The petrology of the Eskdale (Cumberland) granite

17

THE PETROLOGY OF THE ESKDALE {CUMBERLAND} GRANITE. By BRIAN SIMPSON, M.Sc. [Received lIlh July, 1933.] [Read 71h July, 1933.] CONTENTS. I.

II. III. IV. V. VI. VII. VIII. IX.

PAGE

INTRODUCTIOl\ Al\D PREVIOUS LITERATCRE

17

FIELD RELATJONS

18

THE GRANITES

21

MARGINAL VARIETIES

25

CONTAMINATED ROCKS

2()

SERICITIZATION AND CHLORITIZATION

28

THE DYKES THE REAVY MINERALS Su~n.IARY AND CONCLUSION

I. INTRODUCTION AND PREVIOUS LITERATURE. THE Eskdale Granite occupies about forty-eight square miles of picturesque country in Western Cumberland. Clifton Ward, in his early studies of the Lake District, used this mass to illustrate his ideas on the metamorphic origin of granite. Later, it was studied in detail by A. R. Dwerryhouse, whilst its heavy mineral suite was described by R. H. Rastall and W. H. Wilcockson. It is the purpose of the present paper to give the results of more detailed field and laboratory work, particularly with regard to the mineral characters, accessory minerals, marginal modifications and the final consolidation effects of the intrusion. In 1875 the Eskdale Granite was described by Clifton Ward [1]* as a reddish rock resulting from the colour of the felspars, and containing dark mica. In the subsequent extension of his work [2] on the granite he offered a suggestion for its origin which is now of historic interest only, his suggestion being that the granite was produced by the extreme metamorphism of the associated volcanic rocks. Sir].]. Teall, in his" British Petrography," described a granite from Eskdale, but as Dwerryhouse [4] remarked, such a specimen does not represent the granite as a whole. A. Harker [5] placed the granite amongst the biotite granites. Dwerryhouse traced the field-relations of the granite and Borrowdale Volcanic Rocks and stated that the granite was certainly pre-Triassic, since the St. Bees Sandstone covered it unconformably on the western side. He discussed the roof [5, pp. 55-80], having noted outliers of the Borrowdale rocks on Great Barrow and Blea Tarn Hill, also the intense pressure set up by the intrusion. In addition, he noted two • For

u.t of references

PROC. G1WL.

Assoc.,

see p. 33. VOL. XLV., PART I,

1934.

2

18

BRIAN SIMPSON,

types of granite [5, p. 6r}, a pink variety and the Wabberthwaite variety. Many marginal types were described [ll, pp. 64-u9J, but these appear to be quartz porphyries and felsites. J. F. N. Green [6}, in discussing the age of the Lake District intrusions, delimited the boundary of the Eskdale Granite on its eastern side, and discussed the contact metamorphism of the granite. In the same paper Green postulated a thrust plane at Devoke Water; his evidence for this was a small window in the granite, on the south side of Devoke Water, showing a gently dipping plane between the granite and the andesite, and said that the latter shows no evidence of thermal metamorphism. In the opinion of the present writer, however, the absence of brecciation or mylonization in the position of the sole of the thrust, the straight character of the contact (for a sinuous contact would be expected in hummocky country with a thrust-plane of such low dip as Green describes) and the fact that the andesite is regarded as having suffered contact metamorphism militates against there being a true thrust-plane in this region. R. H. Rastall and W. H. Wilcockson [7} described the heavy minerals and noted their rarity. They recorded abundant zircon which they considered increased in abundance southwards from Eskmeals. More detailed work does not bear out this contention. Apatite, tourmaline, garnet and phyrotite were also recorded. T. A. Jones [8, p. 39J described patchy-blue and brown tourmaline from Beckfoot Quarry which he believed to be confined to joint planes and similar structures.

II. FmLD RELATIONS. The boundary is described in a clockwise direction beginning in the neighbourhood of Easthwaite Farm, S.W. of Wastwater. Here the granite is probably in contact with the Ennerdale Granophyre, but the contact is masked by a wide alluvial plain. From here, the boundary runs east over Whin Rigg, but is complicated by faulting. On Whin Rigg, at about 1200 ft., the boundary swings east and then north-east along the flanks. In this region dykes of fifty yards width can be picked out by their rugged nature as compared with the generally smooth grass slopes. The dyke-rock is a coarse granite-porphyry in which both quartz and felspar reach lengths of as much as three inches. The boundary is next seen in Robin Gill, where at about 600 ft. the junction is marked in the stream-bed by a small waterfall. Here the granite is fine-grained and pink. From Robin Gill the boundary swings across Tongue Moor into the Mite Valley, which it crosses at about 570 ft. and then, running south for about Soo yards, cuts along the flanks of Low Longrigg and is in contact with a good hornfelsed andesite carrying biotite and hornblende. At 800 ft. the boundary crosses Low Longrigg and swings northwest by Brats Moss and under Boat Howe to the shores of

PETROLOGY OF T HE ESoKJ);\LE (CU~IBE RL AN D) GRA ~ ITE.

19

~CQ.E. tN

DANO£S 'TE

lEl t

DIDOT 1SE;O I'\N OlS',.r.

~--JM 'L (~.

~ FIG. 2. -

S K E T CH };L~p OF THE E S K D AI.E

BIOT IT E

- HOIlN F'EL3

(CUMBERI.AN D ) GRANITE.

