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SOME LAMPROPHYRES OF THE CHANNELISLANDS.

By H. G. SMITH , B.Sc., F.G.S.

[Rectl t'ed 30th September, 1932.]

[Read 6th J anuary , 1933.)

IN 1884 the Re v. E. Hill [1]' located tw o lamproph yres inGuernsey, one south of th e Bee du Nez and the other

in the bay at Moulin Huet . These were described by ProfessorBonney in an appendix to the same paper. From thesedescriptions one gathers th at th e ev il reputation of th is class ofrocks for ex t reme decomposit ion is fully justifie d. While,however, this does ap ply to the former case, the example a tMoulin Huet is not nearl y so bad as indicat ed; the workersmentioned must have been unfortunate in their choice ofspecimens .

In 1912 Mr. E. Hill [2] gave a list of localities, compiledchiefly from the Transacti ons of the Guernsey Society of NaturalHist ory, for lamprophyres discovered by local worker s. ForJ ersey the chief source of inform ation is the excellent " Geologiede J ersey " by Noury (1886), while Dr. G. H. Plymen [3], in19 21 , discussed an ew the geology of the island and gave a list oflocalities for lamprophyres. Meantime a short account of th eMinettes of J ersey by M. Mau ger [4] had appeared in 1916 ; thispaper includes two chemical analyses.

GUERNSE Y.Moulin Huet.

In the dyke, "six or eight feet broad," described by Hilland Bonney, the largest const it uent minerals are pseudomorphsafte r olivine with a maximum length of about three mill imetres,preserv ed in talc or serpen tine or a mixture of the tw o. Acolourless pyro xene, probably diopside, is fairly abundant an dis in some cases beautifully idiomorphic ; it is very little decom­posed, but has a fringe of dark material which displays a t endencyto penetrat e along the cleavages. Small crystals of biotite arecommon; they are of the usua l t ype found in lamproph yres,showing the pseudo-hexagonal form s and dark margins thatone expects . These minerals are embedded in a pale brownishgroundmass in which minute laths of striated felspar can berecognised; a few small apat ite s are associated with them.

At the western end of the bay, south of the battery, are twoaddit ional lamprophyres, parallel and in close proximity. Theseare sharply contrasted: one , ab out nine inches wide, is hardand compact; the oth er is so friable that it can be crushedbetween the fingers. The association of these two varieties calls

1 For list of References see pa ge 130.

PROC. GEOL. Assoc., VOL. XLIV., P ART 2, 1933.

122 H. G. SMITH,

to mind the occurrence at Pentire Farm, Newquay, although atthat locality the compact variety occurs as rounded masses,each about the size of one's head, embedded in the friablelamprophyre.

Sections of the better-preserved rock show abundant pseu­domorphs after olivine, here preserved in rhombohedralcarbonate, a mineral which is almost entirely absent from thelamprophyre in the middle of the bay. In many cases there isa marked peripheral accumulation of black material. Beautifullyfresh idiom orphic biotite is common; it has only a slight tendencyto the development of a darker colour on its margins. Sometimesthere are lenticular inclusions intercalated along the cleavages;these consist, not of carbonate, but fibrous serpentine, thefibres being arranged transversely. Fairly large crystals ofapatite are not uncommon. Most of them are scattered in thegroundmass, but there is a definite tendency for them to associatewith biotite, the prism edges being parallel to the cleavages ofthe latter mineral. The cloudy groundmass contains manysmall laths of felspar, which seem to be untwinned.

