St. Austell detritals

139 ST.

AUSTELL

DETRITALS.

By ]. G. C. LE ECH . A.R.S.M. [Received Ma rc" 6th, 1928.] [Read December 7th, 1928. ]

IN opinion a recent paper describing the Dartmoor detritals, [1]* the is expressed that each of the West of England

granites ma y prove to be charact erised by a mineral assemblage which, in qualitative and quantitative detail, is distinctive. In respect of the St. Austell mass this opinion is confirmed in many details by an examination of both the granites and the detritals of the area, and the purpose of this paper is to add to the data already on record concerning the mineral composition o f the West of England granites in relation to provenance problems raised by sedimentary formations in the south and west of England. The St. Austell Granites. Like the Dartmoor, Bodmin Moor and the Scilly Isles granites, the St. Austell mass is a composite intrusion showing an intrusive sequence which , in the main, grades from relatively basic to thoroughly acid types. It is characterised by a late stage of severe pneumatolysis and extensive kaolinisation. By a detailed micrometric study of the complex Dr. W. A. Richardson established the identity of three main granite t yp es, and sketched th eir areal distribution on a broad basis [2]. 1. B i 0 tit e - m usc 0 v i t e g ran i t eoo Dominant in the Luxullyan area , and forming the eastern lobe of the mass . It is the local tor-granite. 2. Lit h ion it e g ran i t e. Forming the central portion (Hensbarrow area), together with the most west erly lobe referr ed to as the Retew ar ea. 3. Gil b e r tit e g ran i t e. Associated with an extensive development of chin a-stone varieties, and restricted in th e main to the St . Stephen's area. The struc ture indicated by exposures of these types in valleys , tors and qu arries is th at of a complex intruded beneath a cupola of sedimentaries and at different stages. The earliest and relatively most basic type was probably fissure -fed. The structural planes (jointing, pseudo-bedding, etc.) are due .to causes which have been frequently discussed in recent papers on the granites of this region. For the present purpose it is sufficient to remark that both pseudo-bedding and the gentle dip of minor sill-like sheets injected into the main masses endow the upper parts of the intrusion as a whole with • For list of re ferences see p. 4().

J.

G. C. LEECH,

the features of a laccolite, though the mass, like Dartmoor (1, p. 44) is probably batholithic at depth. The central and western parts of the exposed granite have been extensively kaolinised, and correlation of kaolinised with unaltered types remains uncertain. Nevertheless, the porphyritic biotite-muscovite granite dominant in the Luxullyan area appears to be represented by kaolinised types lying well within the Luxullyan area and by similarly altered types in the chinaclay district to the west. A comparison of the heavy accessory minerals isolated from (a) fresh and (b) kaolinised types carrying up to 20 % of porphyritic felspars reveals differences which, though very marked, are such as might reasonably be expected as the result of the processes involved in kaolinisation, e.g., the decrease or complete disappearance of apatite, the obscuration of dark mica by bleaching, and the possible, though apparently slight, increase in topaz, fluorite and tourmaline, and the advent of secondary minerals such as anatase and rutile, developed at the expense of titaniferous minerals, principally biotite. The gradual passage of one of the main types into kaolinised granite is seen at the Luxullyan china-clay pit; but elsewhere direct correlation of the Luxullyan biotite-granite with kaolinised granites has not been found possible on the basis of field evidence. Nevertheless, such indirect means of correlation as are afforded by suites of accessory minerals afford ground for the belief that the biotite-granite of the Luxullyan area has its representatives among the kaolinised granites in the china-clay district to the west. The present writer fully agrees with Richardson's opinion (2, p. 569) that the precise delimitation of unaltered types cannot be mapped on field evidence, and that there appears to be a gradual transition from the biotite-granite into the lithionitegranite. Richardson regards the Cam Grey quarry-granite as a type intermediate between the two, but in the present writer's opinion the Cam Grey type stands alone-on mineralogical and other grounds-as a distinctive type, closely comparable with the Merrivale type of Dartmoor, a type which is among the " later" granites of that district. The extensive development of china-stone in the St. Stephen's area is certainly the product of a definite and " late" stage of intrusion; the china-stones present marked textural and mineralogical contrasts with the two other main granites, from which they also differ markedly in respect of bulk composition. The china-stones are essentially free from dark mica, iron ores and monazite; they carry little tourmaline; they are essentially free from porphyritic felspars; the plagioclase is almost pure albite, the lime being appropriated to apatite and fluorite.

