Petrography of Spilites, Albite Dolerites and Keratophyres
Both porphyritic and non-porphyritic types occur in the dark green spilites (7920–7967, 7912). In the former, phenocrysts of augite and feldspar (1–2 mm.) are set in a pilotaxitic base of augite prisms (0·2 mm.), feldspar laths (0·6 mm.), iron ore and chloritic minerals (Fig. 5). Larger plagioclase and augite phenocrysts (2–3 mm.) are present in rocks (7931, 7946) transitional to dolerites. The non-porphyritic types consist of a pilotaxitic mosaie only (7935, 7941, 7944). A glassy phase, developed as selvedges in the pillows, is shown in two cases (7910, 7913) where abundant phenocrysts of feldspar and augite are set in a black glassy groundmass, in which plentiful feldspar microlites and pyroxene crystallites can be distinguished, and from which plentiful iron ore granules have crystallised. Variolitic structure is occasionally developed in rocks (7922.
7923) possessing affinities with variolites. In the green albite dolerites (7892–7902), feldspar laths (2·5 mm.) enclose augite crystals (1 mm.) in ophitic texture (Fig. 7). Rarely the dolerites are coarser grained (7902, 7889), and in these cases the feldspar is 4 mm. in length and the augite 2–3 mm. The light green keratophyres (7903–7905) typically display a porphyritic structure in which feldspar phenocrysts (1–2 mm.) are set in a cryptocrystalline groundmass (Fig. 6). Shearing movements have affected several of the spilitic rocks (7928, 7896, 7901), and in some instances epidote has crystallized along the shear zones.
As will be shown in the mineralogy section of this paper, the spilitic rocks are characterised by fresh augite, albitic feldspar, and an abundant development of epidote and pumpellyite.
(ii) Mineralogical Features.
Plagioclase. Feldspar phenocrysts in the spilites and keratophyres, and laths in the albite dolerites do not normally exceed 3 mm. in length, but larger crystals are known from two dolerites (7889, 7902). In the pilotaxitic groundmass of the spilites, the laths are less than 0·6 mm. in length. The majority of the feldspar crystals contain plentiful fine inclusions of epidote and pumpellyite, and their presence may prevent accurate measurement of refractive index, although generally the twinning is not obscured. Much larger epidote grains (greater than 2 mm.) can be observed in the plagioclases of several dolerites (7907, 7985), spilites (7906, 7943), and keratophyres (7903–5). Feldspar free of inclusions is relatively rare (7912, 7922, 7892).
Three methods were used in determining the character of the feldspar: (1) The normal microscopical methods as described by A. W. Winchell (1933, pp. 337–352), particularly the measurement of extinction angles in sections normal to a bisectrix (p. 358); (2) measurement of the refractive indices α and γ with respect to Canada Balsam. usually with the aid of a universal stage, and (3) the standard four-axis universal stage procedure as described by W. Nikitin (1936, pp. 96–103), and F. J. Turner (1940, 1947). These methods indicated that the dominant plagioclase present is albite, typical determinations with the universal stage being:—
spilites An0–10 three crystals (7935). An5 (7932, 7943).
albite dolerites An0–2 (7892). An5 (7893).
keratophyres An10 (7903).
More calcic feldspar has, however, been determined in several spilites—An18–20 (7936), An22–27 (7913), An33–36 (7942).
Pyroxene. Pyroxene is present as colourless prisms usually less than 2 mm. in length in the spilites and albite dolerites, and less than 0·2 mm. in the pilotaxitic groundmass of the former. A characteristic feature of the pyroxene is its fresh appearance, which is in marked contrast to the widespread chloritization of mafic minerals in many spilitic rocks (Dewey and Flett, 1911, p. 203; Gilluly, 1935, pp. 228–233), but on the other hand comparable with pyroxene in spilites described from New South Wales (Benson, 1915, p. 141), Eastern Fennoscandia (Eskola, 1925, p. 21) and the Great King Island (Bartrum, 1936, p. 420).
Following the recommendations of F. J. Turner (1942) the composition of the pyroxene has been determined using Nemoto's method for twinned crystals on a universal stage (Nemoto, 1938). One determination (7932) gave γ°c = 40° ½ and 2V =+50; this suggests a composition of Wo37 En9 Fs55 or that of a normal augite (Benson, 1944, p. 77). Measurement of optic axial angles of other pyroxenes agrees with this result. The pyroxenes of the albite doler-ites also appear to be normal augite.
Olivine. Olivine has not been observed but possible serpentinous pseudomorphs (bowlingite?) occur in three spilites (7913. 7932, 7946).
Iron Ores. Iron ore is present as irregular or platy grains less than 0·1 mm. in diameter, and as densely crowded minute crystals, occasionally in two generations (7916). The ilmenitic character of part at least of the iron ore is shown by the formation of leucoxene (7901, 7894). Pyrite has been introduced in several instances where shearing has occurred (7947, 7952).