North of Jerbourg Point.Here the pseudomorphs after olivine attain a maximum

dimension of four millimetres ; they are preserved in silica andcarbonate. Biotite also occurs in unusually large crystalsalmost as long as the olivines. The crystals are idiomorphic,pseudo-hexagonal, and are definitely darker towards the margins.Some of them are closely adherent to the olivine, suggestingformation as a result of reaction between the magma and thatmineral. Apatite in small crystals is fairly abundant, sometimesenclosed in biotite. Scattered through the slide are numerousopaque crystals, some of which are white in reflected light:these must be ilmenite; curiously, however, some of themtransmit a deep reddish-brown colour, but still have the sameappearance in top light. All these minerals are embedded in agroundmass which is obviously made up largely of more or lessdecomposed felspar. In it are areas of no particular shape,containing quartz, into which project crystals of very fresh-lookingorthoclase, in some cases associated with a rhombohedral car­bonate. These areas are explicable on the assumption ofinfilling from a residual magma rich in potash and silica; thecarbonate may be the result 'of a reaction between the carbondioxide accumulating in this residue, and the oxides displacedfrom certain silicates during the conversion of these into biotite,in the manner demonstrated by Dr. Bowen. Some of thesepatches are approximately circular in outline, but have nowell-defined limiting fringe of biotite as have the ocelli sofrequently occurring in lamprophyres. They are closely com­parable with the nests in some of the rocks of this class found

SOME LAMPROPHYRES OF THE CHANNEL ISLANDS. 123

near Sedbergh [5J. There is the same sheaf-like occurrence ofthe partly decomposed felspar.

North of the Harbour at St. Sampson.This rock contains pseudomorphs after olivine with a

maximum diameter of five millimetres. These now consist ofa mixture of serpentine, carbonate and epidote; they aresurrounded by a narrow rim of fresh, colourless pyroxene, insome cases discontinuous. Similar pyroxene, but with a strongtendency to idiomorphism, is scattered through the rock, un­connected with the olivine. Biotite is abundant, but is notso well formed as in the examples previously described. Hereand there the biotite is associated with beautiful green epidotewhich is usually in lenticles with an elongation parallel with thatof the biotite; it does not, however, displace the biotite laminre ;the cleavages of the latter mineral are perfectly straight lines.Closely associated with the biotite and the epidote, and gradinginto both, is an almost opaque, brownish mineral; the clearerareas in this have a high relief and polarise in first or secondorder colours; the mineral is probably one of the epidote group.

Perhaps the most striking character of this rock is the relativelylarge size of the felspars, indeed the rock is very similar to thatfrom Sale Fell; the felspar is much decomposed, but appearsto be orthoclase. Quartz and carbonate occur interstitially;they also occur in ocelli, which may be three millimetres indiameter, associated with decomposed felspar which appearsto be identical with that of the general mass of the rock. Thereare also patches of quartz which appear to be xenolithic. Theyhave rims of small, well-formed crystals of pyroxene similarto those found elsewhere in the rock.

Also xenolithic in character, and probably fragments of theplutonic rocks into which the lamprophyre is intrusive, areareas consisting largely of quartz and alkali felspars, the lattersomewhat decomposed. Some of the felspars enclose irregularcrystals of colourless kyanite with a maximum dimension offour millimetres, others enclose a felted mass of colourless needleswhich are sillimanite. The kyanite is partly decomposed togreenish serpentine. The formation of these aluminous mineralscan hardly be supposed to be due to excessive temperature of thelamprophyre magma, but must be the result of its proximity.The suggestion here offered is that the magma leached out thealkalies from the pre-existing felspars and introduced themagnesia and water to form serpentine. A similar associationof kyanite and serpentine was found in the xenoliths of thelamprophyre in Backside Beck [5J, near Sedbergh; in thedescription of that occurrence, the explanation submitted wasthat the kyanite had been caught up as such from the foundationrocks of the area. If, as now seems certain, kyanite may be

124 H. G. SMITH,

produced from felspar by this leaching process, that assumptionas to the origin from a rock like kyanite-schist is unjustified.

South of Fort Doyle.Near the north end of the island, a few hundred yards south

of Fort Doyle, a large quarry is worked. On its eastern sideit is in close proximity to the sea, and a lamprophyre can beobserved on the beach and in the quarry; from this latterplace the better specimens are available. Biotite, maximumlength three millimetres, is the most conspicuous and abundantmineral; occasional idiomorphic sections are to be seen, andthe tendency in these is for the margin to be lighter in colourthan the centre. Included in some of the crystals are lenticlesresembling those found at St. Sampson, which there wereattributed to one of the epidotes. These seem to be of a similarnature, but polarisation colours are low, and zoisite is suggested.Not so abundant as the biotite, but still forming a considerablepart of the rock, is diopside; it is very pale green, almostcolourless. Many of these crystals have a deep green, narrow,irregular border, which in places projects outwards at intervalsin a serrated fashion; this darker pyroxene also occurs inter­stitially in nests of sub-radiate needles ; pleochroism is usuallygreen to brown (the latter colour for transverse vibrations),and the mineral is undoubtedly a:girine. It exhibits a tendencyto occur also in needles projecting outward from biotite. Thereare some iddingsite pseudomorphs after olivine, and muchdecomposed felspar forms the groundmass. Apatite occurs,but is scarce; similarly with carbonate and quartz.

