424 A. .l. Barber & M. D. Max

from the PrecambrianMona Complex. Earthplanet. - 1969. The Precambrian of North . In WOOD,A Sci. Lett. 1, 113-7. (ed.) ThePrecambrian and LowerPalaeozoic rocks of MUIR,M. D., BLISS,G. M., GRANT,P. R. & FISHER,M. 1979. Wales, Univ. of Wales Press, Cardiff, 1-18, Palaeontologicalevidence for the age of somesup- -1975. Precarnbrian rocks of Wales. In HARRIS, A.L. et posedly Precambrian rocks in , North Wales. J. al. (eds) 4 correlation of Precarnbrian rocks in the British geol. Soc.London, 136, 61-4. Isles. Geol. Soc. Lond. Spec. Rep. No. 6, 76-82. NATARAJ,T. S. 1967. Glaucophanic metamorphism in Ang- STROGON,P. 1974. The sub-Palaeozoic basement in central lesey. Thesis, Ph.D., Univ. Leeds (unpubl.) pp. 103. Ireland. Nature, Phys. Sci. 250, 262-3. OWN, T. R. 1976. The geological evolution of the British Isles. Born, R. S. 1972. Ocean floor basalt affinity of Precam- Pergamon, Oxford. brian glaucophane schist from Anglesey. Nature,Phys. PEACH,B. N., HORNE,J. GLJNN, W.,CLOUGH, C. T.,HINX- Sci. 240, 103, 164-6.

MAN, L. W., & TEALL,J. J. H.1907. The geological ~ 1974. Aspects of magmatism and in the structure of the north-west Highlands of Scotland. Mem. Precambrian of England and Wales.Geol. J. 8, 115-36. geol. Suru. U.K. WOOD,D. S. 1969. The base and correlation of theCam- RAMSAY, J. G. & GRAHAM,R. H. 1970. Strain variation in brian rocks of Wales. In WOOD, A. (ed.) The Precam- shear belts. Can. J. Earth Sci. 7,786-813. brian andLower Palaeozoic rocks of Wales. Univ. of SHACKLETON,R. M. 1954a. The structure and succession of Wales Press, Cardiff, 47-66. Anglesey and the Lleyn peninsula. Advmt. Sci. Lond. 11, -1974. Ophiolites, melanges, blueschists and ignimbrites: 106-8. early Caledonian subduction in Wales? In Don R. H. - 19546. The structural evolution of North Wales. Liver- Jr. & SHAVER,R. H. (eds) Modern and ancient geosync- pool Manchester geol. J. 1, 261-97. final sedimentation.Soc. Econ. Pal. Min. Spec. Pub. 19, - 1956.Notes on the structureand relations of the 334-44. Precarnbrian and Ordovicianrocks of south-western WOOD, M. & NICHOLLS,G.D. 1973.Precarnbrian Lleyn,Caernarvonshire. Liverpool Manchester geol. J. stromatolitic limestones from northern Anglesey.Nature, 1, 400-9. Phys. Sci. 241, 65.

Read 31 January1979; received 12 March 1979. ANTHONY JOHN BARBER, Departmentof , Chelsea College (Universityof London), 271-3, King Street, London W6 9LZ. MICHAEL DAVIDM, Geological Survey of Ireland, 14 Hume Street, Dublin 2, Ireland.

Discussion

DR R. S. THORPEwrites: Since first proposedby evidence for a late Precambrian metamorphic-igneous Greenly, it has frequently been argued that the Mo- cycle at c. 600 Ma, itis felt that the available evidence nian gneisses are older than, and hence form a floor to indicates thatMonianthe gneisses must be the ‘Bedded Succession’. In spite of Shackleton’s cog- metamorphosed equivalentsof the Bedded Succession. entarguments that the gneisses aremetamorphosed This view is in full agreement with the interpretation equivalents of part of the lower-grade Bedded Succes- of Shackleton. sion,the authors have accepted Greenly’s view and This interpretation has implications for the age and suggestthat the gneisses areArchaean in ageand structural setting of the Rosslare Complex, which has possiblyequivalent to the Rosslare Complex. New beenregarded as older than the adjacent, probable Rb-Sr whole-rock isochron ages for Monian gneisses Monian, Cullenstown Group. In addition, Max (1975, atGaerwen Quarry enable a cleardistinction to be p.101) suggested that ‘Rb-Sr whole-rock isochron madebetween these conflicting hypotheses(Beckin- dataindicate a minimumage of c. 1700-1600 Ma sale & Thorpe1979). The Rb-Sr data indicate that wouldbe appropriate for the second episode in the a major metamorphic gneiss-forming episode, involv- Rosslare Complex but that there is some evidence of ing paragneiss and amphibolite at 595 f 12 Ma(initial incompleterejuvenation of theoldest part of the 87Sr/’‘%r = 0.7061 =t3) was followed by emplacement complex which would probably be no more than about of tonalite,granodiorite and granite gneiss at 562* 2400 Ma’. In view of thelate Precambrian age of 13 Ma (initial 87Sr/86Sr = 0.7081 f 8). These data are high-grademetamorphism and igneous activity, and clearlyinconsistent with an Archaean age for the closeassociation of metamorphicand igneous rocks gneisses. A new Rb-Sr whole-rock isochron indicates and low-grade metasedimentary rocks in Anglesey, it that the granite crystallized at 603 f 34 Ma seems likely that both the Cullenstown Group and the (initial ratio = 0.7086 f9). In view of the relatively low RosslareComplex are late-Precambrian in ageand initial87Sr/86Sr ratios of theolder gneisses (which correspond respectively to the Bedded Succession and precludes a long earlier crustal history) and the clear the gneisses of the Mona Complex.

