Structure and petrology of theTregonning-Godolphin granite, Cornwall

MAURICE STONE

STON E, M. 1975. Structure and petrol ogy of the T regonn ing- God olphin Granite,Cornwall , Prot . Geol. Ass .. 86 (2), 155- 170. Field evidence indicates that the plutonichistor y of the complex bega n with the emplacement of fine-porph yritic biotite granit e(Godol phin gran ite and early granite porphyries). Th is was followed by the passiveemplacement, by cau ldro n subsidence and block stoping, of the lithium-rich Tregonn­ing granite. Different iati on of this mass in situ gave rise to the rocks forming the RoofZone and the gra nite sheets of Tremearne, composed of leucogranites and aplite­pegmatite bodies. Lat e granite porphyries (,elvans') cut the Tregonning granite.Greisenisation and kaolinisation affected all rock types and completed the plutonichistory.

Statistical considerati ons lead to the conclusion th at the chemical data can be con­sidered to be distr ibuted norm ally for the purpose of correlation and factor analysis.Two important associa tio ns are Ti, Fe, Mg, K and Na, u, Mn. The latter increaseand the former decre ase with time in the sequen ce biot ite gra nite, lith ium mica granite(Tre gonning), leucogranite and aplite.

Lithium -mica gra nite is believed to have been derived from biot ite granite byLiAI ~ 2(Mg , Fe) and alkali-ion exchange reactions together with the de­anorthitisation of plagioclase feldspar in the presence of ab unda nt volatiles rich influorine . Such tr an sfor mat ions could result in increased mobilisation through localmelting as compositions approa ch a minimum melt ing point or eutectic.

Some repetition of the exchange processes and an increase in F-ion activity in thefluid phase is invoked to explain the concentration of leucogranites at the roof and theevidence for several genera tions of aplite-pegmatite bodies.

Department of Geology. The University, North Park Road. Exeter EX4 4QE .

CONTENTS

I. INTRODUCTION2. STRUCTURE AND FORM3. PETROLOGY4. CHEMISTRY5. CONCLUSIONS

ACKNOWLEDGMENTSRE FERENCES

1. INTRODUCTION

page

155157161i63i68T69T69

The Tregonning-Godolphin granite is the small granite mass that occurs between Praa Sands andTremearne on the coast and extends inland between Breage and Germoe and forms the higherground of the Tregonning and Godolphin Hills in western Cornwall (Fig. I). This granite, firstrecognised as a composite granite and called the Tregonning-Godolphin granite by Stone (1960),has drawn the attention of few geologists. It has never been adequately described and most of thepreviou s interest has been directed at the spectacular granitic sheets exposed at the MegiliggarRocks. Several earlier workers in this area have referred to the uniformity of mineralogy and tex­ture of the granite that Flett & Hill (1912) and Hall (1930) call the Godolphin granite. Theauthors of the Memoirs (Reid & Flett, 1907; Flett & Hill, 1912) note that the granite is finer­grained than the typical granites of south-west England, but they do not recognise the Tregonning

155

156 M AU RICE STONE

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TREGONNING-GODOLPHIN GRANITE, CORNWALL 157

granite as a variety distinct from the other granites. However, De la Beche (1839,349) observesthat 'The granite extending from Trewavas Head to Godolphin Hill is of a somewhat variablecharacter' and that 'The granite of Godolphin Hill forms another variety ... '. Hall (1930) notesthat there is a tendency for the granite to become feebly porphyritic on Godolphin Hill.

The present writer divided the granite into three major units (Stone, 1960 & Fig. 1): theGodolphin granite-a fine-porphyritic biotite granite exposed near to the summit of GodolphinHill and occurring on mine dumps and in 'clitter' around the hill; the Tregonning granite-a non­porphyritic lithium rich, topaz-bearing granite exposed on the coast between Praa Sands andLegereath Zawn and in several quarries on and around Tregonning Hill; and a Roof Complexcomposed predominantly of leucogranites and aplites with associated pegmatites, together withTregonning-type granite and occasional bands of greisen. These rocks are exposed at the roof ofthe Tregonning granite along the whole of the coast section, where they will be referred to as theRoof Zone and in a series of sheets in the country rocks between Legereath Zawn and the water­fall at Trernearne (Stone, 1969).

The outcrop of the granite is shown in Fig. I. Contacts with the country rocks are well exposedon the coast but are not exposed inland. The mapped contact is based mainly upon the distribu­tion of debris in the fields, and upon the predominant material found on mine dumps,supplemented in two instances by mine records (Dines, 1956, 225, 233). The mapped boundarycorresponds approximately with that shown on the Geological Survey One Inch Sheets (351,358).

=Mylor Beds

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2. STRUCTURE AND FORM

(a) Contacts

Sea-level

(i J External contactsExposed contacts between the Tregonning granite or the Roof Complex and pelitic hornfelses ofthe Mylor Series occur at several localities on the coast (Fig. 2).

Eastern end of Praa Sands: At low tide, pelitic hornfelses can be seen overlying gently dippingRoof Complex. The contact dips 12-15° to the west-south-west. The dominant cleavage in thepelites, formerly called S2 by Stone (1966a) but now relabelled S3 in accord with current usage(Dearman, Freshney, Selwood, Simpson, Stone & Taylor, 1971), dips off the granite subparallelto the contact, but bedding, commonly folded about near horizontal F3 axes, has an aggregatenear-vertical dip. Banding in the Roof Complex dips parallel to the contact with the Mylorpelites. The contact is sharp and there is no intricate veining of the hornfelses by the granite.

Rinsey, north-west contact: A roof pendant of Mylor pelites and psammites is well exposed at

158 MAURICE STONE

Porthcew about 500 m. south by south-west of the hamlet of Rinsey and about the same distanceeast of Rinsey Head. Both north-west and south-east contacts are exposed. The north-west con­tact is sharp and transgresses the dominant flat-lying cleavage (S3) in the pelites. The contact dipsgently, though irregularly, to the south-east but steepens on the northern wall of Porthcew. At thecontact, the granite is coarser than usual, but the coarse layer is underlain by a zone of bandedaplite, leucogranite and pegmatite, in which the banding lies parallel to the contact.

Rinsey, south-east contact: This contact is sharp and nearly vertical. The granite retains itsmedium-grained texture up to the contact. The rocks of the banded Roof Zone overlie Tregonn­ing granite to the south-east and appear to dip towards the pendant, but almost die out before thevertical contact is reached. A thin pegmatite and some tourmaline-rich schlieren lie parallel to thesteep contact.

