testing the reliability of discrimination diagrams for determining...

(2007) 103–125www.elsevier.com/locate/chemgeo

Chemical Geology 242

Testing the reliability of discrimination diagrams for determining thetectonic depositional environment of ancient sedimentary basins

Kieran M. Ryan ⁎, D. Michael Williams

Department of Earth and Ocean Sciences, National University of Ireland, Galway, Ireland

Received 22 May 2006; received in revised form 25 February 2007; accepted 5 March 2007

Editor: R.L. Rudnick

Abstract

The Lower Palaeozoic rocks of the west of Ireland Caledonides provide a novel way to test the reliability of discriminationdiagrams in sedimentary rocks. A comparison between the geochemical signature of the tuff bands and the signature from theadjacent sedimentary rocks reveals that a combination of multi-element diagrams and discrimination diagrams is best suited toidentify the tectonic environment of deposition. The tuff bands from the Ordovician and Silurian formations all carry an activecontinental margin signature. Only three of the sedimentary discrimination diagrams assigned more than 50% of the samples to thissetting. The results from the discrimination diagrams indicate an active margin setting for the Ordovician Mweelrea and RosroeFormations and the Silurian formations, whilst the Derrylea and Sheeffry sedimentary samples have a passive margin signature.Overall the results show that in the case where felsic volcanism occurred on a quartz-rich source region the active continentalmargin field on many established discrimination diagrams is not reliable in identifying this setting. However, the study highlightsone of the key strengths of discrimination diagrams in their ability to be used as a lever to reveal significant information about thesediment source, as in this case to identify sediment that has been transported from different tectonic settings and deposited in onesedimentary basin.© 2007 Elsevier B.V. All rights reserved.

Keywords: Discrimination diagrams; West of Ireland Caledonides

1. Introduction

Discrimination diagrams are useful tools for identify-ing the tectonic setting and provenance of sedimentaryrocks. They have been used for over twenty years (e.g.Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch,1986, 1988) and as the diagrams became more widelyused their validity was called into question (e.g. Floydet al., 1991; McCann, 1991). Studies have been carried

⁎ Corresponding author.E-mail address: [email protected] (K.M. Ryan).

0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2007.03.013

out to analyse the accuracy of the diagrams by comparingthe signature of sedimentary rocks and modern day sedi-ment from known tectonic settings to the signature iden-tified using the established diagrams. This study is uniquein that it uses the geochemical signature of interbeddedtuff bands to test the reliability of sedimentary discrim-ination diagrams. Tuffs coeval with their volcanic originare of course direct indicators of the type of volcanism andtherefore tectonic setting. Although it has been shownthat in active arc terranes the composition of tuffs candiffer from the bulk volcanic source (Roser et al., 2002;Roser and Coombs, 2005). Ten of the most widely used

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http://dx.doi.org/10.1016/j.chemgeo.2007.03.013

104 K.M. Ryan, D.M. Williams / Chemical Geology 242 (2007) 103–125

discrimination diagrams for sedimentary rocks were ana-lysed, these included seven diagrams which employ theuse of major elements and three diagrams which use traceelements. The diagrams tested comprise Bhatia's (1983)TiO2, Al2O3/SiO2, K2O/Na2O, and Al2O3/(CaO+Na2O)against Fe2O3+MgO bivariate diagrams, Bhatia's (1983)discriminant function diagram, Roser andKorsch's (1986)K2O/Na2O – SiO2 diagram, and Roser and Korsch's(1988) discriminant function diagram. The trace elementdiagrams comprised Bhatia and Crook's (1986) La–Th,La/Y–Sc/Cr, and Ti/Zr–La/Sc diagrams. Two otherdiagrams (Zr–Th and V–Sc) were described by Bhatiaand Crook (1986) but they did not have fields discrim-inating tectonic settings. The geochemical signature of thesedimentary rocks was also analysed using upper conti-nental crust normalising values (after Taylor and McLen-nan, 1985).

The tuff bands were analysed using multi-elementdiagrams normalised to mid-ocean ridge basalt values(after Pearce, 1983). The tectonic setting was confirmedusing the Rb–Y+Nbdiscrimination diagram (after Pearceet al., 1984) and Rb–Hf–Ta ternary diagrams (after Harriset al., 1986).

From the earliest studies it was recognized that externalfactors affecting the framework and chemical compositionof sandstones must always be considered (e.g. Middleton,1960). The complexity of sediment transport is one of themain problems when dealing with provenance studies ofsedimentary rocks. Grains can be transported from onetectonic environment setting and deposited in a basinassociated with a completely different tectonic setting(McLennan et al., 1990). Cullers et al. (1988) carried outtrace element geochemistry provenance studies onmodernstream sands derived from igneous and metamorphicsource rocks in order to investigate the extent to whichdaughter sands retained their parental geochemicalsignature.

Recent studies into the behaviour of metals duringcompaction in modern sediments have also highlightedpossible further questions over the validity of some of thediscrimination diagrams. Whiteley and Pearce (2003) re-ported differential behaviour of the heavy metals iron,manganese and copper during burial of contaminatedsediments in Dulas Bay, North Wales. The behaviour ofthe metals during diagenesis is described as being theresult of a complex response to a large number of vari-ables such as porewater concentration changes and smallphysiochemical changes.

Other studies found that the early discrimination dia-grams using major elements could not be universallyapplied. Winchester and Max (1989) examined a lateProterozoic clastic succession in North West Ireland and

found that Bhatia's (1983) discrimination diagrams anddiscriminant function diagram assigned the samples to theactive continental margin setting which contradicted fieldevidence suggesting deposition of the sediments in anintracratonic basin. Multi-element plots of the trace ele-ments were also compared to multi-element profiles forknown tectonic settings. The trace elements, which areusually lessmobile than themajor elements, supported thefield evidence for a passivemargin intracratonic basin andso were more reliable than the major element diagrams.

In 1991 a number of papers in a special publicationentitled “Developments in Sedimentary ProvenanceStudies” (Morton et al., 1991) highlighted studies whichrevealed problems with the discrimination diagrams.McCann (1991) showed that there may be a number ofproblems with Bhatia's (1983) model which were relatedto the small data set used to define the fields, the use of Rb,Sr and Ba whose distribution was shown to be veryvariable, and also the exclusion of the complexity ofgeochemical history of sediments in creating the model.

Floyd et al. (1991) highlighted the complex history thatsediments undergo in which simple binary plots can beaffected by sorting, heavy mineral content and proportionof mafic input. These authors also found that although themodels of Bhatia and Crook (1986) are good discrimina-tors they suffer in that they do not take into account thepossibility that sedimentary sequences can include sedi-ment derived from a number of geologically unrelatedtectonic settings. Thus it appears that the general trend isthat a universal approach or method is not applicable forsedimentary rocks and regions must be interpreted on acase by case basis (Molinaroli et al., 1991). More recentstudies to test the discrimination diagrams have usedmodern day sediments from known tectonic settings(Whitmore et al., 2004; Armstrong-Altrin and Verma,2005). They have found that the discrimination diagramsare particularly poor at identifying the correct tectonicsetting.

Despite these difficulties and the advice that all inter-pretations of original tectonic setting should be cautious(e.g. Roser and Korsch, 1988; McLennan et al., 1990;McCann, 1991; Floyd et al., 1991; Milodowski andZalasiewicz, 1991) the early discrimination diagrams(Bhatia, 1983; Roser and Korsch, 1988) for sedimentaryrocks are still widely used in provenance studies.

2. Geological setting

The geology of South Mayo comprises an Ordovicianvolcanic and sedimentary succession approximately 10kmthick and Silurian terranes which occur to the north andsouth of the Ordovician succession. The Ordovician

105K.M. Ryan, D.M. Williams / Chemical Geology 242 (2007) 103–125

stratigraphy for the region was established by Dewey(1963) and a detailed description of the Silurian andOrdovician Formations was presented by Graham et al.(1989). The Ordovician and Silurian rocks of the westernIrish Caledonides have been studied for over a centuryand record the deposition of sediments and tuff bands inbasins along the Laurentian margin during the closure ofthe Iapetus Ocean. The formations in the Ordoviciansuccession are distributed along the north and south limbsof the Mweelrea–Partry syncline (Fig. 1), in a part ofCounty Mayo that is generally referred to as the Murriskpeninsula. Four of the Ordovician Formations have beenused in this study: the Sheeffry, Derrylea, Rosroe, andMweelrea Formations. The Sheeffry and Derrylea For-mations are part of the conformable succession on the

Fig. 1. Geological map

north limb of the synclinewith theMweelrea Formation atthe top of the succession. The Rosroe is a fault boundedformation on the southern limb and is the youngest ele-ment of the Ordovician South Mayo stratigraphy (Ryan,2005). TheMweelrea Formation consists of red sandstoneand conglomerates interbedded with purple fine-grainedrhyolitic tuff bands. These sediments were deposited dur-ing the rapid erosion of a volcanic provenance founded onquartzose Dalradian continental crust (Williams, 1984;Pudsey, 1984). The Rosroe Formation comprises greencoarse-grained sandstones and conglomerates with inter-bedded massive tuff bands. The Formation is interpretedas being deposited by turbidity currents (Archer, 1977)based on the thick massive and graded coarse-grainedbeds and the lack of shallowwater features. The dominant

of South Mayo.

Table 1Major and trace element concentrations for the Ordovician and Silurian tuff bands

Sample Formation SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Total Sc V⁎ Cr Co

