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Some features of Cornish lamprophyres
260
SOME FEATURES OF CORNISH LAMPROPHYRES. 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 lamprophyre near Ashburton, South Devon, and described some of its xenoliths containing corundum, staurolite, and green spinel. The present contribution is the result of an examination of other west country lamprophyres, undertaken with the object of studying the abnormal features known to occur, at any rate, in some of them. t In locating the several intrusions, the maps and memoirs issued by the Geological Survey were found to be indispensable. Specimens were collected from each, wherever possible, these being obtained so as to include any unusual texture; in other cases, samples of the normal rock were dug out and subsequently broken up in the laboratory, the search for xenoliths being continued.
Lemail Cutting, Wadebridge. This is a fine-grained rock, a lens being necessary to demonstrate the existence of the ferro-magnesian mineral. In sections this is seen to consist of the usual biotite with dark borders, together with much decomposed felspar. There are subordinate amounts of quartz, carbonate and greenish pseudomorphs after some ferro-magnesian phenocrysts. The xenoliths invariably contain felspar and have a maximum diameter of about l inch. In many cases this is untwinned, has a refractive index lower than that of Canada balsam, and is biaxial with negative birefringence. It is undoubtedly orthoclase and presumably is rich in potash. Quartz is associated with this, and the two minerals are frequently intergrown in graphic fashion. The greater portion of the felspar is only slightly decomposed, but where it abuts against the quartz there is much alteration; moreover, in this cloudy zone the appearance of cross-hatching usually associated with microcline is to be observed. The external margin of the xenolith, in contact with the lamprophyre, is usually not quite sharp, there being a zone of feslpar, formed of what appear to be discrete crystals with streaky alteration, but all extinguish simultaneously with the orthoclase of the xenolith proper. There is some biotite in this zone, 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 concentric arrangement, a structure very much like ripple-marks. Some of these pseudo-spherulites are composed of elongated, colourless crystals with a high relief, and polarising in brighter colours than in the case of the felspar, and their occurrence as larger masses, not radiating, but with varying orientations and sizes, gives an opportunity of determining some of the optical properties; the crystal is biaxial with a small optic axial angle, it is positive, and the optic axial plane coincides with a definite cleavage. The mineral is considered to be sillimanite. There are associated crystals of green spinel, some of them almost opaque. In one case there occurs a nest of these dark spinels associated with crystals of pale blue corundum. These latter have a good relief, a maximum absorption for longitudinal vibrations, polarise in first order yellow, show straight extinction and have a positive sign of elongation; there is no doubt as to their identity. Each crystal is embedded in a zone of colourless mica and the lamprophyre in contact with the xenolith is richer in biotite than elsewhere. A fact of some interest is that quartz occurs within two millimetres of corundum and spinel. In one case corundum is embedded in sillimanite. In the lamprophyre itself the felspars are sometimes arranged radially in a manner comparable with that described by Sir Jethro Teall,* and attributed by him to contact action. Another feature of the lamprophyre is the presence of what appear to be vesicles, each surrounded by elongated biotites arranged tangentially. These are very similar to those described and figured in the Ashburton lamprophyre. They are occupied usually by unstriated felspar, carbonate, chlorite, and a kind of quartz with a tendency to symmetrical arrangement of inclusions and a lack of uniform extinction. Similar quartz is to be seen fringing the quartz of the xenoliths where, between crossed nicols, it suggests the appearance of a section through velvet pile. Sir John Flett] described clear quartz grains with a broad brownish zone apparently made of rectangular prisms. They are attributed to the action of a vitreous magma along the rhombohedral cleavages, but in that case these individual prisms extinguish with the clear quartz. The example at present under consideration seems to be of a different nature. Another curious fact about these vesicles is that the felspar within them may be in optical continuity with crystals of the lamprophyre outside them; it is rather striking to observe the 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, striated albite. . The occurrence of a radial structure associated with concentric " ripple" marks in the felspar of the xenolith has already been mentioned. Further curious structures in these felspars are worthy of notice; they seem to be comparable with some found in the lamprophyres of the Lake District.' as described by Dr. Harker. * Some of the xenoliths consist of irregular masses of cloudy felspar and quartz, embedded in fine-grained, somewhat decomposed felspars. Usually it is the larger felspars that abut on the quartz with a complicated outline of bays, promontories and small islands; and there is a strong suggestion that one of the minerals is in a condition of arrested digestion. Within the felspar are larger, apparently globular bodies different in colour from the main felspar, and smaller vermicular bodies consisting of clear