260

SOME FEATURES OF CORNISHLAMPROPHYRES.

BY H. G. SMITH, E.Sc., A.R.C.S., F.G.S., East London College.

[Received 14th August, 1928.]

[Read 7th December, 1928.]

SOME years ago,* I recorded the existence of a lamprophyrenear Ashburton, South Devon, and described some of

its xenoliths containing corundum, staurolite, and green spinel.The present contribution is the result of an examination ofother west country lamprophyres, undertaken with the objectof studying the abnormal features known to occur, at any rate,in some of them. t In locating the several intrusions, the mapsand memoirs issued by the Geological Survey were found to beindispensable.

Specimens were collected from each, wherever possible,these being obtained so as to include any unusual texture; inother cases, samples of the normal rock were dug out and sub­sequently broken up in the laboratory, the search for xenolithsbeing continued.

Lemail Cutting, Wadebridge.This is a fine-grained rock, a lens being necessary to demon­

strate the existence of the ferro-magnesian mineral. In sectionsthis is seen to consist of the usual biotite with dark borders, to­gether with much decomposed felspar. There are subordinateamounts of quartz, carbonate and greenish pseudomorphs aftersome ferro-magnesian phenocrysts.

The xenoliths invariably contain felspar and have amaximum diameter of about l inch. In many cases this isuntwinned, has a refractive index lower than that of Canadabalsam, and is biaxial with negative birefringence. It is un­doubtedly orthoclase and presumably is rich in potash. Quartzis associated with this, and the two minerals are frequentlyintergrown in graphic fashion. The greater portion of thefelspar is only slightly decomposed, but where it abuts againstthe quartz there is much alteration; moreover, in this cloudyzone the appearance of cross-hatching usually associated withmicrocline is to be observed.

The external margin of the xenolith, in contact with thelamprophyre, is usually not quite sharp, there being a zone offeslpar, formed of what appear to be discrete crystals withstreaky alteration, but all extinguish simultaneously with theorthoclase of the xenolith proper. There is some biotite in thiszone, but it is not so abundant as in the lamprophyre.

* Quart. Journ. Geol. soc.• vel. lxxii., 1917, pp. 77-83.t J. B. Hill, Trans. Roy. Geol. Soc. Cornw., vol. xii., 1902, p, 565.

SOME FEATURES OF COH.l'rSH LAMPROPHYRES 26I

In certain cases, within the felspar of the xenolith, is a fan­shaped grouping strongly resembling that seen in spherulites;running across the radii is occasionally seen, with concentricarrangement, a structure very much like ripple-marks. Someof these pseudo-spherulites are composed of elongated, colour­less crystals with a high relief, and polarising in brighter coloursthan in the case of the felspar, and their occurrence as largermasses, not radiating, but with varying orientations and sizes,gives an opportunity of determining some of the opticalproperties; the crystal is biaxial with a small optic axial angle,it is positive, and the optic axial plane coincides with a definitecleavage. The mineral is considered to be sillimanite. Thereare associated crystals of green spinel, some of them almostopaque.

In one case there occurs a nest of these dark spinels asso­ciated with crystals of pale blue corundum. These latter havea good relief, a maximum absorption for longitudinal vibrations,polarise in first order yellow, show straight extinction andhave a positive sign of elongation; there is no doubt as to theiridentity. Each crystal is embedded in a zone of colourlessmica and the lamprophyre in contact with the xenolith is richerin biotite than elsewhere. A fact of some interest is thatquartz occurs within two millimetres of corundum and spinel.In one case corundum is embedded in sillimanite.

In the lamprophyre itself the felspars are sometimes arrangedradially in a manner comparable with that described by SirJethro Teall,* and attributed by him to contact action.

Another feature of the lamprophyre is the presence of whatappear to be vesicles, each surrounded by elongated biotitesarranged tangentially. These are very similar to those describedand figured in the Ashburton lamprophyre. They are occupiedusually by unstriated felspar, carbonate, chlorite, and a kindof quartz with a tendency to symmetrical arrangement ofinclusions and a lack of uniform extinction. Similar quartz isto be seen fringing the quartz of the xenoliths where, betweencrossed nicols, it suggests the appearance of a section throughvelvet pile. Sir John Flett] described clear quartz grains witha broad brownish zone apparently made of rectangular prisms.They are attributed to the action of a vitreous magma along therhombohedral cleavages, but in that case these individual prismsextinguish with the clear quartz. The example at presentunder consideration seems to be of a different nature.