Ordovician samplesDTBA1 Derrylea 57.58 0.64 19.29 6.30 0.08 2.59 0.98 7.01 3.43 0.16 98.1 22.5 121.5 25.6 12.9DTBA2 Derrylea 65.77 0.31 15.34 2.47 0.05 1.35 0.99 6.73 0.74 0.14 93.9 9.7 24.7 21.0 11.4DTBA3 Derrylea 61.92 0.64 19.01 5.75 0.09 1.84 2.16 7.91 1.52 0.16 101.0 23.5 136.4 21.1 12.0DTBB1 Derrylea 55.72 0.81 18.90 8.42 0.11 2.59 1.46 7.12 2.33 0.17 97.6 28.9 183.3 25.8 15.8DTBC1 Derrylea 57.17 0.79 17.82 7.64 0.12 2.42 2.55 6.03 2.67 0.18 97.4 27.3 179.0 28.6 17.7DTBD1 Derrylea 61.18 0.63 16.19 4.85 0.09 1.82 0.76 8.11 1.36 0.18 95.2 21.0 102.0 21.0 10.4MATB03 Mweelrea 72.05 0.38 15.42 2.79 0.05 0.76 1.22 5.19 2.58 0.13 100.6 9.7 43.0 16.0 5.9MWTB5 Mweelrea 78.84 0.30 12.46 1.77 0.04 0.34 0.79 5.02 3.19 0.09 102.9 8.7 17.6 8.0 4.3MWTBA1 Mweelrea 63.90 0.47 16.91 3.65 0.10 1.86 0.63 6.35 1.66 0.13 95.7 14.9 59.0 10.5 9.0MWTBB2 Mweelrea 62.54 0.57 17.49 4.67 0.14 2.68 0.86 6.63 2.38 0.19 98.1 16.5 83.8 23.6 12.9MWTBC1 Mweelrea 63.82 0.57 17.18 4.63 0.08 2.01 0.84 7.04 3.13 0.21 99.5 16.7 75.0 13.7 11.7MWTBC1b Mweelrea 72.18 0.17 13.55 1.26 0.04 0.08 0.20 5.69 2.93 0.03 96.1 6.0 b8 10.0 3.2MWTBC3 Mweelrea 75.64 0.17 14.78 1.11 0.04 0.33 0.49 4.58 3.19 0.03 100.3 4.7 b8 10.2 4.8MWTBE1 Mweelrea 74.44 0.34 14.41 2.45 0.06 1.38 0.62 5.92 1.72 0.11 101.5 10.1 37.7 21.7 5.2MWTBE2Z Mweelrea 60.77 0.55 19.85 4.44 0.10 2.43 2.03 8.06 1.11 0.17 99.5 16.8 74.4 14.4 13.0MWTBG1 Mweelrea 64.05 0.48 17.83 3.80 0.10 1.41 1.60 5.83 3.51 0.14 98.7 15.2 56.4 14.0 7.5MWTBG2 Mweelrea 66.92 0.45 16.79 3.75 0.10 1.51 0.88 6.47 2.06 0.11 99.0 14.9 49.2 12.0 9.3MWTBH1 Mweelrea 66.36 0.43 13.51 3.14 0.07 0.86 3.83 4.15 2.22 0.16 94.7 12.8 47.8 12.0 5.6MWTBi1 Mweelrea 74.92 0.16 13.85 1.11 0.05 0.06 2.30 2.86 2.65 0.05 98.5 5.1 b8 14.3 1.9MWTBJ1 Mweelrea 72.16 0.20 14.72 1.40 0.02 0.38 0.30 4.65 4.10 0.03 98.0 5.9 11.1 5.7 4.2MWTBL1 Mweelrea 64.30 0.52 15.92 3.82 0.12 1.50 2.29 5.20 1.69 0.18 95.5 14.7 77.7 11.9 9.4MWTBL2 Mweelrea 66.28 0.58 17.93 4.62 0.11 2.29 0.91 6.61 2.43 0.19 101.9 16.9 79.0 12.3 10.4MWTBM1 Mweelrea 67.80 0.48 18.00 3.81 0.10 1.63 1.38 6.19 3.27 0.14 102.8 15.1 64.3 12.2 7.9MWTBM2 Mweelrea 63.29 0.49 17.82 3.73 0.11 1.80 1.05 5.38 3.80 0.15 97.6 16.2 58.8 10.9 8.6MWTBP1 Mweelrea 67.47 0.54 14.83 4.14 0.10 2.20 0.96 5.37 3.49 0.20 99.3 16.8 77.9 20.0 11.7RTBA1a Rosroe 64.51 0.64 18.71 5.89 0.12 2.13 3.76 5.20 1.19 0.18 102.3 21.8 112.5 37.8 13.3RTBA2a Rosroe 59.8 0.67 18.16 6.18 0.14 2.38 4.57 5.81 1.10 0.16 98.2 23.6 118.5 20.0 11.1RTBB1 Rosroe 51.91 0.92 16.53 8.32 0.16 3.21 7.05 2.09 2.39 0.18 92.8 38.6 188.6 38.0 19.8RTBB2 Rosroe 50.59 0.82 16.55 7.84 0.14 3.95 3.79 2.35 3.26 0.21 89.5 36.5 182.5 40.0 18.6RTBC1 Rosroe 56.60 0.76 17.02 7.18 0.13 3.15 6.05 3.78 1.03 0.18 95.9 37.0 178.4 29.5 17.9RTBC2 Rosroe 59.73 0.77 18.06 7.09 0.12 3.48. 5.44 2.76 1.73 0.18 99.4 31.8 160.6 25.9 16.6RTBC4 Rosroe 70.08 0.49 12.42 3.52 0.11 1.49 3.57 3.40 1.81 0.13 97.0 15.1 87.6 29.0 10.2RTBD1 Rosroe 59.23 0.62 19.10 5.93 0.12 1.98 6.91 3.25 1.61 0.16 98.9 21.3 118.5 21.2 12.5S/tuff#1 Sheeffry 54.19 1.03 15.48 10.41 0.15 4.78 3.00 4.86 3.05 0.22 97.2 48.4 228.1 50.0 22.4SDTBA1 Sheeffry 70.93 0.05 14.65 0.37 0.02 0.07 0.06 5.11 6.11 0.04 97.4 2.1 b8 10.0 2.8SDTBA2 Sheeffry 69.63 0.05 16.16 0.48 0.02 0.07 0.06 4.56 6.76 0.02 97.8 1.5 b8 8.7 2.9STBA1 Sheeffry 66.92 0.36 16.06 2.95 0.05 1.41 3.29 3.46 3.91 0.14 98.6 5.8 50.0 21.4 42.3STBB1 Sheeffry 66.68 0.26 13.30 2.230 0.02 2.28 0.10 0.92 6.87 0.05 94.8 6.6 23.2 28.0 5.0STBD1 Sheeffry 67.93 0.04 16.67 0.63 0.03 0.28 0.39 1.75 2.48 0.05 90.3 1.5 b8 5.1 1.6STBE1 Sheeffry 51.17 0.70 16.95 6.41 0.09 3.29 0.26 5.03 4.25 0.16 88.3 30.2 147.8 24.0 13.9STBE2 Sheeffry 53.17 0.94 18.13 10.41 0.12 4.19 0.68 4.88 3.77 0.25 96.5 47.2 270.8 51.7 22.7STBE3 Sheeffry 58.81 0.81 18.63 6.31 0.04 3.37 0.26 5.13 5.53 0.19 99.1 30.6 167.1 38.0 14.7STBF1 Sheeffry 67.66 0.40 16.75 2.87 0.04 1.19 2.36 3.80 3.36 0.19 98.6 6.2 44.6 24.0 7.9

Silurian samplesFi1 Bouris 72.77 0.36 15.35 2.56 0.01 1.01 0.28 3.50 3.28 0.02 99.2 5.2 39.1 17.4 6.6Cl14 Strake Banded 83.67 0.26 9.37 0.84 0.01 0.93 0.08 2.98 1.83 0.02 100.0 2.9 15.0 11.3 6.3Cl1a Strake Banded 70.11 0.44 13.66 2.45 0.06 3.91 0.69 2.65 3.17 0.14 97.3 6.2 140.6 26.5 6.7Cl1c Strake Banded 77.88 0.52 9.86 2.95 0.06 2.85 0.76 2.00 2.23 0.13 99.2 7.1 57.6 17.8 9.6Cl3a Strake Banded 64.37 0.76 18.30 3.13 0.04 5.58 0.47 2.88 4.75 0.10 100.4 12.1 56.7 22.1 10.0Ci6b Strake Banded 76.11 0.23 12.87 1.16 0.03 2.35 0.25 3.13 3.02 0.08 99.2 2.8 11.0 4.4 3.5Cl6c Strake Banded 73.24 0.23 15.41 1.49 0.03 2.87 0.12 2.35 4.71 0.08 100.5 2.9 10.4 5.1 2.1Cl6d Strake Banded 76.83 0.26 12.61 1.10 0.03 1.96 0.42 3.63 2.67 0.08 99.6 3.2 12.0 5.9 2.6WTC1 Bunnamohaun 83.80 0.17 9.88 0.98 0.02 0.65 0.25 5.30 0.37 0.05 101.5 3.3 20.4 8.9 5.6

Ni Rb Sr Y Zr Nb Cs Ba La Ce Sm Yb Hf Ta Pb Th U1Total oxide weight percent, low totals reflect loss on ignition.⁎Analysed using ICPAES.


Table 1 (continued)

Ni Rb Sr Y Zr Nb Cs Ba La Ce Sm Yb Hf Ta Pb Th U

74.5 189.3 18.6 52.6 11.3 0.6 1149.0 27.7 59.1 6.3 1.7 1.7 0.6 8.9 5.2 1.314.8 29.8 874.0 18.7 98.4 14.9 0.6 216.0 41.5 83.0 6.4 2.1 3.4 1.3 6.9 14.8 3.38.5 44.9 200.5 20.1 39.3 9.5 1.2 438.3 19.8 43.4 5.6 1.8 1.4 0.5 15.8 3.6 0.98.3 69.0 267.6 20.4 42.3 9.8 1.7 666.7 21.2 47.3 6.4 1.7 1.4 0.4 7.0 2.8 0.714.5 63.3 300.2 18.1 38.1 9.5 0.9 496.4 10.4 26.3 4.1 1.7 1.3 0.4 7.3 2.1 0.65.2 35.2 237.0 13.8 34.4 8.5 0.9 994.0 22.8 49.2 5.7 1.3 1.3 0.5 6.3 4.1 0.84.2 83.7 196.0 16.9 95.4 15.2 3.3 753.0 22.1 45.2 3.9 2.0 3.3 1.3 5.6 15.2 2.61.6 65.7 67.0 24.5 95.7 18.1 0.9 522.0 55.5 112.2 9.0 2.6 3.7 1.0 12.2 19.9 4.04.6 31.1 269.9 19.0 96.4 17.2 0.7 943.8 45.7 91.3 7.7 2.2 3.1 1.0 26.3 11.9 2.712.5 42.6 318.3 25.8 90.1 16.2 0.5 973.1 66.2 136.0 9.6 2.2 2.9 1.0 20.2 12.1 2.86.3 85.0 295.8 22.1 86.9 15.7 0.3 883.9 44.0 86.2 7.3 2.0 2.7 1.0 16.1 11.5 2.51.0 54.9 148.0 24.9 79.4 15.9 0.3 785.0 47.5 106.7 8.6 2.4 3.2 1.3 10.7 15.2 2.47.5 64.7 207.0 20.3 69.5 16.3 6.5 1475.0 47.5 108.2 7.5 2.1 2.6 1.0 8.9 12.8 2.710.6 41.6 165.1 20.3 85.1 15.0 0.8 493.1 40.3 81.6 6.3 2.1 2.7 0.8 22.0 11.0 2.56.1 31.8 325.6 28.6 108.9 18.7 1.3 471.7 63.7 114.9 8.4 2.6 3.4 1.1 20.5 12.6 3.12.6 100.0 589.0 26.5 99.8 16.0 2.0 1126.0 44.7 91.9 8.0 2.5 3.5 1.2 24.3 14.3 2.82.8 42.8 265.0 23.6 54.0 9.7 0.8 713.0 40.0 81.1 7.5 2.4 1.7 0.7 17.9 12.4 2.52.4 65.1 263.0 20.8 84.9 13.1 2.2 629.0 43.4 84.3 6.9 2.0 3.0 1.1 22.1 13.7 2.69.0 82.8 150.1 27.9 68.8 16.4 1.6 772.8 48.7 95.2 7.6 2.5 2.6 1.0 13.4 13.7 2.82.8 89.6 121.5 23.1 82.2 18.0 0.9 1114.0 48.0 96.6 7.8 2.4 3.0 1.1 12.6 14.3 2.86.7 37.4 309.9 22.6 82.8 16.1 1.8 1062.0 38.1 81.1 6.9 2.1 2.7 1.0 23.6 12.2 2.76.4 47.1 222.9 14.9 92.4 16.1 1.2 1439.0 44.5 86.2 6.7 1.7 2.9 1.0 21.4 12.8 2.84.4 81.8 330.3 25.9 95.4 17.2 1.2 1004.0 44.9 90.7 7.8 2.4 3.0 1.0 57.9 11.7 2.74.8 135.3 408.1 28.0 96.1 17.5 2.3 1334.0 46.4 95.5 8.5 2.5 3.0 1.0 30.2 12.1 2.67.3 88.0 236.0 16.8 140.5 16.1 1.5 1115.0 43.4 87.7 7.3 1.8 4.4 1.3 10.2 16.2 3.319.4 39.3 1712.0 22.7 13.9 11.1 1.5 800.3 26.6 55.2 6.3 1.9 1.5 0.6 13.8 4.9 1.23.9 38.2 965.0 22.6 49.6 10.0 1.3 525.0 27.7 57.8 6.6 2.1 1.8 0.7 11.2 6.0 1.48.3 70.1 641.0 29.9 58.6 11.4 2.4 1046.0 23.6 55.0 8.3 2.8 2.2 0.7 10.3 4.5 1.08.9 97.3 381.0 27.3 64.3 10.9 3.4 1530.0 21.7 52.4 8.0 2.7 2.3 0.7 10.5 4.8 1.28.8 43.3 1244.0 31.5 55.7 11.6 2.0 1178.0 23.9 56.3 8.0 2.7 1.8 0.5 12.2 4.4 1.09.3 58.6 1101.0 29.3 78.2 14.3 2.1 627.6 30.4 67.5 8.1 2.6 2.5 0.7 22.2 7.0 1.77.4 64.0 598.0 17.0 47.1 8.2 2.0 634.0 27.1 52.1 4.8 1.7 1.6 0.6 9.5 5.5 1.110.3 57.3 456.3 21.2 45.9 11.3 1.1 554.0 25.5 53.9 5.9 1.8 1.6 0.5 8.9 5.2 1.320.3 270.1 278.0 11.4 48.4 11.2 46.8 437.0 20.4 53.0 9.1 1.2 1.8 0.6 10.0 3.3 0.63.8 156.4 87.0 8.8 77.3 61.3 1.6 515.0 4.1 8.6 1.0 0.6 3.9 6.3 14.0 3.1 8.05.9 160.0 56.4 7.6 59.8 12.2 2.7 777.9 4.2 8.4 0.9 0.6 2.7 1.0 44.4 1.2 8.010.5 171.6 210.9 8.3 169.0 6.7 1.6 468.2 24.4 47.5 2.9 0.7 4.0 0.8 43.4 6.8 1.822.3 154.6 46.0 17.1 143.1 14.7 1.1 1155.0 36.9 71.5 4.8 2.0 4.8 1.4 38.6 16.4 3.53.4 70.5 22.4 12.2 40.2 19.3 1.1 767.5 7.9 18.5 2.2 1.2 2.1 1.7 35.4 4.0 4.67.1 107.1 99.0 13.1 67.5 10.0 1.4 979.0 24.1 55.2 6.8 1.6 2.3 0.7 101.1 6.0 1.313.8 123.2 112.5 10.2 40.2 8.6 2.2 785.5 21.6 52.7 7.6 1.1 1.3 0.4 8.8 2.2 0.710.8 111.1 164.0 18.3 70.5 11.6 0.07 1207.0 26.2 58.4 7.3 1.9 2.4 0.8 6.0 6.3 1.414.8 88.7 280.0 9.8 189.1 9.7 1.3 676.0 29.8 56.8 3.6 0.9 6.4 1.1 14.7 10.4 2.5

