Journal of the Geological Society, London, Vol. 144, 1987, pp. 699-706, 6 figs, 2 tables. Printed in Northern Ireland

An occurrence of shoshonites near Kilmelford in the Scottish Caledonides and its tectonic implications

J.-X. ZHOU Department of Geology, Imperial College of Science and Technology, Prince Consort Road, London SW7 2BP, UK and c/o Professor Wu Liren, 6th Division, Institute of Geology, The Chinese Academy of Science, Beijing, The People’s Republic of China

Abstract: The Kilmelford intrusives were emplaced in Dalradian metasediments and eruptives during the late stages of the Caledonian orogeny. The suiteof rocks includes biotite-hornblende diorite and, to a minor extent, biotite-pyroxeneand biotite-hornblende-pyroxene diorites,together with hornblende-biotite-quartz diorite, granodiorite, porphyrites of dioritic and dacitic composition and volcanic breccia. The major element geochemistry shows that the Kilmelford rock suite is alkali-calcic having petrogenetic affinities with the igneous rocks of the New Guinea ‘continental arc’. The high contents of K, Sr, Ba, Rb, Zr and Ce, together with a high K,O/Na,O ratio, give these rocks their shoshonitic signature, which is of interest as an unusual example of the occurrence of shoshonites in the Scottish Caledonides. The rocks have very steep REE patterns (Ce,/Yb, = 10-28) and do not show a Eu anomaly. The shoshonitic signature of the Kilmelford rocks is interpreted as due to the late orogenic- nature of the magmatism - which tookplace when subduction had abated following the collision of the two continents.

Inthe last two decades calc-alkaline andshoshonitic I I I I 80 6O 60 20 I p associations havereceived much attention from geologists dueto their importance in island arcs, activecontinental margins andcontinental collisionzones. Theterm m Volcanics of Old Red Sandstone age shoshonite was first used by Iddings (1913) and later revived by Joplin(1965). Since then many papers havebeen published on shoshonitic assemblages (Dickinson et al. 1968; 0 100 km ORKNEY U Joplin1968; Jakes & White 1969,1971, 1972; Gill 1970; Jakesand Gill 1970;Lefevre 1973; Hormann et al. 1973; Barberi et al. 1974; Johnson et al. 1976; Arculus et al. 1977). N Ithas been recognized that shoshonitic and high-Krocks maybe related to specific tectonicenvironments although the resolution of this problem is complex. The present paper draws attention to an occurrence of shoshonites in an old orogenic belt and is based on the results of a comprehensive investigation of the geology of the Kilmelfordintrusive series in W Scotland. A brief account of the geology of the series,based upon field andpetrographic observations, is followed by adiscussion of theshoshonitic geochemical character of the rocks and the tectonic implications of this character.

History of research of the area The Kilmelfordintrusive series is located to the south of Oban in Argyllshire, Scotland (Fig. 1). Geological studies in thearea began during the 1800s whenSurvey workers mapped six areas of outcrop and observed the diorite,which was referredto as granite, a large mass of felsite tothe south of Kames,and the Lagalochan agglomerate (which was regardedas the basal conglomerate of theOld Red Fig. 1. Location of Kilmelford and distribution of the Old Red Sandstone) (see Fig. 2 for location). The general geology of Sandstone volcanic rocksin N Britain. Kilmelford and the adjacent areais described in detail in the Memoirs of the GeologicalSurvey relating tothe 1inch werepublished until the work of Knill (1957) who sheets 36 (Peach et al. 1909) and 37 (Hill et al. 1905). After investigated thepre-Tertiary geology of the Craignish- theappearance of thememoirs, no further investigations Kilmelford district anddescribed theintrusions at

