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

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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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/4/703/4891614/gsjgs.148.4.0703.pdf by guest on 25 September 2021 704 J. A. EVANS 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/4/703/4891614/gsjgs.148.4.0703.pdf by guest on 25 September 2021 LOW-GRADEMETAMORPHISM RESETTING Rb-Sr AGES 705 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
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