« PreviousContinue »
The earlier pre-Delhi granite and pegmatite intrusive into the Aravallis, are distinct and distinguishable from the later postDelhi granite and pegmatite.
Description of the Syenites.
The ten masses of syenite are on different horizons, and the largest has an irregular margin, owing to the low and variable dip of the rocks enclosing it, to which it is roughly parallel but to a certain extent transgressive. The four outcrops farthest to the north-east are isolated in, and separated from each other by, alluvium and the real form of the intrusions to which they belong is accordingly unknown.
In texture the syenite is very variable but may be divided into three mutually merging varieties:-a granitoid type in which the minerals are evenly distributed with no orientation in any particular direction, a foliated type in which the small patches of the different minerals are elongated in parallel planes, and a banded type in which layers of rock poor in dark ferro-magnesian minerals, an inch or two wide, alternate with basic bands of about the same thickness, consisting almost entirely of ferro-magnesians. In the first the clusters of amphibole are compact (glomero-porphyritic texture of Judd)1, but in the two other varieties are markedly crushed and broken up, as if by motion in a semi-consolidated magma (glomero-plasmatic texture of Loewinson-Lessing).2
Both foliation and banding follow the foliation of the enclosing Aravalli rocks, the largest mass is very strongly banded or foliated over most of its outcrop, and its foliation dips are at low angles, 20°-40° to north-west and west-north-west, and are notably regular, taken as a whole, though in detail often much corrugated.
In the field the grain of the rock is that of a somewhat coarse, non-porphyritic granite, but as explained below, the grain under the microscope, is much finer than is macroscopically apparent, for what appear to be mineral individuals are in reality aggregates of like minerals.
1 Quart. Journ. Geol. Soc., XLII, (1886), p. 71.
"Geologische Skizze der Besitzung Jushno-Saosersk und des Berges Deneshkin Kamen in nördl. Ural," (1900), p. 208.
In hand specimens, or more clearly on weathered surfaces, it appears to have three mineral constituents: (a) grey, with a greenish tinge and greasy lustre, which weathers more readily than the other two, thus giving rise to recesses and producing the conspicuous pitted surface characteristic of the syenite, (b) in about equal amount to (a), an aggregate of pure white granular crystals with pearly lustre, projecting in relief on weathering and sometimes showing simple twinning to the naked eye, (c) a greenish black mineral, in less amount than either (a) or (b), weathering more readily than (b) and less readily than (a), seen in hand specimens to be made up of interfelted elongated crystals showing a distinct cleavage. (a) The greyish constituent is under the microscope seen to be clusters of nepheline grains with a certain intermixture of felspars, (b) is essentially an aggregate of orthoclase and microcline (probably soda-microcline) with some albite, and (c) is mainly amphibole with sphene and apatite. Biotite and garnet are also present in some modifications.
There is thus visible in the field an almost complete differentiation of the magma into three portions, consisting essentially of felspathoids, felspars and ferro-magnesians. Doubtless had crystallization taken place under static conditions these patches would have produced large crystals, but owing to slight movement in the magma during consolidation, crystallization round certain centres has been interfered with, and a somewhat granulitic rock results of aggregates composed more or less of one mineral. Holland has drawn attention to the prevalence of this tendency (glomero-plasmatic texture of Loewinson-Lessing) in the charnockites and the Sivamalai syenites. Further movement under pressure produces the foliated varieties 2 wherein these aggregates of crystallization are drawn out into lenticles and sheets.
In the syenite, nepheline occurs in two generations, the older of rounded phenocrysts, of small size, and the younger of small allotriomorphic grains scattered through the felspathic ground
As well as phenocrysts of nepheline, there are in the granitoid variety less numerous phenocrysts of microcline which do not show
1 Mem., Geol. Surv. Ind., XXVIII, pp. 152, 241, (1900); Mem., Geol. Surv. Ind., XXX, p. 195, (1901).
Mem., Geol. Surv. Ind., XXVIII, p. 221, (1900); Bonney and McMahon, " Crystalline rocks of the Lizard district," Quart. Journ. Geol. Soc., XLVII, p. 478, (1891).
crystal outlines. The finer groundmass consists largely of felspar, which under the microscope appears as an equidimensional mosaic of clear grains. These are clear and unweathered, and are mostly untwinned and probably orthoclase, but microcline, albite and oligoclase also occur in subordinate amount. The plagioclases were identified by the method of Michel-Lévy--of extinction angles measured from the trace of the albite lamellation.
Perthitic intergrowths of microcline with orthoclase are frequent. Among the felspars cancrinite and calcite are present interstitially in small amounts and are certainly primary constituents. The Becke method of gelatinising with hydrofluoric acid and staining with malachite green was used in an attempt to identify quartz, which appears to be absent.
