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Rock magnetism and anisotropy of magnetic susceptibility studies on charnockites, southern India

Venkateshwarlu Mamilla, Ramesh Babu Nathi, Satyakumar Animireddi Venkata

Acta Geophysica

2023

Vol. 71, no. 2

613--624

Rock magnetism and anisotropy of magnetic susceptibility (AMS) results are reported for the first time from the southernmost (Nagercoil) charnockites of the Southern Granulite Terrane (SGT), south India. Forty-eight oriented block samples from nine sites have been collected and analysed. The integrated results of isothermal remanent magnetization (IRM), hysteresis loops, day plot, and thermomagnetic curves indicate the presence of magnetite with pseudo-single domain (PSD) and multidomain (MD) magnetite particles dominating the studied samples. Rock magnetic investigations reveal that the ratio of remanence (Mrs/Ms) range from 0.04 to 0.53 and the coercivity ratio (Hcr/Hc) between 1.19 and 5.17. The magnetization saturation was at 250–300 mT, and the coercive force ranged from 24 to 41 mT in all the samples. The results of the thermomagnetic study reveal the Curie temperatures between 560 °C and 580 °C, indicating the presence of magnetite. The observed principal AMS axes show mainly two types of magnetic fabrics. The first one shows the maximum susceptibility axes (K1) horizontal to sub-horizontal, and the minimum susceptibility axes (K3) plot near the pole. The second fabric describes that the minimum susceptibility axes (K3) show intermediate to vertical and the maximum susceptibility axes (K1) plot near the horizontal to sub-horizontal. AMS studies reveal that Nagercoil charnockites formed during the late-stage metamorphic event, magma flow (magnetic grains aligned) was sub-horizontal to horizontal, and the magnetic fabric grains are from prolate to oblate in shape.