C4 Luxulyan Quarry (Golden Point, Tregarden)

[SX 054 591]

Highlights

Luxulyan Quarry contains the coarse, megacrystic biotite granite of typical Cornubian type which forms the earliest variety in the complex magmatic sequence at St Austell. Its fresh xenoliths of pelitic and semipelitic sediment provide evidence about the origin of the magma, and there is also evidence of post-magmatic activity in the form of luxullianite in situ.

Introduction

Luxulyan Quarry (which has also been called Goldenpoint and Tregarden) is situated in typical Cornish granite of the St Austell mass, described in detail by Ussher et al. (1909); these authors, however, did not realize that there was a sharp distinction between this eastern rock and the granite types seen a few kilometres to the west. The differences were first recognized by Richard­son (1923) and later Exley (1959), who both concluded that the Luxulyan Granite represented an earlier, separate intrusion which has subse­quently been interpreted as a boss about 9 km across and dated, by the Rb/Sr method, at 285 ± 4 Ma BP by Darbyshire and Shepherd (1985). The boss was emplaced by stoping and subsidence, and xenoliths of the country rock, found in the quarry, have been used by Lister (1984) as evidence bearing on the origin of the magma.

The rest of the St Austell outcrop consists of a second, slightly larger, intrusion which, having been emplaced to the west of the Luxulyan Granite, was itself intruded by a magma of entirely different composition and was altered by a complex interchange of elements in a volatile-rich environment. The changes and mechanisms are noted in general terms in Chapter 2 and the Tetrogenesis' section above, and in detail in the appropriate site descriptions. It is sufficient here to note that, although widespread in the western area, they have affected the Luxulyan area to only a moderate degree (Figure 5.4).

Among the few volatile-induced modifications, however, was the formation of luxullianite. This rock, described by Bonney (1877a), Flett in Ussher et al. (1909) and by Wells (1946), was known only from boulders until found in situ by Lister (1978, 1979a, with a contribution by Alderton (1979)). Lister (1979b) also used material from this site in her study of quartz-cored tourmaline.

Description

The granite at Luxulyan (Figure 5.10) has been described (Dangerfield and Hawkes, 1981) as 'coarse megacrystic' and as Type B (Table 5.1) (Exley and Stone, 1982). It is characterized by biotite and zoned oligoclase (An_25–30), and contains abundant K-feldspar megacrysts between 20 and 100 mm in length. These are generally aligned and commonly include zones (containing quartz, plagioclase and biotite) indicative of growth following a potassium-rich metasomatism process. Other minerals include muscovite, tourmaline and trace quantities of apatite, topaz, andalusite, fluorite, zircon and iron ore. The rock is closely comparable with the other coarse biotite granites of Cornwall and Devon and its mineralogy and chemistry show that, despite its early arrival in the St Austell sequence, it is a highly evolved, high-level variety (Table 5.2).

Evidence bearing upon the emplacement of this granite is seen in the abundant, rounded xenoliths, which are usually about 20 mm across, but range up to about 130 mm. These are mainly composed of quartz and abundant biotite, but sometimes contain andalusite. The majority have lost any foliation that they might have had, but have not yet been sufficiently 'granitized' to develop feldspar. They are clearly of pelitic or semipelitic origin and derived from the stoping of its walls by the magma. Fresh cordierite has been recorded from this quarry (Ussher et al., 1909).

Luxullianite, an attractive rock composed of red K-feldspar, acicular tourmaline and quartz, and formerly used as an ornamental stone, occurs in often discontinuous, near-vertical sheets which sometimes anastomose. These strike approxi­mately ENE–WSW, and are up to a metre or two in thickness. The jointing is both more extensive and less regular than in many Cornubian ex­posures, and some joints show evidence of post-magmatic activity in the form of reddening and veining by quartz and tourmaline. Sometimes, such tourmaline has cores of quartz or feldspar (Lister, 1979b). There are also small pods, up to 0.5 m in diameter, of pegmatite consisting of the chief minerals of the granite but mostly deficient in plagioclase. A major joint, with a veneer of tourmaline striking 070°, serves as the quarry wall beneath the crushing plant.

Kaolinization, not due primarily to weathering, is confined to a zone striking N–S and tapering downwards in width from about 10 m. This separates the north-eastern quarry from the rest of the site.

Interpretation

The granite at Luxulyan is typical of the eastern part of the St Austell outcrop, which, having been recognized as significantly different from the granite in the western part (Richardson, 1923; Exley, 1959), was interpreted by the latter as the first member of a magmatic differentiation series. This interpretation followed from its relative enrichment in Ca and Fe (exhibited in oligoclase and biotite) and impoverished in Na (in albite), Li (in zinnwaldite), B (in tourmaline) and F (in topaz) (Table 5.1) and (Table 5.2). Later work has shown, however, that much of the western granite has a similar texture to that in the east and a composition intermediate between the medium-grained Li-mica–albite–topaz granites (Type E, (Table 5.1)) and the biotite granite. Hence the present interpretation is that the first member of the western intrusion was also a biotite granite, much of which was metasomatized by incoming Li-mica–albite–topaz granite derived at depth (see Chapter 3 and the Petrogenesis' section of this Chapter). The first importance of the Luxulyan site therefore lies in its exposure of the rock held to be the earliest in both hypotheses.