felspar, with a rod-like core of a colourless mineral with high relief, giving the appearance of clearing of the felspar by the core, comparable with the cleared glass around the incipient crystals of the Arran pitchstone. Another example shows the large felspar, here exhibiting twin lamellee, traversed by "lanes" occupied by the fine-grained cloudy felspar; the twin lamellas interrupted by the "lanes" can be picked up on the other side. Altogether, the evidence for digestion of these bodies, xenoliths or phenocrysts, seems to be strong. Pendennis Point, Falmouth. This intrusion is well exposed at low tide. It is a muchdecomposed lamprophyre, but the biotite flakes are comparatively fresh; inclusions of quartz, slate and pegmatite are conspicuous, the latter in ellipsoids with a maximum diameter of several inches. In section, one of the inclusions, less than I inch in diameter, is seen to consist of small, sub-angular quartz grains with some pyrite; it is traversed by several streaks consisting of what appear to be the finer-grained constitutents of the lamprophyre. There seems to be no doubt that it is a xenolith of sandstone almost in its original condition; its texture is very uniform. Another example, about the same size, is not obviously granular. It consists largely of quartz fragments of varying size, and the whole has a spongy appearance, the interstices being occupied by slightly decomposed felspar showing twin lamellae in places. When quartz meets felspar there may be a very narrow zone composed either of recently-added quartz or that mineral in process of conversion to another. In still another example, approaching • Geol. Mag., r892, p. 205.
PROC. GEOL.
FIG.
I.
Assoc.
VOL.
XL.
(1929).
FIG. 2.
SILLIMANITE.
Lemail,
X
14.
PLATE 24.
SILLIMANITE Al'D GREEN SPWEL.
Lemail,
FIG.
3.
GRAPHIC QUARTZ AND FELSPAR.
Lemail,
X
13.
FIG.
4.
X
30.
GRAPHIC QUARTZ AND FELSPAR. 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 minute inclusions and there is some difficulty in distinguishing hetween this mineral and the slightly decomposed, associated felspar. The whole mass of quartz is traversed by a network of that mineral free from inclusions, and between crossed nicols the clear threads are seen to come at the junctions of individual grains of the mineral. These grains do not resemble sand grains; they have a tendency to elongation in a particular direction, each grain extinguishes as an aggregate, and at the junction of two grains (along the clear streaks) there is an accumulation of very minute quartz individuals. One gets the impression of a great amount of shearing which almost certainly antedates the existence of the fragment as a xenolith. The associated felspar, in the identification of which one welcomes the occasional cleavage and twinning, partakes to some extent of the same sheared appearance, but this may be due to its having replaced quartz which had been previously sheared. A few " islands" of quartz in contact with the lamprophyre suggest that they are residues not yet converted. A slide from the pegmatite, examined with a lens, shows a graphic structure between quartz and felspar, but the latter mineral is seen to consist of more decomposed individuals embedded in lighter material. Under the microscope these decomposed crystals exhibit twin lamella, in some cases in more than one direction; they are, in places, in contact with quartz, but as a rule this latter mineral is in contact with the interstitial liner-grained felspar, and in places is moulded on its individual idiomorphic crystals. Many of these smaller felspars are striated, and although they usually form a sort of mosaic, some of them are distinctly elongated. In another slide is some brown glass with a suggestion of flow in places, and containing crystals of sillimanite, spinel, and probably andalusite; this brown substance is considered to be identical with the buchite recorded by Dr. H. H. Thomas in the tholeiite intrusions of Mull. *
Coast below Messack, East of the FaI. This exposure is accessible from a boat at low tide. It contains inclusions of quartz and pegmatite, some of them several inches long. The rock consists of fresh, pale biotite, felspar (some of it not much decomposed), quartz, apatite, and carbonate; the felspar appears to be orthoclase. The quartz of the xenoliths consists of numerous grains of all sizes, some of the larger ones showing undulose extinction. In some cases the large quartz grains interlock, in others there are 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 this graduates into the lamprophyre, the felspar (untwinned) becoming clearer, and separating into distinct crystals without marked elongation; then the biotites appear and the rock becomes normal. In other cases the biotite comes in contact with the quartz. A section through the pegmatite shows quartz with the usual eccentric shapes, and, as in the example from Pendennis Point, larger, much-decomposed felspars with interstitial, fresher, fine-grained material, this latter including occasional radial accumulations. The larger individuals have occasional clear areas in which twin larnellse, faintly seen in the decomposed areas, become beautifully clear; one of these forms a marginal fringe to the felspar in contact with quartz, and one unaltered area with one set of striations can be shown by the Becke test to have a refractive index lower than that of Canada balsamit is undoubtedly albite. Some of the felspar appears to be untwinned, some shows two sets of intersecting lamellse, but all 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, either potash or soda, or both. Quartz abuts on both the larger and smaller individuals of felspar.