The texture of this groundmass is the same as the "mosaic texture illustrated by Holmes1 and Weinschenk and Johannsen.2
The ferro-magnesian minerals are abundant and occur in irregular clusters consisting of both biotite, with small included zircons, and the characteristic amphibole of the rock, with sphene and apatite in considerable amount and in some slides corroded garnets, brown (melanite) and pale pink (almandine). The amphibole is in large ragged plates and may be classified as a hornblende with an abnormally high extinction angle, 36° as a maximum, and pleochroic from deep blue-green to greyish-yellow.
The composition of the foliated variety does not differ materially from the above, except that the minerals have a distinct banded or lenticular arrangement and are, in some slides, but not always, seen to be crushed. The ferro-magnesian minerals make up a larger proportion of this than they do in the granitoid variety and are still more abundant, relatively, in the banded type.
In the latter the phenocrysts of nepheline are sometimes irregular and broken, and surrounded by fine detached particles. The felspathic groundmass may have "mortar" textures, or may be mylonised and granulitic in part.
1 Holmes," Petrographic Methods and Calculations," pl. 4, fig. 6.
2 Weinschenk and Johannsen, "Fundamental Principles of Petrology," pl. 5, fig.,
pl. 6, fig. 1.
3 Weinschenk and Johannsen, op. cit, pl. V, fig. 4.
Felspathoid-bearing rocks have hitherto been recorded from four other localities in India-Sivamalai, elæolite-syenite described by Sir Thomas Holland2; Vizagapatam, a miaskite described by T. L. Walkers; Mount Girnar, Kathiawar, a monchiquite described by J. W. Evans; and from Sarnu, Jodhpur, a tinguaite described by Sir Thomas Holland5.
All of these are too distant from Kishengarh for any magmatic relationship to be likely. The Kishengarh rock appears to correspond more or less with miaskite, grading towards shonkinite in the dark portions.
The Associated Pegmatites.
Traversing the various exposures are one or two veins of microsyenite of small size and differing from the normal rock only in texture but not in composition, and also a few large veins of coarse pegmatite of great interest from the minerals they carry. The commonest
1 Karpinsky, Guide, VII Cong. Geol. Inter., V, p. 22, (1897).
2 Mem., Geol. Surv. Ind., XXX, pp. 169—217, (1901).
3 Gen. Rep., Geol. Surv. Ind., 1902-3, p. 25; Rec., Geol. Surv. Ind., XXXVI, pp. 19-22, (1907).
are composed of the amphibole of the syenite but in large crystals and aggregates, with quartz, microcline and other felspars. These occur in force beyond the margin of the main syenite body, in the low scarp of quartzites to the south-east of it, and are in fact more numerous outside the syenite than within its limits. Another common variety carries large individuals of grey elæolite sometimes more than a foot in diameter with interstitial finely granular material consisting of sodalite and of an intergrowth of elæolite and sodalite, occasionally accompanied by cancrinite. The sodalite is intensely blue or colourless and is otherwise distinguishable from the elæolite by its greater transparency, saccharoidal granularity and by its brighter lustre as compared with the characteristic greasy aspect of the latter. The remarkable fading of the carmine tint of the freshly broken rock takes place in this type of pegmatite when the sodalite is colourless, but by no means all the colourless sodalite exhibits this property. The quartz-felspar-amphibole and elæolite-sodalite pegmatites occur most commonly and in the largest veins; the others are in small and scarce bodies. Perhaps the most striking is the bizarre rock consisting of pure white felspar veined and marbled with deep blue sodalite, bright yellow cancrinite and shining black biotite. The felspar is microcline perthitically intergrown with orthoclase and also the two species in granular aggregates. The sodalite and cancrinite are also granular, in veins. containing both, or of sodalite alone, in the former case the sodalite being colourless, in the latter blue. Near Mandaoria in particular as well as at other points within the syenite masses, are veins of a pegmatite in which cancrinite is the principal mineral, in granular masses and large crystals up to a foot in length, with sodalite and biotite. The cancrinite appears to have crystallised before the sodalite, which is in veins and interspaces between the cancrinite crystals.
Another form of pegmatite is characterised by large idiomorphic pale pink garnets in cancrinite and felspar (albite and oligoclase in part) with biotite, apatite, sphene and sometimes calcite. The felspars are somewhat cloudy, show cleavage clearly, and have irregular and interlocking margins-" sutured" texture1-with much interstitial fine material. In the coarse granitoid rock adjoining this pegmatite the felspars show granophyric structure. This pegma
1 Weinschenk and Johannsen, op. cit., pl. V, figs. 2 and 3.