ST. AUSTELL DETRITALS.

141

These china-stones, and associated gilbertite granites, are not thin sill-like sheets, but extensive masses in which deep and wide quarries have been opened. In these quarries the extremely regular jointing and almost horizontal pseudo-bedding give the quarried rock the aspect of flat-bedded limestone. The depth to which the china-stones extend is unknown. Their lateral extension and limits are obscure. The masses appear to form broad plugs occupying vents opened in the lithionite granite to accommodate residual magma-fractions potentially leucocratic but specially enriched in fluxes, especially water and fluorine, though somewhat deficient in boric acid. The evidence afforded by other leucocratic minor intrusions and complementary melanocratic vein-rocks points to complex differentiation processes locally (as in the china-stone area), involving high concentration of fluorine antecedent to the maximum accumulation of volatile compounds of boric acid. Reference to these processes is irrelevant to the present purpose except in so far as it serves to explain the incoming of topaz, fluorite and white mica in more than normal proportion. The effects of pneumatolysis and tourmalinisation of the main granites are not described by Richardson. Yet it is to such effects that several of the distinctive features of the St. Austell detrital assemblage are to be attributed. Species wholly absent from the normal granites (e.g., brookite and anatase) have arisen only as a consequence of pneumatolytic processes. The general features and effects of pneumatolytic action are in close agreement with those described by Brammall in the case of Dartmoor [3J. though the conditions determining the mineral products of the processes operative on the St. Austell mass must have differed from those operative on Dartmoor in respect of factors concerned with the quantity, colour and crystal habit of the specially characteristic species, brookite and anatase. Here may be mentioned the fact that the following species are liable to be missed in thin sections of the granites, but are found, sometimes in unexpected quantity, by examination and treatment of granite in bulk : Monazite, two contrasted varieties of zircon, garnet, sphene, granular rutile, brookite, anatase, pyrrhotite, autunite, torbernite, ilmenite, and dumortierite. Though the detritals from such a complex might be expected to blend into an assemblage with well-marked "average" features, varying little with locality, this is by no means the case in this district. The detritals now being dispersed from the eastern lobe of the Moor differ in a marked degree from those dispersed from the south and west. The facies from the eastern lobe presents many features identical with those of the south-west Dartmoor facies. The western 51. Austell facies has few characteristics in common with Dartmoor detritals.

J.

G. C. LEECH,

Unlike the Dartmoor mass, the St. Austell "distributive province" affords little data on which to base an opinion concerning the petrological character of the granites which formed the upper parts of the intrusion before the latter lost its capping of country-rock. The existence of hornfelsed types in the original roof must be presumed, from the evidence of the aureole and the presence of hornfelsed sedimentaries as xenoliths in the granite. Beyond this fact it is not possible to indicate whether, or how, the detritals may have varied in facies since early postCarboniferous times. As in the case of Dartmoor the earliest detritals Were probably richer than those of the present day in contact-aureole species (e.g., andalusite). Here may be mentioned the outlying S1. Denis granite, in which Richardson noted an unusual abundance of andalusite. As this small mass, lying just beyond the margin of the main intrusion, may be regarded as having been only recently uncovered, it may afford an indication of what the upper horizons of the main mass as a whole may have been before denudation had bitten deeply into the granite. Moreover, marginal facies of the main mass are locally enriched in either topaz or andalusite. Description of the St. Austell Assemblage. It has already been stated that marked contrasts occur between the eastern and the western facies of St. Austell detritals. The differences observed are in close agreement with those to be expected from the distribution of the three main granite types recognised by Richardson. For the purposes of sampling, the alluvials in the middle part of the Moor are disappointing: they are dominated by " sand" and" white mica" discarded from the china-clay pits, the influence of hard, unaltered rock in situ being on the whole almost negligible. The differences noted between the two contrasted facies are most clearly indicated when expressed in terms of (a) topaz and fluorite, (b) dark and light micas, and (c) andalusite. EASTERN FACIES.