South of Trelissick (W. of the FaI.). Much of this rock is so decomposed that it can be crumbled in the fingers, but there are spheroids, which are hard and exceedingly tough. A section through this harder variety shows the usual minerals of a biotite lamprophyre, but apatite is rather more abundant than usual and there is some undoubted sphene. A large quartz xenolith presents some interesting features. It polarises as a unit and has a marginal zone with a maximum width of about o.ymm., marked on its inner side by a very definite line which separates the clear quartz of the bulk of the xenolith from the marginal zone which contains minute inclusions in great abundance. Within this margin, which extinguishes simultaneously with the rest of the quartz, are crystals of sphene, and a green, slightly pleochroic mineral which seems to be a pyroxene. Traversing the xenolith is a band of quartz, with crowds of minute inclusions, exactly like that of the marginal zone. Here and there along this band, decomposed felspar appears, and this is bordered by a narrow fringe of the quartz with minute inclusions and small green pyroxenes. The hypothesis that the felspar, showing twin lamella: in places, has been manufactured by reaction between magma and quartz, and that the appearance
PROC. GEOL. Assoc., VOL. XL. (1929).
F IG. I. CORUND UM AN D G R EE N SPINEL. Lemail, X 40.
F IG . 2 .
PLATE 25 .
CURIO US S TR U CT U!(ES ALKALI F ELSPAH .
Le rnail,
FIG .
3. N.
GARNET, R UTILE AN D QUARTZ . of Pill Cre ek , x 20 .
FIG .
4.
X
IN
'2 0 .
GR A P HI C QU A RTZ AND FELSP AI{.
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 previously dealt with in that the biotite is inconspicuous and decomposed, while the apatite is much more abundant. Xenoliths about I inch in diameter are fairly common. The darker ones resolve themselves into a mixture of green and brown chlorite; the lighter ones are made mainly of spongy quartz associated with felspar, the latter occupying the interstices. Embedded in these minerals are crystals of garnet and rutile, the latter showing in one case the characteristic geniculate twin. Apatite also is present and there is a little pale yellow-green mica. Some of the interstices are occupied by chlorite. There is a tendency for 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 of interest in the present inquiry. In the vicinity of Newquay several lamprophyres were examined, including that below Pentire Farm, but there is nothing 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 one within the sheet which will repay microscopic investigation, but mentions the existence of corroded quartz xenocrysts in all the other dykes. Two of these xenocrysts were dug out of the face of the quarry south of the Gannel. In section, the quartz was found to be associated with a felted mass of flexuous, narrow, colourless crystals, having a good relief and polarising usually in second order colours; this is considered to be sillimanite. In places, the crystals become more rigid in character and are embedded in what appears to be muscovite. Crystals of green spinel are associated with the sillimanite, which seems to be reluctant to meet the quartz, a film of felspar always being interposed between the two minerals. In addition, epidote and some doubtful andalusite are present. In one case, more or less separated pieces of quartz are embedded in felspar and give a very strong suggestion that they are residual, much of the mineral having been (apparently) converted into the felspar. This idea is considered to be supported by the existence of some curious streaks traversing the quartz. The lamprophyres of Holywell Bay and Towan Head exhibit few 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 existence of small, apparently globular areas about 4 mm. diameter which are coarser than the rest of the rock. These areas are sharply demarcated from the rest of the rock by a narrow belt of what appears to be colourless mica. It rather suggests the existence of globules of magma different in composition from the rest, but of course, other explanations are possible and more likely to meet with acceptance. The lamprophyres near the Tea Cavern are traversed by conspicuous veins of quartz and Carbonate.