There is a variety of the rock, seen in certain sections, inwhich biotite is insignificant in amount. The dominant colouredconstituent is diopside in elongated crystals. lEgirine, sporadicin its occurrence in the normal rock, here occurs as a definiteserrate fringe to all the diopside. Felspar is more abundantand fresher than it is elsewhere, and is probably orthoclase.There is a little interstitial quartz.

The mode of occurrence of the a:girine seems to indicatethat this mineral crystallised from a residual magma, and itis possible that, atter the crystallisation of much biotite,pressure was diminished, water ceased to play an essential role,and the soda-iron compound became insoluble. The biotite­poor variety may be a segregation vein.

JERSEY.South Hill Battery.

In this dyke, intrusive into granite about half a mile south ofSt. Helier, biotite crystals reach a maximum dimension of eightmillimetres. Idiomorphic sections are common; repeated colour

SOME LAMPROPHYRES OF THE CHANNEL ISLANDS. 125

zones are occasionally seen, the darker colour being a rich.reddish brown; enclosed lenticles of carbonate are fairlyabundant. Pseudomorphs after olivine, sometimes almostas large as the largest biotite crystals, are not rare. Some ofthem are completely surrounded by biotite; the outer limit ofthe association is fairly definite, but the biotite crystals projectinwards, into the mixture of fine-grained silica and carbonatein which the pseudomorphs are preserved, in a very irregularfashion. In such cases it seems clear that the peripheral biotiteis the result of reaction between olivine and magma. Certaingroups of small, differently oriented crystals of biotite mayhave had a similar origin, the reactions having affected even themiddle of the original olivine. In other cases the peripheralbiotite is one single crystal, the different parts of the rim beingin optical continuity. Pyroxene [6J, sometimes perfectlyidiomorphic, is pale-green in colour and may be fairly fresh;in many cases, however, it has been decomposed to a chloriticmineral. One of these pyroxenes encloses perfectly-formedsub-parallel crystals of biotite; applying the usual test ofidiomorphism, it would appear that the crystallisation of biotitepreceded that of pyroxene, but one hesitates before taking thatsequence as proved. Orthoclase, almost undecomposed, andsometimes in sheaf-like aggregates, forms most of the ground­mass; there is a little quartz and some carbonate. Apatite isscattered through the rock in crystals of various sizes and isfound as an inclusion in almost every other constituent, even inthe olivine pseudomorphs. Again a suspicion is aroused thatthe idiomorphic enclosure is not necessarily the earlier mineral;it may be due to paulopost changes. Perhaps the mostinteresting constituent, however, is <egirine. It has a modeof occurrence similar to that of the normal rock at Fort Doyle;as needles in the groundmass or as sub-radiate aggregates fringingbiotite.

La Plaine.In this dyke, biotite is the most prominent constituent.

although crystals rarely attain a length of four millimetres.It has a greenish-brown colour in vertical section and has astrong pleochroism. Basal sections, however, have a slightreddish tinge and have poorly-defined dark borders. Theextraordinary fact about the biotite is the associated carbonate.

It occurs as lenticles in the biotite, may be three or four ofthem in one crystal, but followed longitudinally; one or more ofthese is continued beyond the biotite rather like the smoke of asky-writing aeroplane. The metallic constituents of the' car­bonate may represent part of a pre-existing silicate from whichthe biotite was produced by reaction. There are crystals of

Iz6 H. G. SMITH,

pyroxene, probably diopside, some of them idiornorphic, scatteredthrough the rock ; they are in various stages of decomposition.