⁎

8.8 113.0 125.3 6.9 99.8 7.5 2.42 645.0 25.9 48.7 3.0 0.7 3.05 0.66 b3 9.31 2.298.7 8.7 63.0 112.1 17.4 58.8 9.2 1.32 311.5 29.5 67.3 8.0 1.6 2.01 0.74 b3 1.0114.4 103.1 137.9 43.0 108.5 17.6 3.31 679.4 62.1 135.4 14.4 3.5 4.66 1.77 3 8.16 1.6211.4 71.8 67.2 21.1 78.8 10.2 1.90 400.0 32.3 70.7 7.7 1.9 2.45 0.75 11 4.76 1.1915.5 171.3 71.1 23.1 387.9 20.6 5.55 978.5 41.1 89.8 7.5 3.4 9.60 1.33 b3 12.38 5.034.4 81.2 130.0 18.8 85.1 16.8 2.36 620.5 41.7 81.8 5.3 2.3 3.29 1.17 b3 12.26 3.284.2 118.1 75.2 21.7 110.3 21.3 3.48 930.9 41.0 81.6 5.3 2.9 4.44 1.46 b3 16.06 3.854.0 71.3 165.9 15.9 68.2 14.1 2.07 547.8 36.9 73.6 4.8 1.9 2.58 0.94 b3 9.79 2.425.1 73.2 1092.6 16.7 79.0 14.5 3.34 960.0 19.6 39.4 2.2 1.1 2.68 109 3 13.09 3.75


Table 2Major and trace element concentrations for the Ordovician and Silurian sedimentary rock samples

Sample Formation SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Total Sc V⁎ Cr Co Ni Rb Sr Y Zr Nb Cs Ba La Ce Sm Yb Hf Ta Pb Th U

Ordovician samplesDSBA(1) Derrylea 65.18 0.59 9.62 4.77 0.07 5.37 1.69 1.11 1.66 0.12 90.2 10.0 56.5 460.5 29.8 358.1 52.7 86.2 14.6 141.2 11.4 1.77 230.6 23.5 47.3 4.0 1.6 4.1 0.72 27.4 6.6 1.4DSBB1 Derrylea 62.54 0.65 9.45 4.82 0.19 6.94 5.33 1.20 3.98 0.16 95.3 9.5 66.5 314.2 28.1 376.8 186.3 151.4 15.5 120.0 11.9 2.01 274.2 27.10 54.86 4.36 1.56 3.3 0.72 25.5 7.0 1.5DSBD1 Derrylea 69.03 0.61 9.04 4.42 0.25 4.08 5.18 1.22 1.49 0.13 95.4 8.7 65.9 711.5 22.3 265.6 55.4 184.5 16.6 106.0 2.7 1.12 243.3 24.94 50.82 4.27 1.55 2.7 0.13 22.1 6.0 1.5DSBD2 Derrylea 69.65 0.76 10.80 5.30 0.08 5.49 0.49 1.62 1.95 0.16 96.3 10.6 63.4 390.8 27.4 356.1 41.2 93.8 13.5 110.9 11.2 0.56 205.2 22.3 45.7 3.8 1.4 3.3 0.68 10.4 6.1 1.3DSTA1 Derrylea 68.72 0.58 9.88 5.25 0.10 6.13 3.02 1.30 1.43 0.13 96.5 9.3 66.0 274.3 26.9 383.8 77.8 178.6 13.6 119.5 12.1 2.18 256.6 24.60 51.71 4.18 1.39 3.2 0.70 5.5 6.9 1.5DSTB1 Derrylea 65.08 0.62 9.88 5.03 0.10 6.42 2.81 0.81 1.87 0.12 92.7 10.2 471.6 30.6 309.7 55.3 34.4 17.9 196.0 15.3 1.96 224.0 27.8 57.5 4.9 1.9 5.5 0.90 22.2 9.1 1.8MASBA1 Mweelrea 68.96 0.47 14.16 3.65 0.04 1.07 0.38 2.68 2.96 0.11 94.5 12.5 58.4 19.2 11.3 8.8 83.7 74.8 14.1 77.4 13.5 3.02 612.1 30.0 61.2 5.1 1.6 2.6 0.83 10.9 8.4 1.4MAST3 Mweelrea 66.83 13.61 2.99 5.56 2.88 1.66 0.13 1.89 0.13 0.54 96.2MAST2(1) Mweelrea 61.13 0.67 11.53 7.12 0.27 2.16 4.48 1.56 2.51 0.12 91.5 25.5 149.8 36.9 15.7 10.0 70.1 157.6 16.6 76.4 9.5 2.96 812.6 18.5 39.5 4.4 1.7 2.4 0.56 10.8 5.1 1.1MATB1 Mweelrea 71.07 0.29 13.96 2.32 0.03 0.73 0.83 3.00 3.14 0.11 95.5MATB3 Mweelrea 68.93 0.34 15.54 3.01 0.05 0.97 1.65 3.80 2.82 0.08 97.2 9.4 48.6 8.0 7.4 7.0 105.0 194.1 13.3 80.8 14.8 3.90 731.3 17.42 37.69 3.20 1.51 2.6 0.94 6.6 11.7 2.5MWSBA2 Mweelrea 74.43 0.38 10.13 3.97 0.05 0.88 0.21 1.50 2.22 0.08 93.8 7.9 72.7 31.9 8.9 11.6 58.3 53.4 15.0 54.1 1.02 2.02 435.8 34.38 59.60 4.92 1.68 1.9 0.69 14.6 7.5 1.4MWSBB1 Mweelrea 80.70 0.39 10.44 3.66 0.05 1.06 0.17 1.64 2.36 0.08 100.5 7.9 67.7 30.4 8.5 14.8 88.3 53.8 18.1 53.8 11.5 2.14 303.8 35.47 56.78 5.30 1.71 1.8 0.78 16.9 6.8 1.4MWSBC1 Mweelrea 80.12 0.43 9.30 4.10 0.04 0.91 0.13 1.95 2.10 0.06 99.1 7.7 62.6 26.6 8.8 11.2 70.1 43.3 13.5 53.7 12.0 2.50 237.4 21.4 42.8 3.5 1.6 1.9 0.90 15.4 11.0 1.6MWSBD2 Mweelrea 77.96 0.37 10.21 2.60 0.03 0.76 0.11 1.54 4.36 0.06 98.0 6.2 31.0 20.2 7.2 10.9 87.5 28.5 14.6 54.9 12.3 1.58 344.7 19.3 38.6 3.5 1.6 1.9 0.86 14.1 8.4 1.2MWSBG1 Mweelrea 83.31 0.31 10.30 2.74 0.05 1.01 0.17 1.33 2.40 0.05 101.7 7.3 32.4 15.4 7.3 8.5 67.9 42.1 9.9 54.0 9.1 2.07 334.6 18.2 36.1 2.8 1.2 1.8 0.65 9.7 5.3 1.0MWSBG2 Mweelrea 81.69 0.26 0.83 2.19 0.06 0.45 2.91 1.64 1.22 0.05 99.3MWSBG3 Mweelrea 75.84 0.26 10.01 2.31 0.06 0.47 3.01 1.93 1.27 0.05 95.2MWSBH1 Mweelrea 78.80 0.31 10.34 3.10 0.09 0.72 4.38 0.54 1.54 0.06 99.9 8.1 40.2 21.8 7.8 14.0 55.7 890.4 14.0 46.6 4.6 1.32 272.6 21.09 43.29 3.37 1.42 1.5 0.27 28.9 6.7 1.3MWSBi1 Mweelrea 75.18 0.35 9.62 3.46 0.10 0.86 3.19 0.75 2.15 0.04 95.7 7.4 44.9 24.1 9.6 11.6 72.3 265.5 16.6 52.6 11.6 2.48 365.8 22.9 45.9 3.8 1.7 1.9 0.85 29.5 7.3 1.3MWSBJ1 Mweelrea 78.56 0.36 9.30 2.51 0.04 0.53 0.15 2.01 2.26 0.08 95.8 5.6 42.0 24.4 6.7 10.3 71.7 53.9 8.8 49.8 9.8 1.44 377.7 22.78 45.56 3.48 0.94 1.6 0.64 9.5 6.9 1.0MWSBK1 Mweelrea 80.89 0.27 8.56 1.92 0.05 0.65 0.50 2.00 2.46 0.06 97.4 5.4 34.0 14.6 7.9 8.1 67.5 48.9 11.4 56.6 7.7 1.34 520.8 18.0 34.9 2.9 1.3 1.9 0.59 31.8 5.9 1.1MWSBL1 Mweelrea 75.82 0.36 10.36 2.48 0.04 0.62 1.05 0.70 2.94 0.09 94.5 9.3 35.1 27.9 3.9 8.6 108.1 47.6 9.9 34.9 10.7 2.93 285.3 22.30 46.13 3.68 0.99 1.2 0.72 7.8 6.1 1.3MWSBN1 Mweelrea 81.95 0.32 9.41 2.34 0.06 0.73 0.78 1.83 2.69 0.06 100.2 5.7 31.4 18.2 7.6 9.2 71.9 44.7 11.6 55.1 8.3 1.65 309.8 19.3 36.7 3.4 1.3 1.8 0.56 13.0 5.7 1.0MWSBP1 Mweelrea 75.68 0.34 8.26 2.30 0.03 0.60 0.17 1.89 2.47 0.07 91.8 5.7 40.0 17.9 5.3 8.1 65.1 48.1 8.3 48.6 8.6 1.35 404.2 21.7 43.1 3.1 0.9 1.6 0.06 10.4 6.5 1.0MWSTA1 Mweelrea 74.60 0.53 10.28 4.84 0.06 0.91 0.18 0.97 1.87 0.10 94.3 8.8 93.0 35.3 9.6 13.0 52.1 27.2 15.6 51.9 12.6 1.67 443.9 39.88 76.84 5.42 1.60 1.6 0.83 13.9 11.9 1.5MWSTA2 Mweelrea 79.84 0.27 9.43 2.22 0.08 0.83 0.91 1.71 1.96 0.07 97.3 5.9 33.9 25.6 5.8 12.3 64.0 57.2 23.0 48.3 7.7 1.68 332.1 35.68 40.33 5.04 1.81 1.5 0.50 10.5 5.3 0.9MWSTC2 Mweelrea 75.98 0.35 9.45 2.84 0.05 0.92 0.47 1.77 2.11 0.05 94.0 6.5 34.0 23.0 8.1 11.1 60.1 33.5 14.6 57.6 11.9 1.46 380.0 26.1 48.4 4.2 1.6 2.0 0.98 15.2 8.2 1.3MWSTD2 Mweelrea 76.01 0.52 8.93 3.61 0.04 0.85 0.10 1.06 1.89 0.05 93.1 6.8 49.6 32.7 8.6 12.1 74.4 18.6 14.8 51.0 14.0 1.38 302.5 29.0 61.3 4.2 1.6 1.7 1.02 17.6 13.4 1.5MWSTH1 Mweelrea 80.07 0.27 9.12 2.75 0.09 0.65 4.11 0.57 1.25 0.03 98.9 6.4 31.1 27.4 62.0 14.7 41.5 104.1 12.9 52.2 8.6 1.25 1475.8 18.0 35.7 3.0 1.3 1.8 0.89 29.5 5.9 1.1MWSTi1 Mweelrea 78.02 0.40 9.61 3.43 0.05 1.10 0.21 0.66 2.10 0.07 95.7 7.4 43.7 30.7 17.7 16.9 80.0 24.3 14.3 61.3 12.7 2.63 477.4 29.4 57.6 4.8 1.7 2.1 0.99 14.2 7.9 1.5MWSTJ1 Mweelrea 75.47 1.02 10.20 6.84 0.05 0.88 0.21 1.81 2.65 0.13 99.3 10.9 132.5 51.1 8.4 14.8 90.0 59.2 17.0 69.2 23.0 1.93 394.0 59.90 118.23 7.92 2.26 2.1 0.84 22.9 15.8 2.1MWSTK1 Mweelrea 79.79 0.31 8.96 2.34 0.04 0.70 0.67 1.77 3.02 0.06 97.6 5.3 39.7 19.0 6.1 8.0 58.8 37.4 10.8 53.3 7.6 1.11 242.3 15.9 32.1 2.8 1.2 1.7 0.53 9.4 5.0 0.9MWSTL1 Mweelrea 75.51 0.44 10.58 4.28 0.05 1.17 0.14 0.98 2.68 0.09 95.9 8.7 65.7 32.0 11.1 12.5 103.0 32.7 11.2 59.9 13.0 3.62 377.1 30.50 60.68 4.18 1.36 1.9 0.90 11.6 8.3 1.3MWSTL2 Mweelrea 82.53 0.26 9.90 2.15 0.06 0.55 2.27 2.00 1.49 0.06 101.3 5.7 30.9 20.8 5.4 8.9 56.9 816.6 15.8 46.4 9.9 1.35 292.4 19.50 39.64 3.38 1.53 1.5 0.64 17.7 5.8 1.2MWSTM1 Mweelrea 75.86 0.25 9.65 2.18 0.08 0.84 2.24 1.28 2.29 0.05 94.7 5.7 33.9 24.6 6.5 10.4 70.8 45.7 11.0 35.3 7.4 1.99 267.4 20.13 38.27 3.08 1.00 1.1 0.49 8.1 5.6 0.9MWSTM1 Mweelrea 75.86 0.25 9.65 2.18 0.08 0.84 2.24 1.28 2.29 0.05 94.7 5.7 33.9 24.6 6.5 10.4 70.8 45.7 11.0 35.3 7.4 1.99 267.4 20.13 38.27 30.8 1.00 1.1 0.49 8.1 5.6 0.9MWSTN1X Mweelrea 81.17 0.39 7.94 2.66 0.08 0.83 0.67 1.38 2.19 0.07 97.4MWSTN2 Mweelrea 81.16 0.37 8.28 2.45 0.05 0.69 0.78 2.03 3.18 0.07 99.1 7.5 36.0 27.9 9.3 13.3 80.4 43.7 16.1 57.8 11.6 2.03 337.1 31.6 58.3 4.7 1.6 2.0 0.78 13.4 7.8 1.1MWSTP1 Mweelrea 75.96 0.38 9.87 3.05 0.08 1.08 0.86 2.10 2.74 0.08 96.2 7.8 50.3 23.0 8.2 9.7 66.9 62.8 11.2 47.2 9.7 1.82 633.5 20.8 42.4 3.6 1.3 1.7 0.67 10.2 6.7 1.0

