The Age of Blueschist Metamorphism in Anglesey, North Wales: Evidence from 40Ar/39Armineral Dates of the Penmynydd Schists

Journal ofthe Geological Society, London, Vol. 144, 1987, pp. 843-852, 4 figs, 3 tables. Printed in Northern Ireland

The age of blueschist metamorphism in Anglesey, North Wales: evidence from 40Ar/39Armineral dates of the Penmynydd schists

R.D. DALLMEYER’ & W. GIBBONS2 1 Department of Geology, University of Georgia, Athens, GA 30602, USA Department of Geology, University College Cardiff, P.O. Box 78, Cardiff CFl lXL, UK

Abstract: The ‘Penmynydd’ schists of SE Anglesey include several high P/low T lithologies including: (1) metasedimentaryquartz-phengite schist; and (2) mafic blueschist with barroisite/crossite as- semblages that appear to have developed from an earlier actinolitic greenschist facies protolith. One phengiteand three amphibole concentrates have been analysed by incremental-release40Ar/39Ar datingmethods and display discordant age spectra with anomalously young apparentages recorded in low T increments. Ages increase systematically throughout intermediate T portions of the analyses to definehigh T plateaux of c. 550-560Ma (barroisite/crossite-rich concentrates)and c. 580-590 Ma (actinolite-rich concentrate). The phengite concentrate also displays an internally discor- dantspectrum withintermediate T incrementsdefining ages similar to thoserecorded by barroisite/crossite and high T increments giving ages similarto those of actinolite. The amphibole and phengiteages are interpreted as dating post-metamorphic cooling following a regional low-P M1 greenschistmetamorphism (c. 580-590Ma)and a high2 M2 metamorphism (c. 550-560Ma). 40Ar/39Ar ages suggest multiple metamorphism of oceanic crust before the arrival of the blueschist terrane and its accretion to the late Precambrian basement of southern Britain immediately prior to development of the Welsh Basin. An age of c. 550-560 Ma for blueschist metamorphism on Anglesey provides a likely maximum age for- ignimbrites within the pre-late Lower Cambrian Arfon Group exposed in NW Wales.

The mostextensive exposure ofblueschist in the British Harbourand Skerries Groups) of highly deformed, Isles occurs within a narrow belt of schists extending for c. low-grade metasedimentaryand volcanic rocks. The New 20 km across southeastern Anglesey in North Wales (Fig. 1). HarbourGroup hosts small masses of serpentiniteand The schist belt has been regarded as lying stratigraphically meta-gabbro and, at higher structural levels, meta-basalt. beneatha cover of Arenigsediments and ,‘Arfon Group’ (2) A central complex dominated by high-grade, pelitic acid volcanics of probable early Cambrianage (Greenly gneisses andamphibolites, low-grade hornfels, and the c. 1919; Reedman et al. 1984). Fragments of quartz-mica schist 600 Ma Coedana Granite. The southeastern margin of this similar to that associated with the blueschist occur within complex is marked by aductile, high strainzone (Mann Lower and MiddleCambrian sedimentaryrocks on the Welsh 1986)which affects boththe Coedana Granite and a mainland(Nicholas 1915; Greenly 1919; Woodland 1938; structurally overlying succession of metasedimentary and Gibbons 1983). This evidence, combined with the existence mafic metavolcanicrocks referred to as the “Penmynydd of fourpoorly exposed outliers of

843

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ANGLESEY

Fig. 1. Geological map of SE Anglesey and the mainland immediately to the SE. The area includes theNE-SW-trending Menai Strait Fault System within which the most prominent faults are the Berw, Dinonvic and Aber-Dinlle.L, Llanfairpwllgwyngyll; A, outliers of Arfon Groupon Anglesey. The Carboniferous cover to the Gwna melange immediatelyNW of the Berw Fault is not ornamented.

east by retrogressed schists which are in tectoniccontact cores which are partially replaced along the rim by a blue with a low-grade melange. sodic amphibole which ranges between Fe-rich glaucophane The Menai Strait Fault System, which forms the present and crossite (HorBk & Gibbons 1986). At several localities SE boundary to the Monian Terrane, has had a long history along theeastern margin of the schist belt, lawsonite of Phanerozoic faultmovements. However,the initial blueschists have also been described(Gibbons & Mann tectonicjuxtaposition of the Monian Terraneto its 1983). Another lithology of particular interest but restricted approximate present position adjacent to the Welsh Basin occurrence is a coarse-grained, actinolite greenschist which must have occurred prior to an Arenig transgression over occurs in isolatedpatches within several of the basic the Monian schists, gneisses, granite, melangeand meta- blueschist masses. Excellentexposures of this rock type sedimentaryrocks. The age of pre-Arenigtectonothermal weretemporarily revealed during recent construction of a eventsrelated to accretion of the Monian Terrane is new by-pass around Llanfairpwllgwyngyll. Texturalre- poorlyconstrained. K-Ar (Fitch et al. 1969) and Rb-Sr lationships here clearly show that the greenschist formsa whole rock ages (reviewed by Thorpe et al. 1984) have been protolith to the typical column-type blueschist (Gibbons & reportedfor Anglesey. All dates obtainedfrom Monian Gyopari 1986). A pale green, NaM,-poor calcic amphibole rocks span thelate Precambrian-earlyCambrian (c. in the greenschist is sequentially replaced first by a deeper 600-550 Ma).The Rb-Sr dates are from plutonicand greento green-bluebarroisitic amphibole, and then by a gneissic rocks showing onlytectonic contacts with the blue sodic amphibole.Boundaries between actinolite and Monian schists. barroisite areboth optically and chemically gradational, whereas those between barroisite and sodic amphibole are sharper. The transition from greenschist to blueschist is also Petrology of the blueschists marked by the replacement of albite, chlorite and magnetite The Anglesey blueschists were first described from a locality by epidote, quartz and hematite. The extremely low NaM, beneaththe Marquis of Anglesey’s Column at Llan- content of the actinolite suggests thatthe blueschist fairpwllgwyngyll (Blake 1888). Herethe rocks are protolithunderwent an initial metamorphism (Ml) under penetratively foliated, fine-grained, epidote-blue amphibole low pressure, greenschist facies conditions (Fig. 2). This schist with minor quartz,chlorite, sphene, hematite and appearsto havebeen followed by markeda pressure magnetite. Mostamphibole grains have green, barroisitic increase to lower blueschist facies conditions (M2). The