Another curious fact about these vesicles is that the felsparwithin them may be in optical continuity with crystals of thelamprophyre outside them; it is rather striking to observethe extinguished felspars, some millimetres in length, running

• British Petrography, r888, p, 354.t .. Geology Colonsay and Oronsay with part of Ross of Mull," Mem. Geol, Surw, r9II, p. 95.

PROC. GEOL. Assoc., VOL. XL., PART 3, 1929. 18

262 H. G. SMITH,

through the vesicle. In one case these unstriated felspars,somewhat cloudy, are fringed within the vesicle by clear, striatedalbite. .

The occurrence of a radial structure associated with concentric" ripple" marks in the felspar of the xenolith has already beenmentioned. Further curious structures in these felspars areworthy of notice; they seem to be comparable with some foundin the lamprophyres of the Lake District.' as described by Dr.Harker.* Some of the xenoliths consist of irregular massesof cloudy felspar and quartz, embedded in fine-grained, some­what decomposed felspars. Usually it is the larger felsparsthat abut on the quartz with a complicated outline of bays,promontories and small islands; and there is a strong suggestionthat one of the minerals is in a condition of arrested digestion.Within the felspar are larger, apparently globular bodies differentin colour from the main felspar, and smaller vermicular bodiesconsisting of clear felspar, with a rod-like core of a colourlessmineral with high relief, giving the appearance of clearingof the felspar by the core, comparable with the cleared glassaround the incipient crystals of the Arran pitchstone. Anotherexample shows the large felspar, here exhibiting twin lamellee,traversed by "lanes" occupied by the fine-grained cloudyfelspar; the twin lamellas interrupted by the "lanes" canbe picked up on the other side. Altogether, the evidence fordigestion of these bodies, xenoliths or phenocrysts, seems tobe strong.

Pendennis Point, Falmouth.This intrusion is well exposed at low tide. It is a much­

decomposed lamprophyre, but the biotite flakes are compara­tively fresh; inclusions of quartz, slate and pegmatite areconspicuous, the latter in ellipsoids with a maximum diameterof several inches. In section, one of the inclusions, less thanI inch in diameter, is seen to consist of small, sub-angularquartz grains with some pyrite; it is traversed by several streaksconsisting of what appear to be the finer-grained constitutentsof the lamprophyre. There seems to be no doubt that it isa xenolith of sandstone almost in its original condition; itstexture is very uniform. Another example, about the samesize, is not obviously granular. It consists largely of quartzfragments of varying size, and the whole has a spongyappearance, the interstices being occupied by slightly decom­posed felspar showing twin lamellae in places. When quartzmeets felspar there may be a very narrow zone composedeither of recently-added quartz or that mineral in process ofconversion to another. In still another example, approaching

• Geol. Mag., r892, p. 205.

PROC. GEOL. Assoc. VOL. XL. (1929). PLATE 24.

FIG. I. SILLIMANITE.Lemail, X 14.

FIG. 2. SILLIMANITE Al'D GREEN

SPWEL.

Lemail, X 30.

FIG. 3. GRAPHIC QUARTZ ANDFELSPAR.

Lemail, X 13.

FIG. 4. GRAPHIC QUARTZ ANDFELSPAR.

Lemail, X 12.

Ph%micro by H.G.S.

[To face p. 262.

SOME FEATURES OF CORNISH LAMPROPHYRES. 263

I inch in diameter, the quartz is crowded with minuteinclusions and there is some difficulty in distinguishinghetween this mineral and the slightly decomposed, associatedfelspar. The whole mass of quartz is traversed by a networkof that mineral free from inclusions, and between crossednicols the clear threads are seen to come at the junctions ofindividual grains of the mineral. These grains do not resemblesand grains; they have a tendency to elongation in a particulardirection, each grain extinguishes as an aggregate, and at thejunction of two grains (along the clear streaks) there is anaccumulation of very minute quartz individuals. One gets theimpression of a great amount of shearing which almost certainlyantedates the existence of the fragment as a xenolith. Theassociated felspar, in the identification of which one welcomesthe occasional cleavage and twinning, partakes to some extentof the same sheared appearance, but this may be due to its havingreplaced quartz which had been previously sheared. A few" islands" of quartz in contact with the lamprophyre suggestthat they are residues not yet converted.