A small pendant measuring about 25 m. across is exposed at low water 50 m. south of thesouth-cast contact of the Rinsey pendant. Sharp contacts between Roof Zone and hornfelses areexposed continuously on three sides of the pendant, but are not seen on the seaward side. Bandingdips steeply (50°-70°) under the pendant along the whole of the exposed contact.

Eastern end of T requean Cliff. The contact here is sharp and vertical. As at the south-eastRinsey contact, the Tregonning granite retains its medium-grain size up to the contact. A narrowpegmatite and tourmaline-rich 'schlieren' lie close to and parallel with the contact. A roughbanding produced by tourmaline-rich layers is visible on some of the smoothed surfaces ofboulders, but is not readily seen on the rougher surfaces of granite in situ. Banding typical of theRoof Complex is absent. This contact can be traced along the shore for about 120 m. close to lowwater.

Legereath Zawn. A near vertical contact between pelitic hornfelses and Tregonning granite isexposed here. The top of a small dome of Tregonning-type granite occurs at the base of the cliffson the eastern side of the Zawn. It is exposed also in several sea stacks (Hall, 1930, fig. 8B, 131).This small mass appears to be connected with the main Tregonning granite and may be the uppersurface of a sheet (like one at the base of the cliff on the Legereath platform). If this is the case,the main granite contact would be expected to continue dipping steeply, in accord with the gravitydata for the whole of the batholith (Bott, Day & Masson-Smith, 1958). .

Mine records from two localities (Dines, 1956, 225, 233) indicate that the eastern contact of theTregonning-Godolphin granite is gently dipping (about 30°). Contacts at the roof between pelitichornfelses and the Roof Zone are exposed between Praa Sands and Rinsey Head, and betweenRinsey and Trewavas Head on the cliff top and at Carn Clodgy.

iii) Internal contactsContact between the Godolphin and Tregonning granites. The junction between these granites isnowhere exposed: as they were not recognised as distinct types prior to 1960, no distinction isdrawn between them in mine records. The mapped contact (Fig. 1) is only approximate. Duringthe early part of the field mapping, it was thought that the Godolphin granite was confined toGodolphin Hill, but fragments found in mine dumps around Boscrege revealed that the wall rockswere composed of fine-porphyritic biotite granite. Further, material from the old china clay pit atTresowes Green is a porphyritic granite, which, though altered, resembles the granite ofGodolphin Hill.

The nature of the contact is uncertain. It is possible that there is a gradation from one rocktype into the other, although the observed occurrence of distinct rock types both in situ and onmine dumps and the absence of rocks having intermediate characters suggest that this is not thecase. Correlation between the Godolphin granite and the earlier set of granite porphyries(described below), however, determines the age relations.

TREGONNING-GODOLPHIN GRANITE, CORNWALL 159

Contact between the Tregonning granite and the Roof Zone. The banded rocks of the RoofZone are exposed along the whole of the coast section, where they overlie typical Tregonninggranite. The contact is 'conformable' and frequently interbanded; Tregonning-type granite com­monly occurs within the rocks of the Roof Complex.

Contacts between Gr anite Porphyries and the Tregonning granite. The granite porphyriesbelong to two distinct sets:

The earlier set is repre sented by the Rinsey granite porphyry which occurs either as a large in­clusion in the Roof Zone or forms part of the actual roof that is partially immersed in the RoofZone, 600 m. south of Rinsey hamlet. Both the earlier-formed contact with the Mylor pelites andthe later-formed contact with the rocks of the Roof Zone are exposed. The Legereath graniteporphyry is well exposed at the base of the cliffs on the Legereath platform and in the nearbyLegereath Zawn . On the wave cut platform, the granite porphyry is nearly I m. thick and occursas a flat-lying sheet, which, however, transgresses S3 cleavage in the adjacent pelites, At thislocality, the granite porphyry itself is veined by a sheet of Tregonning-type granite. The fieldrelations have been described and illustrated by Hall (1930) although the significance of an earliergranite porphyry set does not appear to have been recognised.

The early granite porphyries are biotite-bearing and are rich in potash. Chemically andmineralogic ally they are simi lar to the Godolphin granite (Table II): indeed, it is highly likely thatthey are apophyses from the Godolphin granite. It follows from such a correlation that theTregonning granite is later than the biotite granites and is, presumably, intrusive into them.

The later set is represented on Tregonning Hill by a near vertical dyke of granite porphyrywhich cut s and is 'ch illed' against the Tregonning granite. The rocks here are kaolinised andgreisened: the alteration clearly postdates the emplacement of the dyke, a relation observed inmany china clay pits in the other granite masses. The Tregonning Hill granite porphyry is similarto the Praa Sands granite porphyry (Stone, 1968) and to the late granite porphyries that occurelsewhere in south-west England.

Aplites within the Tregonning granite. Several metres to the east of the small pendant atRinsey, referred to abo ve, aplites of two different ages occur. At this locality, the Tregonninggran ite lies very close to its Roof Zone and it is possible that the aplites are part of this roof.However, they occur within otherwise homogeneous Tregonning granite. A clear cut aplite dyke,5 em. wide, splits into two parts. Part of the dyke contains margins of pegmatite. Thi s dyke andits bran ch clearly cut the granite and are later. However, close by, fine-grained relict aplite dykesoccur within the medium-grained granite. Their margins are uneven and diffuse and they pinchout along their lengths within a few centimetres. In one example there is some suggestion of inter­nal banding, like that in the aplites of the Roof Complex (Stone, 1969); indeed, it is possible thatthese rocks represent a 'granitised' part of the roof. In any case, these aplites are interpreted asearly 'granitised' aplites, like those described from the Carnmenellis granite by Stone & Austin(1961) . They appear to represent more advanced stages of transformation than has so far beenobserved in the Carnmencllis granite.

(b) Joints

Steep, near vertical jo ints are aligned approximately north-north-west to south-south-east andwest-south-west to east-north-east in common with directions observed for the other plutons ofsouth-west England. The trends of the coastline follow these directions and are clearly governedby master joints.

Bedding (sheet) joints lie parallel to and appear to have been controlled by the planar

160 MAURICE STONE

mega fabric of the rocks of the Roof Zone. In the coast section between Rinsey and TrewavasHead, the sheeting and banding both dip at ca 10° to the west. In the section between TrewavasHead and Trequean Cliff, the sheet joints are either horizontal or dip gently to the south. Closerto the contact at Trequean Cliff, the sheet joints become steeper, but, like the banding, appear todie out before the contact steepens markedly. A similar relation can be observed at the Rinseypendant. It is clear that sheet joints and banding are closely related, and as the banding appears tolie parallel to the contact with the roof, it follows that the sheet joints are related to the surfacebetween granite and country rocks. In this coast section, this surface determines alsq the slope ofthe land surface.