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In reply, the AUTHORS state that they can only reiter- structurally overlying Unit. It is my contention atethe evidence given in thepaper and previously that careful field mapping and petrographic evidence detailed by Baker (1969): wherever members of the showboth of thesediscordances to be of apurely Bedded Succession are juxtaposed to either the gneis- hallucinatory nature. ses orthe rocks of thePenmynydd Zone of There is general agreement that the New Harbour metamorphism, they are separated by belts of mylo- Beds show strong structural contrast to the underlying nite or by faults. The mylonites are metamorphically turbidite-quartzitesequence which theauthors term in the greenschist facies and result from the deforma- theSouth Stack Unit. Inview of thelithological tion and retrogression of high grade gneisses. Nowhere contrast between (a) thinbedded semi-pelitic schists are rocks which are undoubted members of the Bed- and (b) sequencea in whichbasic fold style, ded Succession metamorphosed to a grade above the wavelength and amplitude is governed by the presence greenschist facies, nor can any progressive metamor- of homogeneous massive attaining several phictransition between the Bedded Succession and hundred feet in thickness, some differences are to be highergrade rocks be demonstrated anywhere in expected. It is, however, a fact that all the deforma- either Anglesey or Lleyn. tional phases found in the New Harbour Beds can be Wetrust that Beckinsale & Thorpe (1979) have matched with those found in the underlying sequences madeclear that their isotopic evidence comes from and vice versa. In responseto the same deforming one small isolated occurrence of gneiss in the Aethwy stresses, the fundamental difference between the New region of Anglesey. The gneissforms a highly de- Harbour Beds and the underlying beds was that during formedlens-shaped body l kmin length, limited to the first and most important large-scale deformation, the NW by the Berw Fault Zone and separated from the New Harbour Beds developed a true penetrative the other rocks of the Zone by mylonitic bedding-parallelfoliation, whereas the lower stratig- shear zones. It is not possible to relate this occurrence raphic members developed a new schistosity at a high in any way either to the Bedded Succession nor to the angle to the primary bedding. Subsequent folding thus much more extensive outcrops of gneiss elsewhere in involved the deformation of a sub-horizontal foliation Anglesey or Lleyn, let alone over a distance of more in the one case and of a sub-vertical schistosity in the than 100 km to the Rosslare Complex. In particular, other,thereby producing the observed contrast in the gneisses of the Rosslare Complex provide evidence style.There is no evidencethat cataclasis increases of a complex history of deformation and igneous intru- eitherdownwards or upwards towards the supposed sion (Max 1972), which is in no way represented in the discordance. nearbyCullenstown Formation, nor in theBedded The continental rise metasedimentary rocks of the Succession of Anglesey. Monian possess a stratigraphy which cannot be treated Dr. Thorpe’s far-reaching conclusions appear to be in alayer-cake manner; there are important and premature in view of his limited data, and certainly do sometimes abrupt facies changes on various scales. On notjustify the dogmatic assertion that the ‘Monian the local scale this holds in Holy Island within Barber gneisses must bethe metamorphosed equivalents of & Max’s Unit, where the ortho-quartzites the Bedded Succession’. termedthe by Greenlyare a series of variablethickness grain-flow intercalations PROFESSOR D. S. WOOD writes: I have maintained a within an overall turbidite-greywacke sequence ‘mat- continuinginterest in the geology of Anglesey by rix’. This fact misled Greenly in his original interpreta- means of aresearch program which has included a tion of the Anglesey structure, whichwas only clarified number of doctoraltheses. This work has been di- by Shackleton’s recognition of the true order of sequ- rectedtowards two majoraspects of theMonian, ence and his essential introduction of the term ‘Rhos- namely the structural history of Holy Island and the colynBeds’ forthose rocks immediately underlying tectono-sedimentary history and nature of the Gwna the New HarbourBeds (Shackleton 1954). Also, on Group.It has resulted in some new conclusions,to- thelocal scale in N Anglesey,central Anglesey, SE gether with affirmations of previously accepted truths Anglesey, Lleyn and Bardsey Island, there are several which areat variance with almost all of the views milanges separated by ‘non-mtlanged’ sequences. The expressed in the present communication. milanges are all olistostromes interbeddedwith a vari- The authors envisage that the Bedded Succession of ety of proximal turbidites, distal turbidites and tufface- theMonian rests with majorunconformity upon a ous members such as the Skerries Group, Church Bay gneissicbasement and that the Bedded Succession Tuffs, Fydlyn Group, Gwyddel Beds and Aber Geirch itself consists of three lithotectonic units, termed the Phyllites. I broadly agree with that aspect of Barber & South Stack, New Harbour and Cemlyn Units respec- Max’s interpretaion of N Anglesey whichrecognizes tively. The New HarbourUnit is apparentlyconsi- mklanges separated by a Greywacke Group and with dered to rest in thrust discontinuity on the structurally the lower mtlange in normal stratigraphic contact with underlying South Stack Unit and to be separated by the Church Bay Tuffs. It must, however, be pointed either thrust or unconformable discordance from the out that this is a simplification; there are more than

