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 investigation 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.-Editor.

Paper on 'ZIRCONS FROM THE GRANITIC ROCKS AROUND SAGTALA, PANCHMAHAL DISTRICT, GUJARAT' byD.Bhaskar Rao and G. Venkataraman, published in the Journal of the Geological Society of India, (Vol. 14, No.2, pp. 124-133, 1973).

Comments by B.L. Narayana (School of Studies in Geology, Vikram Univer-sity, Ujjain).

The authors have studied the zircons of the granitic rocks around Sagtala, based on which they have drawn conclusions on the periods of emplacement and the origin of the rocks. Geological investigations on the region by the present writer (1972) prompt him to offer a few comments while agreeing with the magmatic origin of the granites, and to provide some additional data relevent to the problems discussed in the paper. The detailed petrographic studies have led to the recognition of only two types of granitic rocks-granodiorites and adamellites-following the classification of Chayes (1957). Most of the granitic rocks of the area belong to the latter group, though there is textural variation from porphyritic to fine grained types with both pink and grey types in them. Granodiorites are associated locally with xenoliths like biotite-schists and amphibolites. From the modal analysis of quartz-monzonite given in Table I on p. 125 by the authors, it can be seen that the rock is not differentfcom adamellite. Chayes (1957, p. 67) reserved the term 'quartz-monzonite' for rocks containing less than 20% of quartz, which is the proposed minimum for granitic rocks. Hence it is appropriate to name the rocks concerned, carrying about 36% of quartz, as adamellites.

foliation and lineation in the same direction. However, near the Banded gneissic complex it swerves to E-W; it shows intrusive relations with the Banded gneissic complex and the Aravallis. The Aravallis in contact with the granitic rocks possess a thin dark zone indicating contact metamorphic effects.

The outcrop marked by the authors as granodiorites to the north of Bamroli is mostly adamellite with only local patches of granodiorite as mentioned earlier. The adamellites at places possess a high percentage of quartz as at Phangia and to the east of Bamroli near the contact with quartzites, indicating contamination with quartzites at least in the latter case.

Magmatic origin:The intrusive relationship ofthe granite bodies with the country rocks, the presence of thin contact metamorphic zone and the occurrence of xenoliths of country rocks suggest the magmatic origin of the granitic rocks. The presence of fine grained adamellites along joint planes in the form of dykes and the presence of xenoliths of the porphyritic adamellite in the former, reveal the liquid nature of the melt as well as the age relationships of the different kinds of granitic rocks, thus supporting the view expressed by the authors.

The view of magmatic origin is also supported by the evidences like zoning and complex twins in plagioclase, presence of exsolution perthites, systematic sequence of crystallization, acicular apatite, carlsbad twinning and feeble zoning in microcline and development of contact metamorphic minerals (such as garnet, sillimanite, cordierite, andalusite, cummingtonite, and diopside) in the aureole rocks, besides the field obser-vations and petrochemical study (Narayana, 1972). The findings of Narayana (1969a, (l969b) on Godhra area hold good to the granitic rocks in the northern part of the present area as the rocks of both the areas are mapped as continuous outcrops and of the same characters (Gupta and Mukerjee, 1938).

Mode of emplacement: The analytical data of six representative granitic rocks and the plottings on different diagrams revealed their calc-alkaline nature, and mag-matic origin. The gradual enrichment in alkalies from porphyritic granites to the fine grained pink granite found near Kaliakua and around Bamroli suggest gradual variation reflecting on their formation under different physico-chemical conditions. However, the granitic rocks exposed in the southern part of the area do not fit in the systematic variation of the above, indicating their intrusion as separate bodies. Ithas. been supposed that the southern gneissic granite represents meso-epizonal pluton while the northern one (porphyritic granite) is an epi-zonal body. The southern one being gneissic indicates its syntectonic nature while the northern one shows intrusive relations with the former (Rama Rao, 1931, pp. 87-88) and appears to be late tectonic with veins of fine grained granite, pegmatite and aplite of post-tectonic character.

REFERENCES

CHAYES, F., (1957) Aprovisional reclassification of Granites. Geol. Mag., Y.94, pp. 58-68. GUPTA, B. C. and MUKERJEIl, P. N., (1938) The Geology of Gujarat and Southern Rajputana.

Rec. Geo/. Surv, Ind.,Y.73 (2),pp,163·208.

NARAYANA, B. L., (l969a) On the occurrence of sillimanite-quartzite near Vejalpur, South of Godhra, Gujarat State. Jour. Ind. Geosci. Assoc.,Y. 10, pp. 63-66.

- - (1969b) The Precambrian formations around Godhra, Gujarat State. Jour. Inst, Geol.

Vikr, Univ.,Y.2, pp, 85-98.