108K.M

.Ryan,

D.M

.William

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icalGeology

242(2007)

103–125

RSBA1 Rosroe 75.87 0.27 10.55 2.30 0.04 0.87 0.97 2.88 2.04 0.02 95.8 7.6 37.7 15.1 7.6 9.1 54.0 86.5 11.6 43.4 5.8 1.25 474.7 21.50 41.42 2.97 1.22 1.5 0.10 7.6 6.9 1.1RSBB2 Rosroe 76.55 0.36 11.36 2.98 0.02 1.01 0.29 1.44 2.47 0.09 96.6 9.2 46.0 29.6 7.6 14.4 75.2 26.4 15.7 70.6 9.6 2.42 275.2 22.84 44.74 3.62 1.50 2.3 0.63 11.7 7.1 1.3RSBC1 Rosroe 60.90 0.86 15.62 7.58 0.14 3.18 4.08 3.68 1.65 0.20 97.9 37.6 198.4 36.5 18.3 11.6 57.0 943.9 31.1 48.0 11.7 2.71 1127.4 21.54 50.95 7.65 2.63 1.7 0.51 11.8 3.9 0.8RSBC2 Rosroe 70.79 0.49 13.07 4.05 0.09 1.71 1.04 3.25 1.86 0.07 96.4 15.9 80.4 20.1 9.0 8.8 61.2 149.3 15.0 62.2 11.8 1.45 1085.3 15.46 32.98 3.58 1.54 2.1 0.68 8.3 7.8 1.6RSBD1 Rosroe 73.23 0.32 12.85 2.81 0.06 1.06 1.05 2.77 2.34 0.05 96.5 9.7 38.2 20.3 8.4 9.7 46.7 152.2 15.9 56.6 10.1 1.68 642.9 24.27 44.39 3.58 1.59 1.9 0.67 11.1 7.6 1.4RSTA1 Rosroe 74.01 0.36 10.40 2.93 0.06 1.07 0.86 3.57 1.97 0.09 95.3 9.4 54.4 16.4 8.4 6.6 46.1 85.2 12.2 49.4 9.2 0.93 383.7 24.8 49.0 3.6 1.3 1.8 0.63 19.8 6.8 1.1RSTB1 Rosroe 64.80 0.44 12.97 3.53 0.05 1.09 0.56 2.58 2.48 0.10 88.6 16.0 70.6 27.6 11.3 15.3 83.4 69.3 22.0 60.0 7.1 2.61 388.6 26.83 52.07 5.55 1.96 2.1 0.16 12.2 6.2 1.3RSTB3 Rosroe 70.34 0.60 11.39 4.77 0.08 1.63 2.27 2.83 2.54 0.11 96.6RSTC1 Rosroe 66.35 0.45 11.69 3.97 0.05 1.59 0.77 1.70 2.43 0.06 89.1 12.6 69.1 31.7 9.3 12.5 66.8 44.5 18.6 76.3 11.6 1.25 1336.2 27.8 56.5 4.9 2.0 2.7 0.72 73.3 8.5 1.5RSTD1 Rosroe 72.19 0.35 12.84 3.06 0.08 1.09 1.54 3.00 2.22 0.09 96.5 11.2 48.0 24.5 9.2 10.0 62.2 126.4 17.8 61.8 10.1 1.95 618.9 30.6 54.1 4.7 1.8 2.2 0.63 11.7 7.7 1.4SDSTA1 Sheeffry 60.98 0.36 6.70 3.56 0.05 5.51 1.02 0.60 0.80 0.05 79.6 6.7 45.5 223.3 20.1 282.8 24.3 67.8 6.8 52.6 5.8 0.77 143.7 12.60 26.91 2.17 0.77 1.4 0.14 2.5 3.6 0.8SSBB1 Sheeffry 66.70 0.69 9.02 5.37 0.12 7.01 1.67 0.96 1.49 0.14 93.2 9.2 73.4 596.7 29.1 373.5 47.2 44.8 14.4 154.1 11.7 1.57 196.5 23.3 49.0 3.9 1.5 4.4 0.69 7.6 6.9 1.4SSBC1 Sheeffry 70.13 0.55 9.32 5.42 0.06 8.01 1.33 0.90 1.19 0.12 97.0 10.3 64.8 531.5 33.1 455.4 33.0 73.5 14.5 119.6 11.1 1.19 230.2 24.0 48.6 4.0 1.5 3.4 0.67 7.3 6.7 1.4SSBD2 Sheeffry 60.38 0.79 11.12 5.93 0.11 6.67 2.13 1.22 3.60 0.14 92.1 17.7 74.6 32.3 28.5 105.2 117.1 51.1 14.2 94.1 15.1 5.82 635.5 32.2 66.1 5.3 1.6 3.4 0.95 29.0 9.5 2.0SSBE1 Sheeffry 74.18 0.59 10.21 4.94 0.06 5.46 0.42 1.47 2.10 0.11 99.5 9.6 75.5 233.0 22.2 227.8 43.7 40.2 14.9 116.0 11.6 1.28 260.6 24.4 49.4 4.1 1.5 3.6 0.71 13.1 6.9 1.6SSTB1 Sheeffry 65.13 0.67 11.08 5.13 0.12 6.09 3.98 1.53 3.41 0.14 97.3 12.5 70.8 419.0 30.2 350.8 63.8 130.0 19.0 124.6 13.4 0.81 328.0 29.3 59.2 5.0 1.8 3.6 0.83 14.8 8.5 1.5SSTC1 Sheeffry 60.03 0.70 11.18 6.01 0.15 7.02 3.80 1.49 2.30 0.12 92.8 12.4 336.1 37.0 488.0 90.3 155.1 15.7 116.9 13.4 3.63 323.4 26.3 54.8 4.6 1.6 3.5 0.81 13.4 7.1 1.5SSTE1 Sheeffry 68.99 0.74 10.36 5.91 0.06 8.08 0.18 1.27 1.81 0.14 97.5 12.2 85.0 848.1 29.0 388.3 42.3 32.2 17.6 146.6 13.2 0.40 264.2 25.8 53.6 4.5 1.9 4.4 0.81 12.0 7.4 1.6