In just a few cases, at the termination of a diopside crystal,is regirine, the one mineral passing sharply into the other withouttransition. A few of these fragments of regirine are associatedwith agglomerations of carbonate, and this latter subst ance maybe the result of changes in the diopside-s-changes that had noeffect on the regirine. The groundmass is made up of remarkablyfresh orthoclase. Crystals of apatite are sporadic. Occasionalpseudomorphs are preserved in green serpent ine and what appearsto be granular sphene ; each pseudomorph has its fringe ofbiotite, and there is some associated carbonate. Th e occurrenceof an infilled cavity with inwardly-projecting crystals of regirinesuggests a late origin for that mineral.

Certain coarse agglomerations of diopside crystals becomegreen and feebly pleochroic on the margins; biotite, apatite,and wollastonite (?) are associated with these. These groupsare considered to be modified cognate xenoliths. Veins ofquartz traverse the lamprophyre and some of the quartz has beendeposited within the biotite in such a way as to force aside thelaminee of that mineral, with the result that wisps of biotite havescattered and are strewn with different orientations about theslide. Chalcedony and opal accompany the quartz.

Included in the lamprophyre is an accidental xen olith,probably detached by the magma from the country not verydistant from the present situation. The xenolith is anadamellite and within it the contacts of quartz and felsparhave developed a neutral zone which Dr. Hawkes (7], whokindly examined a slide, considers to be comparable with theexample described by him from Kvosafoss, S.E. Iceland. Fusionhas 'taken place at the contact of the two minerals,and fibrousamphibole .and other minerals have been developed in thecontact zones as a result of the introduction of material s fromthe lamprophyric magma.

The Devil 's Hole.In this rock th e biotite reaches a maximum dimension of

four millimetres. It is beautifully fresh and idiomorphic witha dark border, and the extinction angle is high enough t o enablethe twin lamellse to be observed. Enclosed lenticles in thebiotite exhibit an agate structure, and the material must besilica ; as in the case of carbonate of other rocks , it seems tohave forced aside the films of mica in, order to make a placefor itself. This biotite has a rather rich brown. colour , wellseen in basal sections; but there is one example of a nest ofbiotite crystals, the whole about four millimetres diameter, inwhich the colour is slightly greenish, and there is no peripheralchange of colour in the individual crystals. It is probable that

PROC. GEOL. Assoc., VOL. XLIV. (1933). PLATE II.

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PHOTO~nCROGRAPI-IS: LAMPROPHYRES FROM THE CHANNEL ISLANDS.

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SOME LAMPROPHYRES OF THE CHANNEL ISLANDS. I27

the two kinds of biotite had different origins. Some obviouspseudomorphs after olivine are preserved mostly in silica, thoughthis may be accompanied by pyrite.

One example seems to be instructive. The shape is that ofolivine and there is a peripheral zone of silica. The greaterpart, however, is an intimate mixture of silica and biotite,some of the latter being in optical continuity. That the olivinewas converted in part to biotite is clear, and there is no evidenceof any intermediate stage in the transformation.

The groundmass is largely made up of fresh, elongatedcrystals of orthoclase with some interstitial quartz. A fewapatite crystals occur, most of them embedded in orthoclasebut some are found in biotite. One fragment of quartz,five millimetres long, showing polysynthetic structure withan indefinite reaction rim, is undoubtedly an alien body, eithercognate or accidental. Veins of silica traverse the rock.

Crabbe.The greater part of this rock is a fine-grained aggregate

of the usual brown biotite, colourless or pale green diopside,and narrow laths of alkali felspar; all these constituents haveclaims to be regarded as idiomorphic. Embedded in this baseare frequent pseudomorphs in carbonate after olivine, usuallyabout two millimetres long, each of them with its adherent fringeof biotite. Small apatites are common. In some cases thefelspar has aggregated into indefinite nests, where it is associatedwith chlorite and carbonate; a few of these nests have rims ofbiotite, forming ocelli. Xenocrysts of quartz, each with itsreaction rim, occur occasionally, and there is one large, irregular,alien fragment of orthoclase. Some large vesicles are occupiedby carbonate and quartz.

