Resetting of Rb-Sr Whole-Rock Ages During Acadian Low-Grade Metamorphism in North Wales

Journal of rhe Geological Society, London, Vol. 148, 1991, pp. 703-710, 3 figs, 2 tables. Printed in Northern Ireland

Resetting of Rb-Sr whole-rock ages during Acadian low-grade metamorphism in North Wales

J. A. EVANS NERC Isotope Geosciences Laboratory, Keyworth NG12 5GG, UK

Abstract: Ordovicianvolcanic rocks andassociated sub-volcanic intrusions in NorthWales give Devonian, metamorphic Rb-Sr whole-rock ages. Isotopic resetting isnot confined to acid volcanic rocks. Three intrusions preserve Ordovician ages. The closed system resettingis not directly related to metamorphic grade, but appears to correlate with the extent to which the secondary metamorphic mineralassemblage is developed.Rocks giving Devonianages have a well developedsecondary mineralogy whereas those retaining Ordovician ages are less extensively altered. The metamorphic ages combine to give a Lower Devonian mean age and weighted 2a error of 399 f 9 Ma. Metamorph- ism is therefore synchronous with Acadian deformation in the North Wales area.

The Caradoc Series in North Wales consists of sequences of pre-tectonic,having cleaved, hornfelsed margins (Sargent varied volcanic rocks interbedded with fossiliferous sedim- 1924, Tremlett1964; Bromley 1969), and are therefore entary rocks, the latter enabling stratigraphic controls to be pre-Devonian. placedupon the volcanicepisodes. The stratigraphic Severallines of evidencesupport the intrusive rocks divisions of the Caradoc are described by Reedman et al. being of Caradocage. They have geochemical signatures (1985) who documented two major volcanic groups within indicative of formationinan extensional, within-plate theCaradoc Series; the earliest volcanic rocks are the environmentconsidered typical of thetectonic regime Llewelyn Volcanic Group which is separated from the later present during the Caradoc (Croudace 1982; Campbellet al. SnowdonVolcanic Group by thepredominantly sedimen- 1985, 1988; Ball & Merriman 1989, Leat & Thorpe 1989). tary Cwm EigiauFormation. Many small, hypabyssal, The Foe1 Fras igneous complex includes both intrusive and intrusions of intermediate to acid composition are spatially extrusivecomponents. It is demonstrably of Caradocage associated with the Caradoc volcanic rocks (Fig. 1). These because its extrusive phases are interbedded with Caradoc- intrusions arethe sub-surface expression of theCaradoc agedsedimentary rocks of theLlewelyn volcanic group. magmatism (Harker 1889;Beavon 1963; Fitch 1967; (Reedman et al. 1985). Bromley1969; Roberts 1979; Croudace 1982; Campbell et Allprevious attempts to date the Ordovician igneous al. 1985; Reedman et al. 1985). rocks in this area have used the K-Ar method (Fitch et al. Thispaper presents and discusses evidence that the 1969), and have not produced reliable results because of the Rb-Sr whole-rock isotope systems of both the intrusive and overprint effects of Acadian and Hercynian metamorphisms extrusive Ordovician volcanic rocks from North Wales were andpossibly the effect of theTertiary magmatism. K-Ar reset during early Devonian low grade metamorphism. The systemscan be partially or totallyreset, so thatages timescale of McKerrow et al. (1985) is used throughout this obtained are minima and do not necessarily date a specific paper. event.

Constraints on the age of the intrusive rocks Method All theigneous rocks described thisin study are Representativewhole-rock samples of approximately 8 kg were pre-metamorphicand their primary mineralogy is over- jaw-crushed and 100 gm aliquots were ground to a fine powder in a printed to a variable extent by a metamorphic assemblage of tungsten carbide Tema mill. After acid dissolution, strontium was albite + K-feldspar + chlorite + ilmenite f white mica f cal- separated using standard ion-exchange techniques and its isotopic cite f stilpnomelane f prehnite f pumpellyite f actinolite f compositiondetermined in a VG micromass30 automated mass spectrometer.The Eimer and Amendinternational 87Sr/ssSr biotite (Evans 1990). Metamorphic minerals dated at about standard gave0.70808 (20) on thisinstrument during 400Ma using K-Ar (Fitch et al. 1969) give a minimum age f0.00008 analysisof samples from theAber Drosgl microdiorite, Tan y for the assemblage.Roberts (1981) and Roberts & Grisiaumicrogranite and Penmaenmawrmicrodiorite, and Merriman (1985) believe that low grade metamorphism was 0.70807 f O.OOO10 (2a)during analysis of all the other samples. synchronous with regional deformation. Bevins & Robinson Rb/Sr ratios for all samples, except those from the Nanhoron (1988) supporta diastathermal model of metamorphism. granophyre and the Mynydd Mawr microgranite, were determined Thediastathermal model invokes enhanced thermal flux, by XRF (Pankhurst & O’Nions 1973) using an automated Philips during extension of the Welsh basin, to generate low-grade 1450 spectrometer, and the quoted ratios arethe average of metamorphic assemblages before regional deformation and analyses of both sides of a powder pellet. Rb/Sr ratios from these cleavage development. Regional cleavage formed during the other two intrusions were determined by isotope dilution because of climax of LowerDevonian, Acadian deformation (Wood- their low Sr concentrations. cock et al. 1988).Intrusive rocks in Snowdonia and the TheYork-Williamson least-squares regression was performed Nanhoronand Llanbedrog intrusions in Llyn are clearly using uncertainties of *OS% (l-sigma) on the Rb/Sr ratios and