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Kilmelford. In 1976, investigations of the copper minerali- belongs tothe Great Glen Fault set of Watson(1984). zation and the related intrusions at Kilmelford were carried NW-SE and E-W structures also occur in the Kilmelford out by the BGS and by Rickard (1979). Related papers were district andthe E-Wstructural lines are particularly subsequentlypublished by Ellis et al. (1977) and Ellis important because they appear to control the distributionof (1977). The map by Knill (1957) has been adapted to take the igneous bodies. into account the results of the field work carried out during In addition to the Kilmelford intrusions, late Caledonian the present study (Zhou 1985b). igneous rocks in the adjacent area include the Lorne lavas andthe rocks of theappinite-kentallenite suite of SW Scotland. Regional setting Recenttectonic models for the evolution of the Scottish Caledonides are based on the idea that the Iapetus ocean, Geology and petrography separatingalarge European continent from the North The Kilmelfordintrusive suite is composed of aboutten Atlantic continent together with a number of smaller massifs individualsmall bodies of diorite, granodiorite, porphyrite (Dewey 1969), opened about the beginning of the Cambrian and volcanic breccia outcroping in an area of 50 km2 (Fig. periodand closedtowards the end of Silurian(Watson 2). Diorites occur at Tom Soillier, Meall Mor, Ceann Mor 1984). The Caledoniancycle ended in collision orogeny and An Cnap. The Tom Soillier diorite is cut by two large resulting in the closure of the Iapetus ocean. The suture is oblique faults and the Meall Mor diorite by an E-W fault. locatedbetween the Lake District and the Southern The rockcontains 45-70% plagioclase of andesine Uplandsand continues into Ireland via the SolwayFirth composition, 25-40% ferromagnesianminerals (mainly (Phillips et al. 1976). The Kilmelfordarea is therefore biotiteand green amphibole) and 2-5% quartzand situated about 200 km to the north of the suture zone in the K-feldspar. Although the relative proportion of biotite and strongly deformedmarginal part of theformer North amphibolevaries, biotite is oftenmore prominent. Atlantic continent. Pargasitic hornblendeand pyroxene are observed in some It is generallyaccepted that the basement rocks specimensfrom the least evolvedunit. The mineralogy underlyingmuch of theGrampian Highlands probably indicates a rock type of quartz-dioritic composition. resemble the Lewisian complex of NW Scotland, which is a Granodiorite is mainly found to the north and west of complex of Archaeanto early Proterozoic gneissesand Loch Beinn Chaorach (towards the west of the area; Fig. 2). granulites. The coverrocks include the late Proterozoic The rockcontains 60% plagioclase of andesine-oligoclase Moineseries which are interfoldedwith young Moines of composition,strongly zoned, 15-20% quartz, 10-15% similarfacies and with thelate Proterozoic to Cambrian biotite, 8% greenamphibole and 3% opaque minerals. Dalradian supergroup. The Kilmelford area lies to the SW K-feldspar is a minor constituent but may be more than 5% of the lenticular outcrop of Moinian rocks in the Grampian in somespecimens. The mineralogyshows the rock to be Highlandsand within a low grademetamorphic terrain biotite-hornblende-quartz diorite to granodiorite. consisting of Dalradianmetasediments; hasbeenit Porphyritesoccur at Kilbride, AnCnap, Carn Dearg suggested that the Moine layer may be absent in this region andLagalochan. The rockconsists typically of 40-50% and that the Dalradian may rest directly on a basement of phenocrysts setin a fine-grained, micropoikilitic ground- Lewisian type (Plant et al. 1980). mass. The phenocrysts are feldspar of andesine composition, The Dalradian group in the Kilmelford area consists of amphiboles of two pleochroic schemes (straw yellow/brown metasediments interlayered with early basic sills, now in the and light greedolive green) and small amounts of altered form of epidiorites.The sediments are divided into two biotite. The groundmass is composed of feldspar, biotite and formations: the Craignish phyllites (1-4 km thick) and the quartz. The mineralogy indicates a dioritic composition for Crinan grit (0.1-3 km thick). Unconformably overlying the the Kilbride porphyrite. products of theCaledonian orogeny is theOld Red Theporphyrites at An Cnap and Carn Dearg contain Sandstone which formedafter Caledonian folding and 60-70% by volume of phenocrysts. At Lagalochan, 30-50% metamorphism. The Lorne lavas occurring to the north of by volume of the porphyriteconsists of phenocrysts. At both the Kilmelford areaare associated with this terrestrial locations, 50-70% of the phenocryst population is made up sedimentary unit. of feldspar of andesine to oligoclasecomposition. Biotite Achange of tectonicregime after the closing of the makesup 5-15%, andthe remainder is formed by IapetusOcean led to post-collisional strike-slip movement amphiboleand quartz, both in minoramounts. The parallel to the Caledonian belt. Before the post-collisional groundmass is composed of feldspar and smaller amounts of faultingtook place the Dalradian, together with the biotiteand quartz. The mineralogysuggests a dioritic to epidiorites,was folded and metamorphosed. Knill (1957) daciticcomposition for the An Cnap, Carn Dearg and identified the Kilmelford area as a monoform on the western Lagalochan porphyrites. limb of the Loch Awe syncline which was formed during this A profile along the east coast of the Arduaine Peninsula period. Theemplacement of the Kilmelfordigneous suite shows that porphyrites were emplaced along the Dalradian clearly postdates the regional metamorphism and deforma- bedding planes as sheets causing brecciation of the country tion.The principalregional fault at Kilmelford is the rocks; the Dalradian fragments have retained their original Gleann Domhain fault which forms the eastern boundary of beddingdirection. On the other hand, in somelocations, the district andtrends NNE-SSW. Thelate Caledonian porphyritescross-cut country rocks to formcontinuous fractures of the Highlandshave been grouped by Watson masses in whichxenoliths of thecountry rocks are (1984) into three principal sets with NE-SW, NW-SE and disoriented. E-W trends. The Gleann Domhain faultis a continuation of Breccia is widely distributed around porphyritic bodies the NNE-trending Ericht-Laiden fault of SW Scotland and although, in some places, only brecciated country rocks are