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Table l. Typical probe analyses from amphiboles in dated samples

Barroisite Crossite Barroisite Actinolite la lb 3 5

SiO, 49.54 55.23 50.81 53.48 TiO, 0.20 0.11 0.13 0.07 A1203 7.68 6.95 5.60 2.53 Fe0* 16.57 15.40 15.74 14.40 MnO 0.26 0.20 0.29 0.34 a0 MgO 10.93 9.94 12.24 13.95 I 800- CaO 8.00 3.41 8.43 12.22 W L Na,O 3.71 5.74 3.12 0.59 3 v) K20 0.20 0.04 0.17 0.08 Total 97.09 97.02 96.53 97.66 600- Numbers of ions in the formula based on 23 oxygens Si 7.23 7.86 7.41 7.76 AI(1V) 0.77 0.14 0.59 0.24 Al(W 0.55 1.02 0.37 0.19 Ti 0.02 0.01 0.01 0.01 200 Fe3+t 0.60 0.47 0.65 0.06 4001 FeZ+ 1.43 1.37 1.27 1.68 l5 Mn 0.03 0.02 0.04 0.04 L I I I I I Mg 2.38 2.11 2.66 3.01 100 200 300 400 500 600 Ca 1.25 0.52 1.32 1.90 Temperature (“C) Na 1.05 1.58 0.88 0.17 K 0.04 0.01 0.03 0.01 Fig. 2. Petrogenetic grid illustrating a possible prograde PT-time Total 15.35 15.11 15.23 15.07 path for the Anglesey schists, from low pressure greenschist conditions (Ml) to high pressure blueschist metamorphism (“2). Analysesperformed on wavelengthdispersive Cambridge Curves relate to suggested stability limitsof (1) glaucophane, (2) Instrumentsmicroprobe atthe Open University. Accelerating jadeite, (3) phengite with Si4+ site occupancy of6.8 per 8 cations, potential was 20 kV with live counting times of up to 100 S. Peaks (4) greenschist-blueschist transition, (5) barroisite (for details see wereprocessed and measured by iterative peak stripping and Gibbons & Gyopari 1986). corrected using the method of Sweatman &L Long (1969). * Total iron. greenschists in the Anglesey blueschist belt are interpreted t. Amphiboleanalyses recalculated to estimate Fe3+ values as isolated remnants recording a low pressure [ocean floor? assuming total cation content= 13 - (K, Na, Ca). (Thorpe 1972)] metamorphicenvironment (Ml) which existed prior to imposition of high pressure/low temperature metasedimentaryrock dominated by quartz,with subordinate blueschist conditions (Gibbons & Gyopari 1986). phengite, and minor epidote and chlorite. The rock also contains All the samples dated in this study were collected from thin pelitic bands rich in phengite and chlorite, with epidote and new road cuttings. Specimens 1, 2and 3 arefrom an minoralbite, magnetite, hematite, sphene and apatite. A exposure on the north side of the by-pass, immediately crenulation similar to that developed in sample 1 is present. The south of Siglan Farm [GR SH.5317231. Sample 5 was concentrateprepared from this rock is more than 99% phengite collected from the east end of the prominent knoll exposed with very minor contamination by epidote, magnetite and hematite. on the south side of the by-pass c. 400111 ESE of Siglan Theaverage tetrahedral site occupancy for analyses of eight farm.Petrographic descriptions of each of the dated rock phengite grains within the concentrate is6.7 Si per 8 cations. samples are given below: Sample 3. Epidote-burrokite-crossite schist. Afine-grained, Sample 1. Crossite-barroisite-epidote schist. Afine-grained, stronglyfoliated and heated barroisiteschist similar to 1. Itis strongly foliated and heated basic schist dominated by amphibole dominated by greento blue-green barroisite. Blue amphibole andepidote, withsubordinate chlorite, albite, quartz, and minor overgrowsbarroisite cores. Minor phases include sphene, sphene,magnetite and hematite. The amphibole is mostlydeep magnetite,hematite, albite, chlorite, calcite and phengite. The green barroisite, often showing patchy rim replacement by purple concentrate is composed almost entirely of sodic-calcic amphibole, crossite. Most of the rock is composed of a fine-grained groundmass withonly minor rim replacement by blueamphibole and a few of mafic minerals containing thin bands rich in quartz and albite. grains of epidote. All amphibole probe analyses obtained from this Thefoliation iscrenulated although no penetrativefoliation concentrate have been barroisitic (Table 1). developed during this later deformation. Petrographic examination and probe analyses of the mineral concentrate prepared from this sample 5. Epidote-actinolite schist. A medium grained, poorly specimeninclude bothbarroisite andcrossite amphibole foliatedbasic schist dominated by epidoteand green amphibole, compositions. Single mineral grains often contain both amphiboles. withminor barroisite, crossite, chlorite, magnetite, hematite and Representative amphibole probe analyses are given in Table 1. sphene. The assemblage represents a transition between the coarse actinolite greenschists described by Gibbons & Gyopari (1986) and Sample 2. Phengite-quartz schist. Afoliated fine-grained, the typical ‘Column’ type blueschist (HorBk & Gibbons 1986). The