703

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Silurian a1 I Ordovician

Rhyoliteand tuff

acid and intermediate B intrusions

f Fault

Fig. 1. Geological sketch map of North Wales showing sample localities: (a) Llyn Anafon andesite; (b) Cwm Idwal rhyolite; (c) Moel y Penmaen andesite; (d) Boduan andesite; (e) Penmaenmawr microdiorite; (f) Aber Drosgl microdiorite; (g) Bera Bach microdiorite; (h) Mynydd Perfedd microgranodiorite; (i) Ogwen microgranite; (j) Mynydd Mawr microgranite; (k) Tan y Grisiau microgranite; (1) Bwlch Mawr microgranodiorite; (m) Gurn Ddu microgranodiorite; (n) Caergribin felsite; (0)Inner Garnfor microgranite; (p) Outer Garnfor microgranite; (9) Penrhyn Bodeilas microgranodiorite; (r) Garn Boduan microgranodiorite; (S) Garn Fadryn microdiorite; (t) Nanhoron granophyre; (U) Llanbedrog granophyre. Grid line intervalsare 10 km.

f0.1% (l-sigma) on the R7Sr/86Srratio; the decayconstant used is Table 1. Rb-Sr abundance and isotopicdata I= 1.42 X 10-"a-' (Steiger & Jager1977). Data andNational GridReferences are presentedTablein and regression1 diagrams Sample Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/ffiSr are given in Fig. 2a-v. (a) Llyn Anafon andesite (SH 667 636.) JE 69- 83.80.844 288 0.71235 Summary of regression results JE 70 72.20.875 239 0.71230 71 121 230 1.525 230 121 JE 71 0.71662 The regressionresults aresummarized in Table 1.1862. A264 108 JE 72 0.71450 detaileddescription of eachregression is givenin Evans 0.483 470 79.0 JE 73 0.71008 (1990). The data has a bimodal distribution. A few suites (b) Cwrn Idwal rhyolite (SH 643 588) give Ordovicianages but the majority of results3.855 give 89.3 118 JE 2 0.72859 Devonian ages with a wide range of initial 87Sr/R6Srratios.7.183 69.0 170 JE 3 0.74753 4 114 55.0 6.020 55.0 114 JE 4 0.74031 Discussion of regression data 7.131JE 555.2 139 0.74772 6 121 49.3 7.168 49.3 121 JE 6 0.74679 7 83.3 55.7 4.334 55.7 83.3 JE 7 0.73091 Volcanic rocks JE4.073 8 59.9 84.3 0.72978 All the volcanicrocks dated here are stratigraphically 9.304 56.2 179 JE 10 0.75928 constrained to the Caradoc and are therefore between 454 (c) Moel y Penmaen andesife (SH 336 386) L L 145 41.8 521 0.230 521 41.8 145 LL 0.70748 and 442 Maold. The Llyn Anafon andesite gives an L L 146 14.8 531 0.081 531 14.8 146 LL 0.70670 isochronage of 443 f 14Ma (Fig.2a). This can be 0.084 537 15.3 147 LL 0.70678 considereda good estimate of the age of the0.079Llewelyn 528 14.3 148 LL 0.70674 Volcanic Groupin general. However, the rhyolite and 19.8LL 150 504 0.113 0.70690 welded ash flow tuff of the Braich tu du Volcanic 0.153 Formation 578 30.5 151 LL 0.70715 (Evans 1989) and the Cwm Idwal rhyolite from the Lower (d) Boduan andesite (SH 331 387) RhyoliticTuff Formation (Fig. 2b) give Devonian ages 1.452of 106 53.3LL 126 0.71470 405 f 6 Ma, 392 f 5 Ma, and 394 f 5 Ma, respectively.0.596 206 From 42.2 127 LL 0.70969 thestratigraphic constraints it is clearthat the regression 0.884 150 45.7 129 LL 0.71133 ages obtained for these volcanic suites are not the age 1.036 126 of45.1 130 LL 0.71247 extrusion but rather represent a later event that 1.406has 89.1 reset 43.2 131 LL 0.71383

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Table 1. (contd.) Table 1. (contd.)