3

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/424/4885857/gsjgs.136.4.0424.pdf by guest on 30 September 2021 426 Discussion two olistostromal mClanges. The fact that units such as onthe mainland of Caernarfonshire, by theArenig. theChurch Bay Tuffsand Skerries Group are only Elsewhere in eastern Anglesey the Ordovician is dis- locally present is, in part, dependent upon their having cordant upon many members of the Gwna Group. On beenreworked by olistostromal redeposition else- the mainland, it is a simple fact that Arvonian passes where. upwards into the Cambrian with slight unconformityin It seems likely that, on the regional scale, the Gwna theBangor district (Wood 1969). I thereforehave Groupshould be considered as a series of easterly difficulty in appreciating how the Gwna Group can be directed olistostromes and inter-slide members which of Cambrian age. I also believe that the conglomerate become progressively younger towards the NW. The atTrefdraeth in south-centralAnglesey is petrog- Group is diachronous to an extent that, SE from the raphicallysimilar and stratigraphically equivalent to area where Barber & Max have erected their stratig- the lowest Cambrian conglomerates of the mainland; raphicand lithotectonic divisions, the base of the it is certainly not of Ordovician age and rests uncon- Gwna Group transects the postulated discordance be- formably on the Gwna Group (Wood 1969, 1974). tween their New Harbour Unit and Cemlyn Unit. My view of theregional relationships between Although field relationshipsare far from clear in Cambrian,Ordovician and Monian are expressed many places, it is my opinion that, in the Gwyndy and elsewhere (Wood 1974). I cannot avoid the conclusion Llandrygarnareas of centralAnglesey, the gneisses that in an attempt to formulate radically new interpre- are both gradational into the Coedana granite on the tations at all costs, the authors ignore a multitude of one hand, and can be seen to bea metamorphic facies evidence.Manipulation of bothstratigraphic and of the ‘Bedded Succession’ on the other (Wood 1974). structuralinformation results in theirrecognition of They show a normal, though telescoped, metamorphic three illogical tectono-stratigraphic units in a Monian zonal relationship to the enclosing schists in the man- successionwhich, in my view, remains solidly sub- ner suggested by Shackleton (1953). I broadly concur Cambrian. with Shackleton’s view that ‘movements, metamorph- ism, migmatisation and the ascent of granite magma In reply, the AUTHORSstate that they are well aware belong to one plutonic sequence’ (Shackleton 1953, p. of Professor Wood’s long-standing interest in the geol- 264). Althoughit is admittedthat more work is ogy of Anglesey, and regret that much of the detailed needed to clarify some problems of the gneisses and work on which he bases his criticisms of their paper the hornfels surrounding the Coedana granite, there is remainsunpublished. Like Professor Wood, we ap- no evidence whatsoever which would support the view preciatethat the contrast inlithology between the that the South Stack Beds or South Stack Unit rests South Stack Unit and the overlying New Harbour Unit upon gneisses. The gneisses are spatiallyassociated mayaccount in somedegree for differences in their withmuch higher members of theMonian stratig- mode of deformation.However, it is certainly our raphy. contention that a more complex deformational history I am therefore disinclined to accept anyof the lower is to be found in the New Harbour Unit than in the or upperdiscordant boundaries of Barber & Max’s South Stack Unit. Our conclusions are based on the South Stack or New Harbour tectonic units. The upperstudy of relativelyfew localities, particularly in the boundary of the Cemlyn Unit remains to be discussed. neighbourhood of the contact between the two units. Barber & Max extend their Cemlyn Unit upwards Perhaps Professor Wood’s detailed evidence from his until rocks unaffected by Caledonian deformation are comprehensive regional study will be of assistance in encountered. In so doing they ignore the one signific- resolving this problem. The real problem in reconcil- ant break within the 25 000 feet or so of rocks under ing the deformational sequences in the two units is a their consideration, namely the sub-Ordovician uncon- geometricone. How did the same deforming forces formity. This results in their assignation of the Monian produce a sub-horizontal schistosity in the New Har- to the Cambrian. The unconformity in question causes bour Unit, while at the same time producing a sub- the Ordovician to rest, in different places, upon virtu- vertical cleavage in the underlying South Stack Unit? ally all members of the Monian stratigraphy, with the It was to resolve this problem that we have suggested exception of those formations within Barber & Max’s that the two structures are of different ages, with the South Stack Unit. Even the gneisses which are claimed sub-horizontal schistosity produced at an earlier stage, as pre-Bedded Succession older basement are, embarr- particularly as it is folded around the folds, such as the assingly, not exempt. Itis difficult to envisage how this Fold, to which the vertical cleavage is axial can fail to be a break of significance! Throughout NW planar. We will maintain this position until it can be Caernarfonshire,and with progressive overstep to- convincingly demonstratedthat the two structures wardswestern Anglesey, the Ordovician is uncon- could have been formed simultaneously. formable. Weappreciate Professor Wood’s endorsement of I wouldmaintain thatBaron at Hill, near our stratigraphic conclusions, in particular those which , the Arvonian is unconformable upon the relate to the melange units. Again we look forward to Gwna Group and is itself unconformably overlain, as the publication of Professor Wood’s detailed account