Silurian samplesCl11 Strake Banded 68.00 0.78 13.14 5.28 0.07 3.69 2.39 1.77 3.23 0.15 98.5 12.6 81.6 72 16.0 43.8 124.5 83 24.91 154.8 15.62 3.2 545 36.8 76.0 6.7 2.7 4.6 1.12 10 10.79 3.20Cl12 Strake Banded 89.52 0.33 6.09 1.04 0.01 1.00 0.14 2.32 0.82 0.03 101.3 2.7 19.0 14 8.8 8.6 29.3 50 9.47 56.4 7.32 0.2 131 14.4 30.5 2.6 1.1 1.8 0.63 b3 3.48 0.99Cl15 Strake Banded 75.10 0.88 10.64 4.34 0.04 3.30 0.67 2.37 1.75 0.09 99.2 12.9 83.0 92 15.4 40.1 61.1 76 25.60 139.9 13.06 1.3 283 30.2 66.9 5.9 2.7 4.1 1.02 7 9.95 2.12Cl18 Bunnamohaun 71.09 0.79 11.72 4.79 0.09 2.57 2.20 2.48 2.41 0.20 98.3 11.8 75.0 74 16.8 46.4 86.4 121 22.65 148.5 13.86 2.9 376 32.1 66.0 6.0 2.4 4.3 0.96 10 8.69 2.09Cl19 Bunnamohaun 83.75 0.39 5.48 1.44 0.07 0.60 1.71 1.73 1.35 0.16 96.7 3.9 56.0 23 7.1 10.2 39.3 61 12.22 78.2 7.26 0.2 229 15.8 34.9 2.9 1.4 2.5 0.19 5 4.28 1.40Cl21 Bunnamohaun 72.69 0.68 10.63 4.25 0.07 2.82 1.54 2.63 2.00 0.16 97.5 9.9 69.1 69 14.5 41.3 61.3 128 21.89 131.6 12.50 1.8 466 26.7 54.4 4.9 2.2 3.9 0.89 6 7.46 2.64Cl25 Strake banded 88.15 0.28 6.79 1.02 0.01 0.71 0.15 2.68 1.15 0.03 101.0 2.7 23.8 10 6.4 7.7 35.8 48 9.45 54.2 7.15 0.2 184 18.6 37.8 2.9 1.1 1.6 0.59 b3 3.70 0.94Cl26 Strake banded 69.99 0.66 13.09 4.14 0.04 3.75 0.48 1.60 3.65 0.10 97.5 10.0 66.0 55 12.8 33.7 134.4 41 24.02 157.6 16.29 3.4 572 35.9 75.4 6.3 2.7 4.6 1.10 b3 13.42 2.53Cl27 Strake banded 60.30 0.66 22.03 3.55 0.02 2.89 0.18 3.33 6.58 0.04 99.6 11.9 18 7.4 16.0 238.9 81 33.13 340.2 23.01 7.8 1071 74.0 157.0 11.9 3.6 9.3 1.46 b3 15.65 2.36Cl28 Strake banded 64.05 0.81 14.13 5.86 0.08 3.78 2.30 1.63 3.68 0.19 96.5 14.4 89.1 85 18.0 57.0 131.7 87 26.01 144.5 16.19 3.5 626 39.5 76.8 6.9 2.7 4.1 1.31 9 11.09 2.36Cl29 Strake banded 73.85 0.78 13.86 4.17 0.06 1.60 3.05 3.60 3.20 0.14 104.3 6.8 77.3 24 7.7 12.8 111.6 206 15.40 97.9 11.24 3.0 622 33.0 64.9 5.1 1.6 2.8 0.71 6 5.76 1.39Cl2a Strake banded 75.42 0.76 10.70 3.47 0.04 2.64 0.42 2.40 2.29 0.10 98.2 8.3 58.0 41 9.4 20.2 74.1 67 18.94 151.8 13.77 1.8 395 34.5 72.3 6.2 2.1 4.4 0.99 b3 7.91 1.85Cl5a Strake banded 81.65 0.35 9.49 1.64 0.03 1.90 0.94 1.96 2.42 0.08 100.5 4.3 33.1 18 7.0 11.5 76.9 65 14.97 78.0 12.13 1.9 352 29.5 61.8 5.6 1.7 2.8 0.88 b3 6.40 1.19Cl9a Strake banded 90.17 0.30 6.02 1.32 0.06 0.75 3.41 1.66 1.36 0.13 105.2 3.4 23.6 17 6.5 10.2 44.4 131 11.91 62.5 7.45 0.8 278 16.2 32.9 2.9 1.2 1.9 0.58 7 4.19 1.06Cl9b Strake banded 70.55 0.42 6.68 1.71 0.11 1.39 5.19 1.62 1.51 0.25 89.4 4.5 26.0 21 9.5 12.3 51.0 127 15.65 86.4 9.79 1.0 274 21.9 46.3 3.9 1.6 2.6 0.86 7 4.59 1.41Fi2b Bouris 62.99 0.97 17.03 7.73 0.06 5.13 0.36 1.59 3.71 0.13 99.7 20.7 124.0 114 25.6 80.5 142.6 51 17.92 153.7 17.19 3.4 670 38.0 79.5 7.0 2.2 4.3 1.14 b3 11.76 2.75Fi3t Bouris 60.71 0.89 14.53 6.20 0.09 4.35 6.19 1.66 3.11 0.21 97.9 17.0 111.0 107 22.7 63.8 124.7 190 15.92 153.0 17.44 2.9 530 37.2 77.7 6.7 2.2 4.3 1.18 6 10.83 2.49Ou1 Bouris 89.95 0.64 5.27 1.64 0.01 0.59 0.09 1.07 1.74 0.03 101.0 4.6 23.4 25 8.9 11.4 46.7 15 8.66 118.8 12.61 0.3 828 24.7 50.4 4.1 1.1 3.7 0.95 b3 5.38 1.47SBWT Bunnamohaun 69.18 0.77 12.25 4.92 0.09 2.87 2.63 2.32 2.56 0.20 97.8 12.6 78.0 80 16.4 52.1 92.8 112 23.73 153.2 14.78 3.3 323 35.3 69.1 6.6 2.4 4.4 1.01 7 9.28 2.25SH1 Strake Banded 63.97 0.80 13.89 5.56 0.08 3.48 3.80 2.03 3.42 0.18 97.2 13.2 75.2 77 16.5 49.1 128.6 107 26.62 146.4 17.01 4.4 506 37.7 77.2 6.7 2.6 4.0 1.12 12 10.91 2.56SHlVsst Strake Banded 77.02 0.33 6.81 1.61 0.07 1.23 5.42 2.16 1.15 0.16 95.9 3.9 22.0 25 7.9 13.3 39.6 107 13.42 61.8 8.35 0.6 195 16.6 34.1 3.2 1.4 2.0 0.65 3 4.95 1.01

1 Total oxide weight percent, low totals reflect loss on ignition.⁎ Analysed using ICPAES.

109K.M

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D.M

.William

s/Chem

icalGeology

242(2007)

103–125


framework grains in the Rosroe sediments are quartz,feldspars and volcanic lithics. The volcanic lithics com-prise plagioclase laths and occasional remnant glassshards are observed. The tuff bands in the Rosroe For-mation are massive green coarse-grained lithic tuffs withdistinctive black pumice fragments. The Sheeffry andDerrylea sedimentary rocks were interpreted by Pudsey(1984) as deep-water turbidite deposits based on lack ofshallow water features and the uniformity of the fine-grained sediments. The tuff bands in the Derrylea andSheeffry Formations are layered, with coarse layers at thebase overlain by fine-grained ash layers, resulting fromsubmarine settling.

The Silurian rocks are represented by samples from theLouisburgh–Clare Island succession and the CroaghPatrick succession. The Louisburgh–Clare Island succes-sion consists of red siltstones, sandstones and conglom-erates with interbedded tuff bands. Two formations fromthe Louisburgh–Clare Island succession are includedin this study, the Strake Banded Formation and theBunnamohaun Formation. The most recent interpretation(Maguire and Graham, 1996) describes the succession asexhibiting fluvial sedimentation with interbedded tuffs.The Strake Banded samples are generally medium to fine-grained light green tuffs. In contrast the tuffs from theoverlying Bunnamohaun Formation are a distinctive pur-ple colour. The sandstones of the Strake Banded For-mation are described as having two source regions:volcanic and non-volcanic. The samples from the CroaghPatrick succession are part of the Bouris Formation whichconsists of green pelites with interbedded tuffs depositedin a shallow marine environment (Williams and Harper,1988). The Bouris tuff from the Croagh Patrick suc-cession is a fine-grained cream-coloured tuff which has ashear cleavage reflecting the deformation of the CroaghPatrick succession.

2.1. Analytical methods

One hundred and twenty-nine Ordovician and Siluriansamples were analysed for major elements and twenty-one trace elements. Sample preparation was carried out atthe National University of Ireland, Galway and sampleanalyses were carried out at OMAC Laboratories, Gal-way. The major element concentrations were determinedusing solution ICP-AES (Perkin Elmer Optima 3000DV)and the trace element concentrations were determinedusing solution ICP-MS (Thermo Electron X-Series quad-rupole ICP-MS).

The fresh rock samples ranged between 2 kg and 5 kgwere crushed and powdered in an agate mill. The pow-dered samples (0.25 g) underwent three-acid digestion

using HF, HNO3 and HClO4 (5 ml, 3 ml and 1 ml, re-spectively) in screw-top Teflon beakers heated to110 °C for a minimum of 12 h. Following this thesamples underwent open evaporation on a hotplate andthe remaining solids were dissolved in HCl. The effec-tiveness of this dissolution method was confirmed withR2 values of 0.996–0.999 for the international standardswhen compared to published values (Govindaraju, 1994).All the measured concentrations of the samples wereabove the detection limits for the instruments at OMAClaboratories.

The precision and accuracy of the major and traceelement analysis was monitored using international stan-dards: andesite AGV-1, basalt JB-1, granite G-2, and shaleSCo-1 (Appendix A). The standards were analysed asunknowns together with the sedimentary and tuff samples.

The precision of the measurements was analysedusing the 2 sigma standard deviation from the mean ofmultiple analysis of AGV-1. Themajor elements have a 2sigma standard deviation of less than 1 wt.%. The traceelements Sc, Co, Ni, Y, Nb, Cs, La, Ce, Sm, Yb, Ta, andU have 2 sigma standard deviations of less than 5 ppmand three of the trace elements have 2 sigma standarddeviations greater than 5 ppm for the multiple analysis ofAGV-1: Rb (9.4 ppm), Sr (13.9) and Ba (48 ppm).

The accuracy, variation from published referencevalues, was less than 2 wt.% for the major elements(SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O,K2O, and P2O5). The trace elements Sc, Co, Ni, Rb, Y,Nb, Cs, La, Ce, Sm, Yb, Hf, Ta, Pb and U have standarderrors of less than 1 ppm. Cr, Sr, and Zr have standarderrors less than 3 ppm whilst Ba has a higher standarderror of 6.8 ppm.

The majority of the Mweelrea and Rosroe sandstonesare in the coarse-grained range and the Sheeffry andDerrylea sedimentary samples are fine tomedium-grained.The average total for the tuff samples is 97.9 and 96.3 forthe sedimentary samples. The low total for some of thesamples (Tables 1 and 2) is attributed to loss on ignition.Petrographic analysis shows that the samples with lowtotals can comprise up to 22% calcite and over 12%chlorite. The low values for the sample can be explainedentirely through the loss of H2O and CO2 from calcite andthe sheet silicates during evaporation stages of samplepreparation.

3. Results for the tuff bands

3.1. Multi-element diagrams

The tuff samples were normalised using Mid OceanRidge Basalt (MORB) values from Pearce (1983) with


the most mobile elements on the left organised in orderof increasing incompatibility (Fig. 2A and B). Theremaining elements are considered immobile and arearranged in order of increasing incompatibility fromright to left. Sc and Cr have been added to the righthand side using MORB normalising values (afterPearce, 1982). The profiles were constructed using theaverage values from the element concentrations foreach formation.

The four Ordovician tuff profiles have parallel trends,with enrichment in the low ionic potential elementsand the profile decreasing to the right with depletionin the elements with high ionic potential. Overall theMweelrea samples are the most enriched relative to theaverage values for the other three formations with peaksin Th and the rare earth elements (REE). The normalised

Fig. 2. Multi-element variation diagrams (A) Ordovician tuff samples, (B) Silursedimentary rock averages. Diagrams (A) and (B) were plotted using MORB (upper continental crust (Taylor and McLennan, 1985) normalised data. The da(1983).

Silurian profiles have a similar trend to the Ordoviciansamples but there is greater variation between the highionic potential elements (Ti–Yb) and the three SilurianFormations record lower Sc values than the OrdovicianFormations.

3.2. Discrimination diagrams for the tuff bands

The tuff bands from the Ordovician and Silurian suc-cessions have a silica (SiO2) range of 51–82% and thevast majority are classified as having an andesitic to rhy-olitic composition. Therefore the discrimination diagramsfor igneous rocks with a granitic composition have beenused to identify the tectonic setting for the South Mayotuff bands. The discrimination diagrams constructed byPearce et al. (1984) defined the tectonic setting for rocks

ian tuff samples, (C) Ordovician sedimentary rock averages, (D) SilurianPearce, 1983) normalised data. Diagrams (C) and (D) were plotted usingshed line on Fig. 2A is the active continental margin profile from Pearce

Fig. 3. Discrimination diagrams for tuff bands using the actual abundances shown in Table 2. Rb–Y+Nb diagram (after Pearce et al., 1984) for theOrdovician samples (A) and Silurian samples (B).VAG: volcanic arc granites; ORG: ocean ridge granites; WPG: within-plate granites; syn-COLG:syn-collisional granites. Rb–Hf–Ta diagram (after Harris et al., 1986) for Ordovician samples (C) and (E). Rb–Hf–Ta diagram for the Siluriansamples (D) and (F).



with a SiO2 concentration between 56% and 80% whichmatches the SiO2 range for the South Mayo tuffs. Pearceet al. (1984) defined four discrimination diagrams, butonly the Rb–Y+Nb diagram (Fig. 3A and B) is presentedin this paper as all four diagrams were found to yieldsimilar results.

The Ordovician tuff samples plot in the field indicatinga volcanic arc setting. They are skewed towards theboundaries for the within-plate granites and the syn-collisional granites, with two of the Sheeffry samplescrossing the boundary into these fields. The Siluriansamples have an identical distribution to the Ordoviciansamples indicating that the tuffs were erupted in a similarsetting (Fig. 3B).

The ternary diagrams produced by Harris et al.(1986) to discriminate samples with a granitic compo-sition use Rb, Hf, and Ta to differentiate between thesamples (Fig. 3C–F). The Ordovician and Siluriansamples plot in similar positions on the Rb–Hf–Taternary diagrams. In Fig. 3C the Ordovician samples aredispersed between the volcanic arc and within-platefields. Harris et al. (1986) identified this location as theposition for granites from a collisional tectonic setting.The majority of the Derrylea (67%) and Sheeffry (80%)samples plot within the volcanic arc field, close to thearea for collisional granites. TheMweelrea samples plotclose to the boundary and extend into the within-platesetting. The Rosroe samples plot equally on either sideof the boundary. In Fig. 3E the Ordovician samplesdominate the field for late and post-collisional granitesbut are skewed towards the boundary with the volcanicarc field. Seven (29%) of the Mweelrea samples extendinto the within-plate field. The Sheeffry samples exhibitthe greatest variation and are spread between thevolcanic-arc, within-plate, and late and post-collisionalfields.

