Wyniki wyszukiwania - BazTech

Ograniczanie wyników

Znaleziono wyników: 2

Liczba wyników na stronie

Wyniki wyszukiwania

Wyszukiwano:
w słowach kluczowych: Paleoproterozoik

Sortuj według:

Ogranicz wyniki do:

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.

First find of biogenic activity in the Palaeoproterozoic of the Singhbhum craton (E India)

Loon A. J., Mazumder R.

Geologos

2013

Vol. 19, No. 3

185--192

The Palaeoproterozoic succession of the Singhbhum craton in E. India was hitherto considered as almost entirely siliciclastic and partly volcanogenic. Here we describe, from the fine-grained, tidally influenced shale facies of the Palaeoproterozoic Chaibasa Formation (2.1-1.6 Ga), a fine, originally more or less horizontal, wavy to strongly undulating (later locally deformed) lamination. Investigation of these laminae shows that they must be ascribed to the accumulation of fine particles on microbial mats that covered a sandy substrate. The structures must therefore be considered as stromatolites, features that are accepted as proof of the presence of micro-organisms, in this case most probably cyanobacteria. The interpretation of biogenic activity is supported by microscopic analysis. It is the first description of traces left by biogenic activity that took place in the Palaeoproterozoic of the Singhbhum craton.