Theories about the derivation of the Cornubian granite magma agree that it resulted essentially from partial melting of a lower crustal source. However, the extraordinary enrichment of the batholith, relative to average granites, in such elements as Sn, W and Cu, Li, Sr and Ba, U and Th and B and F have led to speculation as to whether their provenance was middle or lower crustal or subcrustal (Simpson et al., 1976, 1979; Watson et al., 1984), and to what extent they were incorporated either from already enriched crustal material or from some subcrustal source. A study of xenolith material, some of which came from Luxulyan Quarry, has suggested that Sn, W, U and Ta were not derived from pelitic sediments, that V, Ba, Sr, Cu and Zn might have been, and that some elements which could easily have escaped from the magma (for instance, Li, Th and F) were in fact retained and concentrated in biotite-rich xenoliths. Those elements not derived from assimilated sediments must have been magmatic. Luxulyan xenoliths are thus of importance in the petrogenetic history of the Cornubian granites (Lister, 1984).

Granitic rocks generally similar to luxullianite have been found in various parts of south-west England, and it is agreed, from textural evidence, that they were formed by the post-magmatic alteration process described as lourmalinization'. Flett (in Ussher et al., 1909), however, when contrasting luxillianite with the tourmalinite of the Roche Rock 6–7 km to the WNW, observed that 'In luxullianite the process of metasomatic replacement has stopped at the half-way stage'. There has also been disagreement over the nature of the replacement and the original mineralogy. Thus, Bonney (1877a) thought that brown tour­maline had replaced biotite, but both Flett (1909) and Wells (1946) believed that biotite had never been present, brown tourmaline occur­ring instead. Again Bonney thought that acicular, blue, secondary tourmaline formed from feldspar, but Flett and Wells considered that it replaced both feldspar and brown primary tourmaline. Lister's (1978) examination of the first in situ luxullianite to be described makes it clear that biotite and primary tourmaline coexist, and that secondary tourmaline came from a hydrothermal generation and did not involve the breakdown of the primary crystals. However, the chemical changes between unaltered granite and luxul-lianite described by Lister (1978) differ from earlier suggestions, principally in showing a decrease in SiO₂ and an increase in K₂O and, following a discussion by Alderton (1979), she agreed that probably there had been a combined process of tourmalinization and K-feldspathization. It is worth noting as Charoy (1982) points out, that there is 'tourmalinization and tourmalinization', and that Lister and Alderton were not comparing like with like. As for tourmaline in veins, Lister (1979b) noted that some from this quarry contained cores of 'polycrystalline quartz and/or feldspar' and attributed this to skeletal growth resulting from undercooling of the tourmalinizing melt.

Regarding other alteration processes, Luxulyan is typical of the eastern St Austell area in showing only minor greisening and kaolinization, although it is interesting that the kaolinized zone in the quarry is of the wedge shape, described by Bristow (1977) as characteristic of such zones found throughout the Cornubian granites, although often on a much larger scale and at such a depth that this shape is revealed only when the zones are worked or from boreholes.

The biotite granite at Luxulyan is typical of Cornubian granites, and is enriched in elements such as Sn, W, Cu, Li, Sr, Ba, U, Th, B and F. It is uncertain to what extent these were contributed by crustal or mantle sources, but research on biotite in xenoliths from Luxulyan suggests that Sn, W and U (as well as Ta) did not come from sediments and were thus not crustal, that B, Sr and Cu (and also V and Zn) could have done so, and that magmatic Li, Th and F were trapped and thus concentrated in the biotite in xenoliths.

A similar type of granite formed the major part of a second intrusion to the west, but this was metasomatized to give the albite–Li-mica–topaz variety now present.

Luxullianite from the quarry has shown that there are two generations of tourmaline in these rocks, that tourmaline and biotite are not neces­sarily mutually exclusive and that the tourmalinization process may be accompanied by K-feldspathization.

Conclusions

Luxulyan Quarry provides an exceptional oppor­tunity to examine the typical Cornubian biotite granite in a locality close to the succeeding lithium- and volatile-rich rocks of the St Austell complex of intrusions. The site shows xenoliths here consisting of metamorphosed, muddy sedi­mentary rocks. It is thought that the incorporation of these rock fragments has made a significant contribution to the final chemistry of the granite. The nearby village gives its name to the rock type luxullianite. This rock, made up of red feldspar, quartz and the dark mineral tourmaline (a complex boron-bearing aluminium silicate), was formed by alteration of the original St Austell Granite by hot fluids associated with the final phases of granite magmatic activity, which, flowing out from the solidifying granite, chemically altered and recrystallized the minerals which made up the granite and the rocks around it.

References