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

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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 re- corded 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 phengite ages 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 <unmetamorphosed, Schist Zone of the Central Region” by Greenly (1919). supposedlyCambrian rocks on Anglesey, has led most (3) TheGwna Melange, which occurs in northern, workers to regard the Angleseyblueschist metamorphism to central and eastern Anglesey, and in SW Llyn (Fig. 1). This be of Precambrian age (e.g. Greenly 1919). There are no unit hasbeen interpreted as deformeda olistostrome stratigraphicconstraints maximumona age for the (Shackleton 1969). Itcontains a wide variety of clast blueschist metamorphism.For this reason,an integrated lithologiesincluding low-grade metasedimentary rocks 40Ar/39Ar dating and petrological study was undertaken to (often similar toparts of thenorthwestern complex), provide more control on the tectonothermal evolution of the quartzite,stromatolitic limestone, pillow basalt,chert, blueschists andtoevaluate their significance in the graniteand serpentinite. Blueschist clasts have notbeen Caledonian evolution of the British Isles. observed. Although Muir et al. (1979) suggest an Early Cambrian age for the melange using microfossil evidence, theirdate was critically discussed by Peat (1984), who Geological setting concluded that itis not well constrained. The Anglesey blueschists form part of a large pre-Arenig (4) A schistbelt in SE Anglesey (referredto as the basement inlierknown as the ‘MonaComplex’ or,more “Penmynydd Schist Zone of the AethwyRegion” by recently,the Monian Terrane (Gibbons & Gayer 1985; Greenly, 1919) which is dominated by elongate, lens-shaped Bevins et al. 1986). This terrane is exposed on Anglesey and masses of metabasite blueschists surrounded by quartz-mica L1@, whereit is separatedfrom the LowerPalaeozoic schists (Fig. 1). Rocks within this belt are the main subject WelshBasin to the SE by themajor, polygeneticMenai of this paper. Strait Fault System (Fig. 1). The Monian Terrane includes a Thefour tectonostratigraphic units arebounded by diverse array of rock types and tectono-stratigraphic units brittle faults andtor ductile mylonite zones. The blueschists which, in Anglesey, may be summarized as follows: in SE Anglesey are bordered on the northwest by a sub- (1) northwesternA complex (SouthStack, New vertical high strainzone (Berw Shear Zone), and on the 843 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/6/843/4888625/gsjgs.144.6.0843.pdf by guest on 01 October 2021 844 R. D.DALLMEYER & W. GIBBONS 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 were temporarily 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/6/843/4888625/gsjgs.144.6.0843.pdf by guest on 01 October 2021 AG E OF BLUESCHISTMETAMORPHISM, OF AGE ANGLESEY 845 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.
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