Discussion

DISCUSSION

This section is intended to provide a forum for the discussion of papers published in our Journal by those working in similar fields of inves.tigation and research. Such a discussion is expected to be of value not only to the actual workers in the concerned field, but also to a wider circle of readers interested in the progress of geological

studies.-Bditor.

Paper on 'THE PLUTONIC CONCEPT' by S. Ray published in the Journal (Vol. 11, No.1, March 1970).

Gomments by M. Ziauddin (Geological Survey of India, Hyderabad)

In his paper' The Plutonic concept' Ray has suggested that 'the metamorphic facies series represented in the Khondalite belt should be treated as a 'type' by itself and be described as the khondalite type of metamorphism.

Hietanen (1967) identified eight different types of metamorphism taking into consideration the entire range of the two physical variables of metamorphism, namely temperature and pressure, besides the bulk chemical composition of the rocks and chemical potential of the mobile components. Each of the types, as depicted in Fig. I (op. cit.) in the form of a curve, crosses both the P-T gradients. As against this, the curve for the proposed type of the author would run parallel to the 700°C isograd and crosses only a few of the pressure levels because the khondalite belt encompasses a narrow range of temperature around 700°C and pressures of 3-7 kb. These P-T conditions would evolve only two major facies; but for purposes of recognising a type, as the very basis chosen by Ray is in disconformity with that adopted by Heitanen, I am unable to reconcile to the suggestion of raising the status of the khondalite belt faCies series to that of a 'type'.

The conclusion to which the author of the paper has reached is possibly due to taking the khondalite belt out of context or isolating it from the larger domain of regional metamorphism. Fringing to the north-west of the khondalite belt lies a region of rocks representing the amphibolite facies passing further in the same direc-tion into that of epidote-amphibolite facies and green schist facies. Adjoining the khondaIite belt, Ray recognises a belt of biotite gneiss but does not extend his studies to fix it up in the P-T fields or determine its relationship with the khondalite.

This zone of biotite gneiss reveals mineral assemblages of amphibolite facies and marks the next lower grade_ metamorphic zone after the garnet zone (amphibolite facies) and sillimanite zone of khondalite,(granulite facies) in the scheme of progres-sive metamorphism. If this adjoining area had been taken into consideration by him he would have found that the khondalite belt, by and large, constitutes a part of the Barrovian type and does not call for any special status to be conferred on it. To substantiate the above contention a few examples are given below.

DISCUSSION 405

KondapaUe. Further towards Jaggayyapeta, that is, near Kanchikcherla, the biotite-almandine sub-facies is present. The amphibolite facies is weakly represented here. Around Nandigama and beyond, the green schist facies makes its appearance up to Jaggayyapeta.

I

30

MAP SHOWING THE METAMORPHIC FACIES IN A PART OF KRISHNA. DISTRICT. ANDHRA PRADESH.