(c) The Roof Zone and its origin

The Roof Zone extends along the whole coast section from the contact at Praa Sands to the Tre­quean Cliff. Banding compares closely in detail with that in the Tremearne granite sheets (Stone,1969). Generally, individual bands range from less than 2 cm. to a metre in thickness. AtTrewavas Head, a 15 em. thick pegmatite, exposed about IO m. below the top of the cliff is takenas the base of the Roof Zone. The rock beneath the pegmatite is homogeneous Tregonninggranite. Above this basal pegmatite are leucogranites together with internally banded aplites andthin pegmatites. Commonly, megacrysts of potash feldspar, measuring up to 2 or 3 em. projectfrom the pegmatites into the adjacent aplite. The sequence of layering found at Trewavas Headseems to be fairly typical of the Roof Zone and although there are variations in the amounts ofthe rock types and in the thickness, the latter is generally about IO m.

The base of the Roof Zone dips generally sea wards, suggesting that the granite contact occursjust offshore, except of course, where it crosses the coast to form the pendants. North ofTrewavas Head, the undulating Roof Zone drops to high-water mark and then rises steeply to thecliff top 300 m. south-south-east of the old engine house at Rinsey. It is here that the bandedrocks lie in contact with both pelites and the early granite porphyry.

Pelitic xenoliths are common in the banded rocks. They are commonly platy parallel to the S3cleavage and may measure up to I m. or more across and some 20 em. thick. Several xenoliths orsmall pendants are oriented with S3 lying parallel to the banding and to the cleavage in the rocksof the envelope. This suggests that they have not been disturbed to any extent during the emplace­ment of the pluton and the development of its Roof Complex.

Detailed relations between the rocks of the Roof Zone and the Tremearne sheets are not seenowing to difficulty of access. The sheets appear to arise from the Roof Zone (Fig. 2) and theyhave similar lithologies and chemical features to the rocks of the Roof Zone. Sheets (or otherbodies) that arise from a lower level (i.e. below the base of the Roof Zone) are composed ofhomogeneous Tregonning-type granite. Relations are discussed in Stone (1969) .

Where contacts between the granite and country rocks are steep, the Roof Zone is either absentor markedly reduced, and the granite retains a medium-grained texture up to the contact: on theother hand, where contacts are Oat-lying, the Roof Zone is well developed. This implies that thebanded rocks are linked to processes that occur at the roof of the granite and nowhere else. Theconcentration of pegmatite in the roof points to the important role of volatiles in the developmentof the Roof Zone. This is also indicated by the enrichment of the leucogranites in particular, inelements like lithium and fluorine, relative to their concentrations in the underlying Tregonninggranite .

Leucogranite is the dominant rock type in the Roof Zone, especially in the highest parts of thecoast section. The megascopic differences between the leucogranites and the Tregonning granite

TREGONNING-GODOLPHIN GRANITE, CORNWALL 161

are small except in those leucogranites that have an aplitic matrix. It is reasonable to suppose thatthe leucogranites have been derived from the Tregonning granite by some process of differentia­tion in situ. accompanied by local mobilisation and the emplacement of some sheets ofaplite/pegmatite within the differentiating roof. This would account for the interbanding ofleucogranite and Tregonning-type granite that occurs at the base of the Roof Zone in manyplaces. If the leucogranites have not been derived directly from the Tregonning granite in situ,they could have been emplaced from some underlying source. However, the absence ofchannelways within the Tregonning granite argues against this. An alternative suggestion that thematerial of the whole Roof Complex was emplaced by movement up the fractures marked by theTremearne granite sheets to the roof of the Tregonning granite is unlikely in view of the evidencefor the lateral differentiation of the sheets from leucogranite to aplite/pegmatite in a directionaway from the main granite contact (Stone, 1969).

Many aplites and pegmatites of the Roof Complex are probably later than the leucogranites.Some aplites cut across earlier-formed banding and must represent local remobilisation (Exley &Stone, 1966). The presence of topaz points to a high content of fluoride, as indicated by theanalyses (Table II), and probably a reduced viscosity at the time of emplacement. Some of theroof banding, especially the tourmaline-rich schlieren, may be the result of deformation of thegranite at the roof owing to the marked ductility contrast between the mobile granite and themore rigid, fairly low-grade pelitic country rocks (cf Berger & Pitcher, 1970).

(d) Form and manner of emplacement of pluton

A diagrammatic cross-section of the Tregonning granite and its associated Roof Complex isgiven in Fig. 2. This is based upon the structure revealed by the continuously exposed coastal sec­tion, which shows the form of the upper part of the intrusion. The Tregonning granite can be con­sidered as a dome-like mass that has an undulating roof and is skirted by a series of outward­dipping sheets. The Godolphin granite appears to have formed a separate intrusion whose formmay not be directly related to that of the Tregonning granite.

The sharp vertical contacts at Rinsey and Trequean Cliff suggest that the emplacement of thegranite here occurred by movement along steeply dipping fractures. The displaced block or blockswould sink to deeper levels and both become dispersed in the granite at depth and contribute tothe production of more granite. The dip of the granite sheets, which cut across S3 at a small angle,may be indicative of some forceful emplacement. However, this must be small as there is little OF

no tilting of S3 cleavage close to the , granite and there is no marked deformation that wouldsuggest that the country rocks had been pushed aside. Alternatively, the granite sheets can beviewed as fractures (subsequently filled with mobile granitic material) that developed as a resultof slight 'foundering ' which occurred in the wall rocks at the time that the main block(s) ofcountry rock foundered. It is concluded, therefore, that the Tregonning granite has beenemplaced fairly passively by block stoping.

3. PETROLOGY

(a) Petrography

The Godolphin granite is a fine- to medium-grained porphyritic rock (grain diameters generally1-3 mm. across) that contains subhedral megacrysts of potash feldspar, measuring 2-4 em. inlength, set in a matrix of anhedral quartz and potash feldspar, subhedral plagioclase, biotite,muscovite and pinite. Zoned plagioclase shows a wide range in composition from An 26 in the core

162 MAURICE STONE

to about An 12 dose to the edge. Commonly, a clear rim having a composition An, occurs at theextreme edges. Zoned anhedral inclusions of plagioclase in potash feldspar range in compositionfrom about An., to An7.1

The late granite porphyries are mineralogically (and chemically) similar to the Godolphingranite, but differ from the latter in containing poikiloblastic andalusite and having a groundmasscomposed of micrographic intergrowths of quartz and potash feldspar. This may be the result ofmetamorphism by the adjacent (and later) Tregonning granite.