Foot of Cant Hill, Padstow. This is a very fine-grained rock in which are visible various masses of quartz, most of them lenticular in form and having a maximum length of about I foot. In section the rock is seen to consist largely of laths of decomposed felspar with pseudomorphs in carbonate after some ferro-magnesian mineral. Magnetite is common, and biotite, never abundant, is now represented only by its decomposition products. One of the quartz zenoliths, with little 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 little trace of shattering. It is very much like a fragment of arenaceous rock from which the cementing material has been leached out. Against this, perhaps, is the very variable size of the grains, but it is possible that there has been some fusion of one into another; this supposition involves some difficulties, but seems to be worthy of consideration in this and other cases. In another slide is an area of about 5 mm. diameter occupied largely by laths of striated felspar which are larger and fresher than those of the bulk of the rock. There are occasional crystals 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 in the Lemail lamprophyre. The quartz is peripheral within the vesicle. A very definite arc of biotite follows the junction of vesicle and coarse felspar for a short distance, then for the rest of its course (about 180 0 in all), it traverses the area occupied by the larger felspars; it does not follow the junction with the normal rock. Another vesicle contains mostly the rhombohedral carbonate with a little of the abnormal quartz. In addition there is a colourless mineral, most of the sections rectangular, showing cleavages parallel to the sides and with a relief inferior to that of the carbonate; polarisation colours are usually second order and twin lamellse run obliquely across some of the crystals. The figure is biaxial and the sign positive. The mineral is identified as anhydrite and the occurrence seems to 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 Rock are of a totally different type and seem to have no bearing on the 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"; idiomorphic crystals of felspar project into the quartz from the margin and there 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 those typically seen in the case of microcline. Whatever the significance of this may be, it is found also in several of the examples here dealt with. Specimens were collected from Halveor, near St. Columb Major; 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 these minerals and structures came into existence. Dr. Harker* says that the diminutive size of the corroded felspars probably indicates that manv others have been entirelv dissolved in the containing magma' and that this solution is' most reasonably referred to the epoch of the injection of the dykes. It appears from his account that the felspar was dissolved as a whole, that there was no selective leaching out of the constituents. Dr. Thomas] (following Teall) attributes the formation of the aluminous minerals to metamorphism of aluminous sediments with a certain amount of selective interchange of constituents. Dr. du Toit, t dealing with the plumasite of -Natal, points out that the dykes penetrate a country consisting of serpentine resting on granulite. So long as the dyke is in the lower formation, it is an ordinary pegmatite, but when it enters the serpentine it becomes narrow, loses all its quartz and acquires corundum. The magma was desilicated by the serpentine, which obtained in addition potash and some alumina. The occurrence is of remarkable interest as showing that the minerals of an intrusive rock may have come into being only because of the chemical peculiarities of the pre-existing rocks in contact. It is possible that some process comparable with this has taken place 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," suggests that in a granitic liquid at the pegmatite stage, quartz may react with the liquid to produce felspar, and sometimes the graphic structure may be a result of this reaction Corning to the facts in the Cornish lamprophyres, we know that they contain fragments of sandstone and slate, and also the nests of quartz, felspars, sillimanite, green spinel and corundum. The presence of fragments recognisable as sediments may indicate either that the aluminous minerals were produced as a result of their modification, the amount of change being dependent on the stage at which they were caught up by the magma, or it may mean that they remain recognisable as such because they were chemically unsuitable for the transformations which others have suffered. If they represent the raw materials from which sillimanite and the rest were produced, was felspar first made by reaction between quartz and magma, or were the minerals made by re-sorting the constituents of an aluminous slate? One would prefer to explain the whole of the phenomena together on a common basis, but at present this seems to be impossible. It is desirable to emphasize the fact that this paper is a result of a search for the unusual and therefore one may get a distorted view of the lamprophyres. There is, however, the possibility that an examination of the abnormal features may throw some light on the lamprophyres as such. If there has been the wholesale resorption postulated by Harker and Bowen, to what extent has the magma as a whole been influenced by the process? Is any part of the lamprophyre to be looked upon as a normal rock ? Thanks are due to Professor Watts for permission to make use of his department for photographic and other purposes. The sections were cut by Mr. E. J. Tallin, who, in some cases, exceeded instructions; he was justified by the results. NOTE.-When this paper was read, it was felt that the amount of work done was not sufficient to justify the expression of a definite opinion regarding the conditions responsible for the occurrence of these abnormalities. A tentative explanation is, however, submitted in a paper dealing with some lamprophyres near Sedbergh, read before this Association on July 5th, 1929.