20

BRIAK SIMPSOK,

Burnmoor Tarn. South of Low Longrigg, near Brats Hill, faulting has isolated a small patch of volcanic rock, and it is from this region that the injection breccia, described below, was obtained. Glacial material and moss conceals the junction from here, until Hardrigg Gill is reached, where the contact is seen at about IIOO ft. Here the granite is pink and fine-grained, and much veining and crushing of the volcanics is seen. From Hardrigg Gill the boundary is traced south into Oliver Gill, where an excellent contact is observed just above the waterfall. From Oliver Gill the contact is traced south along the flanks of Great Howe to Whinscales, the junction being a normal contact much disturbed by faults with a general N.E.-S.W. strike. At Whinscales another injection breccia is found near the sheepfold. South from Whinscales to Stoney Tarn, the boundary is faulted, one set of faults being of a general N.-S. strike, whilst a second cuts them in a N.E.-S.W. direction. At Pcelplace Noddle the boundary is cut off by a strong fault, which throws it half a mile to the east. In this region the country-rock is a bedded ash, well seen in the face of Goat Crag, where its white appearance contrasts effectively with that of the darker andesite. A strong gully marks the position of the main fault and carries the boundary north-east to the bank of Scale Gill. From here it swings south-west by Whahouse Bridge to Birker Force, where the contact can be seen in the stream-bed. From Birker Force, the boundary is traced under Gate Crag to the south-west end of Hartley Crag, in which aplite veins are very well marked : at this point the granite is thrown south by a small fault for about 200 yards. The boundcry then runs south of west to Birker Beck, where a good contact is seen 400 yards north of Whincop Bridge, and from here a fault throws the boundary almost a mile to the south to High Ground Farm, whence the junction runs south-west to the shores of Devoke Water. Two faults with a N.W.-S.E. strike have displaced a little of the pink granite to the south side of the lake. From the west end of Devoke Water the contact is faulted E.W. t,o Raven Crag. The greisen described below is found on the flanks of Water Crag and is conspicuous by its pure whiteness. Veins of aplite can be seen extending into the black andesite. There is, however at the eastern end of Devoke Water an exposure of andesite which is considerably epidotised, as shown by its beautiful green colour. The boundary runs almost due south from Raven Crag to Corney Fell, and is a normal junction dipping eastwards. On Corney Fell it swings east and shows a good contact at the head of Buckbarrow BEck, at about 1200 ft. This stretch of boundary is interesting in that there is either a contaminated granite or one carrying large xenoliths at all visible contacts,

PETROLOGY OF THE ESKDALE (CUMBERLAND) GRANITE.

21

the latter especially well seen near the head of Buckbarrow Beck. Another feature in all the contacts is the zone of crushed rock, often several feet in thickness, indicative of the intense pressure set up by the intrusion. On the eastern slopes of Prior Park the boundary swings south and passes to the foot of Stoneside Hill, and then sharply south-west to Bootle Fell. The country-rock is a fine-grained andesite. The western boundary is the most hypothetical, for, from Bootle Fell to Muncaster Fell, sand dunes and alluvium cover all the contact and a broad coastal plain is seen stretching north to Ravenglass. There is, however, a point 50 yards east of Mumcaster Mill, where an outcrop of flow brecciated andesite can be seen; the granite occurs about 50 yards further east, so that the junction must lie somewhere in this area. Across the foot of the valley alluvium again obscures the contact and it is not until the foot of Irton Fell is reached that it can be seen again. Here the boundary runs north-east across the flanks of Irton Fell to join a north-south fault a hundred yards east of the Pike. The rock of the Pike is a blue porphyritic andesite and shows evidence of the effect of heat. The upper surface of the granite mass and its general shape must of necessity be hypothetical. The surface dips north in the Robin Gill portion, and gradually changes until on Great Howe it dips to the east and continues to do so along the eastern boundary. On Irton Pike it dips west. From these few positions there is little doubt as to the upper surface being dome-shaped. There are two portions of the roof left, one on Blea Tarn Hill and one on Great Barrow. Both these outliers are disturbed by faults, the latter less than the former, and thus afford no evidence of the height of the dome. Little evidence is forthcoming concerning the lower surface of the granite-if such a surface existed. One section, however, on the south side of Devoke Water does afford a little evidence. Here a pink granite has been thrown by two N.-S. faults to the south side of Devoke Water. A stream flowing into the tarn has carved out a window in which is seen a porphyritic andesite whose surface dips at a low angle into the granite mass. Since the granite dips with this plane it seems reasonable to suppose that it shows some evidence of the lower surface of a laccolite. This, as already indicated, is the region where J. F. N. Green places his thrust plane.

III. THE GRANITES. In this description of the Eskdale Granite three types have been distinguished: the Pink type, the Green type and the Grey type. Whilst the types are definite and localised, actual boundaries between them have not been observed. The per-

22

BRIAN

SIMPSO~,

centage composition of the rocks is based on micrometrical analyses of rock slides. Quartz Perthite Plagioclase Orthoclase Sericite Muscovite Iron Ore Biotite

Pink Granite. 32·5

Figures are averages of

Green Granite.

58.5 7·5 2·3

4.

I

I

o

39. 1 34·3

Grey Granite. 27. 2

20.0

10.8

11,9

8·4 6·3 ·5

23. 1 9·4

·4 2.0 100

8·5

traverses.