The Tregonning granite is a medium-grained non-porphyritic rock containing anhedral quartzand potash feldspar, the latter commonly interstitial, euhedral plagioclase feldspar, a pale-brownmica, topaz and dark-brown tourmaline. At many places, the Tregonning granite has undergonesome alteration, mainly incipient kaolinisation and greisening, although locally, on TregonningHill, alteration has been marked.

Leucogranites resemble Tregonning granite in grain size, although there is sometimes areticulate aplitic matrix to a medium-grained hypidiomorphic fabric in the former. These rocksare more leucocratic than the Tregonning granite and contain more plagioclase and less potashfeldspar and tourmaline. The tourmaline is pale brown. Aplites occur as fine-grained leucocraticsaccharoidal rocks (predominant grain diameters <I mm.) in which tourmaline (pale-brown) isscarce. The pegmatites are centimetre-grained, rich in pink potash feldspar and commonly inquartz. Other minerals are plagioclase feldspar, white mica, a pale-brown mica, dark-brown tour­maline and green apatite. They are commonly associated closely with aplites. Plagioclase in allthe lithium rich rocks is close in composition to pure albite (An2-3). Pale brown mica is believed tobe a lithium aluminium mica comparable with the lepidolites from the Meldon aplite (Chaudhry& Howie, 1973). Like the Meldon rocks, the Tregonning granite and its associated RoofComplex are rich in lithium and aluminium and poor in iron (Table II) and compare in theirgeneral chemistry and mineralogy. Even after allowing for the amounts of lithium in tourmaline(that in pegmatite from Tremearne contains 0·06 per cent Li20 by weight) and amblygonite(when present), there is sufficient lithium left to form a normative lepidolite in amounts that cor­respond with modal brown mica in fresh samples. The value of 61 0 for 2Vz in topaz correspondswith about 16 mole per cent of (OH) in the (OH, F) position (Deer, Howie & Zussman, 1962).

Table 1 gives average modes and standard deviations for the principal rock types. TheGodolphin granite (and early granite porphyries) are clearly distinct from the lithium-rich, topaz­bearing rocks. Owing to the small sample sizes the latter have been examined with the aid of non­parametric statistical tests (the Kruskal-Wallis H test and Mann-Whitney U test, as described inSiegel, 1956 and Dixon & Massey, 1969). The tests reveal no significant differences between themodes of the aplites and leucogranites, which mineralogically can be considered as one group, butquite significant differences between these rocks and the Tregonning granite (at the 0·05significance level).

(b) Interpretation of textures

Evidence that quartz and potash feldspar developed late in the crystallisation sequence of theGodolphin granite (and other biotite granites in south-west England) has already been given(Stone & Austin, 1961; Exley & Stone, 1966). Lace-like networks of both muscovite and tour­maline within potash feldspar provide evidence for a replacement origin of these minerals. Suchnetworks are sometimes observed to be in optical continuity with larger distinct 'primary' grainsof the matrix. These observations indicate that much of the presently observed fabric is not the'Compositions based upon the maximum extinction a' i\ (010) in zone [0101. determined with the aid of a Leitz five-axis universal stage.

TREGONNING-GODOLPHIN GRANITE, CORNWALL

TABLE I Average modes

163

Godolphin granite Tregonning granite Leucogranite Aplite

7·3 2·43 9·7 ]·28 8·82·5 0·96 0·8 0·59 0·42·5 0·55 3·9 ]·34 2·20·6 0·]3 0·7 0·74 0·3

100·2 100·0 99·9

7 7 7

Vol. % SO

Quartz 30·2 2·39Potash feldspar 32·7 4·72Plagioclase 24·5 2·11Biotite 4·8 1·13Muscovite 6·2 2·50Lithium micaTourmaline 0·8 0·70TopazApatite and others 0·9 1·04

TOTAL 100·1

No.ofsampks 5

Vol. %

28·022-536·8

SO Vol. %

1·34 26·61·27 19·12·49 39·2

SO Vol. 0/(

2·26 27·01·72 18·43·97 42·8

SO

2·974·264·56

257o·391·280·23

SO is one standard deviation.At least 3 thin sections were counted from each sample (except for the aplites, where I or 2 sections were counted). The

area covered on each slide measured at least 2 sq. ern and in some cases exceeded 8 sq. cm.

result of direct crystallisation from magma. Only plagioclase and biotite could, perhaps, fall intothis category. Most biotite is undoubtedly 'xenolithic' (Stone & Austin, 1961) and although itmay have been modified by equilibrium adjustments with granitic magma, it is unlikely that itever crystallised from magma. There is no textural evidence that would suggest a post-magmaticorigin for plagioclase, although it is possible that plagioclase, even when zoned, does not repre­sent original magmatic plagioclase.

Textures within the lithium-rich granites are more difficult to interpret, but there is commonlyevidence for quartz grain enlargement, for replacement of albite by potash feldspar and for theproduction of albite by exsolution from sodi-potassic feldspar, and for the late development oftourmaline, topaz and apatite (on the inclusion principle and the occurrence of skeletal growths inboth albite and potash feldspar). Some leucogranites contain patches of aplite, commonly inter­stitial to the medium-grained (1-3 mm. grained) fabric: this is interpreted as relict aplite.Pegmatite commonly includes parts of a relict aplitic matrix (Stone, 1969) and grains of quartzand albite from the groundmass may be included within large crystals of perthitic potash feldspar.

4. CHEMISTRY

Average chemical analyses for the principal rock types are given in Table II. Only major andsome minor elements are considered here. Li20, Na20, CaO, MgO and MnO were determined byatomic absorption methods; Ti02, Fe203, MnO, CaO and K20 were analysed by wet methods(modified after Riley, 1958, and Shapiro & Brannock, 1962) and by X-ray fluorescence methodsusing pressed paper discs and concentrated solution 8 (HF-HN03 digestion). Other elementswere determined by colorimetry and X-ray fluorescence (Leake & others, 1969).

A visual inspection of Table II reveals marked ( and significant) differences between the biotitegranites and the other rocks, particularly in the amounts of Ti02, Fe203, MnO, MgO, Li 20 andP20 S' Similar clear differences are not quite so apparent amongst the lithium-mica granites

164 MAU RICE STONE

T1\ BLE II Chemical data : major and minor elements

wt. (.~ s.d . wt . % s.d. wt . g; s.d.