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m r,.wa

%pp” e Fig. 2. Geological map of the Kilmelford area, W Scotland (from Knill 1957; Zhou 1985b). Dashed lines indicate faults.

observed.Alarge mass of breccia is observednear Lagalochan area wasa subvolcanic eruptive centre(Zhou Arduaine,containing breccia fragments of Dalradian 1985b). country rocks and vein quartz fragments of many different sizes ranging from 0.5 to 2 m across. Field evidence suggests that the breccia was formed by the local explosive disruption Whole rock chemistry of country rocks. The brecciation might have been caused About 100 representativesamples were selected for by explosive gas pressure which could have originated from chemicalanalysis. Themajor elements and a selection of within a magmatic body underneath or laterally adjacent to trace-elementswere analysed Philips aon PW1212 the country rocks. It is probable that such a body was of the automatic XRF spectrometerat Imperial College. The porphyrite type. method of preparation is fully described by Parker (1977). A large mass of agglomerate occurs to the north of the The calibration methods employed during the processing of Lagalochan porphyrite in the NE of the district. In the east thedata for both the major and trace elements, and the of this mass,the boundary between porphyrite and types of computer programs used during the data handling agglomerate is displacedtowards thesouth by 50-60 m andreduction are described by Parker(1978, 1979). along the NNE-SSWfault which follows the Lagalochan Rare-earthelements were determined onthe ICP at the valley. Blocks or fragments in the agglomerate are generally Department of Geology, King’s College(University of subangular and derived from the Dalradian country rocks, London).Sample preparation and analysisemployed porphyritesand felsites. Agglomeratefragments entrained conventionalrock dissolution procedures, followed by by agglomerateindicate that there is morethan one cationseparation techniques (seeWalsh et al. 1981 for generation of this material (hence more than one eruption in details). International rock standards were employed in the the area). The agglomerate is generally fragment-supported, construction of thecalibration lines. The quality of the andthe matrix present in some of themore compact analyses was controlled by: (1) running duplicate standards; agglomerates is composed of comminuted material with the (2) running duplicate samples; (3) a precision run which was same composition as the fragments. The agglomerates have made by analysing tenduplicate discs of onesample at been classified by Knill (1957) into four types according to random intervals throughout. the size of the fragments. Knill found no systematic pattern Representativeanalyses are given in Table 1. Average in the distribution of thesetypes, although crude bedding data for some elements arelisted in Table 2 and are grouped was identified during this study in the northern part of the according to evolutionarystages established using field agglomerate outcrop. The bedding is defined by intercala- observations, petrography and the geochemistry of major, tions of fine andcoarse layers in theagglomerate. Field trace, rare earth elements and mineral phases (Zhou1985b). observationsindicate that the agglomerates were erupted The analysesplotted in Fig. 3 arefrom the least altered contemporaneously with the Lagalochan porphyrite and the representative samples. The criteria for minimum alteration

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Table 1. Chemical compositions of representative samples of the Kilmelford shoshonitic suite (major elements in %; trace elements in ppm) Diorites POR D10 GDT PORPOR T S TS CM MM TS MM CM TSTS TS AC BCL CD L N o. ZK199ZK207No. ZK196 ZK281 ZK161 ZK204 ZK134 ZK246 ZK260 ZK55ZK93