WESTERN FACIES.

Topaz constant, but" rare."

Fluorite

II

rare."

inconstant

and

Topaz constant-s-often the dominant species in a nonmagnetic fraction of" heavy" minerals. Fluorite constant, but varying quantitatively. In material dispersed from the china-stone area it is often equal in quantity with topaz.

ST . AUSTELL DETRITALS.

EASTEfu" FACIES.

Biotite is domin ant over mu scovite, which is someti mes scanty. Andalusite

II

rare."

inconstant,

143

WESTERN FACIES .

Biotite subordinate ; locally absent. dominant, Light micas locally to the exclusion of dark mica. Andalusite exceptional.

Of grea ter importance are those contrasts which would serve to distinguish th e St . Aust ell assemblage as a whole from the typical Dartmoor assemblage : DARTMOOR.

Manganiferous garnet constant; only locally does it rank as a rare species. Titaniferous biotite dominant , except in the southwest.where white mica becomes very abundant. Zircons, dominantly th e dusky zoned variety, abundant. The clear variety is scanty, except in marginal leucocratic, and some ot her very localised modifications. Monazite is constant ; locally only slightly sub ordinat e to zircon . Brookite and anatase const ant. Cassiterite constant ; locally abundan t. Ilm enite in excess of magnetite. Th e rarity of fluorit e and topaz, except locally. Amphibole rare.

ST . AUSTELL.

Garnet inconstant and " rare." It is not appreciably manganiferous. East: Titaniferous biotite dominant. West: Dark mica subordinate or even absent. Zircons as a whole are smaller and less abundant; th e clear zircons are numerically only slightly subordinate to the dusky variety. There are distinctive habits. Monazite rare, or absent. Both inconstant. Brookite is distinctive. Cassiterite rare, except in the north and west. Magnetite and ilm enite in roughly equal proportions. In th e west both are subordina te . The abundan ce of fluorite and topaz in the west ern facies; rare in the east ern facies. Amphibole has not been observed.

Distinctive Features. It remains to consider, in detail, St. Austell species which present features sufficiently in cont rast with Dartmoor occurrences to be regarded as distinctive.

J.

G. C. LEECH,

ZIRCON. (Plate 13')

(a) Both the average size attained, and the total proportion of zircon in the St. Austell detritals (and granites) are appreciably less than in the Dartmoor residues. (b) Dusky zoned zircons are dominant, but the size and proportion of clear zircon are greater than in the average Dartmoor residues.

Th e dusky variety (plate 13, figs. 1-7) is distinguished by (i) elongation and (ii) the dominance of a steep bipyramid modified quite near to the poles by a very subordinate low bipyramid ; the typical crystal presents the" torpedo" aspect. Zoning is less conspicuous than in the Dartmoor variety. Prismatic and granular inclusions (zircon, apatite, opaque dust) are common; spherical or irregular cavities are frequent. Some extraordinary vermiform tube-like inclusions are not uncommon.

The clear zircons (plate 13, figs. 8-1 3) are characterised by the simple prism; only occasionally do the prisms of the first and second orders occur in combination. The terminal planes are characteristically those of the di-tetragonal bipyramid, very occasionally associated with the simple bipyramid. Inclusions are often numerous, and like those of the dusky variety. APATITE.

Many prisms show a central grouping of minute opaque inclusions, as well as the usual voids and minute crystalline bodies, both granular and prismatic. In the main, however, this mineral shows no distinctive features. As it is unlikely to survive in detritals it is probably of no value for provenance. TOPAZ.

In the western facies this species is characterised by (a) abundance of minute opaque (black) inclusions, frequently associated with colourless transparent granules and prisms referred to quartz and apatite, rarely fluorite. (b) "Damouritisation" i.e., alteration to a micaceous (sericitic) decomposition product which is irregularly distributed through the grain as feathery wisps, flakes, or clots. As this feature is commonly displayed by topaz in thin rock-sections of the china-stones, etc., there is no reason to suppose that such alteration is subsequent to the entry of topaz into detrital material. In the eastern facies topaz presents no distinctive features.

ST. AUSTELL DETRITALS.

145

BROOKITE. (Plate 13, fig. 17-25.)