71·1 ( ·87 72 ·0 0·35 72 ·6 0 ·970 ·06 0 ·02 0 ·03 0 ·01 0 ·03 0 ·01

16·11 0 ·87 15 ·97 0 ·32 15 · 54 0 ·521· 24 0· 21 0 ·64 0 · 19 o.77 0 ·260 ·07 0 ·0 2 0 · 11 0 ·01 0 ·09 0 ·()20· 09 0 ·03 0 ·04 0 ·03 0 ·08 ()·()60· 59 0·25 O· 32 0 ·14 0 ·27 0 ·073· 73 I· 34 4 ·59 0 ·42 4 ·84 ()·624·84 1·4 2 3 ·91 0· 27 3 ·66 1· 110 ·27 0 · 13 0 ·49 0 ·06 0 ·42 ()·090 ·50 0 ·10 0 ·55 0 ·1 1 o.37 0 .0 31.22 6 1·4P I · 30'

10 6 7

Biotite G ranite

EarlyGodolph in Granite

G ran ite Porph yry\\"1. (;";, wt. (~

Si02 72 ·6 71 ·0Ti02 0 -14 0 ·30AbO) 14 -55 15 ·10Fe20J* 1 ·06 1 ·27MnO 0·04 0 ·03MgO o·35 0 ·40CaO o.57 o.57NmO 1 57 1 · 18K20 5 ·6 8 6 ·25Li20 0 ·04 0 ·04P20 S 0· 16 0 ·19F o· 15' 0 · 11'

n 1 1

Tr egonnin g G ra nite Leucogr an ite Apl ite

n = num ber of sa mples* = to ta l iron as FC203Superscripts give number of an a lyses when these differ from n.Each sample unnlysed at least in dup licate .

themselves. Where app rop riate, the Kruskel-Wallis H and Ma nn-Whitney U statistics have beencomputed in order to test for similar ities between these small sam ples of lithium-rich granites. Aswith the modal da ta, there are no significant differences between the leucogranites and the aplites,but qui te significant differences (at the 0 ·05 level) between each of these and the Tregonninggranite for certain elements, notab ly Lip, TiOt5, Fep3' MgO and Ca O.

TABL E III Major and minor element correlation

CO R RELATI O N MATR IX

Si T i AI Fe Mn Mg Ca Na K Li P

X Si - 0· 02 -o·n 0 · 14 0· 04 0 ·05 - 0 ·61 - 0 · 26 0· 18 0 ·03 --0 · 29e<:: T i 0 - 0 ·43 0 ·84 -0·66 0· 93 0 · 19 - 0· 63 0 · 53 -o·n --0 · 57f-

AI -I -0 ·36 0 ·26 -0 ·41 0 ·44 0 ·5 6 - 0 ·44 0 ·37 O· 57-c -2z Fe 0 2 0 -0 ·62 0 ·75 0·2 1 -0 ·73 0 ·59 - o· n --0 ·53U.l Mn 0 2 0 - 2 - 0 · 74 -0· 40 0 ·59 -0·57 0 · 87 o 51U Mg 0 2 - I 2 - 2 0 ·24 - 0 · 61 o 53 - 0· 75 - O· (l( )z<l: Ca - 2 0 I 0 - I 0 - 0 · 10 0 ·12 - 0· 46 0 · 15U Na 0 2 2 -2 2 -2 0 - 0 ·94 0 ·78 0 ·44G: K 0 2 - I 2 - 2 2 0 - 2 -o·n -0 ·36z Li 0 1 I -2 2 -2 -I 2 - 2 0 ·560

P 0 - 2 1 -2 2 - 2 0 I 0 2(f)

N umbe rs in the significa nce mat rix:

2-Corre lalion sign ifican t a l the 0 ·01 level of sig nifica nce: N ull Hypoth esis rejectedI- Correla tion significant a t the 0 ·05 level of significance: N ull Hypoth esis acceptedO-Corrcla liun only significa nt a bove the O· 05 level of significa nce: N ull Hypothesis accepted

TREGONNING-GODOLPHIN GRANITE, CORNWALL \65

(a) Correlation

Tests for normality based upon the Kolmogorov-Smirnov one-sample test (Siegel, 1956) andmeasures of skewness and kurtosis as well as plots on probability paper indicate that the normaldistribution can be assumed in all elements for the purpose of examining element inter­relationships by conventional correlation procedures.

The correlation matrix for the major and minor elements of all twenty-eight samples is given inTable III. Values of the correlation coefficient (r) have been obtained from the raw analyticaldata. The 1 distribution is used to test the Null Hypothesis that the values of r = O. From thesetests the signi ficanee matrix of Table III has been constructed. In order to allow for both closu reeffects and small departures from the normal distribution, only significance levels of t~O· 0 I areconsidered: these correspond with the figure 2s in the significance matrix. Some obvious andsome interesting relations emerge.

Ti is correlated with Mg and Fe (these occur in the micas and in tourmaline), but surprisingly,these elements all have a negative correlation with Mn. This reflects the enrichment of Mn in thelater terms, and hence its association and positive correlation with Na, Li and P. The negativecorrelation with K is the result of the strong negative correlation between Na and K. The correla­tion between AI and Na, like that between AI and P, appears to be related to the higher overallcontent of AI in lithium mica (brown mica) rocks compared with biotite granites, appearing astopaz in the modes. There is a notable lack of correlation between Ca on the one hand and Mgand Fe on the other. Some of the more significant chemical variations are considered below.

A preliminary factor analysis of the major and minor elements considered here resu Its in thetype of grouping already indicated. Loadings obtained after Varimax rotation give factors thatcorrespond clearly with the Ti, Fe, Mg and the alkali, Mn associations. Another factor includesCa, Al and Si. However, the association Ca-AI is less certain owing to the lack of highly signifi­cant correlation.

+

o

o

o

o 0

+

o

8 9 10 K

+

+

K o Granite porphyry (late) Fm+ Biot ite granite

12 Tregonntng granite 38

• Leucogranite

10 qj x Api ite~.~:

+2+

++

00 0\.~~ (§:' 0

o )x 00 )(008 x Ox

.0 x

x " "I:0

2 4 6 8 10 12 No 2 3 4(0)

Fig. 3. Relations between major alkali elements and between potassium and fernie constituents. Data converted tomembers of cations (as percentages of total cations).