SampleRb(ppm) Sr(ppm) X7Rb/R6Sr "Sr/*'Sr SampleRb(ppm) Sr(ppm) 87Rb/86Sr "Sr/%r

(e) Penmaenmawr quartz microdiorite (SH 7000.73248 755)3.494 57.8 69.4 24 LL 1 93.2 79.1 3.384 79.1 93.2 PM 1 0.72468 0.79013LL12.84 25 18.6 82.1 PM 2 1.802PM 131 81.7 0.71893 26 0.73100LL3.296 50.8 57.6 PM 3 PM 100 1.842 157 0.816300.7189218.78 26.3 168 27 LL PM 4 PM3.218 79.4 88.1 0.72658 153 28 LL 0.8164918.96 23.6 81.8 145 1.629PM 5 145 81.8 0.798710.7180515.73 28.4 152 29 LL 6 71.2 128 1.604 128 71.2 PM 6 0.7971415.650.71797 27.5 147 30 LL PM 7 2.173PM 122 91.6 0.783560.7210812.47 30.6 130 31 LL PM 8 2.094PM 124 90.1 0.72018 (l)Bwlch Mawr microgranodiorite (SH 423 480) PM 9 2.060PM 121 86.4 0.71996 0.71847LL1.799 80 133 82.7 PM 10 84.7 131 1.865 131 84.7 10 PM 0.712340.719360.744 191 49.1 81 LL PM 11 PM2.918 74.3 94.5 0.717760.725501.695 128 74.9 83 LL PM 12 PM3.798 74.5 97.8 0.72723 (m)Gurn Ddu microgranodiorite (SH 396 466) PM 13 PM2.617 109 99.0 0.723060.71046 0.923 294 93.7LL 70 (f) Aber Drosgl microdiorite (SH 667 701) LL 71 0.7103084.90.870 282 LL 86 100 166 1.746 166 100 86 LL 0.71805 72 LL 0.7103381.20.886 265 LL 87 131 143 2.646 143 131 87 LL 0.713760.723051.401 185 90.0 73 LL LL1.661 88 168 96.8 0.710920.717170.971 280 94.9 74 LL LL 89 LL1.802 158 98.6 0.71789 LL 75 0.7115888.51.084 236 91.8 243 1.093LL 90 243 91.8 0.714200.71141 1.076 255 94.7LL 76 L L 91 1.367LL 185 87.6 0.715490.71198 1.163 230 92.4LL 78 LL 92 94.1 173 1.576 173 94.1 92 LL 0.71675 (n) Caergribin felsite (SH 367 440) LL 93 95.3 162 95.3 93 LL 1.70i 0.803530.7174316.11 19.0 105 192 LL L L 94 102 166 102 94 LL 1.783 0.71813 271 LL 104 0.8034016.11 18.9 LL 95 105 135 2.238 135 105 95 LL 0.822770.7202319.48 17.7 108 273 LL (g) Bera Bach quartz microdiorite (SH 672 678) 102 274 LL 0.8149817.84 15.3 58 91.6JE 58 236 0.712651.124 0.8106517.25 18.2 107 313 LL 59 100 235 1.234 0.713231.234 235 100 JE 59 LL 314 106 0.7994615.19 20.5 104 237 1.271 0.71354 1.271 JE60237 104 106LL 315 0.8030915.85 19.6 61 96.8 232 1.209 0.712981.209 232 96.8JE 61 0.7995615.20 20.2 105 316 LL 62 92.2 228 1.172 0.712811.172 228 92.2JE 62 0.8006515.48 20.0 105LL 317 63 95.7JE 63 0.712971.186 234 15.36 20.8 109LL 319 0.8oooO 64 99.3 261 1.098 0.712171.098 261 99.3 JE 64 (0)Inner Garnfor microgranodiorite (SH 360 460, and SH 65 96.4 230 1.214 0.713101.214 230 96.4JE 65 354 455) 66 94.3 267 1.025 0.711961.025 267 94.3 JE 66 LL 1 361 68.4 0.550 0.70824 (h)Mynydd Perfedd microgranodiorite (SH 626 624)376 66.0 2 LL 0.508 0.70796 35 79.5 185 1.243 0.714571.243 185 79.5JE 35 0.708090.533 366 67.5 3 LL 36 97.0 213 1.319 0.715301.319 213 97.0JE 36 0.708140.539 366 68.2 4 LL 37 79.8 213 1.087 0.713911.087 213 79.8 JE 37 0.708320.553 362 69.1 7 LL 38 79.3JE 38 209 0.713871.099 0.708180.536 370 68.9 251 LL 39 108.0JE 39 185 0.717301.687 259 0.70810 LL0.533 371 68.5 40 69.9 156 1.299 0.716391.299 156 69.9 JE 40 0.708110.533 370 68.2 282 LL 41 58.9 192 0.890 0.713280.890 192 58.9 JE 41 (p)Outer Garnfor microgranodiorite (SH 360 460 and SH (i) Ogwen microgranite (SH 654 608) 354 455) 11 205 49.3 12.15 0.7814112.15 49.3 205 JE 11 0.713581.276LL 5 197 89.7 12 212 JE 12 0.8732312.61 49.1 0.713281.226 207 87.8 6 LL JE 13 2120.77512 11.18 55.3 0.712911.223 208 88.1 9 LL 14 221 JE 14 0.8059416.44 39.3 0.714181.373 194 92.4 10 LL 15 218 45.9 13.88 0.7912013.88 45.9 218 JE 15 11 LL 0.7132685.81.262 197 16 207 59.0 10.23 0.7695410.23 59.0 207 JE 16 0.713761.406 186 91.1 177 LL JE 17 2050.77443 11.09 53.8 0.713391.339 192 89.1 178 LL JE 18 2210.80910 17.10 37.8 0.714001.415 181 88.4 179 LL 19 207 52.8 11.41 0.7759911.41 52.8 207 JE 19 0.706260.048 2388 43.2 233 LL 20 186 44.7 12.15 0.7823912.15 44.7 186 JE 20 260 0.71347 LL1.330 194 89.3 (j) Mynydd Mawr microgranite (SH549 551) 0.712801.265 208 90.9 261 LL LL 32 240 10.2 71.30 1.1463971.30 10.2 240 32 LL (4) Penrhyn Bodeilm microgranodiorite (SH 319 421) LL 33 183 23.6 22.72 0.8465122.72 23.6 183 33 LL 0.708480.590 289 58.9 64 LL LL 34 231 18.4 37.39 0.9383937.39 18.4 231 34 LL 0.708400.548 311 58.7 65 LL LL 35 216 20.1 31.72 0.9007431.72 20.1 216 35 LL 0.708780.652 279 62.7 66 LL LL 36 226 4.29 168.2 1.79078168.2 4.29 226 36 LL 0.708840.629 282 61.2 67 LL (k) Tan y Grisiau microgranite (SH 694 453, 0.70844SH 0.590675 453, 296 60.2 68 LL and SH 690 442) (r) Garn Boduan microgranodiorite (SH 303 394) L L 22 0.73923LL4.651 41.6 66.9 137 LL 58.50.70927 0.759 223 L L 23 52.4 31.5 4.839 0.740964.839 31.5 52.4 23 LL 138 LL 59.5 232 0.709190.743

