The Petrology and Geochemistry of Dykes of the Lizard Ophiolitecomplex, Cornwall

J. geol. Soc. London, Vol. 141, 1984, pp. 53-59, 6 figs., 2 tables. Printed in Northern Ireland.

The petrology and geochemistry of dykes of the Lizard OphioliteComplex, Cornwall

G. A. Kirby

SUMMARY. Dykes from the coastal section of the Lizard ophiolite, SW England, have been analysed for both major and trace elements. The dykes vary from fresh olivine dolerites to loweramphibolite grade metadolerites. All are basalticand tholeiitic and possess broadly oceanic characteristics. Three magma groupings can be defined geochemically on the basis of trace element contents, particularly rare earth elements and PN ratios. These magma groupings are thought to have been generated by differing degrees of partial melting and from differing source rocks. Each group is thoughtto have undergone fractional crystallization. Thespatial and suggested temporalseparation of thesegroups supports a pulsatory magma chamber model for the spreading centre at which the Lizard ophiolite developed.

TheLizard Complex consists of anassemblage of Petrography peridotites,gabbros, hornblende schists, banded gneisses,metasediments and dolerite dykes. The Thedykes can be divided petrographically into complex is faultbounded tothe N againstthe metadoleritesand non-metamorphosed dolerites. No Meneagecrush zone. The complex has previously significantdifferences were observed between the beenregarded as the type example of aperidotite metadolerites at Coverack and those at Porthoustock. diapir(Green 1964) but there are many features compatiblewith an ophiolitic origin (Bromley 1973, Non-metamorphosed dolerites 1976; Kirby 1978, 1979; Strong et al. 1975). On the E coast of the Lizard peninsula there is a recognizable These very from aphyric to sparsely porphyritic and although truncated ophiolite sequence passing north- from fine-grained to microgabbroic. The finest aphyric wards (up-sequence) from peridotites through a zone dolerites consist of thin randomly aligned microlites of of complexly interdigitated peridotites, troctolites and often skeletal plagioclase averaging 100 p long, set in gabbros(the ‘transition zone’) into layered and turbid,poorly formed and zoned clinopyroxenes. massive gabbros and finally into the base of a sheeted Whereporphyritic, the olivine phenocrysts are com- dykecomplex at Porthoustock (Fig. 1). The main monlyskeletal and the plagioclases acicular, with concentration of dykes is aroundPorthoustock, some showing evidence of resorption. althoughdykes occur locally throughout the section. Thecoarser examples have a sub-ophitic texture There is a second, much lesser concentration around withimpinging, randomly-aligned plagioclase laths, Coverackin the transition zone of thesequence. often strongly and continuously normally zoned from Coverack and Porthoustock are the two main sampling labradoriteoligoclase.to Clinopyroxene is buff- localities.Dykes trend NW-SE andare steeply coloured,interstitial and granular and often clouded dipping to sub-vertical. Most dykes are metadolerites; or partiallyreplaced by amphiboles.Groundmass however,some fresh olivine dolerites outcrop at olivine,commonly clouded or replaced by opaque Coverack. minerals, is widespread(10-20%) and suggests an If models for the generation of ophiolites are correct affinitywith alkaline dolerites or primitive olivine (Cann1974; Dewey & Kidd1977; Sleep 1975), in tholeiites. Skeletal ilmenite and chromite are present which dykes of the sheeted dyke complex are fed from insmall amounts. Phenocrysts of olivineand plagio- theunderlying gabbroic magma chamber, then the clase aremoderately abundant and often contain dykes at Coverack cutting rocks of the transition zone inclusions of each other demonstrating an overlap of shouldbe later and derived from a different source their crystallization histories. Olivine phenocrysts are fromthose at Porthoustock. No cross-cuttingrela- commonly marginally and normally zoned. tionshipsbetween thedykes aredemonstrable, althoughthe presence of completelyunmetamorph- Metamorphosed dolerites osed dykes at Coverack and their absence at Porthous- tock supports the above suggestion. It was hoped that Many samples retain remnants of an original ophitic anexamination of thegeochemistry of thedykes texture;small randomly-aligned plagioclase laths are would help elucidate this problem. set in granular or granoblasticaggregates of green

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and Nb) of the Lizard dykes were determined from pelleted rock powders using a Phillips PW 1212 automatic X-ray fluoresence spectrometer. Water determination followed the procedure described by Shapiro (1955). Rare earth element analyseswere performed by DrP. J. Potts of theOpen University using neutronactivation analysis. Thedetection limits of the elements Ce, Nd. Sm and Gd is very roughly 5 times the average chondrite abundance. and of Fu, Tb, Yb and Lu approximately 1-2 times thc chondrite average.