SiO, 59.89 61.96 57.42 54.89 56.16 59.02 58.53 59.55 63.47 58.77 63.01 TiO, 0.76 0.54 0.78 0.77 0.80 0.79 0.48 0.61 0.39 0.42 0.35 A1203 17.10 16.53 16.97 17.47 17.75 16.29 16.19 16.81 16.26 15.69 16.58 Fe203 5 .OS 4.16 5.69 6.67 6.05 5.43 4.16 4.98 2.78 3.44 3.25 MnO 0.09 0.06 0.10 0.12 0.12 0.08 0.08 0.09 0.02 0.08 0.07 M@ 2.27 1.84 3.21 3.50 2.66 3.05 2.48 2.37 1.14 1.76 0.83 CaO 4.56 3.33 5.36 5.52 5.28 5.14 3.94 4.72 3.01 3.32 1.93 Na,O 4.01 4.49 4.42 4.01 4.14 3.76 4.28 4.57 4.47 2.58 3.39 K20 3.64 3.88 3.39 2.64 2.95 2.93 3.17 2.79 3.47 5.48 3.94 P205 0.34 0.29 0.44 0.47 0.40 0.32 0.25 0.26 0.17 0.28 0.17 H20 0.15 0.31 0.33 0.61 0.22 0.33 0.18 0.28 0.53 0.58 0.54 L01 1.26 1.53 1.06 2.47 2.75 1.95 5.48 1.98 3.63 6.70 4.62 Total 99.15 98.92 99.17 99.14 99.28 99.09 99.20 99.01 99.34 99.10 98.81 V 114 l0 129 146 121 115 80 92 59 80 44 Cr 81 25 31 79 71 96 98 103 106 19 87 Ni 18 24 34 21 23 34 39 34 17 16 20 Th 8 8 7 8 3 7 3 5 4 12 11 Rb 65 69 56 38 40 45 43 45 29 84 61 Sr 1486 1518 1776 1530 1817 1398 770 1167 1557 841 1043 Y 22 16 21 22 17 19 9 14 4 20 15 Zr 172 158 166 110 90 135 78 92 72 142 137 Nb 12 9 9 6 7 10 3 5 1 7 9 Ba 1648 1718 1867 1369 1236 1284 1260 1359 4461 1597 1041 La 42.2 35.8 45.2 44.4 30.9 32.5 21.0 20.9 15.1 55.6 30.9 Ce 89.6 69.7 88.1 88.2 59.4 59.3 42.4 47.3 28.6 126.5 71.0 Pr 10.2 8.2 11.6 9.9 7.0 7.4 4.5 5.9 4.0 12.7 7.4 Nd 44.2 35.0 51.5 45.7 32.4 34.3 21.5 25.5 17.2 53.2 30.0 Sm 7.9 6.0 8.9 8.5 5.7 6.3 4.0 4.4 2.8 8.8 4.9 Eu 1.84 1.41 2.08 2.04 1.51 1.50 1.14 1.13 0.77 2.02 1.30 Gd 5.04 3.96 5.67 5.58 3.87 4.14 2.88 2.97 1.98 5.40 3.24 DY 3.38 2.73 3.66 3.63 2.69 3.05 1.so 2.18 1.24 3.31 2.14 Ho 0.75 0.65 0.78 0.80 0.63 0.69 0.47 0.52 0.36 0.72 0.52 Er 2.04 1.79 2.14 2.11 1.66 1.77 1.os 1.30 0.90 1.95 1.45 Yb 1.36 1.13 1.26 1.36 1.04 1.16 0.63 0.83 0.48 1.01 0.85 Lu 0.23 0.22 0.23 0.23 0.18 0.20 0.12 0.14 0.10 0.17 0.16 REE 212 167 22 1 212 147 152 102 113 74 27 1 154 Ce/Yb 15.0 14.2 15.9 14.7 13.0 11.6 15.3 13.0 13.5 28.5 19.0

TS, Tom Soillier; K, Kilbride; MM, Meal1 Mor; CM, Ceann Mor; AC, An Cnap; BC, Beinn Chaorach; CD, Carn Dearg; L, Lagalochan; POR, porphyrite; DIO, diorite; GDT, granodiorite.

werebased on an analysis of loss on ignitionvalues for discrimination of rock series but also for recognition of combinedwith petrographic study of thealteration tectonic environment, e.g. as a crude pictorial definition of mineralogy (Zhou 1985b). arc maturity (Brown 1982). Brown (1982) hasalso used a variationdiagram in which the ‘calc-alkali ratio’, CaO/(Na,O + K,O) is plotted against silica. Individual rock Geochemistry of major elements in relation to rock series are well discriminated in the diagram, in contrast to suites the AFM plot where the trend lines of different rock series Various chemical criteria have been used to classify igneous tend to cluster towards the alkali apex of the diagram. The rockseries. Historically, the definition of the calcic, calc-alkali ratio-silica diagram is also useful because, apart calc-alkaline, alkali-calcic andalkaline rock suites was from the above advantages,it shows graphically the value of achievedusing the ‘alkali-limeindex’ of Peacock (1931). alkali-lime index for plotted rock suites. In Fig. 3 the plot Calc-alkalinerocks have an index of between 56 and 61, of log(ca1c-alkali ratio) againstSiO, gives an alkali-lime while the calcic suite has indices greater than 61. The range index of 54 for the Kilmelford series, which demonstrates forthe alkali-calcic suite lies between 51 and 56 andthe that the Kilmelford rocks are an alkali-calcic suite. alkaline suite has indices less than 51. Among other criteria TheAFM plot for the least altered samplesat the AFM plot is commonly applied. It is now not only used Kilmelford (Fig. 4) confirms the alkali-calcic nature of these