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Table 1. (conrd.) Intrusive rocks The Bera Bach microdiorite, the Mynydd Mawr microgran- SampleRb(ppm) Sr(ppm) R7Rb/X6Sr "sI/'~s~ iteand Gurn Ddu microgranodiorite (Fig. 2g, j, and m) LL 139 60.6 221 0.779 221 60.6 139 LL 0.70950 give ages that agree with the generally accepted view that LL 140/1 60.3 223 1.050 223 60.3140/1 LL 0.71095 therocks are of Ordovicianage. These rocks have fairly LL 140/2 59.0 162 0.745 162 59.0140/2 LL 0.70919 consistent initial ratios within the range 0.7045f 0.0010 (2a) LL 141 60.3 234 0.794 234 60.3 141 LL 0.70950 which is taken as representing the Sr-isotopic compositionof (S) Garn Fadryn microdiorite (SH 280 352) the magma at the time of intrusion. Inner Garnfor (Fig. 20) LL0.884 101 278 84.7 0.71164 is included in this group because of its low initial s7Sr/s6Sr LL 102 ,330 LL 1 19790.6 0.71415 ratio,although itsregression age is toopoor tobe LL 103 101 272 1.079 272 101 103 LL 0.71270 diagnostic. LL 104 LL 85.00.864 285 0.71135 Themajority of theintrusions, however, yield results LL 105 LL0.887 191 58.8 0.71257 which are similar to the volcanic rocks insofar as they give LL 106 80.7 322 0.726 322 80.7 106 LL 0.71049 Devonianages with variableerrors, and display a wide 0.71316 LL 107 LL 63.30.892 205 range of initial ratios that are all above 0.7045 (Table 2). 0.71320 LL 157 77.3 184 1.217 184 77.3 157 LL Three of these intrusions have whole-rock Rb-Sr ages that LL 158 LL1.197 179 74.3 0.71293 aredemonstrably reset. The Rb-Sr whole-rock systems of LL 159 96.3 400 0.697 400 96.3 159 LL 0.71035 the Tan y Grisiau intrusion (Fig. 2k) give a Devonian age of LL 160 79.81 0.71 325 0.71040 LL 161 88.0 385 0.663 385 88.0 161 LL 0.71038 384 f 10 Ma, but are reset as this intrusionis stratigrapically LL 162 LL 88.5 267 0.932 0.71156 constrainedto the Caradoc (Campbell et al. 1985). The (1) Nanhoron granophyre (SH 288 330) Llanbedrog granophyre (Fig. 2u) gives a regression age of 203 20.7 62.33NH 1 20.7 203 1.08389 403 f 3 Ma, although field evidence shows that the intrusion NH 2 172 12.0 42.40 12.0 172 2 NH 0.96924 caused soft sedimentdeformation of Caradocsediments LL 37 187 37 LL 42.04 13.2 0.97286 while it was emplaced (Fitch 1967). That the age is reset is LL 38 152 10.1 44.75 10.1 152 38 LL 0.99104 furthersupported by the factthat theLlanbedrog LL 39 155 8.10 57.63 8.10 155 39 LL 1.06399 whole-rockRb-Sr regression gives thesame age as its LL 42 149 42 LL 48.15 9.28 1.00379 chloritewhole-rock age (Fig. 2v). Because thechlorite in LL 43 205 43 LL 41.28 14.2 0.96785 the rock is metamorphic, both ages are therefore interpreted 44 192 15.8 35.92 15.8 192 LL 44 0.93219 as dating the metamorphic event, not the intrusion of the 113 7.81 43.24 LL457.81 113 0.97163 granophyre. Finally, the Bwlch y Cwyion microgranite gives LL 46 173 46 LL 42.53 12.0 0.97437 a Devonian age whereas its hornfels retains the Ordovician LL 47 161 8.66 55.76 8.66 161 47 LL 1.04676 age of emplacement (Evans 1989). (U) Llanbedrog granophyre (SH 330 304) All the intrusive suites, with the exception of the Gurn LL 54 148 36.7 11.79 36.7 148 54 LL 0.78056 D&,Bera Bach, and Mynydd Mawr intrusions give LL 55 157 55 LL 12.60 36.3 0.78475 Devonianages, in agreement with thedemonstrably reset LL 56 160 43.7 10.69 43.7 160 56 LL 0.77400 volcanicages described above and the metamorphic K-Ar LL 57 162 57 LL 14.58 32.5 0.79779 ages of Fitch et al. (1969). The event which reset the Rb-Sr LL 58 162 58 LL 16.9 28.17 0.87657 isotope systems of the volcanics also disturbed and reset the LL 59 161 59 LL 18.68 25.2 0.82050 whole-rocksystems of themajority of microcrystalline 157 14.9 31.14 LL6014.9 157 0.89107 160 15.9 29.61 LL6115.9 160 0.88319 intrusions. The majority of Ordovician acid igneous rocks in LL 62 161 17.3 27.43 17.3 161 62 LL 0.87143 NorthWales had their Sr-isotope systems disturbed and LL 63 163 19.5 23.69 19.5 163 63 LL 0.84858 throughly reset during early Devonian times. LL 132 166 22.4 166 132 LL 11.73 0.78064 LL 133 149 36.6 21.80 36.6 149 133 LL 0.83659 LL 134 163 28.4 11.91 28.4 163 134 LL 0.78136 Application of resetting models to the North Wales LL 135 158 135 LL 16.79 19.6 0.80826 intrusions LL 133 LL 5.458 2.77 5.21 0.74256 Resettinganisochron, that is, causingtwo-stage a development of the Sr isotopes so that the age obtained is notthe age of primaryformation of therock, can be theRb-Sr whole-rock systems. The Moel y Penmaenand considered according to two models; open system and closed Boduan andesites give only imprecise ages, (Fig. 2c and d) system. The open systems involves loss or gain of strontium buteven so, do notfall within error of the Caradoc. It is and/or rubidium. The closed system requires that theSr and clearthat these andesites have also had their Rb-Sr Rb in agiven volume is completelyhomogenized without whole-rocksystems reset. The lack of agreementbetween the loss or gain of any components. In a closed system, the stratigraphicages and Rb-Sr whole-rock ages in rhyolites isotopic clock is reset to the age of the disturbance with a has previously been observed, and discussed particularly in new initial R7Sr/R6Srratio, which is directlyrelated tothe referenceto the use of acidvolcanic rocks in time-scale R7Rb/'6Srratio of the systemand thetime difference calibration (Gale et al. 1979, 1980; Gale & Beckinsale 1983; betweenformation and disturbance. In such a system the McKerrow et al. 1980).In this study it is notonly the regression age will be the time of disturbance, which will be isotopesystems of rhyolites and ignimbrite that are reset, youngerthan theexpected intrusive age, and the initial but also those of the andesites. This shows that resetting is 87Sr/86Srratio will be higher by an amount directly related to not caused by anything inherent in the rhyolite texture such the average "Rb/86Sr of the whole system. The majority of as devitrification of original glass. the regression results giving Devonian ages display features