- - (1972) The Geology of the Precambrian formations around Devgad Baria, Panchrnahals. District, Gujarat State. Ph.D. thesis submitted to Vikram University,

Authors' reply

The points raised by B. L. Narayana pertain mainly to the varieties, field distri-bution and petrochemistry of the granitic rocks and not to their zircon characteristics. The granitic rocks have been classified after Bateman (1961) and not after Chayes -(1957). The term quartz-monzonite is synonymous with adamellite (Joplin, 1964). After innumerable modal determinations, the authors have come to the conclusion that there are granodiorites, quartz monzonites and granites, and there is no room for controversy here.

B. L. Narayana's observation that the tract between Bamroli and Sagtala is -occupied by Banded gneissic complex of pre-A ravalli age is not acceptable to the authors. Here the granitic rocks contain abundant xenoliths of the Champaners which have been incorporated during the emplacement of the granites. Since the sketch map is intended to give an idea about the distribution of granitic rocks only, the xenoliths have been purposely left out.

Since the paper is on the zircon characteristics, petrographic and petrochemical characteristics and contact metamorphic phenomena have not been invoked to support a magmatic origin. Itmay be mentioned here that sufficient chemical differences are noticed between grey and pink granitic rocks to justify considering them as two different units. Further, field characteristics strongly suggest that the pink granites are intrusive into the grey quartz-monzonite. Amongst the grey granitic rocks the zircon characteristics of porphyritic granodiorite and nonporphyritic quartz-monzonite are entirely different, thereby indicating that they are two different units.

REFERENCES

BATEMAN, P. C., (1961) Granitic formations in the East-Central Sierra Nevada Near Bishop,

California, Geol. Soc. Am. Bull., v, 72, p. 1524.

JOPLIN, G. A., (1964) A petrography of Australian igneous rocks, Angus and Robertson Ltd., p.14.

Paper on 'THE STRUCTURAL STATE OF K-FELDSPARSAND REGIONAL METAMORPHISM IN THE PRECAMBRIAN OF S-E MYSORE' by G. V. Anantha Iyer, published in the Journal of the Geological Society of India, (Vol. 14, No.2, pp. 153-162, 1973).

Comments by P. K. Yadav; (Geological Survey oJ India, Mysore Circle)

By making use of the triclinicity of feldspars, Anantha lyer has made an attempt to determine the regional metamorphic picture of the south-eastern part of Mysore, This is a new approach which was first initiated by Heir (1957) by making use of the transformation temperatures as revealed by feldspars found in amphibolite and in the granulite facies. Heir (1957) has shown that transformation from amphibolite facies to granulite facies takes place at 500°C.

the transformation. According to Makenzie and Smith, the transformation takes. place at 300°C starting from 250°C. Tomisaka (1962) found by hydrothermal' experiment that the symmetry transition took place from 360 to 470°C. As suggested by Wright (op. cit) transformation depends on several factors, namely bulk composi-tion of feldspars and the time at which transformacomposi-tion takes place. Geologists. have observed that K-feldspars from migmatite, gneiss and augen gneiss show wide range of triclinicity. Smithson (1962) investigated distribution of obliquity within a single hand specimen of Precambrian granitoid rocks and came to the conclusion that it varies, and these various phases represent phases arrested at certain temperatures (or oscillating temperature intervals) during the vane of metamorphism. Similarly Goldsmith and Laves (1954) have observed diffuse reflections in addition tol::.with a definite value. According to them, these diffuse reflections represent variable6 values within a crystal. Marmo's (1971) statement that the occurrence of orthoclase in recent sediments at a very low temperature indicates that all the orthoclase may not be a product of transformation from microcline. Therefore, different temperatures. of transformations in different physico-chemical conditions and variation ofl::.in single sample suggest that Heir's idea should be applied with caution and it cannot be regarded as geothermal indicator.

Further, Iyer has shown disordered feldspars having [:. '17 near Kunigal in amphibolite facies, but Heir (op. cit) and Steiger and Hart (1967) have shown presence' of disordered feldspars in granulite facies. Iyer has not explained this unusual occurrence. This '17 value of [:. suggests that either the grade of metamorphism is increasing from Eto

w

or the sample is from the contact aureole around pegmatite. Similarly values like [:. '86, 6 '90 and [:. '43 in Kolar area suggest that the grade of metamorphism in this area is increasing from

w

to Ewhich is contrary to the field observations. Iyer has mentioned that it increases from N to s. Similarly, Kabbal Durga gneiss falls in the granulite facies of Pichamuthu (1965) and the presence of

[:.'90 in it as determined by Iyer cannot be explained from the Heir's (op. cit) theory as he has shown ordered microclines in amphibolite facies.

In the Closepet granite areal::.'65, [:. '98, [:. '87, indicate a wide variation in triclinicity. Wide variation of triclinicity has been explained by Heir (op. cit) for Langoy granites, as due to late hydrothermal solutions on the magmatic granite. Iyer has supported metamorphic theory for Closepet granite but does not mention of late hydrothermal solution's reaction.