Greve au Laneon,Several dykes occur in this bay. They are somewhat

variable in character, but each resembles one or other of thetypes already described. One of them, however, presentsfeatures of considerable interest. It is vertical, more thantwo feet wide, and consists of three portions. At each sideis a brownish, compact rock, and in the centre is a varietyexhibiting a " pisolitic" structure, each globose mass standing outin relief and being less than a centimetre in diameter. Sectionsthrough the marginal compact variety show that it is very fine­grained, consisting of a great number of small, idiomorphicbiotite crystals embedded in a pale brownish groundmass,probably orthoclase. Some pseudomorphs after olivine, maxi­mum about four millimetres, are preserved in silica and chlorite.The spheroids of the central variety are exactly similar to themarginal portions of the dyke, but the interstitial material,

128 H. G. SMIT H ,

although quite as rich in biotite, presents a greenish aspe ct onaccount of the abundance of chloritic material in the ground­mass ; olivine pseudomorphs are more abundant and red ironoxide is present in quantity. The line of demarcation betweenthe two varieties is not perfectly sharp ; indeed, one olivinepseudomorph transgresses from one to the other .

A possible explanation of the co-existence of these twovarieties involves the idea of liquid immiscibili t y. On thisassumption , the two liquids separated according t o grav ity,but about the junction , globules had failed t o find t heir properlevel. Material of the lighter fraction ascended the fissureand"was frozen on the walls ; inject ion from a lower level causedinfilling of the centre and it happened that the material camefrom the lower part of the transit ion zone, that is, from whereglobules of the light er fraction were float ing near t he summitof the heavier.

It should be pointed out that this dyke is one of those ment­ioned by Noury, and that it is discussed by de Lapparent [8].Noury's contribut ion as to the cause of the" pisoliti c " structureis "je suppose qu ' au moment de son eruption , cett e veineresta plus longtemps liquide en son centre que sur ses bordes ,ce qui permit au feldspath de se concentrer en granulat ions."

The exam ples here described are those that could be obtainedin a reasonable state of preservation. A greate r number areeither t oo friable to surv ive transport , or when cut, show littleof va lue. Sect ions from the following localities were cut andexamined :-

Guern sey: North of Bee du Nez, south of Bee du Nez,north of St Martin's Point, west of Les Tielles, L'Eree,Fort Richmond, Fort Hommet , Long Port, Grande Rocque,Port Soif.

j ersey : Mont Orgueil , Fliquet Bay, Rosel, Les Piatons,Bouilly Port, Les Malt ieres , Valley des Vaux.

CONCLUSIONS.That lamprophyres present certain exceptional characte rs

is eve rywhere admitted. Harker says (op . cit. , p. ISO) : " Thelamprophyres are a peculiar group of rocks," and aga in , " theirst ructure is in some respect s peculiar." Teall [9] refers t o th emas .. rocks of somewhat exceptional composition." Bowen[10], discussing the olivine-bearing types, says, .. The generalcharacter of the group is a richness in ferro-magnesian con­stituents combined with a richness in alkalies, which combinationof characters is, indeed, rather distinctive of lam prophyres ingeneral."

Many of the fact s mentioned in this communication areexplicable as a result of the application of principles laid down

SOME LAMPROPHYRES OF THE CHANNEL ISLANDS. 129

by Bowen. It may be that the rocks represent crystallisationat two periods, partly from an early, and partly from a muchlater stage of the same magma, some of the early crystals havingbeen materially modified in consequence of reactions with thelater, more alkalic, magma.

The case of the " pisolitic" dyke at the Greve au Lancon,however, presents some difficulties. It is possible that therounded bodies are variolitic in character, comparable withthose occurring in the minette of the Little Belt Mountains,Montana, a detailed description of which is given by L. V.Pirsson [11J. There seem, however, to be several importantdifferences. In the example there described, the varioliticfacies occurs in the saalband; the matrix is extremely rich inbiotite compared with the varioles, and in these bodies thefelspars are arranged radially and even the mica shows thesame tendency. None of these characters is to be observed inthe rock from the Greve au Lancon, and the theory of segregationafter solidification hardly seems to offer a perfect explanation.