Km.IO 0 10 20Km

I I I I I I I I

~~~~~~/GONOWANA ?

E XPL ANATION

IIIIIB.

. . ,Ie it. kaOlin roc •. r;-v--vl Acid to intum.dial,. bosk and

L:.X...:....,,~ 1Iltrobasic c:hornoclltif. with the

2 o, .. oelattd c:hromltt ortbodit'.

~ Garnllsillimonil'-lbiotll.l"onulit.

~ (l,p'ynU.J with banda 0' b •• le

I e".rnooilif. and p,rox,n, gtanulit ...

I.

Wl/JlJ7Jj) Mica cftlorlll phyllit ••

~ ."Ieit. schi,'. quarfl

"'u.coylt • • efti •• I ItC .

Talc actinolit. chlorlt. IChltt ..

Ipldot. ohlorit, I'CltJly:olcii. ~ c"lorit. Ic .. IIJ,Go,;'a. ell

~ 9,onl.ie gn.I •• , blotlt'

5 and hor"S"l.ncf. Ichl,t

~ Gorn.t olftphioljl.

DISCUSSION

A similar situation seems to exist in the Bastar-Jeypore area (Crookshank, 1963) .. Although Crookshank has not delineated the progressive zones of metamorphism it will be seen from the lithology of the area that such a zoning does exist. The' Kondakamberu area which forms a part of the khondalite belt is occupied by the khondalite series and charnockite series. Further north-west, rocks of the amphibo-lite facies and epidote-amphibolit6 facies make their appearance with almandine-andalusite and almandine-cordierite sub-facies.

A few more similar instances can be quoted in support of my contention that the khondalitt;: belt represents only a part of the Barrovian type; but for the' present suffice is to say that it cannot constitute a separate type by itself in addition to those which have been recognised by Hietanen.

REFERENCES

CROOKSHANK, H., (1963) The geology of the southern PlU;t of Bastar and Jeypore area. Mem .. Geol. Surv. Ind., v. 87.

HIETANEN, A., (1967) On the Facies series on varioust ypes of Metamorphism. Jour. Geol .•.

v. 75, pp. 187-214.

RoY, B. C., (1960) Kyanite deposits in Chundi Estate, Nellore district. Rec. Geol. Surv.Ind .• _ v. 86, Pt. 4.

ZIAUDDIN, M., (1963) The Archaean Complex of Krishna District, Andhra Pradesh. Jour.

Deccan Geol. & Min. Soc., v. 3.

ZIAUDDIN, M. and KANNAN, C., (1968) The copper occurrences near Garimenapenta, Nellore-district, Andhra Pradesh. Indian Minerals., v. 22, No.1 (in press).

Authors' reply

I am thankful to Ziauddin for having taken an interest in my paper.

Para I-A field of variable temperatures and pressures must in mathematicaf logic include the limiting cases, too. In fact Hietanen's type I (contact type) is the case to the point, variable virtually in temperature only. The khondalite illustrates another extreme, variable in pressure only. It seems that such metamorphism is now recognised as belonging to 'facies groups' as distinguished from the 'facies. series' (this work published in Denmark by Valence is not yet available to me).

Paras 2, 3, 4-A study in metamorphism must take the two variables, pressure and temperature, into consideration. Failure to take a full appraisal of the third variable viz. time, puts the whole study into a muddle only.

I acknowledge that I am not conversant with any critical petrological study of the rock areas quoted by Ziauddin, where lower grade metamorphism has been recorded, to bring out their temporal relation with khondalite metamorphism. 'Recrystallisation to leptynite' is undoubtedly a later event and might have taken place during any of the 10 different ages recorded for the Eastern Ghats from 2900 m. yrs to about 500 m. yrs. Superposition of metamorphism in one or another of such periods may explain the time relation of the said rocks with the khondalite metamorphism. We must await this clarification. The granulite facies impress in the khondalites seems to be discontinuously distinguished from the metamorphism of other rocks related with it in space.

DISCUSSION 407

()f Hietanen! This fault has not been established, and Pascoe (Geology of India, footnote, p. 60, v. 1) suggested the existence of a low angle thrust. I have an alter-native suggestion of the rise of a crustal warp as found elsewhere in the world.

Ziauddin's comments are welcome. Perhaps much of the confusIon would be dispersed jf instead of 'the khondalite belt', we read 't.he khondalite meta-morphism'. The khondalite problem needs proper scrutiny.

The details of the petrology of the rocks and the mapping out of the meta-morphic rocks of the Eastern Ghats occurring around and within the area of the khondalites, by critical study is long due, age by age. I shaH be thankful to Ziauddin if he can kindly direct me to literature on these points; nevertheless he has my thanks for bringing the khondalites to the anvil.

Paper on 'A SEDIMENTARY PETROGRAPHIC STUDY OF THE ALWAR ROCKS

NEAR DEBARI, UDAIPUR, RAJASTHAN' by R. T. Shukla and B. L.

Sharma, published in the Journal (Vol. 11, No.2, June 1970).

Comments by D. N. Kanungo (National Geophysical Research Institute, Hyderabad, and N. K. Mahalik, Regional College of Engineering, Rourke/a).

Shukla and Sharma's paper on the sedimentary petrographic analysis of the :ancient and deformed rocks of Precambrian age belonging to the Alwars is of great interest to us. Doubts arise whether such ancient deformed and metamorphosed rocks could be utilised for such an analysis to arrive at the original sedimentary -characters. We have the foHowing comments to offer in this regard.

1. The pebbles and grains have been treated for size, shape and roundness analysis. From their paper it is observed that both pebbles and grains have been <leformed. The elongated shapes of pebbles are due to stretching. Transverse mineralised fractures appear perpendicular to the elongation of the pebbles. Trans-verse sections of some elongated pebbles of quartzite show preferred orientation of the crystallographic axis of the individual quartz grains. Other pebbles (rounded, -subrounded) must have also been deformed. So the pebbles have definitely lost at least a large part of their original size, shape and roundness.

2. Speaking of the analysis of grains of the matrix, it is doubtful if the grains have retained their original textural character. The quartz grains show invariably undulose extinction and some show secondary overgrowth. They must have also undergone some amount of recrystallisation during metamorphism.

3. Hence both the pebbles and the grains in the matrix must have lost their miginal sedimentary textural characters. It would have been more useful if the authors had stated at the outset the tectonic and metamorphic history of the rocks of the terrain and given an idea of the deformation these rocks have undergone.

4. For the analysis of the grains the rock samples have been disintegrated. It

DISCUSSION

Authors' reply

We appreciate the comments by Kanungo and Mahalik on our paper and are thankful to them for certain suggestions.

It may be mentioned that our observations are based on a comparatively very small area, and hence the inferences about the metamorphic and tectonic history of the rock types could only be outlined in this paper. We are extending our observa-tions to adjoining areas and we intend to contribute another note in this connection in due course.

The conglomerate in this area occurs in three major bands each containing heterogeneous assemblage of numerous pebbles of quartzite, granite, vein quartz, tourmaline (schorl) etc. varying in size and shape. Some of the pebbles of quartzite no doubt show effects of subsequent tectonism, but such is not the case with other quartzite pebbles nor with any pebbles of other rocks. Further it has been observed that out of the three conglomerate bands, in one elongated quartzite pebbles are dominant, while in the second such pebbles are less abundant, and in the third they are totally absent. This clearly indicates that the tectonic force had a very limited area of influence. Its exact nature cannot be inferred at this stage. Similar obser-vations are noted in case of quartz grains in various bands of conglomerate and intervening arkose bands. Secondary growth as observed in some of the grains is not in optical continuity of the parent grain. Further, primary sedimentary struc-tures are preserved in the arkose and grit bands associated with conglomerates.