General geochernistry Allanalysed dykes are basaltic and tholeiitic in composition(Table 1, Fig. 2). Theyare chemically primitive: Fe, Ti, Rb, Sr and Ba are strongly depleted and Mg, Ni and Cr enriched relative to the 'average' tholeiite (Manson 1967). As many of the dykes sampled are metadolerites, emphasishas been placed on the use of thestable traceelements, particularly Ti, Zr, Y and P (Cann 1974; Floyd & Winchester 1975; Pearce & Cann 1973). These elements are of use in determining the tectonic setting in which basaltic rocks were generated (Pearce I mile I& Cann 1973;Pearce et al. 1975). Dykesfrom the Lizard complex have closest affinities with Mid Ocean Ridge basalts (Fig. 3) and are quite distinct from other greenstones of SW England.This isconsistent with their chondrite-normalized rare earth element profiles

bd Doleritedykes TABLE1: Average geochemistry of Lizard dykes FIG. 1. Simplified geological map of thestudy area. Average anhydrous Average Average pleochroic hornblende, thought to be secondary after Element (all dykes) Coverack Porthoustock clinopyroxene.Partial overgrowth of the margin of ~~~ ~ plagioclase laths by hornblende is common. Granular Si02 50.23 49.22 f 2.0 51.31 f 1.2 ilmenite is confined to the background of hornblende Ti02 1.31 1.03 f 0.16 1.61 f 0.41 and is generally minor (1-10'%). Plagioclase laths are AI203 16.22 16.46 f 1.56 15.97 f 1.2 strongly and continuously normally zoned from oligo- FeKh 9.33 8.86 f 1.0 9.82 f 1.06 clase toandesine. Pseudomorphed ferromagnesian ME0 9.66 11.23 f 4.2 8.00 f 1.16 CaO 10.29 10.36 f 1.4 10.22 f 0.8 phenocrysts often show a variety of mineral assemb- NazO 2.8 2.74 f 0.7 2.86 f 0.5 lagesand zonal arrangements, although frequently K20 0.26 0.14 f 0.08 0.29 + 0.14 with a tremolitic core. The shape of the majority of P 796 450 f 99 1056 f 455 pseudomorphs suggests that they are secondary after Zr 114 70 f 21 140 f 50 olivine.Plagioclase phenocrysts areoften severely Y 38 30 i 6 42 f l0 clouded. Nb 9 8f3 9f3 Theabsence of epidote,and the presence of V 182 136 f 13 205 f 66 moderately calcic plagioclase, hornblende and tremo- Cr 328 390 f 50 290 f 127 litein the groundmass suggests that metamorphism Mn 1240 1217 f 180 1263 f 146 Ni 151 192 f 26 105 f 42 occurredat temperatures nearer amphibolite than cu 69 84 f 24 53 f 30 greenschist grade. The lack of a penetrative metamor- Zn 50 49 f 13 52 f 10 phic fabric shows that this metamorphism occurred in Ga 14 13 f2 16 f 2 the absence of any major directed stress field. Rb 5 3f2 6f5 Sr 192 197 i 52 216 f 27 Geochemistry Ba 91 68 f 17 107 f 39 Analytical methods Major elements in 9%. The major element (Si. Ti,AI, Fe, Mg, Ca, Na and K) and Trace elements in parts per million; ranges quoted are one the trace element contents (P. Ni, Cu. Zn, Ca. Rb. Sr. Y, Zr standard deviation.