The Pink Granite. The Pink Granite, situated in the north part of the mass, occupies approximately half the total area. It is a square cleaving rock in which there is always a greater or less degree of banding. The high silica percentage is at once made apparent by the large and numerous grains of quartz in the rock. The felspar varies in colour from a pale rose-pink to a deep brownpink, and also varies in amount. The ferromagnesian minerals are scattered in small nests, and are frequently only represented by a mass of chloritic material. The average specific gravity is 2.6Ig, and variation within the type is caused by the varying amount of ferromagnesian minerals and basic felspars ; specially heavy specimens are found to owe their weight to coatings of iron oxide. In thin section the rock shows the following minerals : quartz, perthite, orthoclase, albite, albite-oligoclase, oligoclase, biotite, muscovite and iron ore. The quartz occurs mainly as large anhedral crystals, and often constitutes as much as 50°!c, of the rock. It displays the usual characters of the mineral, although the author is unable to agree with Dwerryhouse [9, p. 6IJ that the quartz is remarkably free from strain shadows, since in nearly all observed sections, strain shadows of varying degree are present, imparting a curious velvety lustre to the quartz between cross-nicols. As shown by Dwerryhouse, the dominant felspar is perthite, the appearance of which varies in crystals in the same slide. In all cases orthoclase is the host. In the most abundant variety the plagioclase component presents a vermiform appearance. In many cases the perthite is perfect and, using the twin striation as the index plane, it is possible to measure the extinctions. Dwerryhouse [9, p. 62J, using this method, ascribed the plagioclase to oligoclase; although oligoclase is present in some cases, albite is the most abundant plagioclase inclusion. Twinning is very common, and although most frequently on the Carlsbad law, Baveno twins are not uncommon. In almost every case the felspar is changed to sericite; the

PETROLOGY OF THE ESKDALE (CUMBERLAND) GRANITE.

23

extent of this varies from a few flakes of white mica distributed along the cleavages, to wholly sericitized masses, whose origin from perthite can only be determined by the distribution of the flakes along the twin planes. Sometimes in the latter case the alteration has resulted in the production of radiating aggregates which resemble spherulites, and often occupy the major portion of the altered area. Next in order of abundance is the plagioclase, which, with the increase in the anorthite molecule up to 25°;;), ranges through albite, albite-oligoclase and oligoclase. The crystals are always euhedral but vary considerably in size. Albite is the most abundant species, and in a few cases may exceed all other felspars in amount. The change to sericite presents a peculiar aspect. The crystals show a dark sericitic centre surrounded by a rim of plagioclase, which often shows little change. Frequently portions of the inner felspar are unaltered, and appear to be in optical continuity with the felspar which forms the rim. As an explanation of this phenomenon, we may suppose that the inner felspar was of earlier crystallization than that of the rim, and that it suffered change immediately on crystallization. Subsequent introduction of more of the anorthite molecule, possibly from the Borrowdale Volcanic Rocks, then caused renewed deposition round the already changed individual and in optical continuity with it. Orthoclase is not abundant. It occurs in two generations; first, as small crystals with some idiomorphic form, and second, as large anhedral plates. Twinning is very common both on the Carlsbad and Baveno laws, and frequently a suturing effect is seen along the plane of composition. Small euhedral crystals of plagioclase, which seem always to be albite, are contained as inclusions, as are also flakes of iron ore and occasional prisms of biotite. Sericitization is usually more intense than in the perthite, the sericite passing later into chlorite and epidote. The kaolinization, observed on the outer portions of the granite, is probably due to the action of atmospheric agents. Frequently the crystals are cut up into a mosaic of orthoclase and sericite, and, as in the case of the perthite, spherulitic aggregates of muscovite are present. Green biotite is present and is usually associated with iron ore. The mineral is strongly pleochroic from blue-green to pale yellow-green, and shows pleochroic haloes round small zircons. Other inclusions are acicular white mica and grains of apatite. The presence, in a few cases, of deep brown biotite suggests the possibility that all the mica was originally brown biotite, which, by the loss of the iron, has changed to the green colour. H is noticed that the biotite which is changing to chlorite frequently has a crumpled appearance, the crumpling becoming

BRIAN SIMPSON,

more intense with advancing chloritization. A possible explanation of this is, that although the specific gravity is becoming less with the passage to chlorite, the volume available is constant, thus resulting in a folding of the mineral in an attempt to accommodate the new volume. Muscovite is always present. Its habit suggests that it does not belong to the original mineral suite, but is the result of some later phenomenon. It occurs in spherulitic aggregates and lies in a random manner in the slide, having no definite relation to the rest of the minerals. The origin of the mineral may be either from a change in the felspars, or from the biotite, as suggested by its intimate association with that mineral. Magnetite often occupies the cleavage traces of the mica. Apatite is present in all sections as an accessory mineral, an idiomorphic form being present. The Green Granite. The Green Granite is confined to patches in the northern area; it never covers extensive tracts, but is well exposed in Linbeck Gill and in the strongly marked gully north-west of Eel Tam. The hand-specimen shows a square-cleaving rock of a greenish caste, but the factor varies considerably, many specimens being almost white. The specific gravity is higher than that of the pink granite, being 2.634. Although this rock carries 7% more silica than does the pink variety, the larger percentage of plagioclase and ferromagnesian mineral accounts for this increase in specific gravity. The orthoclase increases at the expense of the perthite, which is 17% less than in the pink granite. The structure, the nature of the rock, and the mineral content are the same as in the pink granite. The Grey Granite. This variety is exposed in the Wabberthwaite Quarries at Broad Oak, and in a small exposure on the left-hand bank of a small stream leaving the upper Stainton Mill Dam. It is a perfectly square-cleaving rock, grey in colour, but with a liberal sprinkling of dark mica in it. The specific gravity is 2.72, which is considerably greater than that of either of the other two granites. This is accounted for in the general increase in the felspar and in the decrease in the quartz. Plagioclase increases and so does orthoclase, at the expense of the perthite and quartz. Dark mica is very abundant, and no doubt plays a large part in the increased specific gravity of the rock. Almandine garnet is present in large quantities in the heavy mineral residues, and still further increases the specific gravity. White aplite veins are abundant in the Wabberthwaite Quarries, and are characterised by the large nests of tourmaline and garnet often several inches in diameter.

