100 SHORTER COMMUNICATIONS

REFERENCES FUCHS, G. and GUPTA, V. J., (1971) Palaeozoic stratigraphy of Kashmir, Kishtwar and Chamba (Panjab Himalayas). Ceol. Verh, B.-A., Wien, Heft I, pp. 68-97. GUPTA, V. J., (1971) Note on the age of Kalhel Limestone, Chamba District H. P. Geol. Verh. B.-A., Wien, Heft 3, PP. 646-647. GUPTA, V. J., MAHAJAN, G., KUMAR, S., CHADHA, D. K., BISARIA, P. c., VIRDI, N. S., KOCHHAR, N. and KASHYAP. S. R., (1970) Stratigraphy along the Manali-Leh road. Publ. Cent. Adv. Stud. Geol., Chandigarh no. 7, pp. 77-84. LYDEKKIlR, R., (1883) Geology of Kashmir, Chamba and Khagan. Mem. Geol. Surv, Ind., Calcutta, v, 22, PP. 1-344. SWIlET, W. C., (1970a) Permian and Triassic conodonts from a section at Guryul Ravine. Vihi District, Kashmir. Palaeontological Contributions, University of Kansas, Kansas, Paper no. 49, pp. 1-10. -- (l970b) Uppermost Permian and Lower Triassic conodonts of the Salt Range and. Trans­ Indus Range, West Pakistan: in Stratigraphic Boundary Problems: Permian and Triassic of West Pakistan, Department ofGeology, Uuiversity ofKansas, Kansas, Publication no, 4, pp. 20;'-277.

QUARTZ-FIBROLITE NODULES FROM BASTAR AND BALAGHAT DISTRICTS.

M. RAMAKRISHNAN Jayanagar, Bangalore Quartz-fibrolite nodules, commonly but inappropriately ca11ed 'tabloids' have been reported to occur in many metamorphic belts of , but very little is known about their tectanogenesis. In the light of recent, research on similar nodular rocks from several parts of the world, the geological significance of such nodules from Jhiram area,Bastar district and Baihar area, , is outlined. The Bengpal Group of Jhiram valley consists ofnodular biotite-muscovite-garnet (± andalusite:) schists, amphibolites and garnet amphibolites. The rocks constitute a homocline forming part of a regional synclinorium. The metamorphism of the rocks is of'andalusite-sillimanite type (Miyashiro, 1961), superposed by thermal metamorphism around granite plutons. The granites with associated pegmatites are of syn- and post-kinematic types with nodule zones around them. The Mansar Formation (?)of. Sausar group in the Baihar valley consists of nodular biotite­ muscovite-garnet (± kyanite, staurolite) schists with associated calc-silicate rock lenses. The rocks constitute a homocline which is a part of the regional refolded complex. The metamorphism is of intermediate pressure Barrovian type. The area is bounded to the east by the Bharweli-Ukua-Bhimlat thrust and to the west by synkinematic granite. Several small bodies of mica pegmatite traverse the schists. The nodule zones are neither spatially nor genetically related to both the thrust and the pluton. A comparison of the geological settings of both the areas suggests that the only common factor favourable for nodule formation is the host rock of appro­ priate composition and the p~T regime which just precedes the significant onset of migmatisation. The general features of the nodules are similar in both the areas. The nodules show up prominently due to differential weathering. Their size averages I to 2 em" SHORTER COMMUNICATIONS 101 although a wide variation in the maxima and minima are noticed. The shapes are diverse ranging from ovoidal to discoidal. The colour is either white or pale shades of green, grey or pink, contributed perhaps by the micas or iron oxide. The nodules are predominantly composed of variable proportions of quartz mosaic impregnated with fibrolite threads, separate sheaves of fibrolite, poikiloblasts of muscovite and biotite, with accessories like zircon, apatite, opaques and tourmaline. Rarely garnet and andalusite are also found as in Jhiram area. The fibrolite is restricted to the nodules only and is seldom seen in the matrix rock. In several instances, particularly in Jhiram area, the nodules are altered to damourite, either due to hydrothermal alteration or retrograde metamorphism. The silica (51 to 77%) and alumina (14 to 33%) contents are variable depending upon the proportion of mineral constituents and degree of damouritisation. The nodule zones are generally irregular and the distribution of nodules within the zones is erratic. Nevertheless, at a given place in the matrix, the nodules are of uniform size, shape and composition, suggesting the importance of the role of P-T and chemical composition of host rock in the formation ofnodules. While commonly the nodules conform to schistosity. discordant relationships are also observed in some instances. This has led some workers to postulate post-deformational crystallisation as a key factor in the nodule genesis (Losert, 1968; Breaks & Shaw, 1973). The present studies indicate, however, that the nodules are formed by syntectonic blastesis, and that the crystallisation outlasted the deformation causing discordant growth. The genesis of nodules has been a matter of great controversy. Several variants of metamorphism, metasomatism, magmatic processes, deformation and boudinage had been proposed (Losert, 1968) to account for their formation. Earlier theories in India identified the nodules with autoclastic conglomerates and emphasized the role of shearing stress in their formation. Subsequent theories invoked metamorphic differentiation, but did not elaborate on the mechanism. In the present study, the geological factors enumerated earlier coupled with recent experimental data have been taken into consideration in evolving a model for nodule genesis. In a pelitic rock of appropriate bulk composition, the critical reaction governing the nodule formation is: muscovite-l-quartz. =K-felspar+sillimanite+water, which takes place around 620 o ± 10°C at 2 Kb with PH,O = P total. (Althaus, et. al., 1970). Sillimanite will be stablished relative to K-feldspar under these conditions, provided there is a decrease in K +activity (de-alkalinisation or potassium leaching) or by a small reduction in FH,O (Eugster, 1970). Another essential requirement for fibrolite formation is the presence of aqueous fluid phase, which is emphasized by several investigators (Pitcher, 1965; Breaks & Shaw, 1973). The fluids may be of magmatic or metasomatic origin. A combination of these conditions creates a certain instabi­ lity in crystallisation, which in turn promotes metamorphic differentiation by concre­ tion principle. This enables the co-segregation of quartz and flbrolite into nodules. This model, involving a special combination of various processes, explains the restricted occurrence of nodules, although high grade metapelites are ubiquitous in geological record.

REFERENCES ALTHAUS, E., KARTOKE, E., NITSCH, K. H. and WINKLER, H. O. F., (1970) An experimental re-examination of the iJpper stability limit of muscovite +quartz. Neus. Jahrb. Mineralogie Monatsch., v. 7, pp. 325-336. 102 SHORTER COMMUNICATIONS BREAKS, F. W. and SHAW, D. M., (1973) The Silent Lake pluton, Ontario: A nodular sedi­ mentary, intrusive complex. Lithos, v. 6, pp. 103-122. EUGSTER, H. P" (1970) Thermal and ionic equilibria among muscovite, K-felspar, and alumino­ silicate assemblages, Fortschr. Mineralogie, v, 47, pp. 106-123. LoSERT, J., (1968) On the genesis of nodular sillimanitic rocks. Rept, 23rd Session. Internat, Geol. Congr. Czechoslovakia, Sec. 4, pp. 109·122. MIYASHIRO, A., (1961) Evolution of metamorphic belts. Jour, Petrol., v, 2, pp. 229-311. PITCHER, W. S, (1965) The aluminium silicate polymorphs in controls of metamorphism. Ed. W. S. Pitcher and G. S. Flinn., Oliver and Boyd.