Densities and magnetic susceptibilities of Precambrian rocks of different metamorphic grade (Southern Indian Shield)
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Densities of 1294 Precambrian rocks from the Southern Indian Shield, confined to low-grade granite-greenstone (Karnataka) and high-grade granulite (Eastern Ghats Belt) terrains are presented and categorized into 16 metamorphic and igneous varieties. In Karnataka, densities of granites (2.65 g/cm3), gneisses (2.68 g/cm3) and granodiorites (2.73 g/cm3) are considerably less than those of greenstone belts (2.84 g/cm3) and dolerites (3.03 g/cm3). An average density of 2.75 g/cm3 is obtained for surface rocks from this terrain. Over the Eastern Ghats belt, densities of charnockites range from 2.71 g/cm3 to 3.12 g/cm3 as they vary from acidic to ultrabasic composition. An average density of 2.85 g/cm 3 is obtained for surface rocks from this belt. The contrast in the average densities of surface rocks from the lowand high-grade terrains may play a significant role in a proper assessment of the regional gravity fields over these terrains. The results compare well with the data from similar terrains in other shield regions. Magnetic susceptibilities for 482 of these samples are also presented. All the rock types include a wide range of susceptibilities, as is to be anticipated in metamorphic terrains. A plot of density versus magnetic susceptibility for thirteen of these rock types suggests a linear relationship.
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