SOME FEATURES OF CORNISH LAMPROPHYRES. 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 lamprophyre near Ashburton, South Devon, and described some of its xenoliths containing corundum, staurolite, and green spinel. The present contribution is the result of an examination of other west country lamprophyres, undertaken with the object of studying the abnormal features known to occur, at any rate, in some of them. t In locating the several intrusions, the maps and memoirs issued by the Geological Survey were found to be indispensable. Specimens were collected from each, wherever possible, these being obtained so as to include any unusual texture; in other cases, samples of the normal rock were dug out and subsequently broken up in the laboratory, the search for xenoliths being continued.
Lemail Cutting, Wadebridge. This is a fine-grained rock, a lens being necessary to demonstrate the existence of the ferro-magnesian mineral. In sections this is seen to consist of the usual biotite with dark borders, together with much decomposed felspar. There are subordinate amounts of quartz, carbonate and greenish pseudomorphs after some ferro-magnesian phenocrysts. The xenoliths invariably contain felspar and have a maximum diameter of about l inch. In many cases this is untwinned, has a refractive index lower than that of Canada balsam, and is biaxial with negative birefringence. It is undoubtedly orthoclase and presumably is rich in potash. Quartz is associated with this, and the two minerals are frequently intergrown in graphic fashion. The greater portion of the felspar is only slightly decomposed, but where it abuts against the quartz there is much alteration; moreover, in this cloudy zone the appearance of cross-hatching usually associated with microcline is to be observed. The external margin of the xenolith, in contact with the lamprophyre, is usually not quite sharp, there being a zone of feslpar, formed of what appear to be discrete crystals with streaky alteration, but all extinguish simultaneously with the orthoclase of the xenolith proper. There is some biotite in this zone, 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 concentric arrangement, a structure very much like ripple-marks. Some of these pseudo-spherulites are composed of elongated, colourless crystals with a high relief, and polarising in brighter colours than in the case of the felspar, and their occurrence as larger masses, not radiating, but with varying orientations and sizes, gives an opportunity of determining some of the optical properties; the crystal is biaxial with a small optic axial angle, it is positive, and the optic axial plane coincides with a definite cleavage. The mineral is considered to be sillimanite. There are associated crystals of green spinel, some of them almost opaque. In one case there occurs a nest of these dark spinels associated with crystals of pale blue corundum. These latter have a good relief, a maximum absorption for longitudinal vibrations, polarise in first order yellow, show straight extinction and have a positive sign of elongation; there is no doubt as to their identity. Each crystal is embedded in a zone of colourless mica and the lamprophyre in contact with the xenolith is richer in biotite than elsewhere. A fact of some interest is that quartz occurs within two millimetres of corundum and spinel. In one case corundum is embedded in sillimanite. In the lamprophyre itself the felspars are sometimes arranged radially in a manner comparable with that described by Sir Jethro Teall,* and attributed by him to contact action. Another feature of the lamprophyre is the presence of what appear to be vesicles, each surrounded by elongated biotites arranged tangentially. These are very similar to those described and figured in the Ashburton lamprophyre. They are occupied usually by unstriated felspar, carbonate, chlorite, and a kind of quartz with a tendency to symmetrical arrangement of inclusions and a lack of uniform extinction. Similar quartz is to be seen fringing the quartz of the xenoliths where, between crossed nicols, it suggests the appearance of a section through velvet pile. Sir John Flett] described clear quartz grains with a broad brownish zone apparently made of rectangular prisms. They are attributed to the action of a vitreous magma along the rhombohedral cleavages, but in that case these individual prisms extinguish with the clear quartz. The example at present under consideration seems to be of a different nature. Another curious fact about these vesicles is that the felspar within them may be in optical continuity with crystals of the lamprophyre outside them; it is rather striking to observe the 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, striated albite. . The occurrence of a radial structure associated with concentric " ripple" marks in the felspar of the xenolith has already been mentioned. Further curious structures in these felspars are worthy of notice; they seem to be comparable with some found in the lamprophyres of the Lake District.' as described by Dr. Harker. * Some of the xenoliths consist of irregular masses of cloudy felspar and quartz, embedded in fine-grained, somewhat decomposed felspars. Usually it is the larger felspars that abut on the quartz with a complicated outline of bays, promontories and small islands; and there is a strong suggestion that one of the minerals is in a condition of arrested digestion. Within the felspar are larger, apparently globular bodies different in colour from the main felspar, and smaller vermicular bodies consisting of clear felspar, with a rod-like core of a colourless