The author has observed that (1) Closepet granite shows flow banding due to arrangement of micropinacoidal faces of microcline. This flow banding frequently changes. (2:)Primary foliation sweeps around the xenoliths of biotite gneiss, amphi-bolite and pyroxene granulite. The flow banding of the granite is oblique to the-foliation of xenoliths in Closepet granite. (3)Itshows sharp contacts with quartzite and pyroxene granulite in Sathanuru area, and (4) it also shows crosscutting relation-ship with migmatite in Sathanuru area. .

These field observations suggest that the granite is magmatic in origin; and this. is also supported by widely varying triclinicity values determined by Iyer, and as. has been observed by Heir in the Langoy granite.

-charnockite in formation as proposed by Pichamuthu (op. cit) if he had determined 6--of feldspars from Kabbal Durga charnockite.

ReFB~BNCES

BARTH. T. F.• (1959) The Feldspars.WileyInterscience,p.117.

HEIR, K. S.•(1957) Phase relation of K-feldspars in metamorphism. Jour. Geol., v,65, p.468.

MARMO, V ••(1971) Granite Petrology and the granite problem, Elsevier Publishing Company, p.170.

PICHAMUTHU, C.S., (1960) Charnockite in the making. Nature,v. 188,no.4745, pp.135-36. - - - (1965) Regional metamorphism and charnockitisation in Mysore State, India. The Ind.

Min.,v.6,no. 1&2,pp. 119-126.

'STEIGER, R. H. and HART,S. R., (1967) The microcline-orthoclase transition within a contact

aureole. Am.Min.,v.52,pp. 87-116.

WRIOHT. T. L., (1967) Transformation in the contact aureole of the Eldora stock, Colorado. Am. Min., v.52.

Author's reply

Yadav points out that transformation temperatures for microcline-orthoclase transition reported vary according to different investigators and so the transition temperature cannot be used as geothermal indicator. It is to be noted that rocks undergo solid state transformation during metamorphism. The PT conditions reported by Tomisaka (1962) for conversion of microcline to disordered feldspar under solid state only has relevance in the presect context. Another important point to bear in mind in the present discussion is the petrographic evidence presented to con-tradict the assumption of crystallographers and others, that microclines with cross-hatched twinning have orthoclase ancestry. So far there is no evidence (field or laboratory) to show that triclinization of othoclases can take place. On the other hand abundant" data is there to show that K-feldspars are formed by microclinization -of plagioclase. Ithas not been possible to synthesise microcline directly, but it can be produced from low albite by alkali exchange in solid state (Iyer et al 1970). Iyer et

at

have also demonstrated how the conversion of oligoclase (from HaJagur) to sanidine takes place under solid state.

Yadav may note that the disordered feldspar from Kunigal area reported is not from amphibolite facies, but is from pink geneiss of charnockitic affinity. He may also refer to the author's paper (1967) for the report of increase in the intensity of metamorphism from north to south in Kolar schist belt which is supported by the structural state of K-feldspars.

The structural state of alkali feldspar from Kabbal does not support the trans-formation of Peninsular gneiss into charnockite in that area. Whether field evidences exist or not for this transformation near Halagur, Yadav can verify.

The field observations which are yet to be published by Yadav are cited in support of the magmatic origin for Closepet granite. But Radhakrishna's field and petrographic studies (1956) and the recent work of Divakara Rao et af(1972) suggest that Closepet granites are of metasomatic origin. Itmay also be noted here that even magmatists regard micrecline granites of Precambrian age as of metasomatic origin,

Molakalmuru, Devarayanadurga, Sivaganga, Savanadurga and Closepet hills. They are all microclines with cross-hatched twinning and are highly ordered. Ifthe occur-rence of disordered K-feldspars in the rocks of charnockitic affinities along the western and southern margins is attributed to the high temperature effects of the intrusive Closepet granite, how then can we explain the formation of low temperature microclines in the granite itself?

The author's preliminary investigations on the structural state of K-feldspars support the metamorphic origin for the granites. The geochemical investigations carried out by the author when published wil\ give a clearer picture on the role of pressure gradients created during metamorphism on the structural state of minerals in the rocks,

The author anxiously looks forward to the publication of the investigations of Yadav in support of magmatic origin for Closepet granites.

REFERENCES

ANANTHA hER, G. V., NARAYANAN KUTTY, T. R. and VASUDEVA MURTHY, A. R., (1970) Current Science,v.39,p. 359.

- - and VASUDEVA MURTHY, A. R., (1967) Proceedings of the Symposium, U.M.P.,

Hyderabad, p.231.

DIVAKARA RAo, V., ASWATHANARAYANA, U. and QURESHY, M. N., (1956) Jour. GeoJ. Soc. India,v.13, p,I.

RADHAKRISHNA, B. P., (1956) Mys, Geol. Assn. Sp, Bull.,no.3.