Magnetic mineralogical variability along Deccan trap basalt borehole (KBH07), Koyna deep continental drilling program,western Maharashtra,India |
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| Authors: | S J Sangode M Venkateshwarulu Rasika Mahajan Vinay Randive |
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| Institution: | 1.Department of Geology,Savitribai Phule Pune University,Pune,India;2.CSIR-National Geophysical Research Institute,Hyderabad,India |
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| Abstract: | A 1248 m long core (KBH 07, 17°18′07″ N; 73°47′28.2″E, 960m above msl) drilled up to basement in the Deccan traps from Koyna region was sampled at ～10m interval for magnetic mineralogical studies.Analysis of routine rock magnetic parameters (mass specific magnetic susceptibility: χlf, frequency dependence of susceptibility: χfd, susceptibility of anhysteretic remanance: χARM, saturation isothermal remanance: SIRM, remanance coercivty: B(0)CR, SoftIRM, HardIRM, S-Ratio, SIRM/χlf, χARM/χlf) and density (σ, gm/cc) depicted significant higher order temporal variation. The χlf varies between 13 and 309 x 10-8m3/kg and is independent of density variation. The χARM, B(0)CR and S-Ratios indicate majority of SD-PSD ferrimagnets with episodes of MD ferrimagnetic concentration and few hard coercivity components. The giant plagioclase lath bearing (GPB) horizons show highest variability of ferrimagnetic concentration marked by anomalous peaks. Overall the variability of rock magnetic parameters independent of lava flow units suggest that the changeover in magnetic mineral concentration, composition and domain size occur at major episodes in magma composition (e.g., primary source, crustal contamination and fractional crystallization). The studied parameters are therefore examined to mark intervals of (i) magma compositional changes, (ii) zones of oxidative conditions and (iii) rapid/slow cooling intervals demanding detailed petrologic studies. We identified one I order trend, four II order cycles and eight III order cycles for the purpose of correlation. Notable peak in χlf at 650–700m, the changeover in rock magnetic parameters at ～930 m and ～280 m can facilitate marker intervals while several higher order variations can be adopted for high resolution correlation to other boreholes in the region. The complex variation in rock magnetic parameters independent of flow units reflect temporal magnitudes of compositional variability, cooling and emplacement history that needs detailed petro-mineralogical attempts; and the present data is useful for high order inter-core correlations under the deep drilling program. |
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