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For P and Ni thesedifferences are statistically significant. Thislends support tothe suggestion lNaZ0 K20 of differingmagma sources at Coverack and Porth- oustock. Considering all analysed dykes there is a very large range in their phosphorous contents (288-2155 ppm). If all dykes were cogenetic and derived by fractional crystallization acting on a magma with a composition approximating to that of the most phosphorous poor Subalkaline member, then 88% crystallization is required to obtain this range. This might be expected to have taken the mostphosphorous rich samples out of the basaltic 'j , spectrum: however, this has not occurred. 0 /; A histogram of the ratio of PIY, two incompatible 4 0 45 40 50 Si02 elements, is presented in Fig. 4a. Three features are noteworthy: a. There is a large spread in values forming a trimodal distribution (labelled 1, 2 and 3). b.Dykes from Coverack plot at lower values than those from Porthoustock. c. Dykes from Coverack show a bimodal distribution. l1 I Whenplotted against Zr (anindex of differentiation), the three groupings of PIY ratios define separate sub-horizontaltrends (Fig. 4b). Dykes within each grouphave similar REE profileswhich are distinct \ fromthose of other groups (Fig. 5). This is strong 1.04 supportingevidence for thereality of thethree groupings as are other inter-element trends (Fig. 6): P Tholeiitic values of group 3 exceedthose of all members of groups 1 and 2, which although having some overlap in theirPcontents, have welldefined and separate inter-element trends. .05 2r:P O5 Thesub-horizontal trends (Fig. 4b) are thought to FIG. Tholeiitic-alkaline2. basalt discriminant plots. TABLE2: Parental compositions of groups 1, 2 and 3

(Fig. 5) whichoften show a slight to moderate light Group I Group 2 Group 3 REE depletion typical of MORB.parent This inparent turn implies parent Element that the Lizard ophiolite was generated at a palaeo- spreading centre, a suggestion compatible with many SiOz 48.45 49.74 51.30 other features of the complex (Kirby 1978, 1979). TiOz 0.8 0.8 1.15 403 17.42 18.91 16.18 Fe203 8.53 8.02 8.56 Detailed geochemistry and petrogenesis MgO 11.23 8.38 8.90 CaO 10.95 11.69 11.14 Thevariations in thetrace and major element Na,O 2.51 2.33 2.74 chemistry'are now examined in greaterdetail in an K20 0.05 0.12 0.13 attemptto defineand toquantify the controlling P 279 421 882 factors in the chemical evolution of the dykes and to Zr 61 55 124 define any magma groupings which exist. Y 30 22 30 Several critical differences emerge in a comparison Nb 7 11 13 Rb - 4 - between the chemistry of the dykes at Porthoustock Sr 130 159 222 andthose at Coverack: Si02, TiO2, Fe203and K20 Ba 62 52 95 are all lower and MgO higher in dykes from Coverack, TaITb 0.196 0.277 0.693 indicating a more primitive nature for the latter group ThiTb 0.218 0.238 0.624 (Table 1). This is emphasized by thetrace element Ta/Hf 0.055 0.090 0.186 contents: P, Zr, Y, Nb, Rb, Sr, Ba andV are all much lower and Cr and Ni higher in the Coverack dykes. Major elements in 7i; trace elements in parts per million.

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d C

Till00 OFB

*Group 1

A Group 2 Group 3 ~~~~~geSW England tholeiite

FIG. 3. Traceelement discriminant plots. Fields: 1. Within-platebasalts. 2, 3. Low-K tholeiites. 3, Ocean floor tholeiites. 3, 4, Calc-alkaline basalts. 5, Ocean floor basalts. 6, Island arc basalts. 7, Calc-alkaline basalts, OFB: Ocean floor basalts. LKT: Low potassium basalts.

resultfrom within-group fractional crystallization of ous-poor member of each group most closely approxi- themain phenocryst phases olivine, plagioclase and mates to the parental composition, andusing the range clinopyroxene; vectors produced by the precipitation of phosphorouscontents within eachgroup, the of these minerals are superimposed on this diagram. amount of fractionalcrystallization which is repre- Thisproposal is stronglysupported by REE data. sented within each group can be estimated as: group1, Withineach group, dykes having the highest Zr 55%; group 2, 46%; group 3, 59%. These values are content have a profile parallel to, but at higher values consistentwith each series remaining within the than, dykes with lower Zr contents. basaltic spectrum. Using the method of Allegre et al. Having established the probable existence of three (1977),estimates of thepercentages of thedifferent magma groupings and their differentiation series, one phases which precipitated during fractional crystalliza- canattempt to modelthe fractional crystallization tioncan be made. The results are, however, to be histories of each series. Assuming that the phosphor- regardedasapproximate, since there are many