PETROLOGY OF THE ESKDALE (CUMBERLAND) GRANITE.

25

The quartz has the same characters as in the other granites, except that the inclusions of gas are very much less numerous. The orthoclase is the most abundant felspar, and generally shows extensive sericitization, the [change taking place from the centre outwards. The crystals are always allotriomorphic. The plagioclase is all of the albite-oligoclase type and occurs in euhedral crystals. The sericitization of these crystals. is much more intense than in the orthoclase, and, as in the other granites, these crystals are often wholly changed to sericite. The biotite is in euhedral crystals and is pleochroic from a deep red-brown to a yellow-brown when unchanged; but there is a certain amount of change to a paler brown variety, and in some cases it alters to chlorite. Inclusions of zircon with pleochroic haloes are common, the zircon showing good idiomorphic form. Apatite in long needles and in rounded grains is common throughout the rock.

IV. MARGINAL VARIETIES. From the large number of varieties on the margin of the granite, three have been chosen which show some episode in the history of the area. In passing, it is well to note that Dwerryhouse considered the granite became more acid as the margin was approached. This fact the author has been unable to substantiate as a generalization, although there are parts of the intrusion which certainly become increasingly acid as the margin is approached. Evidence of the pressure with which the magma was intruded is afforded by the injection breccias which can be seen in a few places along the margin of the granite. The greisen from the Devoke Water area is also described, and constitutes a wellmarked variety. The third type described is a quartz-sericite rock found on Water Crag, and is an excellent example of the end-stage effects. The Eskdale Moor Injection Breccia. Injection breccias are not uncommon in the area, especially fine examples being found at Whinscales near the sheepfold which Dwerryhouse described, and also on Eskdale Moor as described below. In megascopic appealance the specimen shows a number of large inclusions of a fine-grained rock in a mass of white aplitic material. The contact of the aplite and inclusion is sharp and well-defined. Evidence of a lightening of colour is seen round the margins of the inclusions, a feature probably due to the infiltration of the magma into the inclusions. The quartz is allotriomorphic and has a slightly sutured appearance, whilst the felspars are changed to sericitic masses. Much of the secondary mica, which occurs in spherulitic aggregates, has changed to chlorite, and a little epidote is present.

BRIAN SIMPSON,

Apatite occurs in large plates. Chlorite is also pre sent and, under a 1/12" objecti ve, flakes of biotite, rutile and granular sphene are seen . The Devoke Water Greisen. This variety is seen in a small exposure south-west of Devoke Water and sout h of Water Crag. In the field the rock is con spicuous by it s brill iant whiteness. The hand-specimen shows a milk-whit e rock with a splinte ry fracture and ha s a specific gravity of 2.88, so that it is much hea vier than any of the granites. In thin section qu artz and t op az ar e present, givin g a t ypical all the crys t als are allotriomorphic. A crushed greisen; specimen showed an da lusite and rar e fluorite in additio n t o the abo ve minerals. The Quartz-Sericite Rock. This rock occurs on Water Crag a little east of the gr eisen. Th e whole of the felsp ar ha s changed to sericite so that the resultant rock is composed only of quartz and sericite. This rock as shown below is the final pr oduct of end-stage effect s on t he granite. It might hav e been formed by the action of stress, but it s field relations are such as to suggest that this is not the cas e. V. CONTAMINATED ROCKS. In the contamina te d rocks are included the contaminat ed granite and xenoliths. The former is con fined to the sout hern part of the area; only one larg e ex posure is seen , t hat in the Wabberthwaite Quarries. Here the granit e carries large xenolith s and apparently overlies the Grey Granite. Xeno lit hs a re widespread , but are only ab undant in the sout h, wh ere the granite is nearer t o t he roof. In t he north no ex te nsive areas of xenolit hic material are foun d ; bu t patches occur near Irton Road stat ion, and small xenolit hs ar e found in all parts. The Contaminated Granite. A mass of da rk green gra nite occurs along St aint on Beck and in the Wabberthwait e Quarries. This portion of the granite is in close proximity to the roof of the mass. The presence of lar ge xen oliths suggest s that assim ilat ion ha s taken plac e. The contaminated granite is gr een and cleaves irregularly, consist ing of quartz, pla gioclase, chlorit e and brown biotite with zircon and apatite as accessorie s; it differs from the normal granite in the large am ount of chlorite present, the more ba sic fclspars, and the presenc e of quantities of epidote. Perthite is present , but in much smaller quantites than in the normal granite , and usually carries 'blebs ' of quartz as inclusions. Plagioclase is the dominant felspar-s-chiefly albite a slight zonin g effect is seen with the central area more sericit izcd

PETROLOGY OF THE ESKDALE (CUMBERLAND) GRANITE.

