Early morning rains and an exceedingly warm hospitality ofMr. Y.S. Prakash Reddy of Pulivendla forced a late start of the field trip on the 12th. The asbestos mines at Brahmanapalle were visited first. Serpentinisation and contact metasomatism of the dolomitic limestones by the doleritic sill has yielded this extensive asbestos deposit, which is being mined by underground excavations. The TummalapaUe Uranium deposit, one of the rare carbonate-hosted stratabound Uranium mineralisation was visited next. The A.M.D. had arranged for an excellent overview of this deposit extensively explored by it.
Following a late lunch, the field-party returned to Tirupati. The common refrain of the participants on their way back from the Annual Convention was that there is a great deal more to be unravelled and to be integrated to build a convincing evolutionary history of the Cuddapah Basin in its totality.
Department of Geology VIVEK S. KALE
University ()f Porma. PUlle -411 007
"DIAMONDS EVERYWHERE"
] wish to invite the attention of our readers to an important paper which has appeared in the latest issue of Nature (v.378, No.6552, 2nd Nov. 1995, pp.14 and 41-44) which describes the occurrence of diamond in natural form from the 23 km diameter Ries impact ,crater in south German}:.,. The diamond in cubic form is stated iObe1iltergrown with silicon carbide, a type of OCCUlTence not recorded so far. The importance of this find lies in the fact that diamond has not formed in rocks derived from the Earth's mantle or even through shock but form the vapour phase from the vaporized carbon-bearing rocks at the surface of the Earth. The occurrence of diamonds in minute form in meteorites has long been known. They are now claimed to be of interstellar (presolar) origin "having probably formed by vapour condensation in stellar atmosphere". Diamonds are also reported in clays marking the KIf boundary, presenting evidences oflarge scale impact. These diamonds are not parts of the original meteorite but developed due to impact. The diamonds have crustal chemical and isotopic signatures. These new revelations emphasize the need to reexamine all suspected impact structures for the possible occurrence of diamond. Kimberlites and lamproites (mantle derived diatremes) obviously are not the only sources of diamonds. "There arc more forms of diamond, and ways of making them between heaven and Earth than were dreamt of a few years ago".
B.P.R
DISCUSSION
Comment
(Comment on the paper "Granitoid rocks of Wangtu Gneissic Complex, Himachal Pradesh : an example of in situ fractional crystallisation and volatile action" by D. Rameshwar Rao, Kewal K. Sharma and K. Gopalan; Jour. Geo!. Soc. India, v.46(1), pp.5-14).
This paper by Rao et al. is welcome for the overall petrogenesis of the Himalayan granitoids, more so for the widespread numerous Early Proterozoic Himalayan granitoids,
which were remobilised during the Cenozoic Himalayan orogenesis. However; the authors should have taken some more care regarding the available work along the Sutlej valley on these granitoids. Some of our comments are as follows :
1. The authors claim that Fig. 1. has been taken from the geological map published by Sharma (1977). A careful comparison of the published map in this paper with Fig.l of
1000
100
e
0.. .s-..0~
10
Syn ColO
VAG
10
WPO
ORO
100
Y + Nb (ppm)
o
CPG of Rao et al., 1995!::J. FOG of Rao et al., 1995
o Wangtu granitoid of Singh et al .. 1995
1000
Fig.I. Rb VS. Y+Nb discrimination plot (cf. Pearce et af. 1984) of the Wangtu Gneissic Complex rocks, data from Rao et at. 1995 and Singh et at. 1995.
Sharma (1977) and Fig.2 of Singh and Jain (1993) clearly reveal that the southeastern closure of the Rampur Window and the location of a thrust between Sarahan and Wangtu have been actually traced from the latter work without proper acknowledgement.
(1993), who is also one of the authors of this paper, have written that liThe porphyritic granite samples when plotted along with seven augen gneiss samples of the WGC collected by one of the authors (KKS) and analysed by BARC fabricated Mass-Spectrometer housed in Physics Department of Punjab University, Chandigarh, gave isochron age of 2025 ± 86 Ma with initial strontium ratio of 0.7074 (Kumar, 1986)". Interestingly, even the abstract, written by Sharmct et al. (1983) containing the same rock samples and isochron, has not been referred to by them in the paper. What was expected from this work, when a total of 12 samples were analysed, that the existing isotopic age data would be better constrained for the overall tectonics of the NW -Himalaya after considering the Rb-Sr whole-rock work by Kwatra et al. (1986). Re-assessrnentofboth the works is not possible due to lack of details on location of samples.