The Silurian samples have a similar dispersal on theternary diagrams (Fig. 3D and F). The sample from theBunnamohaun Formation has a definite within-plate sig-nature and the Bouris sample has a volcanic arc signature.The samples from the Strake Banded Formation plot onthe boundary for the two fields indicating a collisionalsetting. Similar to the Ordovician samples the Strake–Banded samples plot in the late and post-collisional fieldand are skewed towards the boundary with the volcanicarc setting.

4. Discussion of igneous discrimination diagrams

The MORB normalised profiles for the Silurian andOrdovician tuff samples are almost identical to the patterndescribed by Pearce (1983) for active margin settings

(Fig. 2A and B). The peaks in the Ordovician andSilurian profiles at Ce and Sm are observed in island arcand active continental margin settings around the world.However, Pearce (1983) determined that rock samplesfrom an island arc setting show a profile from Ta to Ybwhich is practically flat and parallel to MORB values.In contrast, active continental margin settings alwaysexhibit enrichment in Ta and Nb relative to Zr and Hf.This enrichment is present in the South Mayo samplesand indicates an active continental margin setting for thetuffs in the formations. Pearce (1983) proposed that theenrichment in Sr, K, Rb, Ba, Th, Sm, and P is the result ofaddition from a subduction component and attributed thedistributions of Ta, Nb, Zr, Hf, Ti, Y and Yb to contri-butions from sub-continental lithosphere. The enrichmentof Ba and Th may originate from crustal contamination.This suggests that the South Mayo volcanism originatedat active continental margins with contributions to thechemistry from subduction melt, sub-continental litho-sphere and crustal melt.

In the Rb–Y+Nb discrimination diagram the Ordo-vician samples plot in the volcanic arc setting but areslightly skewed towards the within-plate setting. Thevolcanic arc granite (VAG) field was defined by Pearceet al. (1984) using samples from oceanic arcs and activecontinental margins whilst the granites used to definethe collision field included granites which develop atcontinent–arc collision zones. It is apparent that the areaon the diagrams where the samples cluster, i.e. close tothe boundary for collision granites and within-plate gran-ites, indicates a contribution of a continental componentto the melt. Therefore the Rb–Y+Nb diagram indicatesthat the tuffs were erupted from melt at an active con-tinental margin, consistent with the interpretation of theMORB multi-element profiles.

The tectonic setting assigned to the tuff samples bythe Pearce et al. (1984) diagrams is supported by a studycarried out by Förster et al. (1997) which evaluated theuse of granitoid geochemistry to discriminate the tec-tonic setting. The study found that granites (and theirvolcanic equivalents) which form at active continentalmargins can plot in the upper portion of the volcanic arcfield close to the boundaries of the collisional granitesand within-plate granites, similar to the results for theSilurian and Ordovician samples. Förster et al. (1997)showed that continental margin granites, which formdue to arc magmatism (subduction) alone, plot in thevolcanic arc field but when arc–continent collision isinvolved the samples cross the boundary and plot in thewithin-plate field.

In the ternary diagrams (after Harris et al., 1986) thesamples are not as well clustered and would appear to


classify the rocks differently; however this is not the case.In Fig. 3D and E the Ordovician and Silurian samples areconcentrated in the late and post-collisional setting rep-resenting group 3 magmatism in Harris et al.'s (1986)sequence. The group 3 magmatism is described asforming similarly to volcanic arc magmas, that is, froma large-ion lithophile enriched mantle wedge (sub-continental lithosphere) above subducted oceanic crust(Pearce, 1983;Harris et al., 1986). The difference betweenthe two magma types is the modification of group 3magmas by lower crustal melting. If the magma whichformed the tuff bands hadmore continental contaminationthan granites with a similar composition then the samplescould be expected to lie on the boundary of the volcanicarc and the late collisional fields.

Harris et al.'s (1986) diagrams show that the tuff bandsfrom Ordovician samples have an increasing continentalcomponent as one moves stratigraphically upwards fromthe Sheeffry samples through to the Derrylea and into theMweelrea samples, which always plot closest to, or in, thewithin-plate field. In both diagrams the Rosroe samplesappear to overlap the Sheeffry, Derrylea and theMweelreasamples. The signature for the Mweelrea Formationindicates a greater contribution from continental crustsupporting a tectonic model with Mweelrea volcanismfounded on Dalradian continental crust.

The Louisburgh–Clare Island Silurian successionreflects a similar pattern of increasing continental com-ponent moving up the stratigraphy. The Strake Bandedsamples have a collision signature and spread into thewithin-plate setting. The Bunnamohaun sample isfrom higher up the stratigraphy and is assigned to thewithin-plate setting, away from the Strake Bandedsamples. Given the field constraints on the position ofthe Mweelrea Formation in relation to the Sheeffry andDerrylea Formations in the stratigraphy (i.e. Mweelreais at the top of the conformable sequence) the conceptof a more evolved melt with a greater continental crust-al component is acceptable. This model, volcanismfounded on ancient continental crust, can be similarlyapplied to the Louisburgh–Clare Island succession.

From the results of the multi-element diagrams andgranitic discrimination diagrams the South Mayo tuffsamples were consistently found to have a geochemicalsignature of an active continental margin setting.

5. Results for sedimentary rocks

5.1. Multi-element diagrams

The sedimentary rock samples were normalisedto upper continental crust values, (after Taylor and

McLennan, 1985) (Fig. 2C and D). The elements areordered with the large ion-lithophile elements (LILE)on the left (K–Sr), followed by the transition metals(Mn–Cr), and the high field strength elements (HFSE)on the right (Sc–Ti).

The profiles for the four Ordovician Formations areseparated into two groups (Fig. 2C), the average valuesfor the Derrylea and Sheeffry samples in one group andthe Rosroe and Mweelrea averages in the other. Themajority of the elements in the four profiles are slightlydepleted relative to upper continental crust values. Theseparation of the profiles into two groups is most ob-vious in the transition elements Mn, Co, Ni, and Cr. Theseparation of the two groups is observed to a lesserextent with the HFSE Zr and Hf. The high Cr and Ni inthe Sheeffry and Derrylea Formations (and high Ti andFe in the underlying Derrymore and Letterbrock For-mations) were previously reported by Wrafter andGraham (1989).

The profiles for the three Silurian samples are closeto the upper continental crust values. The three profilesexhibit a significant depletion in Sr which is followed byenrichment in the mafic elements Co, Ni and Cr. Thismafic enrichment is not as significant as in the Derryleaand Sheeffry profiles.

5.2. Discrimination diagrams for the sedimentary rocks

The results from the ten diagrams are summarised inTable 3. The most obvious result is the greater numberof samples assigned to the active continental marginsetting in the major element discrimination diagrams incontrast to the trace element diagrams.

In relation to the Ordovician samples the TiO2–Fe2O3+MgO diagram is the most successful as it assigns74% of the samples to the defined fields. This diagramalso highlights the decreasing concentration of maficminerals between the Formations of the South MayoOrdovician stratigraphy as the Derrylea and Sheeffrysamples have a much higher weight percent of ferro-magnesian oxides and also a higher TiO2 concentrationrelative to the Rosroe and Mweelrea samples (Table 1).The TiO2–Fe2O3+MgO diagram fails to assign themajority of the Derrylea and Sheeffry samples to the pre-defined fields, with only one of the Derrylea sam-ples assigned to the continental island arc field. TheMweelrea and Rosroe samples are well constrained bythis diagram with 80% and 91% of the samples plottingin the defined fields, respectively. Of the 91% of theMweelrea samples 85% are assigned to the activecontinental margin field with the remaining 6% plottingin the continental island arc field. The Rosroe samples

Table 3Results from the ten discrimination diagrams for the sedimentary rock samples

Bhatia (1983) discriminationdiagrams

Oceanic islandarc — field A

Continental islandarc —field B

Active continentalmargin — field C

Passivemargin — field D

Unclassified % assignedto ACM

Ordovician samplesTiO2 – Fe2O3+MgO 1 4 35 0 18 61Al2O3/SiO2 – Fe2O3+MgO 2 1 27 2 26 46K2O/Na2O – Fe2O3+MgO 1 1 6 6 44 10Al2O3/(CaO+Na2O) –

Fe2O3+MgO6 2 14 8 28 24

Silurian samplesTiO2 – Fe2O3+MgO 7 3 2 5 2 10.5Al2O3/SiO2 – Fe2O3+MgO 4 0 1 5 9 5.3K2O/Na2O – Fe2O3+MgO 0 1 2 1 15 10.5Al2O3/(CaO+Na2O) –

Fe2O3+MgO2 3 2 0 12 10.5

Bhatia (1983) Discriminant func.diagram — Ordo. samples

0 5 41 9 0 74

Bhatia (1983) Discriminant func.diagram — Silurian. samples

0 2 14 3 0 74

Bhatia and Crook (1986)discrimination diagrams

Oceanic islandarc — field A

Continental islandarc —field B

Active continentalmargin — field C

Passivemargin — field D

Unclassified % assignedto ACM

Ordovician samplesLa – Th 1 28 0 0 28 0Ti/Zr – La/Sc 4 0 0 0 53 0La/Y – Sc/Cr 0 0 7 0 50 12

Silurian samplesLa – Th 2 2 0 0 15 0Ti/Zr – La/Sc 0 0 0 0 19 0La/Y – Sc/Cr 0 0 0 10 9 0

Roser and Korsch, 1988discriminant function diagram

P1 P2 P3 P4 Unclassified % assignedto ACM

Ordovician samples 0 1 10 43 0 18.5Silurian Samples 0 0 1 18 0 5.2

Roser and Korsch, 1986K2O/Na2O– SiO2

Passive margin Active continentalmargin

Island arc Unclassified % assignedto ACM

Ordovician samples 29 24 1 2 43Silurian Samples 6 13 0 0 68

Also shown is the percentage of samples assigned to the active continental margin field (ACM) in each diagram.


have a dominantly active continental margin signaturewith six of the ten samples plotting in this field with theremaining samples spread between the continental islandarc and oceanic island arc fields.

A notable feature from the four bivariate diagramsdefined by Bhatia (1983) is the high number of sampleswhich fail to be assigned to the defined fields. In theAl2O3/SiO2 diagram 41% of all the samples are assignedto the defined fields. The Mweelrea and Rosroe sampleswere the most successful with 80% of the Rosroe sam-ples and 53% of the Mweelrea samples indicating anactive continental margin setting.

The K2O/Na2O–Fe2O3+MgO and Al2O3/(CaO+Na2O)–Fe2O3+MgO diagrams are poor discriminationdiagrams as 49–77% of the Ordovician and Siluriansamples fail to be classified.

For the Silurian samples the TiO2–Fe2O3+MgOdiagram is the most successful with 89% of the samplesassigned to the defined fields. Similar to the Ordoviciansamples, the Al2O3/SiO2 diagram assigns the secondhighest number of samples (53%) to the defined fields.

Roser and Korsch's (1988) discrimination diagram(Fig. 4A) divides the Ordovician samples into two dis-tinct groups: one group in the quartzose sedimentary

Fig. 4. Discrimination diagrams for sedimentary rocks. Roser and Korsch (1988) diagram for Ordovician and Silurian samples (A) and (B) respectively,plotted using anhydrous normalised data. Bhatia (1983) discriminant function diagram for the Ordovician and Silurian samples (C) and (D) respectively,plotted using actual abundances as analysed. CIA: continental island arc setting. Roser and Korsch (1986) K2O/Na2O–SiO2 diagram for Ordovician andSilurian samples (E) and (F) respectively, plotted using anhydrous normalised data.



field (P4), defined by the Derrylea and Sheeffry sam-ples and a second group comprising the Mweelrea andRosroe samples which lie on either side of the boundarybetween the P3 and P4 fields. The majority (95%) of theSilurian samples (Fig. 4B) plot in the P4 field and areskewed towards the P3 field. The remaining sample isfrom the Strake Banded Formation and plot within theP3 field.

The Ordovician samples are separated into twogroups on Bhatia's (1983) discriminant function dia-gram (Fig. 4C): the Mweelrea and Rosroe samples inthe active continental margin field and the Derrylea andSheeffry samples in the passive margin and continentalisland arc field. Unlike the bivariate diagrams the dis-criminant function diagram exhibits less overlap andscatter with all the samples assigned to a specific tec-tonic setting. The Derrylea and Sheeffry samples have asimilar distribution extending from the passive marginfield into the continental island arc field with two ofthe Sheeffry samples plotting in the active continentalmargin field.