27

than the rim. Orthoclase occurs in small quantities, and is associated with altered plagioclase. Brown biotite also occurs in small quantities. In basal sections a well developed pyramidal and prismatic parting is seen. The biotite changes to chlorite, the latter mineral simulating the prismatic habit and strong cleavage of the biotite. Closely associated with the biotite is epidote, pleochroic from pale yellow to colourless. Of the accessory minerals, zircon is the most abundant, and along with apatite shows good crystal form. A little magnetite is present in all sections, and appears to develop in the change from biotite to chlorite.

The Xenoliths. The xenoliths are splintery, very fine-grained, and a greenish black in colour. In hand-specimens an inner and an outer zone are frequently seen, and the contact with the granite is always sharply defined. The sections show the xenoliths to be of two kinds, first the Fine-Grained Type, which was probably an ash, and second the Coarse-Grained Type, which was probably an andesite. Both types show a junction with a flaked appearance. In the author's opinion this effect is caused by heat. On immersion in the magma the xenolith expanded rapidly and shelled, much like the layers of an onion. The first type of xenolith, the Fine-Grained, contains large quantities of biotite and chlorite from biotite. The biotite is brown and green, usually displaying a lath-shaped habit. Orthoclase, plagioclase and perthite are all present but are mostly represented by masses of sericite. The second type, the Coarse-Grained, is exemplified by a specimen from the Wabberthwaite Quarries, and is described below. A section shows a microgranitic structure. Albite and oligoclase are the most abundant felspars; their habit is prismatic, both stumpy and elongate; the sericitization is as intense as in all the other rocks described. In some cases the euhedra consist wholly of sericite, with occasional grains of epidote enclosed. Perthite is present in small quantities. Orthoclase, occurring as large anhedral plates, is one of the last formed minerals, and encloses euhedra of plagioclase, prisms of biotite and patches of hornblende: the felspar, although sericitized to some extent, is not so extensively changed as in other parts of the mass; Quartz displays the same characters as in the normal granite, except in showing the curious sutures associated with secondary quartz. A deep brown pleochroic biotite occurs as small prismatic crystals and is abundant in thin section. The ultimate alteration product is a green chlorite, the change taking place first to a pale brown biotite, and finally to chlorite. The hornblende is a pale

28

BRIAN SIMPSON,

green variety resembling actinolite-hornblende. An interesting feature is the intimate intergrowth of the hornblende and biotite, a phenomenon connected with the origin from augite. The chlorite is a pale green pleochroic mineral with the maximum absorption, deep emerald green parallel to Z. The accessory minerals are zircon and apatite, which occur in good crystals, and a little magnetite. As the granite contact is approached there is a noticeable decrease in the amount of hornblende and an increase in the quantity of biotite, which in this position is more chloritized than in the inner zone. The Changes in the Xenoliths. In the changes which have taken place in the xenoliths, subsequent to their immersion in the granite magma, the two types appear to have followed similar lines. From the field evidence and from the resultant character of the xenoliths, it would appear that the original minerals concerned were augite and labradorite, which by the combination of thermal metamorphism and soaking by residual liquors have been reduced to the present character described above. The first change was probably the breaking-down of the augite to hornblende and biotite with the leaching-out of iron oxide. The ultimate effect has been to reduce these minerals to chlorite and to reduce the labradorite to albite and oligoclase. To this end the introduction of potassium and water was necessary, so that the process seems to have been one of soaking by a quartz-potassium rich liquid residuum, which same liquor by virtue of the presence of water reduced both the biotite and hornbiende to chlorite and caused the leaching-out of the iron ore. Some lime from the hornblende has gone to produce epidote. The presence of apatite as an integral part of the slide is evidence of the presence of some phosphates and fluorine in the magma, probably in a gaseous state.

VI. SERICITIZATION AND CHLORITIZATION. In this paper, emphasis has been placed upon the excessive sericitization and chloritization of the felspars. This phenomenon might have resulted from the action of stress alone; had this been the case, it would not have been expected that every slide would show the change, and, furthermore, field evidence would have supported this view. Such changes as these have been ascribed to the action of atmospheric agents, but against this view may be given the following arguments (d. J. L. Gillson) (10]. 1. The country has recently been scoured by ice, and in many cases the boulder-clay is still many feet thick; the upland streams, especially in the southern part of the region, are still

PETROLOGY OF THE ESKDALE (CUMBERLASD) GRANITE.

29

cutting through the boulder-clay. Fresh specimens arc as a result fairly easy to obtain. 2. Sericite, chlorite, and epidote are commonly pneumatolitic or hydrothermal in their origin. In the production of sericite from orthoclase, water must have played an important part, as also in the chloritization and epidotization. F. W. Clarke [11J has explained the production of sericite from orthoclase by the operation of high temperature, chemical activity of water, and mechanical stress acting together, resulting in the production of muscovite, free quartz, and the leaching out of potassium silicate. In the case of plagioclase, Clarke [11] gives a formula which, however, is not proved. The chloritization of the micas is due wholly to hydration; this fact, coupled with the need for much water in the processes of change, would necessitate a high percentage of water being present in the end-stage liquor. To these changes the phrase 'deuteric effects' has been given. Gillson [10J, in discussing the Pende Oreille Granodiorites, notes similar changes to those the writer has described in the Eskdale Granite. Gillson's criteria for the recognition of a deuteric mineral are well born out in this investigation. Firstly," the grain does not fit in the crystal mesh of the rock, but lies in a haphazard or random manner, without regard to the boundaries of adjacent minerals": secondly" the grain is of irregular shape, and sends out arms or finger-like processes into other minerals, or it encloses or partly encloses, several adjacent minerals." The second feature is especially well seen in specimens of plagioclase, orthoclase and perthite, all of which are riddled by arms of sericite.