mineral with high relief, giving the appearance of clearing of the felspar by the core, comparable with the cleared glass around the incipient crystals of the Arran pitchstone. Another example shows the large felspar, here exhibiting twin lamellee, traversed by "lanes" occupied by the fine-grained cloudy felspar; the twin lamellas interrupted by the "lanes" can be picked up on the other side. Altogether, the evidence for digestion of these bodies, xenoliths or phenocrysts, seems to be strong. Pendennis Point, Falmouth. This intrusion is well exposed at low tide. It is a muchdecomposed lamprophyre, but the biotite flakes are comparatively fresh; inclusions of quartz, slate and pegmatite are conspicuous, the latter in ellipsoids with a maximum diameter of several inches. In section, one of the inclusions, less than I inch in diameter, is seen to consist of small, sub-angular quartz grains with some pyrite; it is traversed by several streaks consisting of what appear to be the finer-grained constitutents of the lamprophyre. There seems to be no doubt that it is a xenolith of sandstone almost in its original condition; its texture is very uniform. Another example, about the same size, is not obviously granular. It consists largely of quartz fragments of varying size, and the whole has a spongy appearance, the interstices being occupied by slightly decomposed felspar showing twin lamellae in places. When quartz meets felspar there may be a very narrow zone composed either of recently-added quartz or that mineral in process of conversion to another. In still another example, approaching • Geol. Mag., r892, p. 205.
PROC. GEOL.
FIG.
I.
Assoc.
VOL.
XL.
(1929).
FIG. 2.
SILLIMANITE.
Lemail,
X
14.
PLATE 24.
SILLIMANITE Al'D GREEN SPWEL.
Lemail,
FIG.
3.
GRAPHIC QUARTZ AND FELSPAR.
Lemail,
X
13.
FIG.
4.
X
30.
GRAPHIC QUARTZ AND FELSPAR. 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 minute inclusions and there is some difficulty in distinguishing hetween this mineral and the slightly decomposed, associated felspar. The whole mass of quartz is traversed by a network of that mineral free from inclusions, and between crossed nicols the clear threads are seen to come at the junctions of individual grains of the mineral. These grains do not resemble sand grains; they have a tendency to elongation in a particular direction, each grain extinguishes as an aggregate, and at the junction of two grains (along the clear streaks) there is an accumulation of very minute quartz individuals. One gets the impression of a great amount of shearing which almost certainly antedates the existence of the fragment as a xenolith. The associated felspar, in the identification of which one welcomes the occasional cleavage and twinning, partakes to some extent of the same sheared appearance, but this may be due to its having replaced quartz which had been previously sheared. A few " islands" of quartz in contact with the lamprophyre suggest that they are residues not yet converted. A slide from the pegmatite, examined with a lens, shows a graphic structure between quartz and felspar, but the latter mineral is seen to consist of more decomposed individuals embedded in lighter material. Under the microscope these decomposed crystals exhibit twin lamella, in some cases in more than one direction; they are, in places, in contact with quartz, but as a rule this latter mineral is in contact with the interstitial liner-grained felspar, and in places is moulded on its individual idiomorphic crystals. Many of these smaller felspars are striated, and although they usually form a sort of mosaic, some of them are distinctly elongated. In another slide is some brown glass with a suggestion of flow in places, and containing crystals of sillimanite, spinel, and probably andalusite; this brown substance is considered to be identical with the buchite recorded by Dr. H. H. Thomas in the tholeiite intrusions of Mull. *
Coast below Messack, East of the FaI. This exposure is accessible from a boat at low tide. It contains inclusions of quartz and pegmatite, some of them several inches long. The rock consists of fresh, pale biotite, felspar (some of it not much decomposed), quartz, apatite, and carbonate; the felspar appears to be orthoclase. The quartz of the xenoliths consists of numerous grains of all sizes, some of the larger ones showing undulose extinction. In some cases the large quartz grains interlock, in others there are 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 this graduates into the lamprophyre, the felspar (untwinned) becoming clearer, and separating into distinct crystals without marked elongation; then the biotites appear and the rock becomes normal. In other cases the biotite comes in contact with the quartz. A section through the pegmatite shows quartz with the usual eccentric shapes, and, as in the example from Pendennis Point, larger, much-decomposed felspars with interstitial, fresher, fine-grained material, this latter including occasional radial accumulations. The larger individuals have occasional clear areas in which twin larnellse, faintly seen in the decomposed areas, become beautifully clear; one of these forms a marginal fringe to the felspar in contact with quartz, and one unaltered area with one set of striations can be shown by the Becke test to have a refractive index lower than that of Canada balsamit is undoubtedly albite. Some of the felspar appears to be untwinned, some shows two sets of intersecting lamellse, but all 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, either potash or soda, or both. Quartz abuts on both the larger and smaller individuals of felspar.