3. Though the quality of the geochemical data and values, obtained by Rao et al.
(1995) is not debatable, Rb, Y and Nb values on the Wangtu granitoid from this paper arc replotted on the Rb vs. Y +Nb plot of Pearce et al. (l984~ along with the data on these elements, obtained by us (Fig. 1 ). This plot clearly reveals that the data from Rao et al. (] 995) falls distinctly within WPG field and only a few points lie in VAG. None of the data from Rao et al. (1995) reveal syn-Col. granite at 1850 Ma. This is of vital importance for the overall petrogenetic model of these granitoids, whether these bodies really evolved through fractional crystallisation.
4. The release of the fluid would cast changes in the chemical composition of the melt in some of the volatile elements like K, Sr, etc., thereby, decreasing the concentration of the residual melt. However, as per the spidergram (Fig.3) of Rao et al. (1995), these elements are enriched indicating that no such release of volatiles had taken place.
5. Singh et al. (1993) interpreted the U-Pb zircon concordia diagram and inferred the lower intercept of 48 ± 28 Ma, probably due to lead loss during the Himalayan orogeny, which appears to have been claimed by Rao et al. (1995).
6. Granite crystallisation has been inferred at 2 to 5 kb water pressure and between 670 to 695°C (p.ll). But the generation of granitic melt is considered to have taken place at more than 35 km depth. This is rather unusual, even assuming a maximum load of 300-350 bar/lan, the melt would have been generated at a depth of 15 to 18 km for inferred water pressure.
7. This paper would have been more useful, if the authors have located their samples on the geological map, especially for the whole-rock Rb-Sr systematics. It is, therefore, requested that the location of these rock samples may be described and also given on a geological map through this dialogue to make the work on Himalayan granitoids more valuable.
Department (~f Earth Sciences Ulliversity (~f RIJorkee,
/?(lorkee -247 667
References
SANDEEP SINGII A.K. JAIN R.M. MANICKAVASAGAM
Kwatra, S.K.. Bhanot, V.B., Kakar, R.K. and Kansal, A.K. (1986). Rb-Srradiometric ages of the Wangtu Gneissic Complex, Kinnaur district, Higher Himachal Himalaya. Bull. Ind. GeoI. Assoc., v.19(2), pp.127-130. Pearce, J.A .• Hanis, N.H.W. and Tindle, A.G. (1984). Trace elemental discrimination diagram for the tectonic
interpretation of granitic rocks. Jour. Petro., v.25, pp.956-983.
Rao, D.R .. Sharma, K.K. and Gopalan, K. (1995). Granitoid rocks of Wangtu Gneissic Complex. Himachal Pradesh: an example of ill situ fractional crystallisation and volatile action. Jour. Geol. Soc. India, v.46( I),
pp.5-14.
Sharma, K.K. (1977). A contribution of the geology of the Sutlej valley, Kinnaur, Himachal Pradesh. India. III:
Coil. Intern. CNRS.: Himalaya Sci. de la Terra. CNRS, Paris, v.268. pp.369-378.
SHARMA, KK, RAO. D.R., GOPAlAN, K. and SIVARAMAN, T.V. (1993). Rb-Sr isochron age of early Proterozoic granites from Wangtu Gneissic Complex, Kinnaur Himalaya. VI National Symposium on Mass-spectrometry, lIP, Dehradun, India, Preprint Volume, pp.501-502.
SINGH. S. and JAIN, A.K. (1993). Deformational and strain patterns of the Jutogh Nappe along the Sutlej valley in Jeori-Wangtu region, Himachal Pradesh, India. Jour. Him. GeoI.. v.4(J), pp.4I-55.
SINGH. S., SJOBERG, H., ClAESSON, S .• GEE, D.G. and JAIN, A.K. (1993). New U-Pb Proterozoic data from Chor-Wangtu grani toids of Himalayan crystalline belt from Himachal, India. Sem. Himalayan Geol. and Geophys., WIHO, Dehradun, India, pp.55-56. .
SINGH, S., CLAESSON, S., JAIN. A.K, SJOBERG, H., GEE, D.O .• MANICKAVASAGAM, R.M. and ANDREASSON, p.O. (1995). Geochemistry and U-Pb Geochronology of the Proterozoic Peraluminous granitoids from the Metamorphic belts, NW-Himalaya, Himachal Pradesh, India. Submitted to Precambrian Research.
Reply
We gratefully appreciate Singh et al. for critically going through our paper and making some important comments. The following are some of our views on the points raised by them:
1. As regards the faulted contact between gneissic and schistose rocks south of Nachaur, we have our independent observations made many years ago, which are also confirmed in your map. We however, admit to have seen the geological map of Kumar (1992), particularly the southeastern closure of the Rampur Window, which is similar to the geological map given by Singh et al. (1993). The omission of Kumar (1992) reference in caption of Fig. 1 is regretted.