Amphibole. Fibrous green or bluish green amphibole is present in the mesostasis of several rocks (7923, 7922, 7943, 7912), and in vesicles where the main filling material is epidote and chlorite (7907).
Pumpellyite. Pumpellyite is widely developed in the spilitic rocks and generally forms in aggregates of prismatic crystals less than 0·04 mm. in length. These aggregates have a distinctly granular appearance under low magnification. The mineral is readily recognised by its moderately high refractive index β = 1·70), low birefringence and the distinctive blue green colour for the β vibration direction (7921, 7895, 7946, 7951). As a result of the combination of low birefringence and strong dispersion, brownish purple or blue anomalous interference tints are sometimes developed between crossed nicols (7951, 7946). The pumpellyite aggregates occur in the mesos
tasis of many spilitic rocks, in vesicles (7951, 7929), veins (7895, 7930), and as inclusions in feldspar crystals (7918, 7949, 7934).
It is interesting to note here that pumpellyite has been observed in amygdaloidal lavas from Lake Superior (Palache and Vassar, 1925) and Haiti (Burbank, 1927), in spilitic dolerites from Nassau (Hutton. 1937, p. 530), in spilites and albite diabases from Eastern Borneo (Roever, 1947), and in albitophyres in the Urals (Zavaritsky, 1944). In New Zealand pumpellyite has been recorded in the reconstituted greywackes and low grade schists of Otago (Hutton, 1937b, 1940; Turner, 1939, pp. 37–39), and rarely in quartz-albite segregation veins in the schists (Hutton, op.cit.).
Epidote. Pale yellow epidote is widely developed in the spilitic rocks as plentiful small inclusions in the feldspar crystal and as aggregates of crystals in the mesostasis (7943. 7908, 7894). Veins of epidote have also been noted (7922) and in some cases the lime-rich solutions have crystallized along shear zones (7943, 7923). Vesicles filled with radially arranged epidote are present in several rocks (7903, 7931).
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- A. Spilite. 4 mls. N. West Dome. Mossburn. 7912 (this paper). Analyst: F. T. Seelye.
- B. Metabasalt, Crystal Falls, Michigan. Analyst: H. M. Stokes (U.S. Geol. Surv., Mon. XXXVI, p. 106, 1889, no. 2.)
- C. Spilite, Tayvallich Peninsula, Argyll. Analyst: E. G. Radley. (Dewey and Flett, 1911, p. 206, no. 1.)
- D. Spilite, Great King Island, New Zealand. Analyst: F. T. Seelye. (Bartrum, 1936, p. 417.)
- E. Spilite, Oregon. Analyst: J. G. Fairchild. (Gilluly, 1935, p. 235.)
- F. Average Spilite. (Sundius, 1930, p. 9.)
- G. Average basalt. (Daly, 1933, p. 17, no. 58.)
Zeolite and Carbonate. The centre of a large vesicle in one spilite (7948) is filled with a colourless mineral with the following properties: Refractive index about 1·48, double refraction 0·007, uniaxial and positive, non-fibrous; this mineral is believed to be chabazite. Carbonate has crystallized in one rock, a keratophyre (7903).
Chloritic Minerals. A very detailed investigation would be necessary to determine exactly the nature of the diverse number of chloritic minerals, but it is sufficient to record the main types as follows:-
(a) Interstitial cryptocrystalline or “celadonitic” green or yellow “chlorite” is invariably present in the mesostasis of the spilites and albite dolerites. In the writer's opinion, this material does not represent direct decomposition of ferro-magnesian minerals, but, following Fenner (1929, p. 245), it is believed that it results from the crystallization of residual magmatic ferruginous gels or solutions.
(b) Deep green fibrous chlorite with properties closely allied to penninite (Winchell, 1933. p. 281) is present in several rocks (7894, 7931).
(a) Spilites. The chemical analysis by F. T. Seelye of a Mossburn spilite (7912) is compared with other spilitic rocks in Table I, and a close similarity is revealed. The relative deficiency of the Mossburn spilite in potash is clearly shown in the ternary diagram Or – Ab – An (Fig. 8), and this supports Sundius' conclusion (1930, p. 9) that extreme deficiency in potash is particularly characteristic of splilitic rocks. The calculated normative plagioclase is Ab76An24, whereas universal stage methods have indicated that many of the feldspars have the composition Ab95An5. The excess CaO must be present in the pyroxene and secondary pumpellyite and amphibole, for the Al2O3 and CaO required for these minerals in the mode would be calculated as anorthite in the norm. The Mossburn spilite is also noteworthy for the low titanium content, which contrasts sharply with the high TiO2 value in the average spilite as computed by Sundius.