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a No of samples The steep bounding line at low Zr values (i.e. near the parental compositions of the different groupings) l in Fig. 48 provides a clue. Its steepness suggests that garnet f clinopyroxenemust have been a major 5p4l.... phase, either precipitating out during fractional crys- ...... l tallization or meltingfrom an ultramafic source. No ...... l .l l. otherphase involved in either removal or melting 10 20 30 yy produces such a steep trend, as demonstrated by the b I vectors inFig. 48 (Pearce & Norry 1979). However, there is no severe Cr depletion in passing from groups 30 1 to 2 to 3 which would necessarily result from either suggestedscheme. The REE profilesand absolute contents would also be expected to change systemati- cally from group 1 to 2 to 3, whichis not the case; 20 members of group 2 have lower REE contents than groups 1 or 3 (Fig. 5). Fractionalcrystallization or i: partialmelting of a commonsource are therefore consideredunlikely. Similarly, because of thenon- systematicchanges in REE contents,progressive meltingis not considered alikely possibility. Source l inhomogeneity is thereforesuspected. The LREE depletion of groups 1 and 2 suggests a more depleted 1c sourcethan that required for group 3, which has a slight LREE enrichment. L Zr However,the higher MgO content of group 1 50 100 li0 260 250 parental magma does suggest that it was generated by m Dykes, Porthoustock higherdegrees of partialmelting, as the bulk solid-

0 Dykes. Coverack liquidpartition coefficient for MgO isunlikely to change much, whatever ultramafic source is assumed FIG.4. (a) Histogramof PNratios. (b)Fractionation diagram demonstrating trends of groups 1, 2 and 3. Arrows indicate trends produced by crystallization of olivine,plagioclase, clinopyroxene andgarnet. Rock:Chondrite Cr = ppm Cr in primitive members of groups 1, 2 Group 1 and 3. I

potential pitfalls in this approach: it is dependent on thephases assumed to be precipitating (here plagioclase,olivine and clinopyroxene as suggested by the phenocrystassemblage), the value of thepartition coefficients chosen, and the tightness of definition of Group 2 the inter-element trends. For groups2 and 3 a scheme - involvingprecipitation of plagioclase,olivine and clinopyroxene in the ratio5 : 3 : 2 and 3 : 1 : 1 respective- ly is indicated. Variation diagrams for group 1 are not sufficiently well-defined to allow the Allegre model to beapplied, although sharply rising Sr valueswith differentiation suggest that plagioclase played a lesser role than in groups 2 or 3. Thereremains the question of howthe magma Group 3 groupingsare related to each other. There are four possibilities: a. An earlier phase of fractional crystallization, b.Varying degrees of partialmelting of a common 10 source, c. Progressive melting of a single source, d. Source inhomogeneity. FIG. 5. Chondrite normalized REE profiles.

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- ,\L 560 1000 1500 2000 p

Group 1 p 560 10'00 15'00 20'00 A Group 2 Group 3 FIG. 6. Inter-element trends, groups 1, 2 and 3.

