The Petrology and Geochemistry of Dykes of the Lizard Ophiolitecomplex, Cornwall
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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 Ophiolite Complex, 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. although dykes 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 in small amounts. Phenocrysts of olivine and 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/1/53/4888039/gsjgs.141.1.0053.pdf by guest on 25 September 2021 54 G. A Kirby 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/1/53/4888039/gsjgs.141.1.0053.pdf by guest on 25 September 2021 Petrologyandgeochemistry of dykes,Lizard Ophiolite Complex 55 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 differentia- tion), 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 The sub-horizontal trends (Fig. 4b) are thought to FIG. Tholeiitic-alkaline2. basalt discriminant plots. TABLE2: Parental compositions of groups 1, 2 and 3 (Fig. 5) which often 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).