In the spilites, albite feldspar occurs with pyroxene only rarely altered to chlorite and amphibole. The question arises, therefore, whether the albite is a primary product of magmatic crystallization or whether it has originated from a more calcic plagioclase. The presence of plentiful inclusions of epidote and pumpellyite in most albite crystals, the occurrence of epidote and pumpellyite as filling in vesicles and veins, and the determination in some cases of plagioclase as basic as An 33–36 would indicate that the albite formed from a more calcic plagioclase. Furthermore, considerations of physical chemistry show that it is most unlikely that albite could crystallise from a magma either earlier than or simultaneously with pyroxene (Gilluly, 1935, p. 338).
Although it seems reasonable to consider that the albite is secondary and formed from an originally more calcic plagioclase, it is not clear how this was effected. Two processes can be suggested: (a) A process of saussuritization whereby the albite formed is residual after the CaO and Al2O3, equivalent to the anorthite molecule had either migrated or appeared as epidote and pumpellyite (Sundius, 1915; 1930, p. 3; Gilluly, 1935, p. 342). (b) Metasomatic replacement of the
Fig. 8–Molecular percentage of normative feldspar of Mossburn spilite (1) compared with that of the average spilite as computed by Sundius (2), and the average basalt (3), andesite (4), and Trachyte (5) as computed by Daly.
original anorthite–rich feldspar involving essentially introduction of soda and migration of lime. If solutions are responsible these could possibly represent authigenic hydrothermal solutions formed during the consolidation of the basic magma (Bailey and Grabham, 1909, p. 253; Dewey and Flett, 1911, p. 246; Eskola, 1925, p. 91), or else resurgent water as postulated by Daly (1914. pp. 339–40) and Gilluly (1935, p. 346–7).
The formation of epidote and pumpellyite concomitant with albite in many crystals appears to favour the first process. This hypothesis can be criticised, however, on the grounds that the albite produced is insufficient and that some introduction of soda is required. Meagre chemical data are available in the Mossburn district to test this criticism, but if the original basic magma is assumed to have the composition of Daly's average basalt (Daly, 1933, p. 17, No. 58), and if the one chemical analysis is representative of the Mossburn lavas, then comparison of the two would indicate that introduction of soda is required with complementary removal of lime (Table I). Obviously, this comparison requires to be tested by additional chemical investigations. On the other hand, if metasomatism by solutions rich in soda is the dominant process, it would be expected (i) that the albite is present either in clear crystals or in crystals where albite surrounds remnants of calcic plagioclase (cf. Gilluly, 1935, pp. 232, 342); (ii) that the pyroxene is generally altered to chlorite (cf. Dewey and
Flett, 1911, p. 203; Gilluly, 1935, pp. 228–233) and (iii) that vesicles and veins filled with albite are plentiful. In the Mossburn spilites, clear albite, altered augite, and veins and vesicles of albite are relatively rare, and this suggests that soda metasomatism by solutions is only of minor importance. Another alternative which may be applicable is that the metasomatism is effected largely by migrations in the solid or semi-solid state. Definite conclusions, however, are not warranted on the evidence available.
To summarize, the albitic feldspar in the Mossburn spilites is considered to be secondary and formed from an originally more calcic plagioclase, but there are insufficient data to indicate how this was effected. Probably some process of soda metasomatism is involved.
(b) Albite Dolerites. The arguments put forward to explain the origin of the spilites apply also to the albite dolerites where fresh augite is present in ophitic texture with albite laths in which inclusions of epidote and pumpellyite are abundant. The writer believes that here, too, the feldspar is secondary after original calcic plagioclase. and that probably a process of soda metasomatism is important.
(c) Keratophyres. The volume of keratophyres in the Mossburn area is small compared with that of the spilites. a contrast with the keratophyres, quartz-keratophyres and spilites described by J. Gilluly (1935, p. 347) from eastern Oregon. It is possible, therefore, that the keratophyres originated by the differentiation along trondjheimitic lines, of the basic magma from which the spilites and albite dolerites were formed. The formation of oligoclase and albite in the keratophyres thus does not produce as difficult a problem as the albite in the spilites and albite dolerites. In many instances, the oligoclase and albite are free of inclusions, but the presence of abundant solutions rich in lime is indicated by the widespread occurrence of epidote, both as vesicle filling and as large crystals distributed throughout the rocks and in some feldspar phenocrysts. In the writer's opinion the limerich solutions are dominantly external in origin (cf. Benson, 1915, p. 160), but further study is necessary to determine the extent to which the albite and oligoclase are primary crystallizations.
|Locality||Cobalt in parts per million|
|Serpentinite, south end of Black Ridge||120.3|
|Serpentinite, west side of West Dome||107.1|
|Serpentinite soil, west side of West Dome||283.9|
|" " " " " "||311.5|
|" " " " " "||248.2|
|" " " " " "||385.0|
|" " Black Ridge||205.0|
|" " " "||277.3|
|" " " "||299.8|
|" " ridge east of Coal Hill||265.8|
|" " " " of Windy Hill||194.9|