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Densities and Magnetic Susceptibilities of Precambrian Rocks of Different Metamorphic Grade (Southern Indian Shield)

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Densities and Magnetic Susceptibilities of Precambrian Rocks of Different Metamorphic Grade (Southern Indian Shield) |00000107|| I Geophys (1981) 49: 101-107 Journal of Geophysics Densities and Magnetic Susceptibilities of Precambrian Rocks of Different Metamorphic Grade (Southern Indian Shield) C. Subrahmanyam 1 and R.K. Verma 2 1 National Geophysical Research Institute. Uppal Road, Hyderabad-500007, India Indian School of Mines, Dhanbad, India Abstract. Densities of 1294 Precambrian rocks from the Southern Precambrian rocks in the Southern Indian Shield occur in Indian Shield, confined to low-grade granite-greenstone (Karna­ both the low- and high-grade metamorphic environments, confined taka) and high-grade granulite (Eastern Ghats Belt) terrains are to the provinces of Karnataka, Bastar and the surrounding Eastern presented and categorized into 16 metamorphic and igneous varie­ Ghats Belt (Fig. 1). While Karnataka and Bastar constitute the ties. In Karnataka, densities of granites (2.65 g/cm 3 ), gneisses low-grade terrains of the shield, the Eastern Ghats Belt belongs (2.68 g/cm 3) and granodiorites (2.73 g/cm 3) are considerably less to granulite facies metamorphism. The present study is a compila­ than those of greenstone belts (2.84 g/cm 3) and dolerites (3.03 tion and presentation of the density and susceptibility data for g/cm 3). An average density of 2.75 g/cm 3 is obtained for surface various rocks from these terrains. The significance and implication rocks from this terrain. Over the Eastern Ghats belt. densities of the density data for a proper assessment of gravity fields asso­ of charnockites range from 2.71 g/cm 3 to 3.12 g/cm 3 as they ciated with the metamorphic terrains in shield areas will also Yary from acidic to ultrabasic composition. An average density be discussed. of 2.85 g/cm 3 is obtained for surface rocks from this belt. The The metamorphic rocks involved in the present study range contrast in the average densities of surface rocks from the low­ from greenschist to granulite facies. Similarly, the igneous intrusive and high-grade terrains may play a significant role in a proper rocks range from acidic to ultrabasic. Major rock types include assessment of the regional gravity fields over these terrains. The metasedimentary-metavolcanic sequences (Archaean schists), bio­ results compare well with the data from similar terrains in other tite and hornblende gneisses, granites, granodiorites, amphibolites, shield regions. Magnetic susceptibilities for 482 of these samples dolerites, migmatites of uncertain origin, anorthosites, granulite are also presented. All the rock types include a wide range of facies gneisses, acid to ultrabasic charnockites, khondalites and susceptibilities, as is to be anticipated in metamorphic terrains. other minor varieties. While a reasonable amount of data is avail­ A plot of density versus magnetic susceptibility for thirteen of able for schists, gneisses, and granites from Karnataka (Qureshy these rock types suggests a linear relationship. et al. 1967), data for rocks from the high-grade Eastern Ghats Belt are very meagre. Key words: Densities - Magnetic susceptibilities - Precambrian During the course of gravity and magnetic surveys by one metamorphic terrains of the authors over these metamorphic terrains (Subrahmanyam 1978), about 655 fresh rock samples were collected. Their densities (and susceptibilities for 482 samples) were measured in the labora­ tory. These data are supplemented with density data already pub­ Introduction lished for another 639 samples (Balakrishna and Ramana 1968; The two-fold classification of shield areas into low-grade green­ Balakrishna et al. 1968, 1971 , Qureshy et al. 1967) and the results stone-granite terrains and high-grade granulite terrains is by now are presented here. widely known (Windley and Bridgwater 1971). Regional gravity investigations indicate that these diverse metamorphic terrains are Sampling and Distribution usually characterized by gravity fields which are mutually distin­ guishable (Gibb and Thomas 1976; Subrahmanyam 1978). While In general. rock samples were collected from outcrops situated the long-wavelength components of these gravity fields are often near gravity and magnetic stations. Samples for which data were attributed to deep-seated crustal and upper mantle inhomogenei­ published earlier (639 samples) also cover all the important litho­ ties, the local anomalies are generally accounted for by near surface logic formations in the Southern