A slide from the pegmatite, examined with a lens, shows agraphic structure between quartz and felspar, but the lattermineral is seen to consist of more decomposed individualsembedded in lighter material. Under the microscope thesedecomposed crystals exhibit twin lamella, in some cases in morethan one direction; they are, in places, in contact with quartz,but as a rule this latter mineral is in contact with the interstitialliner-grained felspar, and in places is moulded on its individualidiomorphic crystals. Many of these smaller felspars arestriated, and although they usually form a sort of mosaic, someof them are distinctly elongated. In another slide is somebrown glass with a suggestion of flow in places, and containingcrystals of sillimanite, spinel, and probably andalusite; thisbrown substance is considered to be identical with the buchiterecorded by Dr. H. H. Thomas in the tholeiite intrusions ofMull.*Coast below Messack, East of the FaI.

This exposure is accessible from a boat at low tide. Itcontains inclusions of quartz and pegmatite, some of them severalinches long. The rock consists of fresh, pale biotite, felspar(some of it not much decomposed), quartz, apatite, andcarbonate; the felspar appears to be orthoclase.

The quartz of the xenoliths consists of numerous grains ofall sizes, some of the larger ones showing undulose extinction.In some cases the large quartz grains interlock, in others thereare interposed numerous small grains, free from the minute

* Quart. Journ. Geol, Soc., vol. lxxviii., 1922, p. 240.

H. G. SMITH,

inclusions which are so abundant in the larger individuals.Forming an irregular belt between the quartz and the lampro­phyre is a quantity of much-decomposed felspar, and thisgraduates into the lamprophyre, the felspar (untwinned) becomingclearer, and separating into distinct crystals without markedelongation; then the biotites appear and the rock becomesnormal. In other cases the biotite comes in contact with thequartz.

A section through the pegmatite shows quartz with theusual eccentric shapes, and, as in the example from PendennisPoint, larger, much-decomposed felspars with interstitial, fresher,fine-grained material, this latter including occasional radialaccumulations. The larger individuals have occasional clearareas in which twin larnellse, faintly seen in the decomposedareas, become beautifully clear; one of these forms a marginalfringe to the felspar in contact with quartz, and one unalteredarea with one set of striations can be shown by the Becke testto have a refractive index lower than that of Canada balsam­it is undoubtedly albite. Some of the felspar appears to beuntwinned, some shows two sets of intersecting lamellse, butall of it, as far as can be judged, in the decomposed condition,seems to have a low relief and hence to be rich in alkalies, eitherpotash or soda, or both. Quartz abuts on both the larger andsmaller individuals of felspar.

South of Trelissick (W. of the FaI.).

Much of this rock is so decomposed that it can becrumbled in the fingers, but there are spheroids, which arehard and exceedingly tough. A section through this hardervariety shows the usual minerals of a biotite lamprophyre, butapatite is rather more abundant than usual and there is someundoubted sphene. A large quartz xenolith presents someinteresting features. It polarises as a unit and has a marginalzone with a maximum width of about o.ymm., marked on itsinner side by a very definite line which separates the clearquartz of the bulk of the xenolith from the marginal zonewhich contains minute inclusions in great abundance. Withinthis margin, which extinguishes simultaneously with the restof the quartz, are crystals of sphene, and a green, slightlypleochroic mineral which seems to be a pyroxene. Traversingthe xenolith is a band of quartz, with crowds of minuteinclusions, exactly like that of the marginal zone. Here andthere along this band, decomposed felspar appears, and this isbordered by a narrow fringe of the quartz with minute inclusionsand small green pyroxenes. The hypothesis that the felspar,showing twin lamella: in places, has been manufactured byreaction between magma and quartz, and that the appearance

PROC. GEOL. Assoc., VOL. XL. (1929). PLATE 25.

F IG. I. CORUND UM AN D G REEN

SPINEL. Lemail, X 40.

F IG . 2 . CURIO US S TRU CT U!(ES I NALKALI F ELSPAH .

Lernail, X '20 .

FIG . 3. GARNET, R UTILE AN DQUARTZ .

N . of Pill Cre ek, x 20.

FIG . 4. GRA P HI C QU A RTZ AND FELSPAI{.B elow Messack, x 13.

Photcmicro by H .C.S .

[T o f ace p. 264.

SOME FEATURES OF CORNISH LAMPROPHYRES. 265

of the crowds of minute inclusions is a stage in the transformation,would seem to be worthy of consideration.