(a) K-Na. Regression equations are: K+-O· 74 Na- 10· 70 and Na= 1·23 K+ 13·82. Correlation coefficient=-0·94.Data for late granite porphyries are plotted for comparison but are not included in the equations.

(b) Fm-K, where Fm = Mg + Fe2 + Fe'. Regression equations are Fm = 0·\8 K + 0·03 and K = \·28 Fm + 4·07.Correlation coefficient = +0·49. Only 24 per cent of the total sum of squares is due to regression. This improves to55 per cent by including the late granite porphyries and to over 70 per cent by removing the three data points that areringed by dots. These correspond with two samples of Tregorming granite from contacts with pelite and one sample thathas been markedly greisened and enriched in potassium

166 MAURICE STONE

(b) Major alkali variation

The marked spread of data points showing a strong negative trend between Na and K (Fig. 3'1)could, perhaps, reflect several different processes, but is likely to reflect an important componentthat is the result of alkali ion exchange. Of course, in mineralogical terms, the pattern here islargely the result of the near constant sum of the two feldspars when the anorthite component issmall. This pattern supports the textural evidence for internal metasomatism referred to earlier inthis paper and described by others (Exley & Stone, 1966; Booth, 1968; Hawkes, 1968). Otherprocesses such as gravity differentiation or additive feldspathisation would be expected toproduce a greater scatter of points, or a different pattern of variation, as indicated by Stone(1966b).

(c) Potassium-femics variation

The positive correlations between K and Mg and between K and total Fe lead, as would be ex­pected, to a strong positive trend in a plot of K against femics (total Fe plus Mg). This trend isshown in Fig. 3b. In earlier works, this trend was believed to have resulted from the 'pelitc' effect,that is, the product of pelitic contamination of earlier members of the intrusive sequence (Exley &Stone, 1966; Stone, 1966b ). This hypothesis might be supported further by the lower total alkaliesand higher titanium in the earlier-formed rocks, but is not supported by the slightly loweraluminium content of the biotite granites compared with the leucogranites and Tregonninggranite.

Further, it is unlikely that contamination of rocks corresponding in composition with the laterterms of the complex would produce a biotite granite. The magma that is believed to haveproduced the later (lithium-rich, topaz-bearing) rocks, therefore, is unlikely to be the uncon­taminated parent magma of the biotite granites. Even differentiation by gravitative cleansing(Read, 1924) of magma corresponding in composition with minimum melting compositions of thealkali granite system but contaminated with pelite, would not be expected to yield rocks so rich inphosphorous, fluoride and lithium and the other trace alkalies (Bowler, 1958).

Stone (1963) suggests that the pale biotites found in pelitic hornfelses in contact with the RoofComplex are enriched in lithium. This is indicated by the high lithium contents obtained in theanalyses of such hornfelses (Stone, 1965; Chesher, 1971) as well as the apparent transition intopale-brown mica like that in the granites. Further, it is suggested that lithium metasomatism waseffected by ion-exchange between the fernie constituents and lithium, with some of the ferniematerial, at least, becoming fixed in tourmaline. Actually, exchange in the octahedral sites inbiotite would involve LiAI~2(Mg,Fe). A similar exchange process within the granitesthemselves is strongly suggested by the marked negative correlation between lithium and each ofthe fernie constituents, magnesium and iron.

(d) Calcium

Exley (1959) recalculated calcium contained in fluorite in the fluorite granite of the St. AustellComplex into anorthite. When this is added to the albite present (Ab9(/\n4) it gives a composition(AbnAn2s) that corresponds with the plagioclase feldspar in the early-formed coarse porphyriticbiotite-muscovite granite. The instability of anorthite in the presence of much water is suggested(Exley, 1959).

The Meldon microgranite, on the northern border of Dartmoor, is another lithium-mica­bearing rock (Chaudhry & Howie, 1973). Its CaO content of 1·2 wt. per cent is close to many

TREGONNING-GODOLPHIN GRANITE, CORNWALL 167

Or

(b)

Q

(0)

+1

,,/A'-'-----Fig. 4. Computer plots of the average granite types onAFM and Q-Ab-Or diagrams. A = alkalies, F = total ironas Fe,OJ, M = MgO (as molecular ratios). Q, Ab and Or arenormative Quartz, Albite and Orthoclase (CIPW). Rocktypes: 1 = Godolphin granite and early granite porphyries;2 = Tregonning granite; 3 = Leucogranite;,4 = Aplite

values in the earlier porphyritic biotite granites of Dartmoor (Brammall & Harwood, 1932).Again, plagioclase has a composition in the albite range, about An] and An 2 in the white andbrown aplites respectively (Chaudhry, 1971), and there is locally abundant apatite and fluoritewithin the rock and fluorite along joint surfaces. A fluoride content of I ·40 per cent by weight I

compares with values in the leucogranites of the Roof Complex.The amount of calcium in the Tregonning granite is similar to that in the Godolphin granite,

although the anorthite content of plagioclase feldspar in the former is much lower. It might be ex­pected that the higher aluminium content in the Tregonning granite would favour a higheranorthite content in its plagioclase feldspar. However, it is likely that the presence of fluoride, orfluoro-cornplexes, results in the instability of anorthite and the release of calcium to form apatiteand/or fluorite: aluminium released by this would become fixed in topaz and/or mica. Changesof this type can be extended to the Meldon and St. Austell examples given above. The later termsof the Tregonning-Godolphin Complex, the leucogranites and aplites, have a lower calcium con­tent than the Tregonning granite: the difference may well be represented by fluorite lining thejoints in these rocks.

(e) Triangular diagrams

The AFM plot of the average rocks (Fig. 4a) shows the expected trend with time towards thealkali corner. This trend reflects any process that will result in increased alkalinity and decreasedamounts of fernie material. Such processes may be magmatic or metasomatic. The latter is in­dicated by the information given in the three preceding sections, whilst the former is, perhaps, in­dicated by the positions of the points in the Q-Ab-Or plot (Fig. 4b). Here, the data pass from theQ-Ab-An-Or tetrahedron, on the quaternary cotectic curve (James & Hamilton, 1969; Winkler,1967; Winkler & Lindemann, 1972) into the Q-Ab-Or triangle of the so-called 'granite system'better called the 'alkali-granite system'. Evidence for high mobility and the mechanism ofemplacement together with plots near to the minimum or eutectic regions in this system (Luth,Jahns & Tuttle, 1964) point to a prominent magmatic role in the evolution of these rocks. Amobile, probably magmatic emplacement of the earlier, more calc-alkalic member (Godolphingranite and early granite porphyries) is suggested by the intrusive relations towards the killasshown by the early granite porphyries.'Analysis by R. Fuge, Dept. of Geology, University College of Wales, Aberystwyth.