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Peru. Comparison of the Kilmelford rocks with these suites shows that the Kilmelford geochemical trend on the AFM NB diagram is similar to those of the New Guinea 'continental arc',Thailand and some of thegranitoids of the coastal batholiths of Peru. Comparison of the Kilmelford rocks with the same suites on the calc-alkali ratio-SiO,plot (Fig. 3) confirms the closesimilarity between the Kilmelford suite and the rocks of the New Guinea 'continental arc'. Calc-alkaline rocks generally are thought tobe related to subduction processes. Very little literature exists concerning the nature of igneous rocks formed in periods when active subductionprocesses are abating and collisional processes comeinto play. In order to understand more about the genesis of lateCaledonian magmatism in thetime period 400-415 Ma,Zhou (1985a)has discussed therelation betweenmagmatism and tectonism in parts of the Alpine-Himalayancollision zone. It was concluded that a wholespectrum of igneousrocks from calc-alkaline to alkaline affinity could beformed following the collisional stage of the orogenic cycle.

Trace and rare earth elements, and the shoshonitic character of the Kilmelford suite Baker (1982) and Ewart (1982) have recently reviewed data on igneous suites from shoshonitic and high-K associations. They recognized four volcanic series based on the variation Fig. 3. Comparative calc-alkali ratio-SiO, trends for intrusive of K20 in orogenic magmas. There is notonly a positive suites from Mesozoic and Tertiary magmatic arcs comparedwith the correlation of K,O with SiO,, but also a superimposition of range for normal calc-alkaline andesites (shaded). NB,New systematic trends in this variationfor any range of silica Britain-Solomon Islands arc; NM, New Guinea mobile belt; NC, contents.The four volcanicseries recognized arethe New Guinea continental arc;EP, Early Panama diorites; CA, shoshonitic series, the high-K series, the calc-alkaline series North Caribbean arc; LP, late Panama granodiorites;SN, Sierra andthe low-K series.Although Ewart (1982) hasdefined Nevada batholith; AR, Alaska Range batholith;*, average the shoshonitic series only for rocks carrying less than 63% Thailand granites (diagram adapted fromBrown 1982). K, the SiOz, Baker (1982) referred to the shoshonitic suite from the Kilmelford suite (from this study). Aeolian arc as having a wider range of SiO, contents, from 50 to 70%. rocks. Brown (1982) has given comparative AFM diagrams The greatest chemical differences between the four rock forthree calc-alkalineintrusive suites of Mesozoicand series at any given SiO, content lie in the higher contents of Tertiary age from (a) SE Asia and the western Pacific; (b) K, Ba, Rb, Sr, Zr and the light REE in the shoshonitic and central America and the Caribbean; and (c) California and high-Kseries as compared with the low-Ktholeiite and calc-alkaline series. Baker (1982) also noted that Th and U FEOT contents in shoshoniticassociations are higherthan elsewhere. Ewart (1982) did not discuss these two elements; however, he found that P and Pb are enriched in shoshonitic A suitesand that significant differencesbetween these two elements exist in four volcanic series. The K,O/Na,O ratio for the shoshoniticassociation is usually close to or in excess of 1. In Table 2 chemical data from Kilmelfordrocks are compared with representative data for low-K, calc-alkaline, high-Kand shoshonitic rock series. In order to matchthe SiO, range of Kilmelford,only groups withSiO, contents between 52-54% and 56-63% were used from the four rock seriesgiven in the compilation of Ewart(1982). Table 2 clearly demonstratesthat the Kilmelfordrocks contain higherconcentrations of K, Sr, Ba,Zr and light REE (represented by Ce)as well ashaving higher K20/Na,0 ratiosthan those of thelow-K, calc-alkaline and high-K series of Ewart, and that the Kilmelford concentrations are comparablewith those of the shoshoniticseries. Only Rb contents in the Kilmelfordrocks are lower than in the shoshonitic series, but they are still higher than those of the low-K and calc-alkaline series. Geochemical data therefore indicate the shoshonitic identityof the Kilmelford intrusives.