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of thisinteresting and important deposit. We also is taken into account. Characteristic of the first cleav- concur with Professor Wood that much workis needed age (S') atSouth Stack and Rhoscolyn Head is the toclarify the relationships between the gneisses, the sudden change in attitude as it passes from psammitic hornfels and the Coedana granite. As recorded in the beds, where it is more or less orthogonal to bedding paper, we have found the gneiss to be a typical mig- andthus classicallyaxial planar-convergentto the matitic gneiss with a complex deformational and intru- major folds, to low angle in pelitic beds. In places at sivehistory and unrelated to the slightly deformed both South Stack and Rhoscolyn Head the refraction Coedana granite. The occurrence of mylonite at H&n- is both marked (angle of refraction up to c. 80") and Shop,presumably derived fromgneisses,the such that it no longer bears an axial planar relation- hornfelsed by the granite, shows that the gneisses were ship to major folds. Rather than being markedly di- alreadydeformed and retrogressed togreenschist vergent within folds, it refracts in the same direction facies before the intrusion of the Coedana granite. We onboth fold limbs. This refraction, whichevidently regard the telescoped metamorphic zonal relationships reflects changes in ductility, may be a primary feature postulated by Professor Wood as intrinsically improb- of the first deformation phase, but it is certainly en- able. As we have recorded in the paper, both Baker hanced by thesecondary deformation. Secondary (1969)and ourselves have found examples of this structures (minor folds and crenulation cleavage) are relationship from Lleyn quoted by Shackleton (1969) seen at South Stack and Rhoscolyn Head to deform tobe retrogressive transitional sequences due to the S', a set of quartz-chlorite veins which lie parallel to deformation of the gneisses in marginal shear zones. S', andbedding, primarily within the pelitic layers. Inconnection with the distribution of gneissicbase- They are poorly developed or absent from the psam- ment, it wouldbe interesting tolearn Professor miticbeds. Thesecondary crenulation cleavage and Wood's views on the nature of the basement underly- axialsurfaces of secondaryfolds are low dipping, ing the South Stack Unit in Holy Island. We consider generally to the NW, and are most clearly appreciated itpossible that this lowest structural unit rests on where they fold the quartz veins lying parallel to S'. gneissic continental basement. It is difficult to guess at Thestructural elements within the New Harbour the depth to this basement, but the problem may be Group can bematched most readily withthose de- amenable to seismic investigation and an exploratory scribed above if the Group is regarded as a very thick borehole. pelitic layer. The first cleavage forms the main planar Professor Wood's comments on the Mona Complex element and constitutes a tectonically derived litholog- and its associated rock units suggest that at the time icalbanding (developed by transposition of bedding whenhe prepared his contribution he was unaware andpressure solution). It is characterized,as in the that Cambrian fossils have been found in two units of underlying unit, by a stretching lineation and coplanar theGwna Group (Muir et al. 1979).Unfortunately, quartz-chlorite veins. Its overall low dip is, I believe, thereare no equivalent palaeontological or isotopic the result of superimposed secondary folding and sub- controlson the age of theBaron Hill rocks, the vertical flattening during the second and/or first defor- Arvonian of theBangor district, nor the presumed mation, as it appears to be in the pelitic beds at South Cambrian conglomerates interbedded with the Arvo- Stack and Rhoscolyn. The apparent refolding of ele- nian on themainland. The earliest fossils are found ments in the New Harbour Group around the Rhosco- much higher in the sequence (Wood 1969), where a lyn Anticline can be explained in this way, and thus protolenidfauna in greenslates beneath the themain evidence for different tectonic histories BronllwydGrit indicates an upper Lower Cambrian within the two units is removed. age. The possibility is left open that all the underlying rocks, including the Arvonian volcanics, are of Cam- In reply, the AUTHORS state that they are well aware brianage, since no base to the Cambrian can be that cleavage may be refracted as it passes from psam- defined. Perhaps the evidence we ignored was not as miticto pelitic horizons in theSouth Stack Unit as soundly based as Professor Wood believes. Nor have described by Dr Powell. However, refraction through we ignored the sub-Ordovician unconformity in Ang- angles of the order of 80°, so that cleavage in pelitic lesey,as reference to the full text of our paper will units approaches bedding, is only seen in fold limbs; in confirm. hinge zones, axial plane cleavages have ahigh angle to beddingin both psammitic and pelitic units. As we DR D. POWELL writes: The authors' interpretation of have reported in the paper, in the hinge zone of the differentstructural histories for the New Harbour Rhoscolyn Fold, as well as in many associated minor Groupand the underlyingSouth Stack Beds, folds, the schistosity of the New Harbour Unit passes HolyheadQuartzite and Rhoscolyn Beds on Holy roundthe fold, parallel to the bedding in the South Island is, on the evidence presented, open to question. StackUnit, whilein theSouth Stack Unit itself the Thegeometry of themain structural elements, viz. cleavage is parallel to the axial planes of the folds and bedding, first cleavage, and the main secondary folds, intersects the New Harbour Unit schistosity at a high is similar in both Units if lithology (and thus ductility) angle. It was this relationship which led us to propose