2. One of the suggestions of the reviewers of our paper was to cut-short the references. In the process some important references like Kumar (1986) and Sharma et ai. (1993) as pointed out by Singh et al. were deleted in the revised manuscript.
3. Though we referred only to Pearce et ai. (1984) diagram to suggest the syn-collision nature, we arrived at this conclusion from many other considerations like: the silica range of 63-76% with unimodal distribution, calc-alkaline to alkaline nature, meta-aluminous to permeta-aluminous nature, having NazO/CaO <0.4, NazO/K20 mostly <2 and molar AlzO/Na20+K20 >1.1 and plotting of samples CPG and FOG on tectonic discrimination
diagrams all match with orogenic granite classification scheme of Maniar and Piccoli (1989); many petrographic and geochemical features of CPG and FOG rocks also match with Group II classification defined by Hussein et al. (1982), wherein the granites were attributed to partial meling of the lower crust probably with some addition from the mantle by collision at plate boundaries; further, the low Nb of <40-50 ppm in
cpa
and FOG rocks does not support within plate magmas (cf. Pearce and Gale, 1977). However, we admit that there was slight discrepency in precise plotting of the fields in Rb vs Y +Nb diagram, because of this, the samples which were clustering above the demarcating line in our earlier diagram, were now seen to cluster at the intersection point of Syn-collision granites, VAG and WPG below the demarcating line. At this point we would also like to draw the attention to the comments of Pearce et ai. (1984) on their diagram. They suggested that, the VAG and Syn-collision granite that have accumulated ferromagnesian and minor phases may plot in the WPG field and that the fields suggested in these diagrams may not exactly match the Precambrian rocks.4. The spidergram (Fig.3) substantiates the evolved nature of FGG as compared to CPG and supports the cogenctic origin and fractionation of the magma suggested in Harker's variation diagrams (Figs. 2a and b) in our paper. Moreover, in Fig.3, the elements K and Sr show inverse relation; one is getting enriched while the other is getting depleted, and not both getting enriched as claimed by Singh et al.
The release of fluids is envisaged towards the end stage of magma fractionation which
JOUR .GEOL.SOC.JNDIA, VOL.46,DEC.1995
caused the crystallization of the FGG rocks at the lower level against the volatile rich coarse grained pegmatite phase in the higher level, as observed in the field.
5. The authors have no intentions to Claim the Pb-Pb work done by Singh et al. (1993). Our statement "The lower intercept ages of 48 ± 28 Ma reported for these rocks may however probably indicate lead loss during Himalayan Orogeny" is in reference to their work (Singh et ai. 1993) but contrary to their statement "The upper intercept 1866 ± 10 Ma (2) indicates the primary crystallization age but the lower intercept does not have any geological significance".
6. Under water saturated conditions the temperature of melting in the crust may be below 695°C, as represented by minimum temperature for granitic system on CIPW norms plot. It is however, not likely that the high level felsic intrusives ofWGC were formed under conditions at such low temperatures. Experimental studies of Carmichael (1967) indicated that the temperatures of eruptive granitic liquids to be of the order of 900°C. The mafic mineralogy of granites of WGC, mainly biotite, indicate the fusion of crustal material will take place within the range of temperature and pressure in which the melting of biotite is dominant, which is more than 15 kIn. What we considered in our paper is Rb-Sr log-log plot having relation to crustal thickness. The crustal thickness values suggested by us is substantiated by the low Sri ratio in these rocks. Further, Condie (1973) has demonstrated that crustal thickness in stable areas has remained essentially constant at about 35-38 km from Archaean (>2500 Ma) to the present. Hence, the "melt generation at a depth of 15 to 18 km for inferred water" as commented by Singh et al. represents emplacement depths rather than melt generation depths.
7. We welcome this suggestion of sample locations to be given on geological map. The samples were collected from a few hundred meters across outcrop on the right side of the Wangtu bridge, which is indicated in Fig.
1.
Our experience suggest that such sampling technique is preferred for more meaningful age data. It may be of interest to mention that samples of gneisses dated by Kumar (1~86) were collected between Karcham and Chaura over a strike length of few tens of kilometers.Wadia Institute of Himalayan Geology 33, Geneml Mahadeo Singh Road. Dehra Dun -248 001
National Geophysical Research Institute Hyderabad -500 007
References
D. RAMESHWAR RAO K.K. SHARMA K. GOP'&.LAN
CARMICHAEL, I.S.E. (1967). The iron/titanium oxides of salic volcanic rocks and their ferromagnesian silicates. Contrib. Mineral. Petrol., v.14, pp.36-64.