VII. THE DYKES. The felsitic rocks and quartz porphyries occur as dykes and marginal varieties throughout the area. The dykes have a general direction N. 20 W., and, taking these as the Q-joints, the pressure on the mass must have been either from E. 20.N. or

W. 20. S. In the porphyries, quartz and felspar form the phenocrysts in a groundmass of the same material. In the immediate neighbourhood of the phenocrysts the most finely crystalline part of the groundmass occurs. Micrographic intergrowth of quartz and felspar is common in this type. The quartz, always in two generations, generally shows strain shadows and the usual fluid and liquid inclusions. Frequently the phenocrysts are cracked and show secondary infilling of quartz and orthoclase. The felspars consist of a perthite and orthoclase; they always show sericitization, and the deuteric nature of the resultant muscovite is apparent from its random distribution in the section. The micas are represented by

30

BRIAX S n I PS OX,

muscovite and green biotite. The former presents a radial appearance , and is alwa ys secondary. The latter mineral is partly altered to chlorite, and iron ore has been leached out along the cleavages. Small quantities of epidote are pre sent in all sect ions . Th e average specific gravity of the rock is 2.58. In the felsites the hand-specimens show a dark pink rock speckled with iron ore. The av erage specific gravity is 2.634, a value slight ly higher than that of the quartz porphyries. Th e gra in of the felsite s is slightly porphyritic, but the groundmass is much finer . Th e quartz is of the granite type and generally shows evidence of resorption. Th e felspars include ort hoclase as the chief species, and this is oft en cra cked and seconda ry- infilled by quartz and orthoclase. Perthite, albite, albite-oligoclase with a composit ion An. IS. Ab. 85 and oligoclase also occur. All the fclspars show sericitization , and some of the ort hoclase has form ed epidote. The micas include both muscovite and biotite, the latter frequently chloritized with the leaching-out of iron. Small hrematite veins fr equ ently traverse the crystals. VIII. THE HEAVY MINERALS. The following heavy min erals have been identified : tourmaline , zir con , andalusit e, t opa z, fluorite , apatite, garn et , hrematite, limonite, i1lmenite an d pyrrhotite ; these sp ecies are described below. Tourmaline. Th e first menti on of this min eral was made by T . A. Jones, who .records it from Beckfoot Quarry. He remarked on the radiating aggregat es and suggest ed that the min eral was most abundant near joint-planes, and that alt hough blu e and brown species were present, brown wa s predominant. The present examination reveals the fact that t ourmaline is mor e widespread than ha s hitherto been recorded, esp ecially from the quartz porphyries. The occurren ce is characteri stic, since the largest percentage of crystals are of a fibrou s form in which a basal parting is shown , thus giving each crystal a fasiculate appearance. Larg e anhedra l cryst als do appear, but are subordinat e in amount to the fibrou s variety. The colour varies through blue, green-blue, brown, yellow, pink, to colourless . All types of pleochroism ar e exhibite d , the most common being :-colourless to blue: brown to blu e : pink t o deep blue. The crystals have a patchy appearance, blue generally being the strong colour. In some crystals there is a definite zoning, there being a central blue core and an outer brown margin and vice versa. The coloured varieties always show a st rong striation. Zi rcon. This is one of the most constant minerals of the area. The major portion of the crystals is colourless; coloured varieties range through honey, purple and brown. A slight

PETROLOGY OF TH E ESKDALE (CUMBERU.ND) GRANITE. 31

zonal effect is seen in some specimens, but is not comm on. A well-marked feature of the crystals is their corrosion . Andalusite. So far as t he author has been able to ascerta in this 'mineral is confined to the felsites and the pink granite, and is a major constituent in the Devoke Water greisen. In the two former the appearance is similar: two types are present, first, a colourless variety, and second, a pink pleochroic form . In the latter, large quantities of inclusions are seen; frequently the whole crystal being choked with indeterminate fine dust. There are . specific inclusions of small red flakes of heematite. The cryst als are frequently cracked , and show change along the cracks, thus giving a mesh-like appearance. Topaz. With the except ion of the Devoke Water greisen 'this mineral is confined to the finer grained rocks; it is never abundant. Presenting a bold outline and a glassy appearance, the typical characters ar e displayed. There are few inclusions and no alteration. Although usually colourless, slightly yellow cryst als are seen in an aplite vein in Beckfoot Quarry. The occurren ce of the mineral in the aplite and the felsite suggests the production of the mineral by pneumatolitic action at the time of their intrusion . It is always associated with t ourmaline. Fluorite. This mineral is very rare and ha s ' only been record ed from two pla ces, Beckfoot Quarry and the Devoke Water greisen. It occurs as small cubes , or as anhedral masse s. The colour is of a purplish cast or not pres ent at all when the mineral has a clear glassy appearance. Apatite. Apatite presents a uniform appearan ce throughout the mass, and displays a peculiar character in the abundance of the inclusion. Colourless and glassy varieties are present, but are subordinate in amount to the dusky forms. Garnet. In the investigation of the garnets, it has not been possible to determine any distributive phenomena. Although a constant mineral in the heavy residues, garnet is prolific in one rock only,the Grey Granite and its aplite veins from theWabberthwaite Quarry. The dominant colour is a rich wine-red, but colourless forms are present. With the exception of the Wabberthwaite forms, which are all anhedral, the garnets ar e all et ched and present an echelon appearance. The specific gravity of all forms is 4.I8, the index of refraction Io778 +. A slight reaction for iron was given with the borax bead. 'General Remarks on the Heavy Mineral Assemblage. The striking feature of the heavy mineral assemblage is the presence of so many minerals of pneumatolitic origin or produced by end-stage effects. Four of the suite require the presen ce of fluorine for their formation, and all but one contain alumina. These facts point to the presence of abundant alumina in the