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thatthe two dominant planar structures in thetwo two groups are different, but this is a reflection of the units are of different ages. This interpretation is rein- differing deformation mechanisms inducedby the mar- forced by the differencein thedevelopment of the kedly different lithologies. dominantplanar structure in each unit. The New Forexample, the authors specificallycited several Harbour Beds havea well-developed schistosity with a generations of crenulation cleavage in the New Har- myloniticfabric which affects quartz veins and even bour Group. This structure certainly has a variety of intercalated psammitic horizons, as may be seen in the orientations, but there are sound mechanical reasons CoedenBeds. The New HarbourUnit alsohas an for this. For instance, it is often associated with chev- intense linear structure lying within the schistosity, at a ron folds, whose orientation vary with fine lithological high angle to the fold axes related to the Rhoscolyn changes, and which commonly occurin conjugate pairs Fold, and is frequently seen to pass round these folds. in these fine-banded rocks, quite unlike the underlying Analogous structures are not, in our experience, de- thick-bedded units. It was critical that these cleavages veloped within the pelitic horizons of the South Stack werenot observed incross-cutting relationships, so Unit. thatit was unnecessary to invoke separate tectonic We have carefully considered alternative interpreta- events. tions of our observations. The scale of the dominant Inthe speaker’s view, the five-fold deformation planar structures in the two units makes it difficult to sequence proposed for the New Harbour Group had visualise how they could have formed simultaneously. notbeen substantiated, and there was therefore in- The schistosity of theNew Harbour Unit is sub- sufficient evidence for the Group being allochthonous. horizontalthroughout the whole of northernAng- lesey,while the cleavagein theSouth Stack Unit is In reply, the AUTHORSstated that: Dr Maltman be- sub-verticaleverywhere in HolyIsland. If thishigh lieved that no episodes of deformation are recorded in angularrelationship is diagnostic of foldlimbs, then the New Harbour Unitwhich are notalso present in the the whole of northern Anglesey must form the right South Stack Unit. There is no detailed account of the way uplimb of a giantrecumbent fold, as Greenly structures seen in the South Stack Unit to match his originallyproposed. There is noother evidence for own account of those in the New Harbour Unit (Malt- such a large scale fold, and it does not account for the man 1977) by which this statement could be judged. linear structure in the New Harbour Unit which must In his account of the structures in the New Harbour be at a high angle to the postulated fold axis. Another Unit,Dr Maltman found himself confronted by the possibilty is that the Rhoscolyn Fold is a second phase same problems for which we have proposeda solution: structure in the South Stack Unit, to which the axial thatthe schistosity of the New HarbourUnit was plane cleavage of a first phase fold is only coinciden- folded around the Rhoscolyn Fold, to which the only tally axial planar. This explanation seems to us equally cleavage in the South Stack Unit is axial planar. His unlikely and also fails to account for the N-S orienta- solution was to suggest that the schistosity and linea- tion of the linear structure in the New Harbour Unit. tion in the New Harbour Unit was produced by bed- Theconcept that the differencein competence of ding plane slip. Although schistosity may develop as a the two groups is responsible for their differences in result of contact strain in the hinge regionof folds (e.g. deformation is not an adequate explanation in itself,as Ramsay1967, fig. 7.82), andslickensides may de- it does not account for all the observed relationships; velop at rightangles to the hinge by beddingplane our model does explain these relationships. slip, this explanation does not adequately account for the scale and extent of the schistosity and lineation in DR A. J. MALTMANthanked the authors for a clear the New HarbourUnit. These structures are seen exposition of a number of stimulatingpropositions, throughout Holy Island and are developed equally in but would like to take issue with one of them, namely the limbsand the hinge zones of thefolds and the alleged allochthonous nature of the New Harbour throughoutthe whole of northernAnglesey, repres- Group. He felt that some of theevidence had been enting a vertical thickness of several km. Again we do presented ina rathermisleading way; the drawings not think that this interpretation accounts adequately andselected photographs had exaggerated points for the observations. appropriate to the authors’ thesis, whereas in the field some of thelocalities mentioned are at very best DR D. E. B. BATES writes: Themagnitude of the ambiguous. sub-Ordovicianunconformity inAnglesey and the However,the keystone of theargument was the Lleyn Peninsula is much greater than the authors have supposed difference in structural history between the claimed. Firstly, the Arenig sandstones and conglom- New Harbour Group and the underlying rocks. The erates do not everywhere rest on the upper mtlangeof speaker believed that there were no episodesof defor- theirCemlyn Unit: it rests in centralAnglesey on mationrecorded in the New Harbour Group which gneisses, the Coedana Granite, and its hornfels; near were not also represented in the underlying sequence. Llangwllog on the Penmynydd zone; and in the Lleyn The nature, style, and orientation of structures in the Peninsulaon gneisses.Secondly, many of the basal