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typical of closed system resetting. The more geochemically nous and therefore do not support a diastathermal model for evolvedrocks generally have thehigher initial87Sr/86Sr metamorphism,in which cleavage post-dates the develop- ratios,and all buttwo of thesuites generate geologically ment of the metamorphic assemblages. realisticinitial R7Sr/s6Sr ratioat 445 f 5 Ma(Table 2), showing there has been no significant loss of radiogenic Sr Metamorphic grade during resetting. The majority of theserecalculated initial K7Sr/86Srratios fall between 0.704 and 0.7076 and are similar Resetting occurred during Lower Devonian metamorphism, tothe initial87Sr/R6Sr ratios of suitesthat have not been but it is not directly related to metamorphic grade because disturbed nor reset during metamorphism (Table 2). Loss of the intrusions giving Ordovician ages are not in the areas of radiogenic strontium during metamorphism may explain the lowest metamorphic grade. The metamorphic grade across lowinitial ratios of theTan y Grisiau microgranite and Snowdonia and Lljm varies from below prehnite-actinolite Nanhoron granophyre. facies to lowestgreenschist facies (Roberts 1981). The Mynydd Mawr and the Bera Bach intrusions are locatedin a The effect of low grade metamorphism on the part of Snowdoniawhere themetamorphic grade is igenous rocks relatively high, yet they retain Ordovician ages. By contrast the Rb-Sr whole-rock systems of the Llanbedrog granophyre Timing of metamorphism are reset despite the intrusions being located in an area of prehnite-pumpellyite facies metamorphism. Themetamorphic ages give a near normal distribution of ages with a mean and weighted 2a error of 339 f 9 Ma (Fig, 3). This is within Lower Devonian times according to the The role of metamorphic water timescale of McKerrow et al. 1985. The asymmetry of the Low grademetamorphism of igneousrocks involves the curveshows that the peak of isotopicclosure was during hydration of high temperature silicates (Roberts1981; earlyLower Devonian times. This age for low grade Bevins & Rowbotham1983; Merriman et al. 1986;Bevins metamorphism in North Wales coincides with the climax of & Merriman1988). The amount of water with which the Acadian deformation in Wales (Woodcock et al. 1988) and rockshave reacted determines the extent to whichthe regional cleavage formation in the Lake District (Soper & metamorphicassemblage is developed.All of the igneous Kneller1990). Isotopic datafrom North Wales show that rockscharacterized by Devonianages show a well- metamorphismand deformation were essentially synchro- developed,secondary, low grademineral assemblage of

