest on 26 September 2021 Wealden detrital tourmaline 275 (New Red Sandstone?) elements. Delicate crystal-aggregates form up to 34% of the tourmaline grains, some being stellate-acicular and penetrating quartz or orthoclase. Green, blue or bluish green in colour, they are clearly secondary (Exley & Stone 1964, p. 172). Their frequencies in the heavy fraction and the whole rock rise steadily with total tourmaline. Parti-coloured grains, perhaps hydro- thermally altered (Power 1968), reach 15 % of total tourmaline. Weald Clay members deprived of Cornubian-type materials are typified by the Newdigate and Gossops Green pebble beds (Worssam 1965, 1966 ). These have yielded no pebbles of tourmalinized rocks (> I oo black 'lydites' sliced), and grains of K-feldspar and tourmaline form less than 3 % and 15 % of the whole-rock and heavy fractions respectively. B) WESSEX Westwards through Hampshire into Dorset, i5o km nearer Cornubia, the 'non- Cornubian' influences become curiously insistent. Thus the lower Wealden sand- stones (Hastings age?) carry less orthoclase (max. = 9 % at Chilcombe Down, 5 % at Lulworth)* and plagioclase than the Sussex maxima, more metaquartzite and deformed quartz, and little or no optically dispersed biotite (?Permian). Grains of tourmaline reach 88 % of the heavy fraction at Durdle Door, where orthoclase is minimal (cf. Weald). Though more abundant than in the Weald (max. 64 %), they include fewer hydrothermal-type crystals (max. 5 %, compared with 12 %). Statistically, the same ratios of fine aggregates (max. 32 %, compared with 34 %) recur. This is significant, because they would have been higher if they had been tied to total-tourmaline frequency in the same manner as in the Weald. Thus the Wessex situation is paradoxical and distinct: contributions of tourmaline- rich sand from non-Cornubian sources (metamorphic; quartzose; lower in feld- spar, d-biotite and hydrothermal tourmaline) seem stronger and more persistent than in the Weald. Associated pebbles are confirmatory. Numerous 'lydites' (~-~i7% of >5 mm grade) from the Dorset Coarse Quartz Grit (Arkell ~947) prove to be foliated meta- quartzites ('psammites') heavily charged with post-foliation tourmaline. Lacking feldspar, red-staining and signs of bleaching, they have no known Cornubian counterparts and recall the rare pebbles of Sussex. + Poor sorting, high angularity, and suspected lateritic materials (possible diaspore at Durdle Door: Dr. A. Parker in litt.) suggest an inheritance from soils on a nearby upland. Secondary derivation through New Red Sandstone pebble beds seems unlikely. Reddened pebbles of other rock-types do occur, but rarely (e.g. ~'~3% of >5 mm grade at Durdle Door). They include aeolian quartzites (e.g. S6o6IBI ~'), silicified ferroan dolomites (some previously partly calcitised (e.g. $6o84BI)), silicified pyrite ores (e.g. $6o83BI) and silicified ?baryte rocks (e.g. $6o79BI). * K-feldspars may have been more abundant originally because the < 2/z fractions at Lulworth Cove (oil-sands on E side) contain K-jarosite (Na20 < 0.8%) > kaolinite >~ quartz and feldspar (Parker: Appendix A). t Reading University Geology Department catalogue number. +*Also the well-known ?dinosaur ?gizzard stones in the I.o.W lower Wealden (e.g. White 1921 , p. 20). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/3/273/4884473/gsjgs.128.3.0273.pdf by guest on 26 September 2021 276 P. Allen Present evidence suggests to Mr. D. J. Shearman that matches might be found among British or Irish Carboniferous and Permo-Triassic formations. Kaolinite-formation and silicification might well have resulted from post- orogenic fumaroles and hot springs on Cornubia, postulated by Dangerfield and Hawkes (i 969). 3- Isotopic ages Isotopic dating (K-Ar) of the Wealden detritals was carried out by Dr. J. A. Miller and Mr. F.J. Fitch. (For details see Appendix B.) This was made possible by Mitchell's (I 968) development of the 4°Ar/39Ar method. Tourmaline had for long been an attractive prospect. Persistently potassic and highly stable, it seemed likely to be reasonably argon-retentive.
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