168 MAURICE STONE

5. CONCLUSIONS

The field evidence presented above leads to the conclusion that the Tregonning component of theTregonning-Godolphin granite complex has been emplaced passively by a process that involvedcauldron subsidence with, perhaps, some block stoping. From a consideration of the outcroppatterns and contacts of the other granite masses and the gravity data of Bott & others (1958), itcan be concluded that they have been emplaced in a similar manner, though, perhaps, with somelate forceful emplacement in some cases (Dearman & others, 1971). The build-up of the wholebatholith by successive intrusions, in much the same way as that indicated by Cobbing & Pitcher(1972) for the Peruvian Coastal batholith, has been suggested already (Dearman & others, 1971).Two such bodies are the Godolphin granite and its attendant veins and the Tregonning granitewith its Roof Complex.

Within the Tregonning-Godolphin granite complex, a marked differentiation, revealed by adecrease in modal quartz and potash feldspar together with a corresponding increase inplagioclase feldspar and a change from biotite to pale brown (lithium) mica, corresponds with theage order of emplacement and the development from biotite granite, through Tregonning graniteto leucogranite and aplite. Chemically, this differentiation is reflected in an increase in sodiumand lithium and a decrease in potassium, iron and titanium. Some of these trends with time areapparent in the chemical variation diagrams (Figs. 3 and 4).

The evolution from porphyritic biotite granite to lithium aluminium mica granite is not easy tovisualise. Magmatic differentiation within a magma chamber poses great problems if differentia­tion in situ cannot be demonstrated from the observed field relations. Fractionation by partia Imelting at depth might be expected to yield successive magmas that lie progressively closer to theternary minimum of the alkali granite system, but fractionation of granites containing initiallysufficient normative anorthite to give oligoclase or andesine would not be expected to yield con­siderable volumes of such end members. However, the abundance of pegmatite close to the roofand the high fluoride content of the lithium-rich members of the complex point to the importantrole played by the volatile constituents in the evolution of these rocks. The high content offluoride and the alkalies, including the trace alkalies, in the country rocks (Bowler, 1958;Chesher, 1971) demonstrate that mobile constituents were being carried into the country rocks byvolatiles that contained fluoride in large concentrations. There is also good evidence for theinternal movement of major, and probably trace alkalies within pegmatite-aplite bodies (Stone,1969) and within the granites themselves (Stone & Austin, 1961).

It is suggested that a marked increase in volatiles carrying the major alkali elements andlithium together with other constituents occurred close to the roof of a body of biotite granite.The Godolphin granite may represent a part of this granite that was unaffected by this volatileconcentration, but it is much more likely that the source rock was more deeply seated. Within thisrock mass, metasomatism resulted in LiAI~ (Mg, Fe) and alkali ion-exchange reactions and thede-anorthitisation of plagioclase feldspar. Such changes could take the bulk composition of thesilicates into the alkali granite ternary system and towards the ternary minimum of that system,especially if some of the calcium and fernie constituents were removed effectively from thesystem. It is possible in this way to produce local melting (corresponding with a low temperaturemelt in the ternary system) provided that the earlier biotite granite system was at a temperaturenot far beneath the solidus when metasomatism occurred. Further, a possible increase in the par­tial pressure of water vapour and the effects of a large partial pressure of fluorine would aid theproduction of melt, even if the temperature had fallen slightly. It is believed that such a partialmelt, together with solid phases, was emplaced by cauldron subsidence as the Tregonning granite.As this body crystallised, volatiles again accumulated at the roof and the processes of

TREGONNING-GODOLPHIN GRANITE, CORNWALL 169

metasomatism together with an increased concentration of fluorine in the fluid phase resulted inlocal partial melting and remobilisation in situ to give the several intrusive phases that are in­dicated by cross-cutting relations of different ages. The products were leucogranites and severalgenerations of aplite and accompanying pegmatite that form the Roof Zone. Continued upwardpressure associated with the development of this roof is believed to have opened fractures that dipaway from the main granite contacts. Roof material moved down these fractures, undergoingdifferentiation as it travelled away from its source, to form the granite sheets of Tremearne(Stone, 1969).

The present fabric of the Tregonning granite resulted from recrystallisation and internalautometasomatism: the textural changes largely obliterated aplite dykes that may have beenassociated with the early development of the Roof Zone. Similar changes occurred within theleucogranites of the Roof Complex and metasomatic differentiation occurred within each lategeneration of aplite to give composite aplite-pegmatite bodies.

The final stages of the plutonic history of the Tregonning-Godolphin granite include theemplacement of the late granite porphyry dykes followed by greisening and kaolinisation. Thedyke emplacement and, perhaps, the late-stage alteration is associated with the hypothermalmineralisation (Hosking, 1962).

The source of the aqueous phase present during the magmatic processes is likely to be bothmagmatic and meteoric. Burnham (1967) and Jahns & Burnham (1969) provide evidence tosuggest that late-stage aqueous or aqueo-silicate fluids rich in alkalies can be evolved directly bycontinued magmatic differentiation. Fluoride is almost certainly magmatic and together withwater may result in the production of alkali-rich fluids by extraction, thus leaving a rock deficientin alkali (several per cent of corundum in the norm). However, much of the water is likely to havebeen derived from the associated metasedimentary rocks both during palingenesis and during theupward emplacement of the bodies of granite magma.

ACKNOWLEDGMENTS

Thanks are due to E. M. Durrance for reading the manuscript and to D. L. Dallow and J.Harding for much of the chemical data. Part of the cost of fieldwork was defrayed by researchgrants from the University of Exeter.

REFERENCES

BERGER, A. R. & W. S. PITCHER. 1970. Structuresin granitic rocks. A commentary and a critique onGranite tectonics. Proc. Geol. Ass., 81,441-61.

BOTT, M. H. P., A. A. DAY & D. MASSON-SMITH.1958. The geological interpretation of gravity andmagnetic surveys in Devon and Cornwall. Phil. Trans.R. Soc .. 251A, 161-91.

BOOTH, B. 1968. Petrogenetic significance of alkalifeldspar megacrysts and their inclusions in Cornubiangranites. Nature, 217, 1036-8.

BOWLER, C. M. L. 1958. The distribution of alkalisand fluorine across some granite-killas and granite­greenstone contacts. Abs. Proc. Con! Geol. andGeomorph., S. W. England, 20-1.