(Table 2). A combination of differing degrees of par- peridotite and gabbro of this ophiolite complex, andof tial melting and differing source rocksis therefore indi- fresh olivine dolerites in group 1, suggests that these cated. wereintruded later than members of group 3. The As a consequence of the differinginter-element suggested source inhomogeneity and higher degrees of trends, groups 1, 2 and 3 form almost non-overlapping partial melting for group 1 than group 3 may suggest clusters on severaltectonic environment discrimina- thatthe spreading centre at whichthis ophiolite tiondiagrams (Fig. 3). Group 3 just straddlesthe developed was at an early stageof development with a continentaloceanic divide on the Ti-P-Kdiagram generallyincreasing, although variable amount of suggesting a transitional nature. This is supported by partial melting acting on an inhomogeneous uprising itsslight LREE enrichment, atypical of MORB (but mantle.This suggested early stage of development stillmuch..less than intypical continental tholeiites). may be consistent with the small size of the ophiolite Group 1 has strong oceanic affinities and group 2 has and itsassociation with metamorphosed, apparently characteristics generally intermediate between groups continentally derived sediments (the Old Lizard Head 1 and 3, although it does possess the strongest LREE Series). depletion. Theexistence of threegroups, allhaving broadly oceanicaffinities and their spatial and suggested temporal variations, does not support a model for the Implications of the suggested generation of thisophiolite sequence involvinga magma groupings steady-state magma chamber (Cann 1974), but rather a pulsatory magma chamber andior jumping spreading Members of group 3 occur exclusively atPorthous- centre. tock. Groups 1 and 2 occur at both Porthoustock and Coverack, although they are concentrated at the latter locality. This spatial separation of chemically different Conclusions groups explains the previously noted chemical differ- encesbetween the two localities. The presence of 1. All dykes are tholeiitic. members of groups 2 and 1 at Coverack, cutting both 2. Onthe basis of incompatibleelement ratios,

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References

ALLEGRE, C. J.,TREUIL,M,, MINSTER,J. F., MINSTER, B.& 199-204. ALBARDE,F. 1977. Systematic use of trace elements in - 1979. The LizardComplex as anophiolite. Nalure, igneousprocesses. Part 1. Fractionalcrystallization London, 282, 58-61. processes in volcanic suites. Contrib. Mineral. Petrol. 60, MANSON,V. 1967. Geochemistry of basaltic rocks. In: HESS, 57-75. H. H. & POLDERVART,A. (eds.). The Poldervart BROMLEY,A. V. 1973. Thesequence of emplacement of Treatise onrocks of basaltic composition. 215-69. basicdykes in theLizard Complex, South Cornwall Interscience Publishers. (Abstr.) Proc. Ussher Soc. 2, 508. PEARCE,J. A. & CANN,J. R. 1973. Tectonic setting of basic - 1976. A new interpretation of the Lizard Complex, S. volcanic rocks determined using trace element analyses. Cornwall, in the light of theocean crust model. Proc. Earth planet. Sci. Lett. 19, 290-300.

geol.Soc. London, 132, 114. ~ & NORRY, M. J. 1979. Petrogenetic implications of Ti, CANN,J. R. 1974. Amodel for oceanic crustal structure Zr, Y, and Nbvariations in volcanicrocks. Contrib. developed. Geophys. J. R. astron. Soc. 39, 169-87. Mineral. Petrol. 69, 33-7. DEWEY,J. F. & KIDD,W. S. F. 1977. Geometry of plate PEARCE,T. H., GORMAN, B. E.& BIRKETT,T. C. 1975. The accretion. Bull. geol. Soc. Am. 88, 960-8. TiO2-KZO-P2Osdiagram: amethod of discriminating FLOYD, P.A. 1976. Geochemical variations in greenstones of between oceanic and non-oceanic basalts. Earth planet. S. W. England. J. Petrol. 17, 522-45. Sci. Lett. 24, 419-26.

~ & WINCHESTER,J. A. 1975.Magma type and tectonic SHAPIRO,I. 1955.Rapid determination of water in silicate settingdiscrimination using immobile elements. Earth rocks. Analyt. Chem. 27, 560-4. planet. Sci. Lett. 27, 211-18. SLEEP,N. H. 1975. Formation of oceanic crust: some thermal GREEN, D. H.1964. The petrogenesis of the high tempera- constraints. J. geophys. Res. 80, 4037-42. ture peridotite in the Lizard area, Cornwall. J. Petrol. 5, STRONG, D. F.,STEVENS, R. I., MALPAS,J. & BADHAM,J. 134-88. P. N. 1975. A new tale for the Lizard. Proc. Ussher Soc. KIRBY,G. A. 1978. Layered gabbros in the Eastern Lizard, 3, 352. Cornwall,and their significance. Geol. Mug. 115,

Revised typescript received 7 September 1983. G. A. KIRBY,Institute of Geological Sciences, Keyworth, Notts. NG12 5GG.

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