North of Pill Creek.This is a narrow intrusion a little to the south of the foregoing

occurrence. The rock differs from the examples previouslydealt with in that the biotite is inconspicuous and decomposed,while the apatite is much more abundant. Xenoliths aboutI inch in diameter are fairly common. The darker ones resolvethemselves into a mixture of green and brown chlorite; thelighter ones are made mainly of spongy quartz associated withfelspar, the latter occupying the interstices. Embedded in theseminerals are crystals of garnet and rutile, the latter showing inone case the characteristic geniculate twin. Apatite also ispresent and there is a little pale yellow-green mica. Some ofthe interstices are occupied by chlorite. There is a tendencyfor the flakes of mica to arrange themselves in parallel position.

The rock on the banks of Truro Creek, below Penpoll Wood,was found to be too much decomposed to yield anything ofinterest in the present inquiry.

In the vicinity of Newquay several lamprophyres wereexamined, including that below Pentire Farm, but there isnothing to add to the description of the rock by Sir John Flett.*South of the GanneI.

Flett remarks that the rock at Pentire Farm is the only onewithin the sheet which will repay microscopic investigation, butmentions the existence of corroded quartz xenocrysts in all theother dykes. Two of these xenocrysts were dug out of the faceof the quarry south of the Gannel. In section, the quartzwas found to be associated with a felted mass of flexuous, narrow,colourless crystals, having a good relief and polarising usuallyin second order colours; this is considered to be sillimanite. Inplaces, the crystals become more rigid in character and are em­bedded in what appears to be muscovite. Crystals of greenspinel are associated with the sillimanite, which seems to bereluctant to meet the quartz, a film of felspar always being inter­posed between the two minerals. In addition, epidote and somedoubtful andalusite are present. In one case, more or lessseparated pieces of quartz are embedded in felspar and give a verystrong suggestion that they are residual, much of the mineralhaving been (apparently) converted into the felspar. This ideais considered to be supported by the existence of some curiousstreaks traversing the quartz.

The lamprophyres of Holywell Bay and Towan Head exhibitfew features of interest. Those examples near the Tea Cavern.

• U Geology of the Country near Newquay," Mem. GeoI. Surv., 1906, p, 60.

266 H. G. SMITH,

Towan Head, seem to be worthy of notice because of the existenceof small, apparently globular areas about 4 mm. diameter whichare coarser than the rest of the rock. These areas are sharplydemarcated from the rest of the rock by a narrow belt of whatappears to be colourless mica. It rather suggests the existenceof globules of magma different in composition from the rest,but of course, other explanations are possible and more likely tomeet with acceptance. The lamprophyres near the Tea Cavernare traversed by conspicuous veins of quartz and Carbonate.

Foot of Cant Hill, Padstow.

This is a very fine-grained rock in which are visible variousmasses of quartz, most of them lenticular in form and having amaximum length of about I foot. In section the rock is seen toconsist largely of laths of decomposed felspar with pseudomorphsin carbonate after some ferro-magnesian mineral. Magnetite iscommon, and biotite, never abundant, is now represented only byits decomposition products. One of the quartz zenoliths, withlittle trace of heterogeneity in ordinary light, breaks up between.crossed nicols into an aggregate of grains, very variable in size.Some of the grains show undulose extinction, but there is littletrace of shattering. It is very much like a fragment of arenaceousrock from which the cementing material has been leached out.Against this, perhaps, is the very variable size of the grains, butit is possible that there has been some fusion of one into another;this supposition involves some difficulties, but seems to beworthy of consideration in this and other cases.

In another slide is an area of about 5 mm. diameter occupiedlargely by laths of striated felspar which are larger and fresherthan those of the bulk of the rock. There are occasionalcrystals of biotite and the interstices are occupied by quartz.Within this area is what seems to be a vesicle occupied by car­bonate and the curious quartz similar to that already met inthe Lemail lamprophyre. The quartz is peripheral within thevesicle. A very definite arc of biotite follows the junction ofvesicle and coarse felspar for a short distance, then for the restof its course (about 1800 in all), it traverses the area occupiedby the larger felspars; it does not follow the junction with thenormal rock. Another vesicle contains mostly the rhom­bohedral carbonate with a little of the abnormal quartz. Inaddition there is a colourless mineral, most of the sections rect­angular, showing cleavages parallel to the sides and with arelief inferior to that of the carbonate; polarisation coloursare usually second order and twin lamellse run obliquely acrosssome of the crystals. The figure is biaxial and the sign positive.The mineral is identified as anhydrite and the occurrence seemsto be one not very commonly observed..