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greenactinolitic amphiboles often show partial replacement by 40Ar/39Arincremental-release methods. The analytical data barroisiteand crossite. The concentrate prepared for this rockis arepresented in Tables 2 and 3 andportrayed as age and mostly composed of green amphibole (sometimes with darker green apparent K/Ca spectra in Figs 3 and 4. andblueish rims) and minor epidote. All amphibole probe analyses The techniques used during 40Ar/39Ar analysis of thesamples obtainedfrom this separate represent actinolitic compositions generally followedthose described in detailby DdmeyerBr Keppie (Table 1). (1987). In the present study, mineral concentrates were prepared from.crushed and sizedrock powders using heavy liquid and @Ac/’~A~analytical methods magneticseDaration techniaues. Mineral concentrates were Three amphibole concentrates and one phengite concentrate wrapped in aluminium-foil packets, encapsulated in sealed quartz were prepared from samples 1, 2, 3 and 5 and analysed by vials,and irradiated for 40 h at 1000 kWin the centralthimble

Table 2. 40Ar/3%r analytical data for incremental heating experiments on blue amphiboles from the Penmyndd schist zone, southeastern Anglesey, Wales

39Ar Percentage 39Ar Release percentage Release of 40Ar 36 Arca Apparent temp. (“C) (40Ar/39Ar)’temp.(“C) (36Ar/39Ar)’ (37Ar/39Ar)2 of total n~n-atmos.~ (%) (Ma)4age

Sample l; J = 0.009485 450 73.76 450 0.19992 1.882 1.68 20.11 0.26 237.7 f 36.3 500 27.22 0.03902 2.985 4.76 58.51 2.08 254.3 f 3.6 550 28.95 0.01284 0.959 14.21 87.14 2.03 387.2 f 2.7 600 33.80 0.00713 0.883 13.30 93.96 3.37475.2f 2.6 650 37.50 650 O.Oo640 0.674 15.85 95.08 2.86525.7f 2.5 700 39.74 700 0.00707 0.858 16.51 94.90 3.30551.9f 1.2 725 40.39 725 0.00810 1.834 12.60 94.43 6.16557.6f 1.5 750 41.53 750 0.01339 3.587 6.33 91.15 7.28 554.5 f 5.6 800 44.42 800 0.02666 6.848 3.61 83.49 6.99 545.8 f 10.4 Fusion 50.39 0.06989 70.280 11.14 70.21 27.35543.0f 4.9 Total 38.55 0.02083 8.588 99.99 87.11 6.08 492.8 f 4.1 Sample 3; J = 0.00932 450 105.16 450 0.29050 2.437 1.11 18.55 0.23 301.9f 30.3 500 32.80 500 0.04638 3.455 4.48 59.05 2.03 300.1 f 5.0 550 34.28 0.01635 2.273 10.79 86.43 3.78 440.7f 1.9 600 39.28 0.01408 1.249 10.01 89.65 2.41 512.6f 2.6 650 40.52 650 0.01096 1.038 9.58 92.20 2.57 539.5 f 3.7 675 40.55 675 0.00689 1.015 13.33 95.16 4.01 554.8f 2.6 700 40.99 700 0.00694 2.000 14.10 95.37 7.84 561.3 f 1.0 750 42.25 750 0.01188 5.259 14.81 92.68 12.04 563.1 f 2.4 800 46.19 800 0.04206 54.637 6.44 82.59 35.33 565.9f 5.6 875 44.19 0.02138 21.998 13.45 89.69 21.99 574.2 f 2.2 Fusion 94.31 0.26849 251.266 1.90 37.28 25.45 593.9 f 26.3 Total 42.62 0.02555 14.056 100.00 87.11 11.01527.7f 4.0 Sample 5; J = 0.009150 450 672.90 450 2.11959 9.435 0.65 7.03 0.12 652.2 f 86.3 500 137.10 500 0.38604 19.937 3.04 17.96 1.40 370.8 f 12.3 550 57.53 0.10733 5.006 8.34 45.56 1.27388.9f 8.6 575 46.29 575 0.06488 5.054 6.48 59.45 2.12 406.3 f 18.9 600 47.87 0.06171 5.712 5.78 62.85 2.52 440.1 f 14.9 625 57.80 625 0.08190 8.558 6.38 59.31 2.84 494.6 f 9.0 650 63.83 650 0.10355 21.063 4.23 54.70 5.53 506.1 f 16.2 675 56.93 675 0.08661 42.442 3.92 61.03 13.33 509.9 f 16.2 725 60.83 725 0.10472 93.176 8.27 61.43 24.20 559.1 f 8.2 775 52.89 775 0.06354 71.356 20.19 75.33 30.55 583.4f 1.9 825 52.05 825 0.04868 42.102 22.14 78.85 23.52 588.6f 4.3 850 80.64 850 0.16382 80.297 4.65 47.96 13.33 576.7 f 11.7 875 97.46 875 0.26065 204.036 3.23 37.79 21.29 590.1 f 20.6 1100 197.58 1100 0.54906 166.666 2.71 24.66 8.26 729.5 f 31.4 Total 67.21 0.11844 51.117 100.00 62.23 16.28 532.4 f 10.5

Measured. *Corrected for post-irradiation decay of 37Ar (35.1day half-life). 340[ Artot - (36Ar,,,,)(295.5)]/40Ar,, X 100. Calculated using correction factors of Dalrymple et al. (1981); two-sigma intralaboratory errors.

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Table 3. 40Ar/3%r analytical data for incrementalheating experiments on phengite from the Penmynydd schist zone, southeastern Anglesey, Wales

PercentagePercentage Release 40Ar of total Apparent36 Arc, temp.(40Ar/39Ar)’(“C) (36Ar/39Ar)’ (37Ar/39Ar)Z 39Ar non-atmo~.~ (%) (Ma)4 age