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Table 2. Average contents of selected elements (SiO, and K in %; shoshoniticcharacter of the Kilmelfordrocks is of great otherwise ppm) of the Kilmelford rocks compared with four rock interest as they constitute a prime exampleof this suite in an series from Ewart (1982) old orogenic belt-the Scottish Caledonides. The REE patterns of the Kilmelforddiorite and K8/ granodiorite(Fig. 5) arecharacterized byhigh REE SiO, K SiO, Sr Rb Ba Zr Ce Na,O abundances(average total REE = 169 pprn),lack of a significant Eu anomaly,and a high LREE/HREEratio. KD stgl 52-66 3.09 1438 51 1545 133 81 0.77 Theyhave CeN=52-113 and YbN=4-7. The ratio KD 1-2 52-66 3.61 1177 57 1567 135 45 0.81 KD stg2 52-72 3.53 1051 65 1548 127 86 0.93 Ce,/Yb,(ll-16) is high, as shown by the steep slope of the patterns in Fig. 5. TheREE patterns of the Kilmelford LKSWP 52-54 0.38 241 5 116 37 5.61 0.16 porphyrite(Fig. 6) are alsocharacterized by high REE LKSWP 56-63 0.62 287 8 174 60 9.48 0.22 abundances (average total REE = 197 ppm) and enrichment CA SWP52-54 1.15 476 21 324 103 38.6 0.39 of LREE relative to HREE. They have Ce, = 69-140 and CA SWP56-63 1.49 425 37 391 122 34.6 0.44 YbN = 5-14 and high ratios Of CeN/YbN (13-28). HKSWP 52-54 2.11 742 49 578 145 48.3 0.60 HKSWP 56-63 2.59 782 65 760 185 59.4 0.69 A SS SWP52-54 3.47 1030 73 753 137 63.7 1.01 '* SS EUSA 56-63 4.25 902 139 2017 394 200 1.05

KD, Kilrnelford intrusives; LK, low-K series; HK, high-K series; SWP,SW Pacific; stgl, 1-2, stg2, evolutionarystages; CA, calc-alkaline series; SS, shoshonitic series; EUSA, eastern zone of W USA.

Baand Sr values are unusually high andapproach the averages of thesetwo elements in the eastern shoshonitic zone of the western USA, although the amounts of Rb and Zr do not match those in this zone. The recognition of the

Lo Ce Pr Nd Sm Eu Gd Dy Yb Lu Fig. 6. Chondrite-normalized REE pattern of the Kilmelford porphyrite. X, ZK 260; 0,ZK 55; A,ZK 93.

Discussion Ewart (1982)recognized a progressive decrease of K/Rb from 600-800 for the low-K series to 200-500 for high-K, shoshoniticand many calc-alkaline eruptives. However K/Rb ratioscannot discriminate shoshonitic rocks from calc-alkaline and high-Ksuites. A largeoverlap of K/Rb ratiosbetween these three groups occurs in thevariation 11 II I II Ill II diagrams of Ewart (1982). La Ge Pr Nd Sm EuGd Dy Yb Lu Zhou (1985b)established set a of mathematical Fig. 5. Chondrite-normalized REE pattern of the Kilmelford expressions to describe the variation of trace element ratios diorite and granodiorite. V, ZK 196; 0,ZK 281; 0,ZK 199; duringfractional crystallization in terms of the difference 0,ZK 204; +, ZK 207; m, ZK 161; A,ZK 246; A,ZK 134. betweenthe bulkdistribution coefficients of the two