BRAMMALL, A. & H. F. HARWOOD. 1932. TheDartmore granites: their genetic relationships. Q. Jlgeol. Soc. Lond., 88, 171-237.

BURNHAM, C. W. t967. Hydrothermal fluids at themagmatic stage. In Geochemistry of HydrothermalOre Deposits (Ed. Barnes, H. L.). Holt, Rinehart &Winston, New York 34-76.

170 MAURICE STONE

CHAUDHRY. M. N. 1971. Feldspars from the MeldonAplite, Devonshire. England. Mineralog . Mag .. 38,179-85.

CHAUDHRY, M. N. & R. A. HOWIE. 1973. Lithium­aluminium micas from the Meldon Aplite. Mineralog.Mag .. 39, 289-96.

CH ESH ER, S. E. 1971 . The Metasomatic Effects oftheIntrusion of the Tregonning -Godolphin Granite intothe Mylor Slates , South Cornwall . Unpublished Ph. D.Thesis, Exeter University.

COBBING. E. J. & W. S. PITCHER. 1972. The coastalbatholith of Central Peru . JI. geol. Soc.. 128,421-60.

DEARMAN, W. R., E. C. FRESHNEY, E. B.SELWOOD. S. SIMPSON, M. STONE & R. T.TAYLOR. 1971.Symposium on the structure ofS.W.England. Proc. Ussher Soc.. 2. 220-63.

DEER, W. A.• R. A. HOWIE & J. ZUSSMAN. 1962.Rock Forming Minerals. Vol. I. Longmans, London.

DE LA BECHE, H. T. 1839. Report on the Geology ofCornwall. Devon & West Somerset. Longman, Orme,Brown, Green and Longmans, London.

DINES, H. G. 1956. The metalliferous mining region ofsouth west England. Mem . geol. Surv. U.K .

DIXON W. J. & F. J. MASSEY. 1969. Introduction toStatistical Analysis. 3rd Edit. McGraw-Hill , NewYork .

EXLEY, C. S. 1959. Magmatic differentiation andalteration in the St. Austell granite. Q. JI geol. Soc .Lond.. 114, 197-230.

EXLEY, C. S. & M. STONE. 1966. The granitic rocksof south-west England. In Present Views of someAspects of the Geology ofCornwall and Devon. RoyalGeological Society of Cornwall 150th AnniversaryVolume, 131-84.

FLETT, J . S. & J. B. HILL 1912. The geology of theLizard and Meneage. Mem. geol. Surv. U.K .

HALL, S. 1930. The geology of the Godolphin granite . Astudy of the coastal geology between Perranuthnoe andLooe Pool. Proc. Geol. Ass ., 41,117--47.

HA WKES, J . 1968. Dartmoor granite and aplitic rocks.In Geology of the Country around Okehampton, byE. A. Edmonds and others . Mem . geol. Surv. U.K .

HOSKING, K. F. G. 1962. The relationship between theprimary mineralization and the structure of south­west England. In Some Aspects of the Variscan FoldBelt (Ed. K. Coe). Manchester, 135-54.

JAHNS, R. H. & C. W. BURNHAM. 1969.Experimental studies of pegmatite genesis: I. A modelfor the derivation and crystallisation of granitepegmat ites. Econ. Geol., 64, 843-64.

JAMES, R. S. & D. L. HAMILTON. 1969. Phaserelations in the system NaAISiJOs-KAISiJO.-CaA 12SilO.-Si02 at I ktlobar water vapourpressure. Contr . Mineral. Petrol .. 21. 111--41.

Received 4 June 1974Revised version received 23 October 1974

LEAKE , B. E., G. L. HENDRY, A. KEMP,. A. G.PLANT, P. K. HARVEY, J . R. .~ILSON, J . S.COATS, J. W. AUCOTT, T. LUNEL & R. J .HOWARTH. 1969. The chemical analysis of rockpowders by automatic x-ray fluorescence. Chern. Geol,5. 7-86.

LUTH, W. c., R. H. JAHNS & O. F. TUTTLE. 1964.The granite system at pressures of 4 to 10 kilobars. J.geophys. Res .. 69. 759-73 .

READ , H. H. 1924.On certain xenoliths associated withthe contaminated rocks of the 1-1untly Mass,Aberdeenshire . Geol. Mag .. 61. 433--44.

REID, C. & J. S. FLETT. 1907. The geology of theLand's End district. Mem . geol. Surv. U.K .

RILEY, J. P. 1958. The rapid analysis of silicate rocksand minerals. Analytica Chim . Acta. 19.413-28. .

SHAPIRO, L. & W. W. BRANNOCK. 1962. Rapidanalysis of silicate, carbonate and phosphate rocks.U.S. Geol. Surv . Bull.. I I44-A, 56pp.

SIEGEL, S. 1956. Nonparametric Statistics for theBehavioural Sciences . McGraw-Hill, New York .

STONE, M. 1960. The Tregonning-Godolphin granite.Abs. Proc . Con! Geol . and Geomorph.. S. W.England . 13.

STONE, M. 1963. Lithium in the Tregonning-Godolphingranite complex. Proc. Ussher Soc.. 1. 50-3.

STONE, M. 1965. Structure and petrology of theTregonning-Godolphin Granite . Unpublished Ph.D.Thesis, Birmingham University.

STONE, M. 1966a. Fold structures in the Mylor Beds,near Porthleven, Cornwall. Geol. Mag .. 103,440-60.

STONE, M. 1966b. Some aspects of van ation in graniticrocks with particular reference to the granites of S.W.England. Proc. Ussher Soc.. 1. 208-12.

STONE, M. 1968. A Study of the Praa Sands Elvan andits bearing on the origin of Elvans. Proc. Ussher Soc ..2. 37--42.

STONE, M. 1969. Nature and origin of banding in thegranitic sheets of Tremearne, Porth leven, Cornwall.Geol. Mag.. 106. 142-58.

STONE, M. & W. G. C. AUSTIN. 1961. Themetasomatic origin of the potash feldspar megacrystsin the granites of south west England. J. Geol., 69,464-72.

WINKLER, H. G. F. 1967. Petrogenesis ofMetamorphic Rocks. 2nd Edit. Springer-Verlag,Berlin

WINKLER, H. G. F. & W. LINDEMANN. 1972. Thesystem Qz-Or-An-H20 within the granite systemQz-Qr-Ab-An-H20. Application to granite magmaformation. Neues Jahrb . Min . MH, 49--{j1.