Sample 2; J = 0.009475 525 32.42 0.00493 0.058 3.48 95.50 0.32464.1 f 0.6 575 36.85 0.00158 0.017 6.19 98.72 0.29534.3 f 0.9 600 38.54 0.00124 0.021 5.46 99.03 0.47556.8 f 1.6 625 39.20 0.00124 0.022 6.97 99.04 0.48565.1 f 1.2 650 39.30 0.00109 0.018 11S8 99.16 0.46567.0 f 0.5 675 39.58 0.00107 0.006 12.13 99.19 0.15570.5 f 0.5 685 39.93 0.00118 0.001 11.11 99.11 0.03574.4 f 0.5 700 40.45 0.00145 0.007 8.55 98.93 0.14579.9 f 0.9 715 40.74 0.00168 0.022 6.30 98.77 0.36582.7 f 0.9 725 41.22 0.00233 0.039 3.61 98.33 0.45586.3 f 0.9 750 41.03 0.00171 0.067 4.97 98.77 1.06 586.2f 1.7 775 40.99 0.00160 0.108 5.74 98.85 1.84 586.3 f 1.6 825 40.76 0.00124 0.139 8.11 99.11 3.03584.7 f 1.6 875 41.40 0.00327 0.505 3.32 97.75 4.20585.7 f 1.0 Fusion 42.91 0.00638 0.257 2.59 95.64 1.09592.6 f 1.5 Total 0.00167 39.68 569.5 0.520.045 98.74 100.00 f 1.0

Notes as for Table 2.

position of the U.S. Geological Survey TFUGA reactor in Denver, Dalrymple et al. (1981) for the reactor used in the present study. Colo.Variations in the flux ofneutrons along the length ofthe Apparent40Ar/39Ar ages were calculated from the corrected irradiationassembly were monitored with several mineral isotopicratios using the decay constants and isotopic abundance standards, including MMhb-l (Alexander et al. 1978). The samples ratios listed by Steiger & Jager (1977). Two-sigma, intralaboratory were incrementally heated until fusion with an RF generator. Each uncertaintiesin each apparent age are reported. Inter-laboratory heating step was maintained for 30min. Measured isotopic ratios comparisonsmay beapproximated by consideration of c. were corrected for the effectsof mass discrimination and interfering 0.25-0.5070uncertainties in the appropriate ‘J value’. Total-gas isotopesproduced during irradiation using factors reported by ages have been computed for each sample by weighting of the ages

- r 11 r e0 0.80- -080- -008- Y - 0.60- -006- l- 0.60- z l K 0.40- -0 04 2 a - a 0.20- - 0.02-

1 I 50 600- - 600- 600 -

W 0 a 500- - 500- l- z W

~ 2 400- - .400 a a 492.8r4. I Mo 527.7r 4.OMo 532.4t10.5Ma U L 300 300 0 20 40 60 80 I00 CUMULATIVE YO3’Ar RELEASED

Fig. 3. 40Ar/39Ar apparent age and K/Ca spectra for amphibole concentrates from blueschist within the PenmynyddSE schist,Anglesey. Sample numbersare plotted above the relevant age spectra. Abscissa coordinates are indicatedat lower left. Analytical uncertainties(two sigma, intralaboratory) are represented by vertical thickness of bars. Experimental temperatures increase from to left right. Total-gas ages are listed on each spectrum.

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intrasample variations in apparent K/Ca ratios suggest that several phases probably contributed gas at various stages in the analyses. The concentrates prepared from samples1 and 3 consist predominately of amphibole and very subordinate epidote.The amphibole grains arepredominately green barroisite with rare, lighter green cores (actinolite?). The grains show variable and minor peripheral alteration toblue crossite. Replacement is more extensive in the concentrate preparedfrom sample 1. It is proposedthat both the systematically increasing apparent agesand K/Ca ratios recorded in 450-625 "C portions of the 40Ar/39Ar analyses arerelated to a changing mixture of gas evolved from relatively non-retentive crossite and more retentive barroisite.The systematically decreasing apparentK/Ca ratios a 14 I observedthroughout higher temperatureportions of the analyses areinterpreted to reflect zoning resultingfrom incompletereplacement of initialactinolite by barroisite. 569.521.0 Ma The c. 550-560 Ma apparent ages recorded in intermediate- temperature gas fractionsare considered closely todate aa400 cooling throughtemperatures required forintracrystalline a0 20 40 60 80 l00 argonretention in barroisite.The slightly olderapparent CUMULATIVE YO3'Ar RELEASED ages recorded in the highest temperatureportions of the analysis of the concentrate prepared fromsample 3 probably Fig. 4. 40Ar/39Arapparent age and K/Ca spectra for a phengite reflect an increasing contribution of argon from the slightly concentrate from blueschists within the Penmynydd schist, SE older and more refractory actinolite cores. Anglesey. Data plotted asin Fig. 3. The concentrate prepared from sample 5 consists largely of actinolitegrains whichshow marginal replacement by and calculatingthe uncertainty of eachtemperature increment. green barroisite and blue crossite. The internally discordant Analyses of the MMhb-l monitorindicate that apparentK/Ca 40Ar/39Ar age spectrumis interpreted to reflect experimental ratios may be calculated as 0.518(~0.0005)X (39Ar/37Ar)correctcd. evolution of gas fromboth phases. The systematically increasing apparent agesand decreasing apparentK/Ca Results ratios observed in 550-725 "C portions of the experiment are considered to reflect a changing mixture of gas evolved from relatively non-retentive crossiteandmore refractory Amphibole actinolite.The relatively constant apparent K/Ca ratios The three amphibole concentrates recordtotal-gas "OAr/39Ar recorded in the 775-878°C increments suggest gas was ages of 492.8 f 4.1 Ma (sample l), 527.7 f 4.0 Ma (sample evolvedfrom a similar source, and the 576.7 f 11.7 to 3) and 532.4 f 10.5Ma (sample 5). They display very 590.1 f 25.7 Ma apparent ages reflected in these increments discordantage spectra of generally similar character.In are therefore interpreted to date cooling through tempera- each, anomalously young apparent ages are recorded in gas tures appropriate for argon retention in actinolite. increments evolved at the lowest experimental temperatures (450-500 "C). Apparent agessystematically increase throughout intermediate-temperature portions of the three Phengite analyses. Theapparent agesdefined bygas increments The phengite concentrate prepared from sample 2 displays evolved at high experimental temperatures show only minor aninternally discordant 40Ar/39Ar age spectrum which intra-samplevariation, ranging between 543.0 f 4.9and defines a total-gas date of 569.5 f 1.0 Ma. An anomalously 551.9f1.2Ma (l),561.3f1.0 and 574.2f2.2Ma (3), and young apparent age of 464.1 f 0.6 Mais recorded in the 576.7 f 11.7 and 590.1 f 25.7Ma (5). The fusion increments initial low-temperature increment (525 "C). Apparent ages of samples 3 and 5 record anomalously older apparent ages. increase markedly over the 575 and 600°C increments to a ApparentK/Ca ratios displaygenerally consistent value of 565.1 f 1.2 Ma in the 625 "C fraction. The apparent variation patterns in the amphibole analyses. Samples 1 and ages increase slightly and systematically overthe next six 3 record very similar patterns, with relatively low values in increments from 567.0 f 0.5 Ma (660 "C) to 586.3 f 1.6 Ma the low T gas fractions,systematically increasing to (725 "C). Apparent ages recorded in the next four maximumvalues in the 600-625"C increments.The increments (750-875"C) aremutually similar, and range apparent K/Ca ratios decrease to very low values in higher between 586.2f 1.7and 585.7 f 1.0Ma. The fusion temperature portions of the analyses. Apparent K/Ca ratios increment yields ananomalously older apparent age of are relatively much lower throughout the analyses of sample 592.6 f 1.5 Ma. 5. Starting at values of c. 0.02-0.05 in the 450 and 500 "C Theapparent K/Ca ratiosrecorded by the various increments, there is a marked increase to a maximum value increments display a systematicvariation. After initially of c. 0.09 in the 550-575"C increments.The ratio drops fluctuating in the 525 and 575 "C increments, values remain markedly over the 600-675°C increments to much lower and constant and low in the 600-685 "C increments. The ratio relatively constant values throughout the remainder of the increases abruptly in the 700"C incrementand then experiment. systematically decreases over the remainderof the analysis. Optical characteristics of the concentrates and consistent Although the phengite concentrate is optically uniform,