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elements. K/Rb ratios do not vary much during fractional zones,a feature which is considered by J. Tarney(pers. crystallizationexcept in thepresence of biotiteand comm.) to be ubiquitous for this tectonic environment. K-feldspar. If amphibole is present,thedegree of fractionation (F) must exceed 28% in order to maintain an Sincere thanks go the the late Professor Janet Watson who greatly unchanged K/Rb ratio. The small difference in K/Rb ratio influenced the author. R. J. Howarth and G. Borley are thanked for betweenthe three groups of rocks thus suggeststwo theiradvice and the author is particularly grateful to J. Knillfor possibilities. Firstly, if theyare genetically related by makingavailable theresults of hisdoctoral research on the fractionalcrystallization, biotite and K-feldspar crystal- Kilmelford area. C. Hallsis thanked for critically reading the lization cannothave been important. If amphibolewas revisedmanuscript and R. N.Thompson for advice atan early present, F must have been larger than 28%. Secondly, other stage. The author is indebted to G. C. Brown for his critical review processesmay have been important in theformation of and generous help. Comments from another anonymous reviewer theserocks. These mayinclude the tectonic environment are gratefully acknowledged. The full set of analytical data will be (e.g. depth to the underlying Benioff zone), the time when stored in the UK-IGBA datafile from which it can be retrieved via the Kilmelford suite formed relative to tectonic events, the theNational Geochemical Databank of theBritish Geological Survey. nature of the overlyingcrust or eventhe stratigraphic position within a particular volcanic province or sequence. The recognition that shoshonitic and high-K magmatism References may be related to the tectonic environmentis of interest and importance.Jakes & White(1972) observed thatwhere ARCULUS,R. J., DELONG,S. E., KAY, R. W., BROOKS,C. & SUN,S. S. 1977. The alkalic rock suite of Bogoslof Island, Eastern Aleutian Arc. Alaskan Tertiary shoshonitic rocks occur, for example in Japan and Journal of Geology, 85, 177-86. New Guinea, the age of the basement rocks and associated BAKER, P. E. 1982. Evolution and classification of orogenic volcanicrocks. sediments is usually pre-Mesozoic: the shoshonitic composi- In: THORPE, R.S. (ed.) Andesites. Wiley, New York. tion is thoughtto be characteristic of alate stage in the BARBERI,F., INNOCENTI,F., FERRARA,G., KELLER,J. & VILLARI, L.1974. Evolution of Eolian arc volcanism, southern Tyrrhenian Sea. Earth and evolution of agiven eruptivesequence. Ewart (1982) has Planetary Science Letters, 21, 269-76. pointed out that the relative proportions, among the erupted BROWN,G. C. 1982. Calc-alkalineintrusive rocks: their diversity, evolution magmas, of high-K andshoshonitic types are greater in and relation to volcanic arcs. In: THORPE, R. S. (ed.) Andesites. Wiley, continental orogenic regions than in island arc settings and New York. thatthe proportions of theseK-enriched magmas are DEWEY,J. F. 1969. Evolution of theAppalachian/Caledonian orogen. Nature, 22, 124-9. particularly enhancedamong dacitic to rhyoliticcomposi- DICKINSON,W. R.,RICKARD, M. J., COULSON,F. I., SMITH,J. G. & tions.Barberi et al. (1974)suggested thatthe shoshonitic LAWRENCE,R. L. 1968. Late Cenozoic shoshonitic lavas in north western suitefrom the Aeolian arc is derived by lowdegrees of Viti Levu, Fiji. Nature, 219, 148. partial melting during the closing stage of the evolution of ELLIS, R.A. 1977. Disseminated copper in Caledoniana calc-alkaline intrusion, Argyllshire, Scotland. Transactions of the Inrtitute of Mining the island arc over a rapidly deepening Benioff zone. and Metallurgy, Section B, 86, B52-B54. Zhou (1985b)also studied the activity of waterin - et al. 1977. Investigation of disseminated copper mineralisation near relation to the order to crystallization of biotite, pyroxene Kilmelford,Argyllshire, Scotland. IGS mineralReconnaissance Pro- and hornblende at Kilmelford, and concluded that this may gramme Report No. 9. EWART,k. 1982. The mineralogy and petrology of Tertiary-Recent orogenic have been low at the time when the Kilmelford rocks were volcanicrocks: with specialreference to the andesitic-basaltic formed. It is well known that high water activity would be compositional range. In: THORPE,R. S. (ed.) Andesites. Wiley.New maintained in atectonic environment where subduction York. processes areactive because wetsediments are constantly GILL, J. B. 1970. Geochemistry of Viti Levu,Fiji, and its evolution as an island arc. Contributions to Mineralogy and Petrology, 44, 179-203. beingbrought down by subduction,and metamorphic HILL, J. B., PEACH,B. N., CLOUGH, C. T. & KYNASTON.H. 1905. The processesin thesubducting oceanic slab wouldinevitably geology of MidArgyll (Expl. of sheet 37). Memoir of the Geological release H,O at depth. The water supply could be expected Survey of Scotland. todecrease when two continents come into collision HORMANN,P. K., PICHLER,H. & ZEIL,W. 1973.New dataon the young volcanicsin thepuna of NW Argentian. GeologischeRundrchau, 62, following the consumption of available oceanic crust and the 397-418. suppression of subduction.Thus it is suggested thatthe IDDINGS,J. P. 1913. Igneous Rocks, Vol. 2. Wiley, New York. tectonicenvironment for the formation of the Kilmelford JAKES,P. & GILL,J. 1970. Rare earth elements and the island arc tholeiite rocks may be one in which subduction was not active. The series. Earth and Planetary Science Letters, 12, 224-30. - & WHITE, A. J. R. 1969. Structure of the Melanesianarcs and K-Ar age of the Kilmelford rocks, 418 Ma, determined on correlation with distribution of magma types. Tectonophysics, 8, 223-6. biotite by Ellis et al. (1977), appearsto support this -& -1971. Composition of island arcs and continental growth. Earth suggestionbecause subduction in Scotland is thoughtto and Planetary Science Letters, U,224-30. have ended at about 420 Ma (Watson 1984). -& - 1972. Major and trace element abundance in volcanic rocks of Theshoshonitic signature of the Kilmelfordrocks is orogenic areas. Geological Society of America Bulletin, 83, 29-40. JOHNSON,R. W., WALLACE,D. A. & ELLIS, D.J. 1976. Feldspathoid-bearing therefore interpreted as reflecting an intrusive series formed potassic rocks and associated types from volcanic islands off the coast of at a stage when subduction was abating and the continents New Ireland, Papua New Guinea: a preliminary account of geology and were in collision; therefore such suites may be characteristic petrology.In: JOHNSON,R. W. (ed.) Volcanism in Australia. Elsevier, Amsterdam, 297-316. of late orogenic magmatism. Thisis in full agreement with the JOPLIN,G. A. 1965. The problem of the potash-rich basaltic rocks. suggestion thatmagmas related to continental collision Mineralogical Magazine, 34, 266-75. processes usually have high contents of alkalis and large-ion - 1968. The shoshonite association: a review. Journal of the Geological lithophile (LIL) elements.Very steep REE patterns Sociefy of Australia, 15, 275-94. KNILL,J. 1957. The pre-Tertiary geology of the Craignish-Kilmelford District, (Ce,/Yb, = 10-28) and the lack of a Eu anomaly are also Argyllshire. PhD thesis, University of London. thought tobe important characteristics for these rocks. LEFEVRE,c. 1973. Les caracteres magmatiques du volcanisme plioquaternaire Similar patterns for rare earth elements have been found in desAndes dans le Sudde Perou. Contributionsto Mineralogy and magmatic rocks formed in oceanic ridge-continent collision Petrology, 41, 259-72.