SOME FEATURES OF CORNISH LAMPROPHYRES. 267

The intrusion at Halwyn, on the opposite side of the Camel,presents no features of interest, while the intrusions at Rockare of a totally different type and seem to have no bearing onthe present investigation.

Hick's Mill, near Polyphant.This very fresh biotite-lamprophyre contains occasional

grains of quartz, which show interesting marginal phenomena.The outline of the grains is at first sight a very regular ellipse,but in detail there are numerous irregularities"; idiomorphiccrystals of felspar project into the quartz from the margin andthere are also smaller vermicular growths of a similar nature.Moreover, the quartz does not extinguish uniformly; it shows,very faintly, spindle-shaped lamellse comparable with thosetypically seen in the case of microcline. Whatever the signi­ficance of this may be, it is found also in several of the exampleshere dealt with.

Specimens were collected from Halveor, near St. ColumbMajor; Gloweth, Boscolleth, and Shortlanesend, near Truro;and Malpas, The occurence near Colan is no longer accessible.

It is not easv to visualise the conditions under which theseminerals and structures came into existence. Dr. Harker*says that the diminutive size of the corroded felspars probablyindicates that manv others have been entirelv dissolved in thecontaining magma' and that this solution is' most reasonablyreferred to the epoch of the injection of the dykes. It appearsfrom his account that the felspar was dissolved as a whole, thatthere was no selective leaching out of the constituents. Dr.Thomas] (following Teall) attributes the formation of thealuminous minerals to metamorphism of aluminous sedimentswith a certain amount of selective interchange of constituents.Dr. du Toit, t dealing with the plumasite of -Natal, points outthat the dykes penetrate a country consisting of serpentineresting on granulite. So long as the dyke is in the lowerformation, it is an ordinary pegmatite, but when it enters theserpentine it becomes narrow, loses all its quartz and acquirescorundum. The magma was desilicated by the serpentine,which obtained in addition potash and some alumina. Theoccurrence is of remarkable interest as showing that the mineralsof an intrusive rock may have come into being only becauseof the chemical peculiarities of the pre-existing rocks in contact.It is possible that some process comparable with this has takenplace in the examples described above. Dr. Bowen,§ in his

• Loc. cit., p, 205.t Loc, cit., p. 250.t Trans. Geol, Soc., S. Africa, vol. xxi., 1919, pp. 53.62.§ ]ouro. Geol., vol, xxx., 1922, p. 197.

268 H. G. SMITH,

paper on "The Reaction Principle in Petrogenesis," suggeststhat in a granitic liquid at the pegmatite stage, quartz mayreact with the liquid to produce felspar, and sometimes thegraphic structure may be a result of this reaction

Corning to the facts in the Cornish lamprophyres, we knowthat they contain fragments of sandstone and slate, and alsothe nests of quartz, felspars, sillimanite, green spinel andcorundum. The presence of fragments recognisable as sedimentsmay indicate either that the aluminous minerals were producedas a result of their modification, the amount of change beingdependent on the stage at which they were caught up by themagma, or it may mean that they remain recognisable as suchbecause they were chemically unsuitable for the transformationswhich others have suffered. If they represent the raw materialsfrom which sillimanite and the rest were produced, was felsparfirst made by reaction between quartz and magma, or were theminerals made by re-sorting the constituents of an aluminousslate? One would prefer to explain the whole of the phenomenatogether on a common basis, but at present this seems to beimpossible.

It is desirable to emphasize the fact that this paper is aresult of a search for the unusual and therefore one may get adistorted view of the lamprophyres. There is, however, thepossibility that an examination of the abnormal features maythrow some light on the lamprophyres as such. If there hasbeen the wholesale resorption postulated by Harker and Bowen,to what extent has the magma as a whole been influenced bythe process? Is any part of the lamprophyre to be looked uponas a normal rock ?

Thanks are due to Professor Watts for permission to makeuse of his department for photographic and other purposes. Thesections were cut by Mr. E. J. Tallin, who, in some cases, exceededinstructions; he was justified by the results.

NOTE.-When this paper was read, it was felt that theamount of work done was not sufficient to justify the expressionof a definite opinion regarding the conditions responsible forthe occurrence of these abnormalities. A tentative explanationis, however, submitted in a paper dealing with some lamprophyresnear Sedbergh, read before this Association on July 5th, 1929.