HUlA:>; SlMPSO",

liquid residuum, large quantities of free silica, and fluorineboron gases. The fluorine-boron minerals arc not widespread, but occur sporadically; this would be expected since the gases would tend to be trapped in pockets rather than to be widespread. The presence of almandine garnet in the Grey Granite is henattributed to an end-stage concentration effect. The presence of andalusite was at first attributed to an excess of alumina, resulting from the assimilation of sedimentarymaterial. The pure whiteness of the andalusite-bearing greisen militates against this view, for had such assimilation occurred the rock would have been of a darker caste, whilst some biotite and iron ore would have been expected in the resultant rock. Furthermore, assimilation would not be expected to be so complete as to have absorbed the whole of the pelitic material, and it would have been expected that some unchanged portions would be present; in the present research no such portions have been observed. Even in the main mass of the granite where andalusite is found, both biotite and iron are are relatively scarce. \Ve may compare these andalusite-bearing granites with the sillimanite-bearing granites of the Strath Hallidale Complex described by Read L12-~. He says" The presence of sillimanite in these leucocratic granites and aplites is not, in the writer's opinion, to be attributed to an excess of alumina arising by the assimilation of pelitic country rock. The sillimanite bearing granites have none of the characters of mixed or contaminated granitic rocks. These latter are invariably rich in biotite (surmicace). The occurrence of sillimanite in these mafic-poor rocks recalls the muscovite-sillimanite veins in certain rocks of Loch Choire Complex. The sillimanite and the replacing or late muscovite of the Kildonan rocks are genetically connected, and both most likely arise by the break up of aluminium halides, alkali aluminates, and similar compounds contained in the residual portions of the granitic magma. In the writer's opinion the operation of similar solutions in the formation of certain types of sillimanite-bearing country rocks, such as faserkiesel, is not impossible." Whilst this explanation is applicable in most cases in the Eskdale Granite where andalusite occurs, it is difficult to account for the presence of the dark opaque inclusions which occur in some of the andalusite grains. IX. SUMMARY AND CONCLUSION. The granite mass of Eskdale was intruded under great pressure into a mass of Borrowdale volcanic material. 2. The shape of the mass is laccolitic. 3. Three granites arc recognised, and arc cut by dykes of quartz-porphyry and felsite. 4. There has been assimilation of the country-rock, as evidenced by the contaminated granites and xenoliths. I.

P ETR OLOGY OF T H E ESKD.\LE (CU MBE RL AN D) GR ANIT E .

33

5. The widespread sericit izat ion and chlorit izat ion suggest the activity of end-stage factors rather than those of weathering. This, coupled with the heavy min er al assemblage, suggest s that end-stage effects played an important part in the history of the granite. The age of the granit e is problem atical. The only evidence is the fact that it is intruded into Borrowdale Volcani c Rocks and has apparently taken on the lines of faulting of the Carboniferous-Permian movem ents. The age may thus be Devonian . The wid ely distributed sericitizatio n , chloritization and pneumat olysis, suggests that the granite has not yet suffered mu ch erosion and that the true granit e lies beneath the present crus t. In conclusion , the aut hor wishes to express his best than ks for much help and suggest ion during t he course of the work t o : Prof. H. H. Read; Prof. P. G. H. Boswell ; Dr. E. Neaverson ; Mr. 1. S. Double, M.Sc.; Dr. G. H . Mitchell; Mr. A. Stuart , M.Sc.; Mr. R. M. Shackleton, B.Sc., an d to Miss H . Simpson an d finally t o the vari ous landowners and estate agent s who have at all t imes allowed him free access t o a ll parts of the a rea. REFERENCES. 1. 2.

3· 4. 5. b. 7.

8. 9. 10 . II.

12 .

"VARD, CLIFTON. 18 75 . Qua rt . f ourn, Geol. S oc., \' 01. xxxi ., pp. 568-602 ; 18 76 , ibid, vol. xxxii. , pp . 1-34 . , 18 76. T he Geol ogy of the N or t he rn pa rt o f the E nglish L a k e Di st r ict . Mem , Geol. Survey . TEALL, J. J. 1888 . British P etrogr aphy . Pp. 32 2- 323. DWERRYHOUSE, A. R. 1909. Qua rt. [ ourn, Geol, S oc., vol. lxv .• p . 56. HA RKER, H. 1906 . Petrology fo r St ud e nt s . GREEN, J. F. N . 1927 . Pro c. Geol , Assoc., \' 01. lxi v. RA STALL, R . H ., a nd WILCOCKSON, \V. H. 19 15 . Quart . jOUY1l. Geol. S oc., vol. lx x i . JO NES. T . A . 191 5 . Proc. L iverp ool Ceol. S oc., vol. 12, p . 139 . D WERRYHOUSE, A. R . op, cit., p . 6 1 GILLSON, J. L. 192 7. j ou rn al of Geology. Vol. xxxv ., p p . 1- 3 'CLARKE, F. W . D a t a of Geochemist r v . U. S . Geol. S ure , B ull . 69 ; . REA D, H . H . 193 1. The Geology of Centra l Sut he rla n d. lWelil. Geolc S u ru. S cotlan d.