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sediments are shallow water grits and sandstones, with tostromes which closely resemble the Gwna mClange brachiopod-trilobitefaunas similar tothose of the (Bates 1972, p. 50). Shelve and St. David’s districts. This may also be true forthe conglomerates at Ogof Gwynfor,the large quartziteblocks being derived from theadjacent MR W. GIBBONSwrites: The paper by Barber & Max Gwnamtlange. The contact here is quiteirregular, is important in two ways. Firstly, it emphasizes Baker’s and local cliff lines may have been present. Therefore, original assertion (in Lleyn) that the contact between although I would agree with the authors that there is the Gwna Mtlange and other Monian rocks is charac- noprofound structural break between the Gwna terised by mylonitic textures (Baker 1969) rather than Group and the Ordovician,I would argue that there is a prograde metamorphic transition (Shackleton 1956). a majorunconformity present, with a considerable Secondly, if thegneisses cannot be shown to have period of sub-aerialdenudation during which the been derived from the Bedded Succession, then they Gwna Group (if it was originally deposited as a con- may instead represent basement slices caught up in the tinuous cover over the region) was stripped completely shearing. It remains debatable, however, whether any from central Anglesey and parts of SE Anglesey, and of the ‘gneisses’ form ancient basement. the Lleyn Peninsula. Someinterpretations in the ‘New Look’however, I wouldagree with the authors that the Fydlyn are based on one or two ‘crucial’ exposures and seem volcanic Formation could well be of Caradocian age. not to accord with the majorityof the field evidence. However,it could equally well belongto the Arvo- nian. 1. Age and deformation of the Gwna Group Barber & Maxsuggest that the Gwna Group has In reply, the AUTHORSwould like to apologize to Dr undergone the same deformational history as the Or- Bates for their failure to emphasize the importance of doviciansediments, and that the unconformity ex- the sub-Ordovician unconformity over much of Ang- posed at (378948) represents a much shor- lesey during the presentation of the paper. This was ter time break than suggested by Greenly (1919). In dueto the pressure of time, which permittedonly many places, however, a gradation from Gwna Group presentation of some of the more controversial aspects intowhat Greenly called the ‘Penmynydd zone of of their interpretation. The unconformity is adequately metamorphism’has been recognized by Greenly described in the published text. (1919), Matley (1928), Shackleton (1956) and Baker The Authors questioned Dr Bates’s assumption that (1969). The existence of thistransition is strongly the basalOrdovician sediments were deposited ina supported by my mapping in Lleyn and general obser- shallow water environment. The Arenig grits at Ogof vations in Anglesey. Nowhere is this relationship more Gynfor, for example, are coarse, roughly bedded, un- graphicallydisplayed than in Lleyn,where at every sortedgrits with no indication of winnowing orre- exposurebetween Gwna Group and other Monian working such as might be expected in shallow water rocksthere isa narrow‘Penmynydd’ belt of intense sediments. They contain angular quartz chips similar shearing deformation. This is seen on the coast W of tothose seen in the underlyingGwna mtlange, and Parwyd (148246 to 150243); along the stream section alsocontain large blocks of quartzitefar removed at Llangwnadl (359332 to 210329) andat Penrhyn from any possible cliff line. Some of the quartz frag- Nefyn (296412-296410). The Gwna Group has been ments are rounded to form pebbles; these have evi- similarlyaffected by thePenmynydd deformation at dentlybeen transported by rivers orrounded on a localities in Anglesey. Porth Cwfan (337678) provides shorelinebefore arriving in theirpresent position. a good illustration, and Greenly (1919) has described These deposits of mixedorigin appear to have been several others. Nowhere has the Ordovician of North transported by mass flow. The simplest explanation for Wales been affected by the Penmynydd deformation. the nature of these deposits is that they were derived The Gwna Group has therefore suffered at least one from the same source area as the Gwna mtlange, but more deformational event than the Ordovician sedi- thesource area had been uplifted and subject to ments. subaerial erosion to produce the sandy matrix of the TheArenig sediments inLleyn lie unconformably grits,rather than the pelitic matriz of themtlange. upon the ‘Penmynydd Schists’ (Matley 1932), the Sarn Similarly the trilobite-brachiopodfauna may not be Granite(Matley & Smith 1936), andthe Monian in situ but swept off some nearby shallow water shelf gneisses(Shackleton 1956). Some of thecoarser to be deposited in deeper water. Overlying Ordovician Arenig beds contain many fragments of Monian rocks deposits represent a shallowing upwards sequence, as (Matley 1928, 1932) and clasts of typical ‘Penmynydd localdepositional troughs were filled withsediment. mica-schists’ (Greenly 1919) canbe identified in Higher in the Ordovician succession, the Porth Corwgl Arenigsandstones S of PorthMeudwy (164254). andPorth Padrig conglomerates indicate renewal of Clastswith Penmynydd lithologies have been iden- uplift of horst blocks and the formation of new olis- tifiedwithin theLower Cambrian sediments of St.

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Tudwals Peninsula (Nicholas 1915). Lower Cambrian If aposition most favourable to the progressive microfossils have been discovered in the Gwna Group metamorphism case is assumed and it is supposed that (Muir et d. 1979). the dominant cleavage in the Gwna Group is equival- TheGwna Group therefore was formed in the entto the schistosity in thecataclastic rocks, the LowerCambrian, sheared, and brought back to the conclusion is reached that before this phase of defor- surfacebefore the end of theLower Cambrian-an mationthe Gwna Group rocks were unmetamorph- epoch lasting 30 m.y. If the Gwna Group on Lleyn is osed, while the ‘Penmynydd Schists’ were already high the same age as the Gwna Group in N Anglesey, the grade gneisses. unconformity at Llanbadrig represents a break of at As pointed out in the paper, Ordovician rocks rest least 50 m.y. unconformably on mylonitized rocks in Anglesey. Pre- suming that the mylonites were all formed in the same 2. The‘tectonic unit’ concept period,this may also be the case on Lleyn. On the otherhand, there is detailedevidence from Ogof The differencesin structural history between the Gynfor that rocks of Greenly’s Gwna Group pass up ‘New Harbour Group Tectonic Unit’ and the ‘South into the Ordovician without a major structural break, Stack Tectonic Unit’ may be more apparent than real. and that both groups of rocks were deformed together Peliticlayers in theRhoscolyn Formation show a in end- times. It is not intrinsically impossible similardeformational history to parts of theNew that elsewhere the Gwna Group was deformed before Harbour Group, including a strong, folded lineation. thedeposition of theOrdovician, particularly if the By contrast,psammitic layers in the Rhoscolyn For- Gwna Group is diachronous, as claimed by Professor mation are less deformed and sometimes display only Wood.We look forward to the publication of Dr onefoliation. The structural contrasts between the Gibbons’sdetailed account of therelationships in New HarbourGroup and the Rhoscolyn Formation Lleyn. thereforecan be more simply explained in terms of Relationshipsbetween the South Stack and New lithologicalcontrasts rather than the invocation of a Harbour Units have already been adequately discus- major tectonic discordance between the two. The ‘tec- sed in the replies to Professor Wood, Dr Powell and tonicunit’ concept is alsounlikely to applyto N Dr Maltman. With reference to the Cemlyn Unit, we Angleseywhere agreat variety of lithologiesand haveevidence of thecomplex deformational history structuralstyles may be found. For example, the referredto by DrGibbons. The rocks of our AmlwchBeds on the W side of WylfaHead Power ‘Greywacke Group’ are pelites and greywacke grits in Station (347936-350943) showa complicated defor- which the pelites show a single slaty cleavage, while mationalhistory quite unlike anything seen in the the grits are virtually undeformed. Ordovician. The ‘tectonic unit’ approach to Anglesey, basedon the recognition of ‘significant’ contrasts in MrM. F. OSMASTONwrites: Thereintroduction by structural state, needs much firmer support before it Barber & Max of major thrust dislocations into the struc- can reasonably be used to erect a new stratigraphy. tural interpretation of the Monian rocks could prove to be extremely important, for it raises the possibility Inreply, the AUTHORS state that they are not sur- that Anglesey is the site of an early Cambrian major prised that Dr Gibbons is confused by Greenly’s con- ophiolite-emplacing event, responsible both for depos- cept of the ‘Penmynydd Zone of metamorphism’. As ition of theGwna MClange andfor an immediately shown on his map the zone includes a wide range of ensuingMonian metamorphism (as indicated in SE metamorphic rocks, from hornblendic and acid gneis- Anglesey by unconformability beneath Arvonian (and ses, through amphibolite facies psammites and pelites Arenig)rocks and by therecently obtained Rb-Sr to greenschist facies mylonites. The term ‘Penmynydd isochron age of 595 * 12 Ma for Monian gneisses (R. Schists’ appears to have been used by Greenly (1919), S. Thorpe & R.D. Beckinsale,pers. comm. 1979)). Matley (1928) and Shackleton (1956) to mean catac- Comparisons with known major ophiolite occurrences lastic material occurring in shear zones. Some of these leadto the following structural interpretation, which shearzones, as at Porth Nefyn (described in the avoidsBarber & Max’s resortto a separate, much paper),lie between gneisses and rocks of theGwna later, deformation and metamorphism in the N and W Group. In every case examined, the Gwna Group is of Anglesey. Would they consider it a viable alterna- composed of obvious sediments, tuffs and pillow lavas tive? withonly singlea cleavage while the mylonitic WhenMaltman (1975) inferredfrom the igneous ‘schists’, in which the origin is not immediately obvi- contactsand metamorphic aureoles of thenow- ous, are strongly foliated and lineated and commonly serpentinized mafic/ultramafic bodies in the New Har- show refolding. In favourable cases, as at Porth Hefyn, bour Group that they had not been emplaced tectoni- atransition may be seen from the mylonites into cally,he seemed to have been unaware of thethen sheared gneisses from which they were evidently de- already accumulating, and now widespread, evidence rived and thence into relatively undeformed gneiss. that in major ophiolite occurrences the ophiolite slices