Table 2. Regression results tabulated in order of age

Age 87Sr/86Sri MSWD R7Sr/%3(,5)

Suites giving Devonian ages with variable errors PenrhynBodeilas granodiorite 325 f 127 0.7058 f 11 0.70472.1 f 1 Moel y Penmaenandesite 356 f 75 0.7063 f 1 0.70620.2 f 7 TanGrisiau ymicrogranite 384 f 10 0.7135 f 9 17.50.7011 f 10 Penmaenmawr qz microdiorite386 f 24 0.7090 f 7 0.70697.8 f 20 B oduan andesiteBoduan 387 f 59 0.7065 f 9 15.20.7067 f 1 Aber Drosglmicrodiorite 391 f 33 0.7080 f 8 11.00.7068 f 2 B raich tu du tuff* 392tuff* du tuBraich * 5 0.7093 f 3 0.70611.7 f 3 Garn Fadrynmicrodiorite 392 f 32 0.7065 f 4 0.70694.8 f 1 Bwlch y Cwyionmicrogranite* 392 f 11 0.7112 f 13 0.70387.6 f 7 C wm Idwal rhyolite 394rhyoliteIdwal Cwm f 5 0.7068 f 4 0.70232.2 f 5 C aergribin felsite 395 felsiteCaergribin f 17 0.7137 f 40 0.70201.6 f 13 Garn Boduanmicrodiorite 400 f 41 0.7050 f 5 0.5 0.7043 f 1 Outer Garnfor microgranodiorite 400 f 27 0.7060 f 5 10.60.7089 f 3 Llanbedroggranophyre403 f 3 0.7129 f 7 0.70552.6 f 1 Llanbedrog whole-rock:chlorite 404f6 0.7111 f6 0.0 BwlchMawr microgranodiorite 405 f 13 0.7081 f 3 0.70700.8 f 2 Braich tu du rhyolite* 405 f 6 0.7089 f 3 0.70201.9 f 14 Mynydd Perfeddmicrogranodiorite 407 f 24 0.7075 f 4 0.70672.6 f 1 Nanhorongranophyre 409 f 23 0.7246 f 146 0.69757.6 f 34 O gw en micrograniteOgwen 410 f 16 0.7099 f 28 0.70383.2 f 10 Suites giving Ordovician ages with variable errors MynyddMawrmicrogranite 438 f 4 0.7040 f 2 1 2.1 Llyn andesiteAnafon443 f 14 0.7070 f 2 2.6 B wlch y CwyionBwlch yhornfels* 454 f 20 0.7090 f 3 1.5 B era Bach za-microdioriteBachBera 456 f 52 0.7052 f 9 1.6 Gurn Ddu microgranodiorite 456 f 24 0.7045 f 4 1.8 Inner Garnfor microgranodiorite 507 f 276 0.7043 f 21 0.2

The uncertaintieson the 87Sr/86Sr ratios are quoted to the fourthdecimal place. The K7Sr/86Sr(,,) ratios are calculated using the median values of each suite. * From Evans 1989.

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0.770 -87~rP~r 0.708 87~r86~r 0.7W -- a) Llyn Anafon andesle b) Cmldwal rhyolite 0.715 0.750 .- -- t 0.740 0.710 0.730 .- Age = 443 i 14 ( 2 0) I Ma lnltlal rabo = 0 7068 t 0 0004 lnltlal ram = 0 7070 0.0002 0'707 356 f 75 Ma (2a) f MSWD = 2 2 0.705 MSWD 2 6 L.lnlbal rabo = 7063 0 0001 - 1 0 t MSWD I 0.2 "RbP6Sr 87Rb86Sr 87Rb86Sr 0.700 0.7W'~"~~"''~0.706 0.0 0.40.2 0.6 1.60.81.4 1.2 1.0 0123456789100.2 0 0.1 0.3

0.716 T 87Sr86Sr

Age = 391 f 33 Ma (2a) lnlllal ab0 = 0.7080 f 0 WOB 0.710 Agelnltla~rabo=~~~ = 387 f 59 Ma (2a) Age = 386 f 24 Ma :(20) ~ , : , InW rat10 = 0 7090 0.0007 0.710 MSWD - 11.o enhancederrors v:MSWD = 15 2 enhanced errors MSWD = 7 8 enhanced errors 0.708 0.710 0.715v- 87Rb86Sr 87Rb86Sr "Rb86Sr 0.706 0.705 0.00.4 0.2 0.61.6 1.40.8 1.2 1.0 2 1.5 0 1 0.5 4 2.5 3.5 3 0 1 2 3