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PARKER,R. J. 1977. Factorsaffecting the quality of major element rock PLANT,J., BROWN,G. C., SIMFSON,P. R. & Shum, R. T. 1980. Signatures of analysisby X-ray fluorescencecombined with flux-fusion sample metalliferous granites in the ScottishCaledonides. Transactionsof the preparation. Technical ReportXRF-2B, Department of Geology, Instiiute of Mining and Metallurgy E, 198-210. Imperial College, London. RICKARD,A. J. R. 1979. Geochemistty andpernogenesis of copper-bearing - 1978. An iterative method for determining background intensities used diorites near Kilmelford, West Scotland. PhDthesis, University of in XRF calibrationlines for flux-fusionsilicate rock analysis. X-ray Strathclyde. Spectrum, 7, 38-43. WALSH, J. N., Bucla~~,F. & BARKER,J. 1981. The simultaneous

~ 1979. A computer-based system of XRF analysis and its application to a determination of therare-earth elements in rocks using inductively geochemical study of Vulsini volcano, central Italy. PhDthesis, coupled plasma source spectrometry. Chemical Geology, 33, 141-53. University of London. WATSON,J. 1984. The ending of the Caledonian orogeny in Scotland. Journal PEACH,B. N.,KYNASTON, H. & MUFF, H. B. 1909. Thegeology of Le of the Geological Society, London, 141, 193-214. seaboard of Mid-Argyll (Expl. of sheet 36). Memoir of the Geological ZHOU,J.-X. 1985a. The timing of calc-alkaline magmatism in parts of the Survey of Scotland. Alpine-Himalayan collision zone and its relevance to the interpretation PEACOCK,M. A. 1931. Classification of igneous rockseries. Journal of of Caledonian magmatism. Journal of the Geological Society, London, Geology, 39, 54-67. 142, 309-17. PHILLIPS, W.E. A., STILLMAN,C. J. & MURPHY,T. A. 1976. Caledonian plate -1985b. Geochemistry of the Kilmelford intrusives, W Scotland, in relation tectonic model.Journal of the Geological Society, London, U2,579-609, to the Caledonian orogeny. PhD thesis, University of London.

Received 27 August 1985; revised typescript accepted 15 January 1987.

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