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were initially emplaced, whilestill at avery high ones in western California, in that the latter have no temperature,onto and into coevally deposited basic thermal aureole and exhibit quite different structural lavasandsediments, producing thermal and and metamorphic relations with underlying rocks. metamorphicaureoles up to several hundred metres thick (Church & Stevens 1971; Church 1972; Williams In reply, the AUTHORSstate that the contribution by & Smyth1973; Glennie et al. 1974;Dewey 1974; Osmastondemonstrates that the re-examination and Coleman1977; Malpas 1977; Bluck 1978). These re-interpretation of the geology of Angleseyhas aureolescommonly attain hornblende granulite/ thrown the tectonic evolution of the island wide open pyroxenehornfels grade near the contact, implying to speculation. For60 yearsBritish geologists, with extremely steep temperature gradients, and in Oman the notable exception of Shackleton, have been inhi- andwestern Newfoundland show evidence of poly- bitedfrom tampering with the framework left by phase deformation. The heat and temperatures involvedGreenly by the sheer volume of his observations, as arefar too great to have been caused by sliding exemplified by his comprehensive memoir and impres- friction (Glennie et al. 1974; Malpas 1977). Continued sivegeological map.Our work has shown that sliding of such ophiolites, with their welded-on aureole Greenly’s framework is open to question, that manyof material, apparently initiated successive dtcollements theobservations on whichhis synthesis was based in theoverridden sedimentary deep basin and shelf havenot been correctly interpreted and that his sequences, so that (in the -so faras is known- geological map is in some important respectspositively extreme case of Oman) as many as 12 such underlying misleading.With presently available information a slices were ultimately gathered beneath and in front of wide range of interpretations is possible. the ophiolite, but probably not more than4 at any one There is a real need for a new phase of research into location. all aspects of the geology of Anglesey, in view of its A feature of this process is that the lower slices may criticalposition in thePrecambrian and Lower containsediments and olistostrome deposits derived Palaeozoichistory of the BritishIsles. This research from the partly-assembledfromthe allochthon as it must commence with remapping of the whole island, approached. This, in Anglesey, would tend to validate with, in many cases, the re-definition of stratigraphical Greenly’s careful clast-derivation studies without lead- and structural units, backed up by the whole range of ing to his erroneousconclusion that the beds are moderntechniques, systematically applied. Only by regionally inverted. On thisbasis, the highly sheared suchacomprehensive approach will a new,more New Harbour Group would need to contain the zone soundly based synthesis of the geological evolution of of shearing between the ophiolites emplaced in it (and the island of Angleseybe established. Meanwhile, perhaps an even higher ophiolitic slice) and the under- Osmaston’sinteresting speculations will bekept in lying slices. This is consistent with its apparently nor- mind. malsedimentary contact with the top of theSouth Stack Group (Shackleton1969). Its presence only in the N and W of the island would suggest emplacement fromthe NW, which is consistentwith the SE ver- References gence of folds both here and in the Lleyn Peninsula BAKER,J. W. 1969. Correlation problems of unmetamorph- (Shackleton1969), and with depositional evidence osed Precambrian rocks in Wales and south-east Ireland. that the South Stack Group was deposited at the foot Geol. Mag. 106, 246-59. of NW-facinga slope (Wood 1974). High grade BATES, D. E. B. 1972. The stratigraphy of theOrdovician metamorphismshown by the Gwna Group outcrop- rocks of Anglesey. Geol. J. 8, 29-58. ping SE of the New HarbourGroup (Shackleton BECKINSALE,R. D. & THORPE, R. S. 1979.Rubidium- 1969)suggests that hot ophiolite formerly extended strontiumwhole-rock isochron evidence for the age of metamorphism and magmatism in the Mona Complex of over these rocks, but that the much less metamorph- Anglesey. J. geol. Soc. London, 136, 433-40. osed Gwna Group rocks lying yet further to the SE BLUCK, B.J. 1978. Geology of a continental margin, 1: The were never overlain directly by a hot allochthon, and Ballantrae Complex. In BOWES,D. R. & LEAKE, B. E. may constitute a structurally lower slice. This is sup- (eds) Crusral Evolution in Northwestern Britain and Adj- ported by the presence of glaucophaneschists along acent Regions. Geol. J. Spec. lss. No. 10, 151-62. the boundary between the two,in the form of ‘enorm- COLEMAN,R. G. 1977. Ophiolites: Ancient Oceanic Lithos- ous lenses bounded by tectonic slides’ (Wood 1974, p. phere? Springer-Verlag, Berlin. 338).The present duplication of thismetamorphic CHURCH,W. R. 1972. Ophiolite:its definition, origin as zonation on either side of the Berw Fault zone could oceaniccrust, and mode of emplacementin orogenic belts, with special reference to the Appalachians. Pubs. very well not be original, but due to some 25 km of Earth Phys. Branch, Ottawa, 42, (3) 71-85. pre-Carboniferous sinistral displacement along it. - & STEVENS,R. K. 1971. EarlyPalaeozoic ophiolite It should, perhaps, be pointed out that the foregoing complexes of theNewfoundland Appalachians as remarksare based on ophioliteoccurrences which mantle-oceaniccrust sequences. J. GeophysRes. 76, differ markedlyfrom those like the much-discussed 1460-66.