Moreover, the analogy between spherulites and variolitesis itself under suspicion. Cole and Gregory [12J, studying thevariolitic rocks of Mt. Genevre, conclude that the variolitestands in the same relation to the basic lavas as pyromeridedoes to those of acid character. They mention, however, thatLoewinson-Lessing regards such bodies as globular formsanalogous to porphyritic felspars, and object that such a viewis calculated to lead the mind away from a number of mostimportant comparisons. It is just the value of those com­parisons, however, that is in question.

One gets the impression that Dr. Harker [13J is not quitesatisfied regarding the analogy; he says that concerningspherulitic structures in basic rocks our information is muchmore scanty and their true nature may be greatly obscured bysecondary changes, as in the so-called variolites. It would,therefore, seem desirable to suspend judgment regarding thevariolitic origin of these globular structures in lamprophyres.

The magnificent work done by Dr. Bowen on theoreticalpetrology makes one pause before again advancing the theoryof limited miscibility, and it is not claimed that this contributionoffers any proof in support of it. The occurrence of numerousenclosed fragments, however, in this class of rocks, as demon­strated by Sir John Flett [14], Prof. H. H. Read [15] and others,does seem to be a point in favour of the existence of two distinctliquids in the reservoir from which some lamprophyres were fed.Such a liquid-contact might be expected to act as a trapable to cause the accumulation of xenolithic material of anydensity whatever. Subsequent injection from this level would:then involve the two liquids, together with the alien fragments;

130 SOME LAMPROPHYRES OF THE CHANNEL ISLANDS.

and after the work of the authors mentioned above there seemsto be little doubt that such fragments are more abundant inlamprophyres than in other igneous rocks.

May not some of the peculiarities in the chemical com-,position, the structures, and the characters of the minerals beexplained on the same basis ?

My thanks are due to Dr. Plymen for introductions toextremely useful residents. From these gentlemen, particularlyMr. A. J. Robinson, M.Sc., of St. Helier, I received every possibleassistance. Dr. H. H. Thomas very kindly examined andcommented on several slides.

REFERENCES.I. HILL, REV. E. 1884. Quart. Journ. Geol . Soc., vol. xl., p. 418.2. HILL, REV. E. 1912. Quart. Journ. Geol, Soc., vol. lxviii., p. 46.3. PLYMEN, G. H. 1921. Proc. Geol, Assoc., vol. xxxii., p. 169.4. MAUGER. 1\1. 1916. Comptes Rendus Acad. Sci., vol. clxiii., p. 293.5. SMITH, H. G. 1930. Proc. Geol, Assoc., vol. xli., p. 336.6. HARKER, A. 1908. .. Petrology for Students," 4th ed., p. 154.7. HAWKES, L. 1929. Min. Mug., vol. xxii., pp. 163-173.8. DE LAPPARENT. 1892. Ann. Soc. Sci. de Brux., vol. xvi., part 2,

p. 29·9· TEALL, J. J. H. 1888. .. British Petrography," p. 350.

10. BOWEN, N. L. 1928. .. Evolution of the Igneous Rocks," p. 259.II. PIRSSON, L. V. 1898-99. zoth Ann. Rep., U.S.C.S., part 3.

pp. 463- 58 1.12. COLE, G. A. J., and GREGORY, J. ""V. 1890. Quart. Journ. Ceol.

Soc., vol. xlvi., pp. 295-332.13. HARKER, A. 1909. .. Natural History of Igneous Rocks," p. 275.14. FLETT, SIR J. 1929. .. The Iverbeg Lamprophyre." Sum. Prog,

Geol. Surv. for 1928, part 2, pp. 29-35.15. READ, H. H. 1926. Explanation of Sheet 102, Mem, Geol. Sur»,

Scot., p. 168.

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EXPLANATION OF PLATE II.Biotite Lamprophyre, Devil's Hole, Jersey. x 31.Augite-Biotite Lamprophyre, South Hill Battery, St. Helier, Jersey.

x 40.Pseudomorph in biotite and silica, Devil's Hole, Jersey. x 40.Nest of orthoclase and quartz, N. of Jerbourg Point, Guernsey.

x 31.Kyanite with a narrow rim of decomposed felspar; in the lower

part of the figure, serpentine is interposed between the two.St. Sampson, Guernsey.' x 31.

Sillimanite. St. Sampson, Guernsey. x 40.