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variationin apparent K/Ca ratios suggests intracrystalline eventethe older reflecting initial M1 growth of white mica zoning. The 625-685 “C incrementsrecord constant associated with intrusion of basic igneous rocks beneath the apparent K/Ca ratios and may reflect gas evolved from a seafloor, and the younger data representing M2 blueschist single phase. Apparent ages reflected by theseincrements metamorphism.Alternatively, the earliermetamorphism range between 565.1 f 1.2 and 574.4 f 0.5 Ma, generally may have been related to a widespread regional greenschist similar to those ascribed to argon retention in the barroisitic event affecting both sediments and basic rocks. Whatever amphiboles. Apparent agesrecorded in the 715-875 “C the metamorphicsetting for Ml, both amphibolesand increments are mutually similar, ranging from 582.7 f 0.9 to phengites record similar ages and appear to have had nearly 586.3 f 0.9 Ma. This range is comparable to that suggested the same thermal evolution. for the time of cooling through temperatures required for The 40Ar/39Arresults suggest that blueschist meta- argon retention inactinolite. The observedvariations in morphismoccurred in the Penmynydd Schist Zone of SE apparent K/Ca ratios, which decrease systematically in the Anglesey at c. 550-560Ma. On most published time-scales high-temperature fractions, may therefore indicate two this correspondsapproximately theto Precambrian- phases of mica growth. Cambrian boundary and providesa maximum age for Cambrianrocks exposed in northwestern Wales. Unde- formed rocks correlated with theArfon Group unconfor- Discussion mably overlie the Penmynydd Schist zone on Anglesey and Thethree amphibole spectra are characterized by must post-date the c. 550-560 Ma blueschist metamorphism, systematically increasing apparent ages in low- and initial whereas in NW Wales the Arfon Group is comformably intermediate-temperature portions of the analyses. A overlain by Lower Cambrian sedimentary rocks. Unless the changing mixture of gas evolved from non-retentive crossite Precambrian-Cambrianboundary is younger than is and either actinolite (sample 5) or barroisite (samples 1 and generally accepted (cf. Odin 1982; Cope & Gibbons, 1987), 3) appears to have dominated increments liberated during the Arfon Group is probably of early Cambrian age. This these portions of the experiments. The nature of the spectra would require redefinition of the base of the Cambrian discordancy at low T suggests thatthe variousmineral sequence, traditionally taken at the topof the Arfon Group. systems experienced minor, post-crystallization diffusive loss The tholeiiticgeochemistry of the basic schists in of radiogenic argon as a result of geologic reheating (e.g. southeastern Anglesey (Thorpe 1979) andtheir blueschist Turner 1970; Dallmeyer, 1975; Dallmeyer et al. 1981). metamorphic overprint, combine to favoura model Although less marked, the phengite spectrum displays the involving the subduction andtor obduction of oceanic crust. same systematically increasing apparent ages in gas fractions Thisorogenic activity clearly occurredbefore the Lower evolved at low experimental temperatures. On the basis of Palaeozoicsubduction of oceanic crust along the south- these relationships, it is suggested that a mild, early-middle eastern side of Iapetus which resulted in the generation of Palaeozoic thermal overprint affected partial diffusive loss of Ordovician,arc-related magmas presently exposed in argon in both phengite and the very non-retentive crossite. southern Britain (e.g. Bevins et al. 1984; Leat et al. 1986). Although the blueschists record no obvious metamorphic Instead, the Monian blueschists are interpreted as relating retrogression,they are cut locally by chlorite- and toan earlierdestructuve plate margin. The dominantly actinolite-bearing quartz veins, and the early metamorphic calc-alkaline, late Precambrian basement exposed southeast fabric has been deformed by southeast-vergingfolds. This of the blueschists in southern Britain(e.g. Thorpe 1979) folding also affected Ordovician and Silurian rocks on could representan arc constructed in relationship to a Anglesey and hasbeen interpretedas late Caledonian SE-dippingsubduction system. However,there are few (Acadian: early to middle Devonian) in age (e.g. Bevins et constraints on this late Precambrian active plate margin, and al. 1986). The area probably also experienced at least local platetectonic modelling must emphasise the likelihood of abnormally high heat flow duringthe early Cambrian major late Precambrian-early Cambrian strike-slip faulting eruption of the nearbyArfonian ignimbrites and during (Gibbons 1983). It is particularly difficult to define the Ordovician(particularly Caradoc) volcanism. There is tectonic significance of the Monian blueschists because they palaeomagnetic evidence for a mild but widespread thermal lie entirely within the Menai Strait Fault System where they reheating of thearea duringCarboniferous times. The occur as a tectonic slice surrounded by shear zones. Overall, diffusive loss of radiogenic argon exhibited by the minerals the Monianrocks of Anglesey ingeneral, and the analysed inthis study could haveoccurred during one or blueschists in particular, are best viewed as part of a more of these Palaeozoic thermal events. displaced terrane (or as several terranes within one Monian The cooling ages recorded by the blueschist amphiboles composite terrane) movedalong an active plate margin are consistent with the petrogenetic history inferred from during the late Precambrian-early Cambrian. The accretion texturesand variationsin mineral chemistry. Gibbons & of this terrane took place immediately prior to the eruption Gyopari (1986) suggested thatthe blueschist protoliths of the Arfonian ignimbrites, the subsidence of the ‘Welsh developed from oceanic lithosphere produced along a late Basin’ area to the southeast, and the deposition of marine, Precambrianspreading ridge. This underwent sub-seafloor sedimentary rocks in southern Britain. metamorphism (Ml) prior to entrainmentin an accretionary We are grateful to Andy Tindle (Open University) for helpwith the wedge and resultant high pressure metamorphism (M2) in a microprobe, and to Jana Hor6k (National Museum of Wales) for subduction zone. The relatively small difference in apparent help with drafting Fig. 1. This work has been achieved in response 40Ar/39Ar datesrecorded by M1 actinolite and M2 to IGCP 233 (Terranes in the Circum-Atlantic Paleozoic Orogens). barroisite/crossite are compatible with thismodel. The phengite schists areinterpreted asrepresenting sediment References associated with the igneous protolith of the blueschists. The ALEXANDER,E. C., g., MICHELSON,G. M. & LANPHERE,M. A. 1978. two ages recorded by phengite may record two metamorphic MMhb-l: a new dating standard. In: Zartman, R. E. (ed.)