CONDIE, K.C. (1973). Archaean magmatism and crustal thickening. Geol. Soc. Amer. Bull., v.84, pp.2981-2992. HUSSEIN, A.A.A., Au, M.M. and RAMLY, M.F.EI. (1982). A proposed new classification of the granites of Egypt.
Jour. Vol. Geother, Res., v.14, pp.187-198.
KUMAR, A. (1992). Calibration of fission track dating system and its application:; in geothermochronometry of a part of NW Himalaya in Zanskar and Himachal Pradesh. Unpubl. Ph.D. Thesis, Kurukshetra University, Kurukshetra, l80p.
KUMAR, S. (1986). Rb-Sr geochronology studies of some granitic and gneissic rocks of Himachal and Kashmir Himalayas, India. Unpubl. Ph.D. Thesis, Dept. of Physics. Panjab University, Chandigarh, 200p. MANIAR, P.O. and PICCOLI, P.M. (1989). Tectonic discrimination of granitoids. Geol. Soc. Amer. Bull., v.101,
pp.635-643.
PEARCE, 1.A. and GALE, G .. (1977). Identification of ore-deposition environment from trace-element geochemistry. In: Volcanic Processes in Ore Genesis. Inst. Min. Metall. and Geol. Soc. Lond., pp.14-24.
PEARCE, I.A., HARRIS, N.B.W. and TINDLE, A.G. (1984). Trace elemental discrimination diagrams for the tectonic interpretation of granitic rocks. Jour. Petrol., v.25, pp.956-983.
SHARMA, KK., RAMESHWAR RAO, D., GOPALAN, K. and SIVARAMAN, T.V. (1993). Rb-Sr isochron age of early proterozoic granites from Wangtu Gneissic Complex, Kinnaur Himalaya. Sem. 6th National Symposium on Mass Spectrometry, held at lIP, Dehradun, Abstract, pp.501-502.
SINGH, S., SJOBERG, H., CLASSON, S., GEE, D. and JAIN, A.K. (1993). NewU-Pb Proterozoic datafromChor-Wailgtu granitoids of Himalayan crystalline belt from Himachal, India. Sem. Himalayan Geol. and Geophys., WIHG, . Dehradun, India, Abstract, pp.55-56.
BOOK lREVIEW
MAN·LAND RELATIONSIDPS DURING PALAEOLITHIC TIMES IN THE KALADGI BASIN, KARNATAKA, 1994. By Raghunath Pappu and Sushma G. Deo, Deccan College, Pune - 411 006, 92pp, Price Rs.
160/-The Precambrian intracratonic Kaladgi Basin, north Karnataka, has long been recognised a key area for palaeolithic research in peninsular India, for it preserves a continuous record of palaeolithic culture succession in a veriety of geologic and geomorphic contexts. In this work Pappu and Deo have attempted to reconstruct man-land relationships primarily on the basis of a morphometric analysis of landforms in the Kaladgi Basin. In addition to the study of topographic features they have also documented a series of Quaternary deposits with which the Palaeolithic material has a significant association. While the morphometric analysis has facilitated in establishing the relationship between terrain characteristics and Palaeolithic human activity loci, the descriptive study of Quaternary deposits has helped in reconstructing the palaeoenvironment as well as the Quaternary history of the basin. This volume presents a summary of prehistoric and Quaternary research in the area that were carried out during the last four decades both by the authors themselves and other workers.
The Malaprabha and Ghataprapha valleys constitute the major drainage network of the region. Furthermore, the physiography of the basin facilitates easy accumulation of water in the form of pools and ponds. While the rivers are a source of water only during the monsoon, the pools and ponds ensure a perennial source of water and consequently other natural resources attracting hominid occupation. The region otherwise abounds in the geological source rocks suitable for stone tool manufacture. The region even today presents a semi~arid savanna environment, though much degraded.
The authors observe that the major part of the basin lies below 400m MSL and that exteremely low to moderate relief are confined to the eastern part of the basin, About 73 % of the area is occupied by a low relief indicating the maturity of the landscape. The dissection index is low towards the lower reaches of the river valleys and these observations are also complemented by hypsometric and altimetric frequency analyses. With the increase in the absolute relief, there is a progressive decrease in the density of Palaeolithic sites and there is tendency to cluster the settlements ~ in areas of low and moderate dissection index. Centrography of the basin also reveals that the mean centre of human activity was located in the lower reaches of the river based on the density of sites, core, marginal and adverse zones within the Kaladgi Basin, perhaps provisionally.