Indian Shield. Special attention lithologies and their surface distribution. Thus, the interpretation has been paid to charnockites and other granulite facies rocks of local gravity anomalies in crystalline areas requires adequate from the high-grade Eastern Ghats Belt as little data have been knowledge of the densities of the various exposed metamorphic available from this province uptill now. and igneous rocks over the terrains. This point has been stressed hy Marshall and Narain (1954), Woollard (1962), Gibb (1968), Density Measurements and Smithson (1971 ), among others. Density data for various rllck types from the crystalline terrain are important, not only Laboratory density measurements were based on the formula f1 Jr interpreting the gravity anomalies, but also for estimating the mean density and composition of the crust in different parts of the continents. 0340-062X/8l/0049/0101/$01.40 0 86° ss 220740 76° s2° 22° KEY MAP \~) 20° ~ , 20 / J0[ L H't r "- '-:::.r;:.-,,...;'> 8 A Y \ -,, O F 8 ENG~ L 0 1s GEOLOGICAL MAP OF 0 SOUTH ER N INDIAN SHIELD 14 L EGEN D 0 0 12 ~ SEDIMENTS · Tef t1ory-Recent 12 ~ DECCAN T RAPS Mu o zole-Tefllory E3 PRECAMBRIAN SEOIMENTS ~ TRAPS · Pre -Comb r101\ fil\iJ 0 ~~~~~~~l~EN~T~~~ig;V~~~ANICS 0 10 ~ GRANITES - Arc hoeon 10 c:::::J UNCL ASSIFIED GRANI TES 6 GNEISSES ~ CH ARN OCKITES [!] EASTERN GHAT BELT [U KARNATAKA IOHARWAR ) PROVI NCE 80 Fig. I. Geological map of the Southern Indian srf 8 0 Shield where p is the density, W1 is the weight of the sample in air, Table I. Densit ies of igneous and metamorphic rocks from South India W2 is we ig ht of the sample in water. All the samples measured were either igneo us or metamo rphic rocks having little o r no SI. Rock Type No. Mean s.o. Range 3 3 3 pore space so the density values obtained are very close to the No. Samples (g/cm ) (g/cm ) (g/cm ) actual g rain density of the rock. The accuracy of the results is 3 ex pected to be within ±0.02 g/cm or about I% . I. G neisses and 544 2.67 2.54- 3.24 granites (undivided) Low-grade terra in Results I. Granites 37 1 2.65 0.02 2.54--2.83 2. Biotite gneisses 135 2. 68 0.05 2.55- 2. 80 The 1294 rock samples have been categorized into sixteen majo r 3. Granodiorites II 2.73 0.03 2.70- 2.78 metamo rphic and igneous varieties; these varieties <ind the density 4. Archean schists 82 2.84 0.20 2.48- 3.35 values are summarised in Table I . Fo r rock types with sample 5 Amphibolites 54 3.01 0.10 2.8 1- 3.35 numbers exceeding twenty, histograms were also prepared and 6. Dolcrites 82 303 0.07 2.74- 3. 19 are shown in Figs. 2-4. Descriptio ns of the rock types and the High-grade terrain densities obtained are discussed below, for the low- and high-grade metamorphic terrains sepa rately. 7. Syenitcs 4 2. 66 0.04 2.59-2. 70 8. Anorthosites 64 2.85 0.13 2.58 3.28 9. Gneisses 38 2.89 0. 11 2. 70- 3.24 10. Migmatites 21 2.89 0 .1 0 2.70- 3.14 Rocks .fi-0 111 Loir-Grade Terrains 11. Khondalites 25 2.89 0.18 2.59- 3.33 12. Horn blendites 5 3.21 0.15 3.05- 3.47 13. Acid charnockitcs 140 2.71 0.07 2.57-2.89 Gra11i1es. The da ta for granites are mostly fro m the publi shed work 14. Intermed iate 11 4 2.78 0.08 2.66- 3.15 of Qureshy ct al. (1967) a nd Bala krishna ct al. ( 1971). In Ka rna­ charnockites taka . these granites intrude bo th the gneisses and schists. A concen­ 15. Ba sic charnockitcs 124 2.99 0. 15 2.67- 3.41 tration at 2.60- 2. 70 g/cm 3 is clearly seen in the histogram (Fig. 2) 16. Ultra basic 24 3. 12 0.13 2. 84- 3.36 3 charnock ites with the average value being 2.65 g/cm . Li ke the biotite gneisses, these granites do no t show a wide distributio n o n the histogram Total 1294 which pro bably indicates a uniform compositio n. 102 |00000109|| GRANITES of 3.03 g/cm 3 is obtained with a range 2. 74-3.19 g/cm 3 for these ::ON= 371 180 l'ove = 2 65gm/cm3 rocks. But the histogram (Fig. 2) shows a concentration towards 3 160 the basic end (2.95-3.15 g/cm ) with a tail towards the acidic end. 140 BIOTITE GNEISSES ::ON= 13~ fo11e = 268om/cm3 Rocks from High-Grade Terrains N These are mostly confined to the Eastern Ghats Belt with its 30 60 regional granulite-facies metamorphic grade. It is believed that N 20 40 this belt may represent a deeper crustal domain (Subramaniam 10 20 1967) as it is characterized by the widespread distribution of the charnockite-khondalite suite of rocks, anorthosites, garnetiferous 0 2 4 26 2 8 2 7 29 gneisses, migmatites and other minor varieties. In the present sam­ ,P = density ,P =density pling, special attention has been paid to the charnockitic rocks. SCHISTS The categorisation of charnockites into four varieties, acidic to 20 ::0 N = 82 ultrabasic (Holland 1900) is retained here.
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