DISCUSSION. MR. T . EASTWOOD re m arked that a s the m embers of t he Cu m b ri a n unit during p art o f 19 32 resu r veyed the Gosforth She et, which in cludes t he n orth-western portion of t he E skd a le Granite o utcrop , t hey wer e naturally interested in the wo rk of Mr. Simpson. Dr. Holling worth surveyed a small portion of the gra ni te near Irton , while Mr. Rose dealt with the r est as far south as Corney, and also visited some of the exposure s beyond the limits of the actual resu r vey. A preliminary ac count of t h eir work h a s b een published in .. Su m mary of Progress for 193 2 ." Mr. Eastwood stat ed that h e was familiar with the work of 1\Ir. R ose, a nd in the latter's a bsence would e ndeavour t o put forwa r d so me of Mr. R ose 's v iews . 1\11'. R ose has found , a s d id D werryhou se in 19 0 8, tha t t h ere is a fin e- gr a ined gran it e mor e acid t h an u su al in so me p laces a lo ng the m a rgi n. This is P ROC. G EOL. A sso c ., VOL. X LV., PA RT I , 19 34 .

3

34

PETROLOGY OF T HE ES KDALE (CU~IBERLAND) GR ANITE.

particula rl y well dev eloped at the so ut h-wes t end of Muncaster F ell , whe n it forms a belt 1 0 0 ft. or more in thickness. It is a slight ly porph yrit ic m icro-granite ofte n ri ch in muscovite, and in pl aces so acid as to be almost pure qu artz. Alt hough charact erist ic of parts of the margin, the acid type is also present as irregular patches within the granite mass -as, for example, at Beckfoot Quarry, Eskdale. Although there is a fairly sharp line in pl ac es be tween the finer grained type a nd the normal granite there is no definite junction and no evidence of chilling either a t Be ckfoot or elsewhere between these t wo types. Mr. Eastwood enquired if the author had any vie ws t o offer as to the or igin of the finer gr ained acid type, and, if so, d id he regard it as a late phase of int rusion . Me. Simpson had state d tha t contami nat ed rock s wer e confined to t he so uthern part of the area ; had he noticed those of Newtown Knott n ea r Muncaster, or the intensely shea re d basified r ocks a little farther sout h? Mr . Rose h ad also visit ed D evoke Water and was in ag ree me nt wi th the a ut h or as to the t wo fau lts of N .\V. trend bounding the sm all er m ass of gr anite on the south side of the lake. Me. Rose believ es, however, t hat the andesite of Ri gg B eck is a faulted wedge and do es not think t here is any evidence for a low ang led junction, thrust or otherwise, be tw een t he granite and andesite. Mr . East woo d remarked that as a field-ma n he was particularly interested in the field relationship of the types, and regret t ed that Me. Simpson d id not ap pear to have attempted to m ap the granite in detail. DR. S. E . HOLLINGWO RTH welcomed the paper as a contribution to the p etrology of this granite mass, particul a rly for the author's description of t he various types and for his study of the heavy minerals and of the widespre ad late-stage alteration effects. From mapping a sma ll a rea of t he gran it e la st autumn a nd a casual hammering ex tendi ng ov er many yea rs he was of t he opi nion t hat in t his ver y va ria ble granite numerous gra da t ions existed between the author 's three types. By some such pro ce d ure as the sor t ing of a large nu mber of specimens into gro ups a nd a su bsequent a ve raging of the micrometric analyses, an unreal d ist inct iveness to the typ es could arise . T he reality of the types would bes t be t ested by more det ail ed mapping of t heir outcrops. As regards the form and structural relations of the mass, the cr ux t o him appe ared to b e t he relation of t he int r usion to t he fold in g in t h e a d jacent B orrowd ales. If, a s seems probable, it tra nsgresses pre-existi ng s t ruct ure s as does, for ex a mple, the E n ne rdale Granophyre st oc k to the north , the attribution of a lac colitic form is scarcely justified b y the d isputed Devoke water sections . The author's conclusion that the gr anite, as exposed, is but the upper part of a les s modified gr a nit e in depth is m ore in accordance with a st ock form. Steep junctions re corded by Mr . R ose on the west and the more gentle northerly and easterly d ipping j unc ti ons in the east would also fit in with this view. Till' AUTIlOR, in r eply, thanked the members for their k indl y r eception of the paper . In reply to Mr . T . E astwood , he would remark t ha t the present p aper was in the hands of the Association before the publication of the Geological Survey work, with the result that there had been n o r efer enc e to those investigations. He agreed that there were localities wher e the Granite be came exc eedingly fine-grained and acidic, but d isagreed with Dwerryhouse's generalisation that the acidity increased a s the margin was approached. H e had come to no definite conclusions r ega r din g the history of this phenomen on, but believed it to be a late stage in t he consolidation . With r egard to the distribution of the contaminated ro cks , he regretted that M e. E astwood had understood him to confine them to the south of the area , a s he was well aware of their presence in the n orth. In reply to Dr. S . Hollingworth, he agreed that the shape of the m ass was not certain from the evid ence a nd allowed of different interpretat ions from the one he had assumed.