0.850 - 87Sr86Sr 0.720 - 87Sr86Sr 0.716 -87srhr i) Ogwen .- h) MynyddPerfedd 0.714 g) braBach microgranite 0.800'- microdiorle -- 0.712 .' 0.715

0.710 .. Age = 407 t 24 Ma (2a) ~ge= 456 f 52 Ma (2a) Initial rals = 0 7075 f 0 WO4 lnltlal ratlo = 0 70% f 0 0028 lnltial rata - 0.7052 f 0 ooo9 MSWDMSWD F 2.6 errors= 3 2 enhanced WSWD - 1.6 87Rb86Sr 87Rb86Sr "Rb86Sr 4 0.704 . 1 0.705 ' 0.700' 0.0 0.60.2 0.4 1.4 1.20.8 1.0 0 0.5 2 1 1.5 0 5 10 15 20

0.720 87Sr~Sr 1 go-^ 87srPsr OB=- 87srPsr k) Tan y Grisiau I) Bwlch Mawr 1.70.-microgranite i) MynW Mawr microgranite -- microgranodiorle 1 so'- 0.800.- 0.715

l .30- Age=405+13Ma(2~) Age = 438 f 4 Ma (20) lnlbal lab0 = 0 7Mll f 0 0003 lnltlal rabo = 0 7Mo i 0.0021 Age = 384 f10 Ma (2a) MSWD = 0 B MSWD=21 ratto lnltlal = 0 7135 t 0 OOW MSWD = 17 5 enhanced ems "Rb8'Sr 87Rb86Sr "Rb86Sr , 1 0.7W' 1 0.705 2 0 50 100 150 200 0 15 5 10 200 0.5 1 1.5

albite + K-feldspar + chlorite + ilmenite f white mica f cal- dueto its emplacementinto Cambrian rocks whichwere cite f stilpnomelane f prehnite f pumpellyite f actinolite f previouslydehydrated during the Tremadoc (Roberts & biotite (Evans 1990). In contrast, those intrusions that give Merriman 1985) and were, therefore, a poor source of fluid. Ordovicianisotopic ages show limited secondary hydrous Bera Bach is a porphyritic microdiorite. It has phenocrysts mineralogyand retain much of theirprimary mineralogy. of clinopyroxene, orthopyroxene and plagioclase present in The Mynydd Mawr microgranite is extremely fine-grained, anoriginally glassy matrix.Alteration is dominated by its saccharoidaltexture of alkalifeldspar, quartz, and growth of extremely fine-grained white mica in the matrix. opaque minerals having an average grain size of <0.1 mm. Thephenocrysts are moderately well preservedalthough Quartz and K-feldspar phenocrysts between 1-2 mm length there is somewhite mica and carbonate replacement of are sparselydistributed throughout thegranite, and plagioclase and replacement of mafic minerals with chlorite. riebeckite occurs as dark-blue to yellow pleochroic net-like Thepreservation of pyroxene,however, demonstrates the patches.Alteration is limited;phenocrysts show little incompletenature of theoverprint. The Gurn Ddu evidence of recrystallization, but white mica and calcite are microgranodioritehas large phenocrysts of plagioclase disseminatedthroughout the groundmass. The limited (<4 mm),clinopyroxene (<2 mm)and rare hornblende secondary hydrous mineralogy in this intrusion is probably along with accessoryapatite and ilmenite set in a

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0.716- 87~~86~~ 0.709 - '?SrPsr 0.900 - 87srPsr 0.714.' m) Gurn Ddu 0.708 -- 0) InnerGarnfor -- n) Caergribin felsite o,712~~ microgranodiorite 0.850 rnicrogranodiorite 0.707 -- 0.710'~ .- 0.m .- Age = 456 f 24 (20) 0.706 lnltlal ratlo = 0 7045 f 0 ooo4 lnlbal rab = 0 704.3 i 0.0021 MSWD= l B MSWD=16 MSWD=O2

87Rb86Sr "Rb86Sr "Rb86Sr l 0.704' ~ I ~ ~ I ~ ' 0.700 , 0.704' 0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 5 15 10 20 25

o,71 '3SrPsr

0.716 87srp6sSr o.709.. q) Penrhyn Bodeilas r) Garn Boduan microgranodiorite 0.714 .. p) Outer Garnfor 0.708'~ 0.712 -- micmgranodiorite

0.710 -- 0.707- Age=325*127&(2a) Age = 400 i 41 Ma (20) lnlhd raPo = 0 7058 f 0 W1 1 lnlhal ram - 0 7050 f 0 0005 lnlarab = 0.7060 t o mm MSWD = 2 1 MSWD=O5 MSWD = 10.6 enhancad MW

"Rb86Sr "Rb86Sr ' 0.705' I L I I ~ ~ -I 0.704 I 0.0 0.2 0.4 0.6 0.8 1.0 12 1.4 1.6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.2 0.4 0.6 1.0 0.8 1.2

0.716 ''S~PS~ .20- a'SrPSr

0.714.- S) Garn Fadryn 1.10.- 1) Nanhoron microdiorite granophyre U) Llanbedmg granophyre 0.712- 1.oo-~