South of Trelissick (W. of the FaI.). Much of this rock is so decomposed that it can be crumbled in the fingers, but there are spheroids, which are hard and exceedingly tough. A section through this harder variety shows the usual minerals of a biotite lamprophyre, but apatite is rather more abundant than usual and there is some undoubted sphene. A large quartz xenolith presents some interesting features. It polarises as a unit and has a marginal zone with a maximum width of about o.ymm., marked on its inner side by a very definite line which separates the clear quartz of the bulk of the xenolith from the marginal zone which contains minute inclusions in great abundance. Within this margin, which extinguishes simultaneously with the rest of the quartz, are crystals of sphene, and a green, slightly pleochroic mineral which seems to be a pyroxene. Traversing the xenolith is a band of quartz, with crowds of minute inclusions, exactly like that of the marginal zone. Here and there along this band, decomposed felspar appears, and this is bordered by a narrow fringe of the quartz with minute inclusions and small green pyroxenes. The hypothesis that the felspar, showing twin lamella: in places, has been manufactured by reaction between magma and quartz, and that the appearance
PROC. GEOL. Assoc., VOL. XL. (1929).
F IG. I. CORUND UM AN D G R EE N SPINEL. Lemail, X 40.
F IG . 2 .
PLATE 25 .
CURIO US S TR U CT U!(ES ALKALI F ELSPAH .
Le rnail,
FIG .
3. N.
GARNET, R UTILE AN D QUARTZ . of Pill Cre ek , x 20 .
FIG .
4.
X
IN
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GR A P HI C QU A RTZ AND FELSP AI{.
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 previously dealt with in that the biotite is inconspicuous and decomposed, while the apatite is much more abundant. Xenoliths about I inch in diameter are fairly common. The darker ones resolve themselves into a mixture of green and brown chlorite; the lighter ones are made mainly of spongy quartz associated with felspar, the latter occupying the interstices. Embedded in these minerals are crystals of garnet and rutile, the latter showing in one case the characteristic geniculate twin. Apatite also is present and there is a little pale yellow-green mica. Some of the interstices are occupied by chlorite. There is a tendency for 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 of interest in the present inquiry. In the vicinity of Newquay several lamprophyres were examined, including that below Pentire Farm, but there is nothing 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 one within the sheet which will repay microscopic investigation, but mentions the existence of corroded quartz xenocrysts in all the other dykes. Two of these xenocrysts were dug out of the face of the quarry south of the Gannel. In section, the quartz was found to be associated with a felted mass of flexuous, narrow, colourless crystals, having a good relief and polarising usually in second order colours; this is considered to be sillimanite. In places, the crystals become more rigid in character and are embedded in what appears to be muscovite. Crystals of green spinel are associated with the sillimanite, which seems to be reluctant to meet the quartz, a film of felspar always being interposed between the two minerals. In addition, epidote and some doubtful andalusite are present. In one case, more or less separated pieces of quartz are embedded in felspar and give a very strong suggestion that they are residual, much of the mineral having been (apparently) converted into the felspar. This idea is considered to be supported by the existence of some curious streaks traversing the quartz. The lamprophyres of Holywell Bay and Towan Head exhibit few 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 existence of small, apparently globular areas about 4 mm. diameter which are coarser than the rest of the rock. These areas are sharply demarcated from the rest of the rock by a narrow belt of what appears to be colourless mica. It rather suggests the existence of globules of magma different in composition from the rest, but of course, other explanations are possible and more likely to meet with acceptance. The lamprophyres near the Tea Cavern are traversed by conspicuous veins of quartz and Carbonate.