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ShortPapers ofthe Fourth InternationalConference, Geochronology, Wales-the preservation of ancient blueschists. Geology, 11, 3-6. Cosmochronology, Isotope Geology. U.S. GeologicalSurvey Open-File GREENLY,E. 1919. The geology of Anglesey. Memoir ofthe Geological Report 78-701, 6-8. Suruey of Great Britain (2 vols). HMSO, London. BEVINS,R. E., GIBBONS,W., HARRIS, A. L. & KELLING,G. 1986. The HOR~K,J. M. & GIBBONS,W. 1986. Reclassification of blue amphiboles from Caledonian rocks of Britain. In: Fettes,D. J. & Harris, A. L. (eds) Anglesey, North Wales. Mineralogical Magazine, 50, 533-5. Synthesb of the Caledonian Rocks of Britain. Reidel, Dortrecht, 1-27. LFAT, P. T., JACKSON,S. E., THORPE, R. S. & STILLMAN,C. J. 1986. BEVINS,R. E., KOKELAAR, B. P. & DUNKLEY,P.N. 1984. Petrology and Geochemistry of bimodal basalt-subalkaline/peralkaline rhyoliteprov- geochemistry of lower to middle Ordovician igneous rocks in Wales: a inces within the Southern British Caledonides. Journal of the Geological volcanic arc to marginal basin transition. Proceedings of the Geologists’ Society, London, 143,259-74. Association, 95, 337-48. MANN,A. 1986. Geological studies withinthe Mona Complex ofcentral BLAKE,J. R. 1888. Theoccurrence of aglaucophane-bearing rockin Anglesey, North Wales. PhD Thesis, University of Wales. Anglesey. Geological Magazine, 5, 125-7. MUIR, M. D., BLISS, G. M,, GRANT, P. R. & FISCHER,M. 1979. COPE,J. C. W. & GIBBONS, W.1987. The Precambrian-Cambrian boundary Palaeontological evidence for the age of some supposedly Precambrian in SouthernBritain: new evidence for therelative age of theErcall rocks in Anglesey, NorthWales. Journalof the Geological Society, Granophyre. Geological Journal,22, 53-60. London, l36,61-4. DALLMEYER,R. D. 1975. “AK/~~A~ages of biotite and hornblende from a NICHOLAS, T.C. 1915. The geology of the St. Tudwal‘s Peninsula. Quarterly progressively remetamorphosedbasement terrane: their bearing on Journal of the Geological Sociery of London, 71, 83-143. interpretation of release spectra. Geochimica et Cosmochimca Acta, 39, ODIN,G. S. 1982. The Phanerozoic time-scale revisited, Epbodes, 3, 3-9. 1655-69. PEAT, C.J. 1984. Comments on some of Britain’s oldest microfossils.Journal -& KEPPIE,J. D. 1987. Late Paleozoic tectonothermal evolution of the of Micropalaeontology,3, 65-71. southwestern Meguma Terrane, Nova Scotia. Canadian Journal of Earth REEDMAN, A.J., LEVERIDGE,B. E. & EVANS, R.B. 1984. The Arfon Group Sciences (in press). (‘Arvonian’) of North Wales. Proceedings of the Geologbts’ Association, -, ODOM, A.L., O’DRISCOLL,C. F. & HUSSEY,E. M.1981. London, 95, 313-22. Geochronology of the Swift Current granite and host volcanic rocks of SHACKLETON,R. M. 1969. The Precambrian of North Wales. In: Wood, A. the LoveCove Group,southwestern Avalon zone, Newfoundland: (ed.) The Precambrian and Lower Palaeozoic Rocks of Wales. University evidence of a late Proterozoicvolcanic-subvolcanic association. Canadian of Wales Press, Cardiff, 1-22. Journal of Earth Sciences, 18, 699-707. STEIGER,R. H. & JAGER, E. 1977.Subcommission on geochronology: DALRYMPLE,G. B., ALEXANDER, E. C. LANPHERE,M. A. & KRAKER, G. P. convention on the use of decay constants in geo- and cosmochronology. 1981. Irradiation of samples for ’Ar/”Ar dating using the Geological Earth and Planetary Sciences, 36, 359-62. Survey TRIGA reactor. U.S. GeologicalSurvey, ProfessionalPaper SWEATMAN,T. W. & LONG,J. V. P. 1969. Quantitativeelectron-probe 1176, 55 pp. micro-analysis of rockforming minerals. Journalof Petrology, 10, FITCH, F. J., EVANS, A. L.,GRASTY, R. L. & MENEISY,M. Y. 1969. Isotopic 332-79. agedeterminations on rocksfrom Wales and the Welsh Borders. In: THORPE, R.S. 1972. Ocean floor basalt affinity of Precambrian glaucophane Wood, A. (ed.) The Precambrian and Lower Palaeoroic rocks of Wales. schist from Anglesey. Nature, Physical Science, 240, 164. University of Wales Press, Cardiff. -1979. Late Precambrian igneousactivity in S. Britain. In: Harris, A. L., GIBBONS,W. 1983. Stratigraphy,subduction and strike-slipfaulting in the Holland,C. H. & Leake, B. E. (eds) The Caledonides ofthe British Mona Complex of North Wales-a review. Proceedings of the Geologists’ Isles-Reviewed. SpecialPublication of the GeologicalSociety of Association, 94, 147-63. London, 10, 579-84. -& GAYER,R. A. 1985. British Caledonian Terranes. In: Gayer, R. A. -, BECKINSALE,R. D., PATCHETT,P. J., PIPER,J. A., DAVIES, G. R. & (ed.) TheTectonic Evolution ofthe Caledonide-AppalachianOrogen. EVANS,J. A. 1984. Crustal growthand late Precambrian-early EarthEvolution Sciences InternationalMonograph Series, Vieweg, Palaeozoic plate tectonic evolution of England and Wales. Journal of the Braunschweig and Wiesbaden, 3-16. Geological Society, London, 141, 521-36. - & GYOPARI,M. 1986. A greenschist protolithfor blueschiston TURNER, G.1970. Thermal histories of meteorites: In: Runcorn, S. K. (ed.) Anglesey, U.K. In: Evans, B. W. & Brown, E. H. (eds) Blueschists and Paleogeophysics. Academic Press, London, 491-502. Eclogites. Geological Society of America Memoir 164, 217-28. WOODLAND,A. W. 1938. Petrological Studies in the Harlech Grit Series of - & MANN, A. 1983.Pre-Mesozoic lawsonite in Anglesey,North Merionethshire. Geological Magazine, 75, 366-82, 440-54, 529-39.