0.710- Age = 409 i 23 Ma (20) Age + 392 32 Ma (20) t lnlbal raho = 0 7246 f 0 0146 lnlual ralbo = 0 7065 i 0 WO4 lnltld ratlo 0 71x3 + 0 0007 MSWD = 7.6 enhandemrs 0.7 - MSWD = 4 8 enhanced errors MSWD = 26 "Rb8'Sr "Rb86Sr "Rb8'Sr 0.706' I 0.70' I 0.7 I 0.0 0.2 0.4 0.61.41.2 1.0 0.8 0 10 20 30 40 50 60 70 0 5 10 15 20 25 30 35

0.851 v) whole-rock:chlorite /

0.75 lnltlal ratto = o 71 11 i o 0006 MSWD=OO

87Rb86Sr 0.70 l Fig. 2. York-Williamsonleast-squares 87Sr/R6Sr-K7Rb/86Srregression diagrams (tri- 0 5 10 15 20 25 angles represent samples not included in the regression calculation).

granophyricmatrix. The feldspars are replaced by albite, Conclusions white mica, prehnite and calcite. Chlorite and stilpnomelane The Rb-Sr whole-rock ages of both volcanic extrusive and partiallyreplace mafic mineralsbut in this intrusion, as intrusive rocks were reset during Acadian, Lower Devonian with BeraBach, the preservation of clinopyroxeneshows IOW grade metamorphism. Resetting of these rocks is not, thatthemetamorphic reactions have not gone to therefore,a feature confined to acid volcanics nor is it completion. It is likely therefore that water required for the related to devitrification. low gradehydration reactions did not penetrate tothe Resetting is not directly related to metamorphic grade; centres of thesebodies, from which the samples were rather it appears to be related to the extent of development collected. In the absence of water, the primary mineralogy of the secondary metamorphic mineralogy which, in turn, is hasbeen better preserved, haveas theSr isotope most probably controlled by the availability of water. The systematics. most obvious source of water for the metamorphic hydration

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-, REEDMAN,A. J. & HOWELLS,M. F. 1985. Regionalvariations in cleavage and fold development in North Wales. Geological Journal, U), 43-52. CROUDACE,I. W. 1982. Thegeochemistry and petrogenesis of theLower Palaeozoic granitoids of the Lleyn peninsula, North Wales. Geochimica et Cosmochimica Acta, 46, 609-622. EVANS,J. A. 1989. Resetting of the Rb-Sr whole-rock isotope system of an Ordovicianmicrogranite during low-grade metamorphism. Geological Magazine, l26, 615-619. - 1990. Resetting of Rb-Sr whole-rock system during low grade metamorphism, North Wales. PhD thesis, London University, FITCH,F. J. 1967. Ignimbrite volcanism in N. Wales. Bulletin Volcanologique, 30, 199-219. -, MILLER,J. A., EVANS, A. L., GRASTY, R. N. H. & MENEISY,M. Y, 1%9. Isotopicdeterminations of on rocksfrom Wales and the Welsh borders. In: WOOD, A. (ed) The Pre-cambrian and LowerPalaeozoic rocks of North Wales. University of Wales, Cardiff, 23-45. GALE, N. H. & BECKINSALE,R. D. 1983. Comments on the paper ‘Fission track dating of British Ordovician and Silurian Stratotypes’ by Ross and others. Geological Magazine, 120, 295-302. Carbonrferous Devonran Sdurlan Ordowcian i --, & WADGE,A. J. 1979. An Rb-Sr whole-rock isochron from the Stockdalerhyolite of the English LakeDistrict and a revised Fig. 3. Non-parametric cumulative frequency curvesas used by mid-Palaeozoic time-scale. Journal of the Geological Sociery, London, Kelley & Bluck (1989) constructed from data given in Table 2 for l36, 235-242. --, & - 1980. Discussion of a paper by McKcrrow, Lambert and suites giving metamorphic ages. Shaded area represents 399 f 9 Ma Chamberlain on the Ordovician, Silurian and Devonian timescales. Earth which is the mean age and weighted error of the data. 2a and Planetary Science Letters, 51, 9-17. HARKER,A. 1889. The Bala Volcanic series of Caernavonshire and associated rocks. Cambridge University Press. reactions is thesedimentary pile into which themagmas KELLEY,S. 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The Chonology of the Geological Record, Geological Society London Memoir 10, 73-80. This work comprises part of the BGS project ‘Caradoc Volcanism in MERRIMAN,R. J., BEVINS,R. E. & BALL, T. K. 1986. Petrologicaland Snowdonia’ and is part of aBirkbeck College registered London geochemical variations within the Tal y Fan intrusion;a study of element University thesis. Many thanks to all the BGS group members for mobility during low grademetamorphism with implications for theirsupport and helpful comments through the duration of this petrogenetic modelling. Journal of Petrology, 27, 1409-1463. work. My thanks go also to R. D. Beckinsale who introduced me to PANKHURST.R. I. & O’NIONS,R. K. 1973. Determination of Rb/Srand 87Sr/““Sr ratiosof some standard rocks and evaluation of X-Ray Welshgeochonology, to R. Cliff and R. Bevins for their fluorescence spectrometry in Rb-Sr geochemistry. Chemical Geology, U, constructivereviews and especially to B. Roberts for his 127-136. supervision. 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