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Mr. F. D. Adams (1929, p. 240) talks of movement of the charnockite magnfas in the final stages of their consolidation, and I have shown how severely the rocks of the charnockite range in southern Jeypore have been crushed.
All these writers agree also that the chief movement took place towards the end of the charnockite period.
There is accordingly every reason to suppose that mountain building was active in Ceylon, Southern India, and in the Eastern Ghats in late charnockite times. In the course of this the thick mass of aluminous sediments, which now forms the khondalites, was probably folded into the partly liquid charnockite, and may there have received its high-grade metamorphic imprint. Some reaction may have taken place between the magma and the sediments, leading to the final development of hypersthene in the charnockite rather than the usual pyroxene.
The whole charnockite region has been exposed to denudation since Archæan times. It is not surprising, therefore, that the only remains of the original sedimentary series are a few synclines of khondalites.
As regard the age of the khondalites there is of course no definite evidence, but andalusite-schists are found in great abundance among the Archæan sediments of the region lying to the west of the Eastern Ghats, and I think it possible that they are the same series as the khondalites, but in a lower grade of metamorphism.
Sir L. L. Fermor (1935, p. 48) insists on the necessity of relative uplift of the charnockite region of the Eastern Ghats with respect to the Archæan rocks further to the west, because he considers its high grade metamorphic imprint could only have been acquired at a great depth, although now it is actually higher than the region further to the west. He considers that a fault along the western margin of the Eastern Ghats is the most probable way in which such relative elevation could have occurred.
I think it likely that faults and overthrusts would occur as the result of mountain-building forces, and that the khondalites may in some cases have been elevated by these, but neither I nor any of the other field workers in the khondalite region have taken Fermor's view of the origin of the high grade of metamorphism found there. The commonest view (see pp. 403 and 404) is that the metamorphism is thermal; the influence of stress is also emphasized.
The author visited southern Jeypore in 1934 and in 1935 to map a part of the western margin of the Eastern Ghats. The chief object was to ascertain whether the Eastern Ghats are separated from the region further west by a boundary fault. The mapping has shown that the boundary of the great charnockite mass of the Eastern Ghats is everywhere irregular, and that numerous intrusions belonging to the charnockite series occur in the country immediately to the west of the main range. The boundary is considered to be a normal intrusive one. There is no reason whatever to suppose that the margin of the charnockite is a great fault.
The various older rocks lying along the western border of the main charnockite boss are described shortly, and it is shown that their grade of metamorphism is low to medium, with increasing stress phenomena as the Eastern Ghats are approached. The newer dolerites of the same area are also described.
Next, the charnockites in the main range are discussed, and their metamorphism is shown to be similar to that of the older gneisses further to the west, but with even more marked stress phenomena.
An account, with analyses, is then given of the hypersthenegneisses and gneissic soda-granites which occur as intrusions in the older gneisses, and it is shown that they closely resemble Holland's charnockite, and are besides indistinguishable from some of the accepted charnockites of the main range. It is concluded that they are undoubted members of the charnockite series.
The hypersthene-gneisses are proved to be of igneous origin. They have been slightly crushed after their consolidation, and some of the minerals of which they are composed have been corroded and partially replaced by a subsequent invasion of alkaline solutions. Their grade of metamorphism is, however, low.
As hypersthene in metamorphosed igneous rocks is confined to rocks of the highest grade, it is concluded that the hypersthene in these rocks is not metamorphic but pyromorphic.
The interesting effects brought about by the passage of alkali solutions through all the rocks west of the main range are described, and it is shown that these solutions were of magmatic origin, and were given off by the main charnockite mass in the final stage of its consolidation.
The last chapter is devoted to the khondalites which lie further to the east, and were not mapped in detail. It is thought that
they are ancient aluminous sediments involved in mountain-building, and later invaded by huge igneous intrusions, the charnockites. It is probably to the heat from these intrusions that their admittedly high grade of metamorphism is to be ascribed. It is considered that they may be the high grade metamorphic representatives of the less altered aluminous sediments so abundant in the Archæans lying further west in Bastar State.
The Geology of Ceylon,
The Evolution of the Igneous Rocks.
SMITH, F. H., 1900.
STILLWELL, F. L., 1918
The Metamorphic Rocks
Petrological Study of Rocks from the Hill Tracts, Vizagapatam District, Madras. Rec. Geol. Surv. Ind., XXXVI, Pt. 1, pp. 1-18.
WASHINGTON, H. S., 1916. The Charnockite Series of Igneous Sci.,
Rocks. Am. Journ.
The Geology of
WALKER, T. L., 1900
WALKER, T. L. AND
VREDENBURG, E., 1918
Preliminary Report on the Geology of the Ganjam District. Gen. Rep. 1899-1900, Geol. Surv. Ind.. pp.
of Adelie Antarctic
XIV.-EXPLANATION OF PLATES.
PLATE 20, FIG. 1. Slightly crushed charnockite. Note the linear arrangement
of the various minerals.
FIG. 2. The centre shows one of the felspathic patches in Fig. I under crossed nicols. The granulation and discontinuity of the twinning planes is due to crushing and movement.
FIG. 3. --Charnockite similar to that shown in Fig. 1, but more intensely crushed. Most of the hypersthene is altered to a feathery aggregate of scaly hornblende.
FIG. 4. Completely crushed charnockite, consisting of narrow bands
of hornblende (dark), felspar, etc. (light) and garnet (G). A few crystals of hypersthen (H) always seem to have survived the crushing.
PLATE 21, FIG. 1.-Typical hypersthene-gneiss.
FIG. 2-Typical soda-granite.
FIG. 3.-Biotite-gneiss with porphyritic crystals of orthoclase (Or). FIG. 4 -Same slide as Fig. 3, with nicols crossed, showing myrmekite (M) corroding the orthoclase (Or).
PLATE 22, FIG. 1.-Myrmekite corroding microcline.
FIG. 2.- Dactylites of biotite and a myrmekitic intergrowth of biotite and albite.
FIG. 3.-Dolerite with margins of augite crystals uralitized and in some cases altered to garnet.
FIG. 4.-Pyroxenite with allanite.
PLATE 23. Variation diagram of the charnockite series.
PLATE 24.-Geological map of a part of Southern Jeypore, Orissa.