Received 17 September 1986; revised typescript accepted 20 November 1986.

Discussion of Dallmeyer andGibbons, giving a maximum age of 550 Ma. For the final 20% gas the ages decrease to 517 Ma, Dr M. D. Max & Dr J. C. Roddick write: Dallmeyer and a phenomenon seen in other amphibole analyses (Harrison Gibbons (1987) have providedevidence onthe timing of & McDougall 1981) and possibly related to the degassing of blueschist metamorphism in eastern Anglesey. We present a a high Ca region within the crossite, as reflected in this case further 40Ar/39Ar amphibole mineral analysis that supports by the low K/Ca ratio (Fig. Dl). The peak age of 550 Ma is this timing and examine the wider implications of the data. considered to provide a minimum age of metamorphism of Figure D1 is anage spectrum for amphibole from a this sample. The low temperaturesteps, increasing from massive crossite-epidote rock (Geological Survey of Ireland 300Ma, provide further confirmation of a later reheating specimen 77-1745) taken from a quarry along the northern event during mid-late Palaeozoic times. side of the old Bangor-Holyhead road about 250m to the Dallmeyer andGibbons suggest that the Anglesey ESE of the Marquess of Anglesey’s Monument (537 715 on phengiterecords two metamorphic events and a later 1 :50 000 sheet 114, Anglesey). This is about 1 km from the reheating.However, Harrison (1983) has discussed, and localities sampled by Dallmeyer and Gibbons. The rock was Wijbrans & McDougall (1986) have documented, the composed of about 70% felted, schistose, light blue to light difficulty of discerning even two metamorphic events in age violet crossite, up to (rarely) 1 mm in length, with about spectra. A realistic interpretation of the phengite spectrum 22% epidote group minerals and about 8% chlorite. There is that only the maximum age may record a geological event. was neitherapparent retrogressionnor intergrowth with This age (c. 586 Ma) is the same as that of actinolite sample other amphibole species. The analysed separate consisted of (5) and, asDallmeyer and Gibbonsconclude, probably 74 mg of 100-250 pm grains of essentially pure crossite. dates sea-floor volcanism or greenschist M1 metamorphism. The age spectrum is broadly similar to that of sample 1 It is possible that blueschist M2 metamorphism closely