In respect of varietal features the following contrasts are specially noteworthy : Dar t moo r. S1. Austell. Colour. Colour. (1) Common in the eastern (1) Dominantly bright yelfacies. low brownish or sherryyellow to orange. (2) Subordinate: pale yel(2) & (3) Dominant: particularly so in the low to almost colourmiddle and western facies. less. (3) Exceptional: dusky or smoke-coloured ("Cairn gorm " tint).

Clarity: Few tablets are free from inclusions of dust-particles; clots of opaque matter are common, but rarely so large as to render a tablet opaque over so much as half its area. Striations: (I) Parallel to the c-axisgeneral; present in the great majority of the tablets. (2) Additional transverse strire seen in a minority of the tablets.

Clarity: Corresponding to (2) & (3) above: Opaque dust is not only abundant; it is characteristic, and may obscure nearly the whole of a tablet. (Figs. 20-22, 25). Striations: (I) Singularly uncommon, except in the eastern facies. Close packed strire showing " chain-dot " discontinuity (fig. 17). (2) A rare feature.

ANATASE.

Both tabular and bipyramidal anatase occur, but present no unusual features as regards crystal habit. Patterning such as that described (1, p. 31) for the Dartmoor species is common-s-occasionally striking. In the St. Austell species the tabular variety is dominantly blue-s-often deep blue to almost opaque. The octahedral variety is often quite opaque; the colour of transparencies being dark blue, dark green, or umber-brown, with occasional patti-colouring. Pale yellow and pale brown tablets are singularly rare-a feature in striking contrast with Dartmoor occurrences. RUTILE.

Apart from the minute needles, common in altering biotite and occasionally seen also in quartz, rutile occurs as brownish

ST. AUSTELL DETRITALS.

yellow or very dark brown granules. (Plate 13, figs. 14-16). As in Dartmoor these granules are derived mainly from hydrolysed or tourmalinised granites, and are often intimately associated with Schorl (fig. 16). DUMORTIERITE.

Only about identified.

SiX

minute splinters of this mineral have been

Summary. The characteristic features of the St. Austell assemblage are as follows : 1. The association of titaniferous dark mica (and muscovite) with zircon (both zoned and clear), ilmenite, and magnetite, with tourmaline, brookite, anatase, topaz and fluorite. 2. The local abundance of (a) andalusite, (b) topaz and fluorite. 3. (Species arranged in order of average relative abundance.)

Western Facies. Eastern Facies. Index fig. (a) Light mica, tourmaline. 10 (a) Dark mica, tourmaline (b) Zircon, magnetite and 7-4 (b) Apatite, zircon, magneilmenite. tite and ilmenite (c) Apatite, topaz, fluorite. 3-2 (c) Muscovite, " pinite." (d) Anatase, rutile, pyrites, 2-1 (d) Anatase, pyrites. andalusite, brookite, and cassiterite. Rare (e) Brookite, rutile and (e) Monazite. cassiterite rarer (f) Garnet, sphene, topaz, (f) Garnet, sphene. fluorite. Andalusite very variable in relative abundance. Dumortierite and molybdenite extremely rare. Autunite very local; abundant at Cam Grey. The author's best thanks are due to Professor W. W. Watts for the provision of laboratory facilities and to Dr. A. Brammall for continual consultations whereby similarities and contrasts presented by Dartmoor and St. Austell mineral assemblages have been established. References. I.

2.

3.

A. Dartmoor Detritals. Proc. Geol, Assoc., vol. xxxix (1928). pp. 27, 48. RICHARDSON, W. A. A Micrometric Study of the St. Austell Granite, Quart. Journ. Geol, Soc., vol. lxxix, pt. 4 (1923), pp. 546-576. BRAMMALL, A., and HARWOOD. H. F. Tourmalinisation in the Dartmoor Granite. Min. Mag., xx. (1925), pp. 319-33°. BRAMMALL,

PROC. GEOL. Assoc., Vor., XL. (1929).

PLATE 13

6

/I

/2

/0

8

/5

/7

2/

24

Typical Crystals and Fragments of St. Austell Zircon (figs. 1-13), Rutile (figs. 14-16) and Brookite (figs. 17-25). To II>'" page 146