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DEWY, J. F. 1974. Continental margins and ophioliteob- London, 6, 97-101. duction: Appalachian-Caledoniansystem. In BURKC. MUIR, M. D., BLISS, G. M., GRANT, P. R. & FISHER,M. A. & DRAKEC. L.(eds) The Geology of Continental 1979. Palaeontologicalevidence for the age of some Margins. Springer-Verlag, Berlin. 933-50. supposedlyPrecambrian rocks Anglesey,in North GLENNIE,K. W., BOEUF, M. G. A., HUGHES CLARKE, M. W., Wales. J. geol. Soc. London, 136, 61-4. MOODY-STUART,M., PILAAR, W. F. H. & REINHARDT, NICHOLAS,T. C. 1915. The geology of theSt. Tudwal’s B. M. 1974. Geology of the Oman Mountains. Verh. K. Peninsula. Q. J. geol. Soc. London, 71, 83-143. Ned. geol. mijnbouwkd. Genoot, No. 31 (3 vols). RAMSAY,J. G. 1967. Foldingand fracturing of rocks. GREENLY,E. 1919. The Geology of Anglesey. Mem. geol. McGraw-Hill, New York. Sum. U.K.(2 vols.), London. SHACKLETON,R. M. 1953. The structural evolution of North MALPAS,J. 1977. Petrology and tectonic significance of New- Wales. LiverpoolManchester geol. J. 1, 261-97. foundlandophiolites, with examples from the Bay of 1954. The structure and succession of Anglesey and the Islands. In: COLEMAN,R. G. & IRWIN, W. P. (eds)North Lleyn Peninsula. Ado. Sci. London, I1 (41), 106-8. American Ophiolites. Bull. Oregon Dept. Geol. Min. In- - 1956. Notes on the structure and relation of the Pre- dustries, Portland, Oregon. 95, 13-23. Cambrian and Ordovician rocks of south-western Lleyn MALTMAN, A. J. 1975. Ultramafic rocks in Anglesey-their (Carnarvonshire). Liuerpool Manchester geol. J. 1, 261- non-tectonic emplacement. J. geol. Soc.London, 131, 97. 593-603. - 1969. The Precambrian of North Wales. In: WOOD, A. -1977. Serpentinites and related rocks of Anglesey. Geol. (ed.) ThePrecambrian and Lower Palaeozoic Rocks of J. 12, 113-28. Wales. Univ. of Wales Press, Cardiff. MATLEY, C. A. 1928. The Pre-CambrianComplex and as- WILLIAMS,H. & Shfnn, W. R. 1973. Metamorphic aureoles sociated rocks of south-western Lleyn (Carnarvonshire). beneath ophiolite suites and alpine peridoties: tectonic Q. J. geol. Soc. London, 84, 440-504. implications with West Newfoundland examples. Am. 1. -1932. Thegeology of the country around Mynydd Rhiw Sci. 273, 594-621. and Sarn, south-westernLleyn, Carnarvonshire. Q. J. WOOD, D. S. 1969. The base and correlation of the Cam- geol. Soc. London, 88, 238-73. brian rocks of Wales. In WOOD, A. (ed.) The Precam- -& SMITH, B. 1936. The age of the Sarn Granite. Q. J. brianand Lower Palaeozoic Rocks of Wales. Univ. of geol. Soc. London 92, 188-200. Wales Press, Cardiff. Mm, M. D. 1972. Two Precambrian complexes in Ireland. - 1974. Ophiolite, mtlanges, blueschists, and ignimbrites: Bull. geol. Sum. Ireland, 1, 99-105. early Caledonian subduction in Wales? In Don, R. H. - 1975. Precambrianrocks of south-east Ireland. In: & SHAVER,R. H.(eds) Modem and Ancient Geosyncli- HARRIS,A. L. et al. (eds). A correlation of the Precam- nal Sedimentation.Spec. Publ. Soc. econ. Paleont. Min- brianrocks in the British Isles. Spec. Rep. geol.Soc. eral. Tulsa, 19, 334-44.

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