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0.51 I I I I l body of related Precambrianbasement rocks, variably l- 4 metamorphosed and deformed in a single event (Shackleton 1975), is no longer tenable. The exciting question that is now posed is, ‘How many terranes comprise Anglesey and what is their history of amalgamation?’. * 0.2 0.31- 830° 0.1 p00 Dr R. D. Dallmeyer andDr W. Gibbons reply: Max and Roddick report a 40Ar/39Aranalysis of what they term “. . . 9 100- 600 - essentially pure crossite”. We find this remarkable in view of the difficulties we had in obtaining ultra-pure amphibole concentrates from similar rocks. Unfortunately, Max and Roddick report no compositional data for their concentrate 500 t to allow direct comparison with ours. They interpret their ‘peak’ age of 550 Ma asa minimum datefor M2 metamorphism in the same way that we suggested that the c. 550-560 apparent agesrecorded intermediate- in temperature gasfractions evolved from our polyphase amphibole concentrates closely dated post-M2 cooling 3000 20 40 60 80 100 through temperaturesrequired for intracrystalline Ar 1retentioninconstituent barroisite. Max andRoddick CUM. % ’‘~1 contend that we suggested thatour phengiteresults Fig. D1. 40Ar/39Ar release spectra for crossite77-1745 and K/Ca “. . . record two metamorphic events and a later reheating”. ratios in the heating steps. Temperaturesof the heating steps are Infact, our interpretation was thatthe variations in noted for several gas fractions. Errors to2a limits. Sample apparent K/Ca ratios observed in the phengite analysis may irradiation, gas extraction and data processing techniques are indicate two phases of mica growth. Max and Roddick cite described in Roddick er al. (1980). Tables of analytical 40Ar/39Ar Harrison (1983) and Wijbrans & McDougall (1986) to and electron microprobe data forthe crossite have been deposited documentthe difficulties in discerning poly-metamorphic with the British Library at Boston Spa, W. Yorkshire, UK, as historiesfrom 40Ar/39Ar results. We, of course, heartily Supplementary Publication No. SUP 180501, arealso available from concur with expressing caution in such interpretation of the Society Library or from J. C. Roddick, Precambrian Division, 40Ar/39Arage spectra.However, neither Harrison (1983) Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, nor Wijbrans & McDougall (1986) discussed the type of Canada. apparent K/Ca variations we observed. Since the realization thatthe concept of displaced followed the M1 event. The minerals have lost a significant terranes can usefully be applied to Monian geology in amount of argon, do not display ageplateaux and rise to Anglesey (Gibbons 1983), it hasbecome apparentthat slightly different maximum ages (550, 558, 574Maeffects previous interpretations of a stratigraphically coherent possibly caused by Palaeozoic reheating. The samples may ‘Mona Complex’ (cf. Greenly 1919; Shackleton 1975) need also belong to differenttectonic slices subducted or to bereassessed. Particular emphasis needs to be placed obducted at different times. The M2 event is constrained to upon: (1) prominenttectonic boundaries such as the L19n between 550 and about 586 Ma. and Berw shear zones thatseparate Monianunits with The‘Mona Complex’ is composed of ut least two differing geological histories(Gibbons 1987), (2) critical completely different terranes.The blueschists andrelated analysis of evidence for any previously suggested correlation rocks in the Penmynydd metamorphic zone were subject to betweenMonian units such as the Penmynydd schists in a subduction-related tectonothermal event for the first time central and SE Anglesey. Wetherefore, of course,agree up to50Ma after the c. 600 Ma emplacement of the that researchin Anglesey shouldconcentrate onthe Coedanagranite (Shackleton 1969) into gneisses of the identification of terranes and their history of amalgamation: Mona Complex. The Penmynydd zone itself may bea that is why we dated the blueschists. composite terrane. Bodies of schist and deformed migmatite gneisses in the Penmynydd zone (imbricate zone of Barber & Max 1979, Barber et al. 1981) are distinct from the Additional references blueschists and may be fragments of otherterranes. Tectonically isolated bodies of rock in the Mona Complex, BARBER,A. J. & Mm, J. D. 1979.A new look atthe Mona Complex (Anglesey, N. Wales). Journal of rhe Geological Society, London, 136, such as the 595 f 12 Ma (Rb-Sr WR) lozenge of gneiss at 407-32. Holland Arms(Thorpe et al. 1984), which lies within the -, MAX, M. D. & BRUCK,P. M.1981. Field meeting in Anglesey and BerwFault Zone along the westernboundary of the Southern Ireland. Proceedings of the Geologists Association, London, 92, Penmynydd ‘terrane’, may have been tectonically trans- 269-91. DALLMEYER,R. D. & GIBBONS,W.1987. The age of ilueschist ported a great distance and should not be used to infer the metamorphisminAnglesey, North Wales:evidence from Ar/3qAr age of tectonothermal events in different bodies of rock now mineral dates of the Penmynnyddschists. Journal of the Geological nearby. Geographically isolated gneisses and schists such as Society, London, 144,000-OOO. the basic and granitic orthogneisses in central Anglesey and GIBBONS,W. 1987. The MenaiStrait Fault system: an earlyCaledonian paragneisses within theNebo inlier in northcentral terrane boundary in North Wales. Geofogy (in press). HARRISON,T. M. 1983. Some observations on the interpretation of some Anglesey (Greenly 1919) might also represent a collage of 40 Ar/”Ar age spectra. Isotope Geoscience, 1, 319-38. terranes. - & MCDOUGALL,I. 1981.Excess 40Ar in metpor hic rocks from The concept that the Mona Complex comprises a single Broken Hill, New South Wales:implications for Ar/ E Ar agespectra

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and thethermal history of theregion. Earthand Planetary Science et al. (eds) A Correlation of the Precambrian rocks of the Bririrh Isles. Letters, 55, 123-49. Special Report of the Geological Societ& London, 6,7642. RODDICK,J. C., CLIFF,R. A. & REX, D. C. 1980. The evolution of excess WIJBFANS,J. R. & MCDOUGALL,I. 1986. Ar/39Ar dating of whitemicas argonin Alpine biotites-a 40Ar-39Aranalysis. Earthand Planetary from an Alpine high-pressure metamorphic belt on Naxos (Greece): the Science Letters, 48, 113-7. resetting of the argon isotopic system. Contributiom to Minerology and SHACKLETON,R. M. 1975. The Precambrian rocks of Wales. In: Harris, A. L. Petrology, 93, 187-94.

M. D. MM, Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland. Present address: Naval Research Laboratory, Code 5110, Washington, DC 20375, USA. J. C.RODDICK, Department of Earth Sciences,University of Leeds,Leeds, UK. Present address:Precambrian Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada. R. D. DALLMEYER,Department of Geology, University of Georgia, Athens, GA 30602, USA. W. GIBBONS,Department of Geology, University College, Cardiff, P.O. Box 78, Cardiff CFl lXL, UK.

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