An active continental margin setting is indicated forthe Silurian samples with 74% of the samples plotting inthis field (Fig. 4D). Five of the Silurian samples plotoutside the active continental margin setting: three in thepassive margin setting and two in the Continental Islandarc setting.

The K2O/Na2O–SiO2 diagram (Fig. 4E) assigns thesedimentary samples differently to the previous majorelement diagrams with 51% of the samples in the pas-sive margin setting, the majority of these samples arefrom the Mweelrea Formation. The remaining samplesare assigned to the active continental margin setting.Within this field the Rosroe samples are separated fromthe Derrylea and Sheeffry due to their lower K2O/Na2Oratios.

The Silurian samples plot along the boundary forthe active continental margin and passive margin fields(Fig. 4F). The Bunnamohaun and Bouris samples areconfined to the active continental margin setting whilstthe Strake Banded Formation samples are dividedbetween the two fields with 32% in the passive marginfield and the remaining 68% in the active continentalmargin setting.

Only one of the trace element discrimination diagrams,La/Y–Sc/Cr, assigned a small percentage (12%) of thesedimentary rock samples to the active continental marginsetting.

The Ordovician and Silurian samples have similardistributions on the La–Th discrimination diagrams(Fig. 5a and b) with the samples clustered at the bound-ary of field B indicating a continental island arc setting.

The clustering is reflected in the similar La/Th ratios forthe samples. The Ordovician formations have averageLa/Th ratios of: 3.41 for the Mweelrea Formation, 3.65for the Rosroe Formation, 3.58 for the DerryleaFormation and 3.49 for the Sheeffry Formation. TheSilurian Formations have slightly higher La/Th ratioswith average values of 4.0 for the BunnamohaunFormation, 3.97 for the Stake Banded Formation and3.76 for the Bouris Formation.

Less than 7% of the Ordovician samples are assignedto the fields defined by Bhatia and Crook (1986) on theTi/Zr–La/Sc diagram (Fig. 5c). The Derrylea andSheeffry samples have an average Ti/Zr ratio of 33and 32 and La/Sc average of 2.7 and 2.9, respectively.These are within the range of values for the continentalisland arc setting of 10–35 for Ti/Zr and 1–3 for La/Sc.The Rosroe and Mweelrea samples all have higher Ti/Zrratios than the Derrylea and Sheeffry samples and arealso more scattered. The Silurian samples fail to be as-signed to the defined fields on the Ti/Zr–La/Sc diagram(Fig. 5d) and are widely scattered due to variations in theLa/Sc ratios.

The La/Y–Sc/Cr diagram (Fig. 5e) assigns 12% of theOrdovician samples to the active continental marginsettings. All samples in the active continental marginsetting (Field C) are from the Mweelrea Formation. Theremainder of theMweelrea samples plot within the Sc/Crrange for the active continental margin setting but have amuch wider La/Y range and as a result are outside thedefined field. The diagram separates the Derrylea andSheeffry samples from the remainder of the Ordoviciansamples and forms a tight cluster with little variation.The Derrylea and Sheeffry samples plot close to thepassive margin setting which is defined by Sc/Cr valuesof less than 0.2.

The La/Y–Sc/Cr diagram (Fig. 5f) assigns 53% ofthe Silurian samples to the passivemargin setting and failsto reflect the volcanic signature present in the sampleswhich was observed in the major element discriminationdiagrams.

The Ordovician and Silurian samples were also plottedon two diagrams which do not have defined fields buthave been described as useful discriminators by Bhatiaand Crook (1986).

The V–Sc diagram (Fig. 6A) groups the samples to-gether but with one obvious variation. There is a positivelinear trend in the Rosroe samples with increasing Vand Sc. Three of the Mweelrea samples exhibit a simi-lar trend, with the enrichment more in vanadium. TheSilurian samples (Fig. 6C) define a positive linear trendsimilar to the trend observed in the TiO2+–Fe2O3+MgOand Al2O3/SiO2–Fe2O3+MgO diagrams and appears to

Fig. 5. (a) La–Th diagram with the Ordovician samples plotted. (b) La–Th diagram with the Silurian samples plotted. (c) Ti/Zr–La/Sc diagram with theOrdovician samples plotted. (d) Ti/Zr–La/Sc diagramwith the Silurian samples plotted. (e) La/Y–Sc/Cr diagramwith theOrdovician samples plotted. (f ) La/Y–Sc/Cr diagram with the Silurian samples plotted. The fields on the diagrams were defined by Bhatia and Crook (1986): A=oceanic island arc; B=continental island arc; C=active continental margin; D=passive margin. Diagrams were plotted using actual abundances of the trace elements as analysed.


Fig. 6. (A) V–Sc diagram for Ordovician samples. Arrow indicates the linear trend defined by samples with an increasing volcanic component. (B) Zr–Thdiagram for Ordovician samples, (C) V–Sc diagram for Silurian samples, (D) Zr–Th diagram for Silurian samples. Diagrams were plotted using actualabundances of the trace elements as analysed.


represent an increase in the volcanic component in thesamples. The samples, in Fig. 6C, are separated into twogroups with one group defined by low vanadium andscandium values.

The Zr–Th diagram (Fig. 6B) clearly separates theOrdovician samples into two groups. The average Zr/Thratio for theMweelrea (7.56) and Rosroe (9.15) samplesis similar to average ratio for the active continentalmargin setting described by Bhatia and Crook (1986).The Derrylea (19.5) and Sheeffry (16.5) samples haveZr/Th ratios closest to the ratio for the Passive Marginsetting. The ratios for the Silurian Formations (Fig. 6D)overlap one another and are similar to the ratios forContinental Island Arc (CIA) and Passive Margin (PM)settings.

6. Discussion of sedimentary discrimination diagrams

The active margin setting identified from the SouthMayo tuff band signatures is not conclusively reflectedby the sedimentary discrimination diagrams. The re-sults for the sedimentary rocks from the OrdovicianFormations show that 6 of the 10 discrimination di-agrams indicate an active continental margin settingfor the Mweelrea samples. The Rosroe samples weremore successful with 8 of the 10 diagrams indicatingan active continental margin setting or continentalisland arc setting. The Derrylea and Sheeffry sedimen-tary rock results were similar in the ten diagrams: threeof the diagrams failed to assign any of the Derrylea andSheeffry samples to a defined field and five of the


diagrams assign the samples to the passive marginsetting. The results of the discrimination diagrams forthe Silurian samples are less accurate than those for theOrdovician samples with five of the ten discriminationdiagrams indicating an active margin setting for thefour formations. Only three of the diagrams classifymore than 50% of the samples to the active continentalmargin setting.

When the tectonic model for the region is consideredthe most accurate diagrams are those which use discrim-inant functions to plot the samples (Bhatia, 1983; Roserand Korsch, 1988). Table 3 shows that only 18.5% ofthe samples were assigned to the field for the activemargin setting on Roser and Korsch's (1988) prove-nance diagram (Fig. 4A). The accuracy increases to78% when the tectonic model for the region is con-sidered. The Rosroe and three of the Mweelrea rocksamples have a chemistry which places them in the P3field indicating a felsic igneous provenance. Roser andKorsch (1988) reported that this source would includean active continental margin setting which is thesignature identified from the South Mayo tuff bands.Although there appears to be a separation between theRosroe and the majority of the Mweelrea samples, thelatter are skewed towards the P3–P4 boundary sug-gesting that the volcanic contribution to the Mweelreaand Rosroe sediments is similar. The position of theMweelrea samples is interpreted as a true reflection ofthe provenance with active volcanism founded on aquartz-rich source region. This type of setting for theMweelrea Formation has previously been described intectonic models for the region (e.g. Williams and Rice,1989; Dewey and Mange, 1999). The Silurian samplesin the P4 field (quartzose sedimentary provenance)(Fig. 4B) plot in the same location as the Mweelreasamples in Fig. 4A indicating a similar setting for thesource region of the Silurian formations.

Bhatia's (1983) discriminant function diagram (Fig. 4Cand D) correctly assigns 74% of all the samples tothe active continental margin setting. The Derrylea andSheeffry samples are separated from the other Ordovi-cian samples in both discriminant function diagrams(Fig. 4A and C) and are assigned to the passive marginfields. This separation of the Ordovician samples isrepeated throughout the discrimination diagrams and isclearly evident on the multi-element trace element di-agram (Fig. 3C). This separation is attributed to a sep-arate source region contributing to the sedimentation ofthe Derrylea and Sheeffry Formations and is discussedin more detail below.

The difficulty in accurately identifying the tectonicsetting from the sedimentary rocks was well illustrated

by the variation in the results from Bhatia's (1983)discrimination diagrams. The TiO2–Fe2O3+MgObivariate diagram was the most successful for theOrdovician Formations assigning 74% of the samplesto defined tectonic settings, although only 61% of thesewere assigned to the active continental margin field.The remaining three diagrams were less successful.The Al2O3/SiO2, K2O/Na2O, and Al2O3/(CaO+Na2O)diagrams assigned 46%, 10%, and 24% of the sam-ples to the active continental margin setting, respec-tively. The Silurian samples were less well classifiedwith only 10.5% of the samples assigned to the activemargin setting on the TiO2, K2O/Na2O and Al2O3/(CaO+Na2O) diagrams. The Al2O3/SiO2, diagram as-signed 5% of the Silurian samples to the active marginsetting.

In the diagrams which used K2O, Na2O and to a lesserextent CaO the samples (Ordovician and Silurian) werescattered on the diagrams. This was clearly evident on theK2O/Na2O and the Al2O3/(CaO+Na2O) diagrams wheresamples from the same formation exhibited a wide vari-ation. A comparison of the signatures from the Ordo-vician and Silurian samples from Roser and Korsch's(1986) K2O/Na2O–SiO2 diagram to signatures from theother diagrams shows that the use of the K2O/Na2O ratioas a discriminator appears to only work for one set ofdata. The Ordovician samples are assigned to differenttectonic settings on the K2O/Na2O–SiO2 diagram than inthe discrimination diagrams defined by Bhatia (1983)whilst the Silurian samples are assigned to similar set-tings in the bivariate diagrams. These findings are inagreement with McLennan et al. (1993, 2003) who ad-vise caution when using sodium and potassium as dis-criminators due to their high concentration and residencetime in ocean waters, reflecting their mobility. Wronkie-wicz and Condie (1987) have shown that sodium is alsosusceptible to leaching through weathering. Therefore thereliability of the diagrams, which employ K2O, Na2O,and CaO in correctly discerning the tectonic setting isquestionable.

Bhatia (1983) used Fe as a discriminator as it wasconsidered to be immobile. However, Whiteley andPearce (2003) have shown that metals such as Fe andMn can become mobile during diagenesis, thereby al-tering their concentrations and making them less ef-fective as provenance discriminators. In the bivariatediagrams the major scatter is due to variation along they axis and not in relation to Fe and Mg (on the x axis)so element mobility is not considered a major factor forthese samples. The failure of the Derrylea and Sheeffrysamples to be assigned to the defined fields in theTiO2–Fe2O3+MgO appears to be the result of variation


along the x axis. This variation is interpreted as areflection of the mafic enrichment in the samples whichis confirmed by the enrichment in ultramafic trace ele-ments on the multi-element profiles (Fig. 2C), and not aresult of the mobility of Fe or Mg.

The trace element bivariate discrimination diagramswere less accurate than the major element diagrams inassigning the sedimentary samples to the active con-tinental setting. In the La/Th and Ti/Zr–La/Sc diagramsnone of the Ordovician samples were assigned to theactive margin setting whilst only 17% of the sampleswere assigned to this setting on the La/Y–Sc/Cr dia-gram. The Silurian samples also failed to be assignedto the active margin setting on the three trace elementdiagrams.

From these results it is clear that the defined fieldson the trace element diagrams are less reliable thanthe major element diagrams. However, the elementschosen as discriminators were efficient in highlightingdifferences between the samples which can be usefulin elucidating the history of the sediment. The traceelement diagrams clearly highlighted differences be-tween the Rosroe+Mweelrea samples and the Derry-lea+Sheeffry samples. In the second group the traceelement signature continuously trends towards a pas-sive margin setting whilst an active margin setting isindicated for the Rosroe–Mweelrea group. This

Fig. 7. QFL diagram for a selection of the South Mayo Ordovician and Silurian(1985).

separation was also identified on the major elementdiagrams and was supported by petrographic analysisof a selection of the samples (Fig. 7). The Ordovicianand Silurian samples have La/Th ratios rangingbetween 3.41 and 4.00. These values fall in therange between the continental island arc field, 2.4, andthe oceanic island arc field, 4.0, (Bhatia and Crook,1986) and show that despite the failure of the definedfields to correctly identify the tectonic setting of thesamples, the trace element discriminators were effi-cient in identifying the volcanic component present inthe samples.