Foot of Cant Hill, Padstow. This is a very fine-grained rock in which are visible various masses of quartz, most of them lenticular in form and having a maximum length of about I foot. In section the rock is seen to consist largely of laths of decomposed felspar with pseudomorphs in carbonate after some ferro-magnesian mineral. Magnetite is common, and biotite, never abundant, is now represented only by its decomposition products. One of the quartz zenoliths, with little 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 little trace of shattering. It is very much like a fragment of arenaceous rock from which the cementing material has been leached out. Against this, perhaps, is the very variable size of the grains, but it is possible that there has been some fusion of one into another; this supposition involves some difficulties, but seems to be worthy of consideration in this and other cases. In another slide is an area of about 5 mm. diameter occupied largely by laths of striated felspar which are larger and fresher than those of the bulk of the rock. There are occasional crystals 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 in the Lemail lamprophyre. The quartz is peripheral within the vesicle. A very definite arc of biotite follows the junction of vesicle and coarse felspar for a short distance, then for the rest of its course (about 180 0 in all), it traverses the area occupied by the larger felspars; it does not follow the junction with the normal rock. Another vesicle contains mostly the rhombohedral carbonate with a little of the abnormal quartz. In addition there is a colourless mineral, most of the sections rectangular, showing cleavages parallel to the sides and with a relief inferior to that of the carbonate; polarisation colours are usually second order and twin lamellse run obliquely across some of the crystals. The figure is biaxial and the sign positive. The mineral is identified as anhydrite and the occurrence seems to 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 Rock are of a totally different type and seem to have no bearing on the 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"; idiomorphic crystals of felspar project into the quartz from the margin and there 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 those typically seen in the case of microcline. Whatever the significance of this may be, it is found also in several of the examples here dealt with. Specimens were collected from Halveor, near St. Columb Major; 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 these minerals and structures came into existence. Dr. Harker* says that the diminutive size of the corroded felspars probably indicates that manv others have been entirelv dissolved in the containing magma' and that this solution is' most reasonably referred to the epoch of the injection of the dykes. It appears from his account that the felspar was dissolved as a whole, that there was no selective leaching out of the constituents. Dr. Thomas] (following Teall) attributes the formation of the aluminous minerals to metamorphism of aluminous sediments with a certain amount of selective interchange of constituents. Dr. du Toit, t dealing with the plumasite of -Natal, points out that the dykes penetrate a country consisting of serpentine resting on granulite. So long as the dyke is in the lower formation, it is an ordinary pegmatite, but when it enters the serpentine it becomes narrow, loses all its quartz and acquires corundum. The magma was desilicated by the serpentine, which obtained in addition potash and some alumina. The occurrence is of remarkable interest as showing that the minerals of an intrusive rock may have come into being only because of the chemical peculiarities of the pre-existing rocks in contact. It is possible that some process comparable with this has taken place 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," suggests that in a granitic liquid at the pegmatite stage, quartz may react with the liquid to produce felspar, and sometimes the graphic structure may be a result of this reaction Corning to the facts in the Cornish lamprophyres, we know that they contain fragments of sandstone and slate, and also the nests of quartz, felspars, sillimanite, green spinel and corundum. The presence of fragments recognisable as sediments may indicate either that the aluminous minerals were produced as a result of their modification, the amount of change being dependent on the stage at which they were caught up by the magma, or it may mean that they remain recognisable as such because they were chemically unsuitable for the transformations which others have suffered. If they represent the raw materials from which sillimanite and the rest were produced, was felspar first made by reaction between quartz and magma, or were the minerals made by re-sorting the constituents of an aluminous slate? One would prefer to explain the whole of the phenomena together on a common basis, but at present this seems to be impossible. It is desirable to emphasize the fact that this paper is a result of a search for the unusual and therefore one may get a distorted view of the lamprophyres. There is, however, the possibility that an examination of the abnormal features may throw some light on the lamprophyres as such. If there has been the wholesale resorption postulated by Harker and Bowen, to what extent has the magma as a whole been influenced by the process? Is any part of the lamprophyre to be looked upon as a normal rock ? Thanks are due to Professor Watts for permission to make use of his department for photographic and other purposes. The sections were cut by Mr. E. J. Tallin, who, in some cases, exceeded instructions; he was justified by the results. NOTE.-When this paper was read, it was felt that the amount of work done was not sufficient to justify the expression of a definite opinion regarding the conditions responsible for the occurrence of these abnormalities. A tentative explanation is, however, submitted in a paper dealing with some lamprophyres near Sedbergh, read before this Association on July 5th, 1929.