Bhatia and Crook (1986) attributed La and Th abun-dances in sediments to a granitic or recycled detritussource rock. For the Rosroe and Mweelrea Formationsthe felsic volcanic signature of the tuff bands andvolcanic lithics observed during petrographic analysissuggests that the high La/Th ratios are related to theigneous origin rather than recycled detritus.

Bhatia and Crook (1986) proposed that the Zr/Thratios can be used to distinguish between an activecontinental margin and passive margin setting. For theOrdovician samples (Fig. 6B) the Zr/Th values forDerrylea–Sheeffry samples have a similar ratio to thepassive margin setting (19.1±5.8) identified by Bhatiaand Crook (1986). Given that all the samples have asimilar range of Th values, the high Zr/Th ratio is

sedimentary samples (From Ryan, 2005). Fields defined by Dickinson


attributed to the Zr abundance within the samples. Thisis characteristic of the passive margin settings where thehigh Zr value can be attributed to a concentration ofheavy minerals in a recycled sedimentary rock. Theaverage Zr/Th ratios for the Rosroe and Mweelreasamples (9.15 and 7.56, respectively) are closest to theratio for sedimentary rocks from an active continentalmargin (9.5±0.7— Bhatia and Crook, 1986), support-ing the signature identified from the discriminationdiagrams. It is more difficult to assign the Siluriansamples to a particular tectonic setting as the Zr/Thratios from the four formations are in the range for thecontinental island arc and a passive margin setting. Thischemical signature for the Silurian samples could resultfrom two source regions contributing to the sediments, apassive source region and a volcanic source region, orderivation of the sediments from an active continentalmargin setting.

The V–Sc diagram (Fig. 6A and C) is particularlyuseful in highlighting samples which have an increasedvolcanic component. Bhatia and Crook (1986) describeV and Sc as residing in the mafic component ofgreywackes. Given the mafic element enrichment in theDerrylea and Sheeffry samples these elements wereexpected to act as good discriminators of the SouthMayo samples. Fig. 6A shows that the elements donot separate the samples into the groups observed inthe other discrimination diagrams. The Derrylea andSheeffry are slightly more enriched in Vand Sc, whilst anumber of the Rosroe and Mweelrea samples define apositive linear trend. Petrographic analysis revealsthat the samples in the linear trend contain a higherproportion of volcanic lithic fragments. In the case ofthe Mweelrea samples these were expected to havean increased volcanic component as sampling occurredwhere tuff-sediment interaction was evident. The sepa-ration of the Silurian samples into two groups also ap-pears to be reflecting an increased volcanic componentand is similar to the distribution observed on the TiO2–Fe2O3+MgO diagram. The distribution of the Ordovi-cian samples on Fig. 6A suggests that the ultramaficenrichment in the Derrylea and Sheeffry samples,which is confined to Cr, Ni, and Co, appears to reflectthe weathering of an ultramafic source (Wrafter andGraham, 1989) rather than the result of weathering of avolcanic component at the source region.

The separation of samples with an increasedvolcanic component is also observed in the Ti/Zr– La/Sc diagrams (Fig. 5c and d), however the samples withan increased volcanic component are assigned to theoceanic island arc field which is improbable. Theseparation of the Rosroe and three Mweelrea samples is

due to the higher abundance of Sc, resulting in a lowerLa/Sc ratio. The low La/Sc ratio from an increasedvolcanic component is confirmed by the Silurian sam-ples. In this diagram (Fig. 5d) the group which haspreviously been shown to have an increased volcaniccomponent plots close to the oceanic island arc setting(Field A), similar to the Rosroe samples. The groupingof the Derrylea and Sheeffry samples on the diagram(Fig. 5c) is due to the higher abundance of Zr resultingin the lower Ti/Zr ratios. As discussed above for the Zr–Th diagram (Fig. 6B), the abundance of Zr is attributedto a passive margin source. This suggests that a sig-nificant portion of the sediment in the Derrylea andSheeffry Formations was derived from a source awayfrom the active margin.

In the La/Y–Sc/Cr diagram (Fig. 5e) the Mweelreaand Rosroe samples appear to be linked to the activecontinental margin field. The very low Sc/Cr values(0.01–0.04) for the Derrylea and Sheeffry samples are aresult of the high Cr concentrations in these two for-mations. These ratios are closest to the values definingthe passive margin field. The results support Bhatia andCrook's (1986) finding that the Sc/Cr ratio decreasesmoving from an arc setting to a passive margin setting.As with previous diagrams which used La and Sc asdiscriminators, a wide scatter is seen for samples with ahigher volcanic component e.g. Rosroe and Mweelreasamples. In contrast to the signature reflected in previousdiagrams, 56% of the Silurian samples are clusteredin the passive margin field of the La/Y–Sc/Cr diagram.The samples are from the Bouris, Bunnamohaun, andStrake Banded Formations which were grouped with thevolcanic rocks in the V–Sc and Zr–Th diagrams (Fig. 6Band C).

The overall results highlight the major problemwith the diagrams in which the majority of the ele-ments used are good discriminators but the samplesfail to be assigned to the correct setting. A similarfinding has been reported in other studies (e.g. Floydet al., 1991; Armstrong-Altrin and Verma, 2005)which questioned the validity of defining the fieldboundaries in the original discrimination diagramsusing a small data set.

Whitmore et al. (2004) studied the effects of grainsize on the mineralogy and geochemistry in modernsediments derived from a modern day arc–continentcollision zone and used Bhatia's (1983) discriminantfunction diagram to test the geochemical signatures.They reported that most of the samples were mis-classified on the diagram as over half the samples wereassigned to the passive margin field yet none were fromthat setting. The greater accuracy in assigning the South


Mayo sedimentary samples using this diagram may bedue to their Lower Palaeozoic age and the diagenesiswhich the samples have undergone, as the samples usedto define the fields in Bhatia's (1983) diagrams andBhatia and Crook's (1986) were of a similar age.

The geochemical signature from the Rosroe andMweelrea sedimentary rocks and the Silurian Forma-tions matches that for the volcanic rocks, whilst theDerrylea and Sheeffry sedimentary rocks appear to havea very different signature to the tuff bands. The lack of avolcanic component in the Derrylea and Sheeffry sedi-mentary samples may be a result of a contribution to thesedimentation from a source unaffected by volcanism.This is supported by the high ultramafic signature in thesediments and is in complete contrast to the felsicsignature of the interbedded tuff bands. This enrichmentis attributed to the weathering of an ultramafic sourcefounded on the Laurentian crust away from the activecontinental margin.

7. Conclusions

Previous studies have been carried out to test theaccuracy of discrimination diagrams for sedimentaryrocks (e.g. Floyd et al., 1991; Armstrong-Altrin andVerma, 2005). However these studies used modernsediments and sedimentary rocks from known tectonicsettings and applied them to the diagrams. The geo-chemical analysis carried out in this study is novel as thegeochemical signature of the sedimentary rocks iscompared directly to the signature from the interbeddedtuff bands. The latter agrees with the active marginsetting indicated from field studies of South Mayo.

The overall results show that the principle behindthe diagrams is sound as the elements chosen as dis-criminators are effective in highlighting differencesbetween samples, which is useful in elucidating thehistory of the sediment. However, the results also sup-port findings from previous studies that the fieldsdefined on the majority of the established discrimina-tion diagrams for sedimentary rocks cannot be uni-versally applied. In addition the study identified thefollowing:

• The discrimination diagrams which employ the useof discriminant function analysis were found toaccurately reflect the tectonic setting of the samples.Roser and Korsch's (1988) discrimination diagramwhich assigned the samples to a particular prove-nance worked well, but it was found that the bound-ary between the P3 and P4 fields should be viewed asgradational to allow for sediments derived from a

highly siliceous volcanic source to be assigned to avolcanic provenance.

• The active continental margin signature in theRosroe and Mweelrea samples and the passive mar-gin signature for the Derrylea and Sheeffry samplesshow that it is possible to identify a combination ofsediment provenances at a particular tectonic setting.This highlights the effectiveness of combiningmulti-element diagrams, discriminant function dia-grams and bivariate discrimination diagrams to beused as a lever to reveal significant informationabout the sediment source.

• The use of Na2O and K2O as discriminators on majorelement bivariate diagrams (after Bhatia, 1983) re-sulted in a wide scatter of the samples; this is attributedto the mobility of these elements during sediment–water interaction.

• The fields defined by Bhatia (1983) and Bhatia andCrook (1986) on the bivariate discriminationdiagrams for major and trace elements cannot beuniversally applied. In relation to the South Mayosamples the diagram using TiO2 as a discriminatorwas the most successful, assigning 74% of thesamples to the defined fields. However, only 48% ofthe samples were assigned to the correct tectonicsetting. On the trace element diagrams the majorityof the samples failed to be assigned to the definedfields.

• The study confirmed that the trace elements V and Scare particularly good indicators of sediments enrichedin a volcanic component. In addition the ability ofZr and Th to differentiate between an active marginprovenance and samples with a passive marginprovenance was confirmed.

• The poor results from the use of single discriminationdiagrams shows that individually they cannot ade-quately identify an active continental margin settingfrom the geochemical signature of sedimentary rocksusing the fields as they are presently defined. This isa reflection of the different provenances which cancontribute to the sedimentation at the active marginsetting which results in mixing of the geochemicalsignatures.

Acknowledgements

We thank IRCSET for funding for KMR, theMillenium Fund, NUI Galway for DMW, and H.O'Donnell for assistance with the geochemistry andOMAC Laboratories. Also, J. Menuge, B. Roser and R.Rudnick for their helpful suggestions which improvedthe manuscript.


Appendix A

Sco-1
Sco-1 AGV-1 AGV-1 JB-1 JB-1 G-2 G-2 Measured Published Measured Published Measured Published Measured Published
SiO2
62.3 62.78 59.86 58.84 52.90 52.17 69.62 69.14 Al2O3 13.95 13.67 16.63 17.15 14.62 14.53 14.95 15.39 CaO 2.71 2.62 5.21 4.94 9.69 9.29 2.07 1.96 Fe2O3 5.15 5.14 6.51 6.77 8.74 8.97 2.59 2.66 K2O 2.86 2.77 3.08 2.92 1.52 1.43 4.65 4.48 MgO 2.72 2.72 1.58 1.53 8.04 7.73 0.77 0.75 Na2O 0.94 0.90 4.59 4.26 2.96 2.79 4.23 4.08 P2O5 0.24 0.21 0.54 0.49 0.29 0.26 0.16 0.14 TiO2 0.57 0.63 1.08 1.05 1.31 1.34 0.50 0.48 MnO 0.06 0.05 0.11 0.09 0.17 0.16 0.04 0.03 Sc 12.10 10.8 12.29 12.2 28.56 27.2 – 3.5 Cr 73.65 68 9.80 10.1 403.69 400 – 8.7 Co 11.15 10.5 15.32 15.3 37.82 38.7 – 4.6 Ni 26.16 27 18.33 16 133.35 139 – 5 Rb 122.11 112 67.94 67.3 41.07 41.2 – 170 Sr 183.48 174 669.72 662 459.63 435 – 478 Y 20.92 26 18.26 20 24.10 24.4 – 11 Zr 118.12 160 237.88 227 150.64 143 – 309 Nb 13.26 11 15.67 15 37.44 34.5 – 12 Cs 8.20 7.8 1.33 1.28 1.20 1.19 – 1.39 Ba 567.37 570 1268 1266 495.11 490 – 1882 La 29.57 29.5 37.80 38 36.92 37.9 – 89 Ce 57.18 62 68.03 67 66.65 66.7 – 160 Sm 5.18 5.3 5.98 5.9 5.10 5.07 – 7.2 Yb 2.09 2.27 1.67 1.72 2.15 2.16 – 0.8 Hf 3.39 4.6 6.97 5.1 3.99 3.4 – 7.9 Ta 0.99 0.92 1.14 0.9 3.12 2.7 – 0.88 Pb 32.91 31 37.75 36 10.30 11.5 – 30 U 3.10 3 1.92 1.92 1.79 1.7 – 2.07
The measured and published (Govindaraju, 1994) concentrations of the major and trace elements for the analysed international standards (SCo-1,AGV-1, JB1, and G-2).

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