Magneto-mineralogy and interpretation of magnetic fabrics of the high grade rocks from saltora, bankura, West Bengal

Abstract

The high-grade rocks of Saltora Area, Bengal Anorthosite, Chhotanagpur Gneissic Complex are ideal terrains for the study of uplift related magnetization because the rates of uplift in these regions were much slower than those of phanerozoic orogenic belts. Anisotropy of Magnetic Susceptibility and the rock magnetic studies of the high-grade rocks of the Bengal Anorthosite have two important contributions to make to the geophysical study of the continental crust: 1. First the Proterozoic crustal rocks invariably possesses a strong and stable record the ancient magnetic field acquired at stages of the early tectonic history of this terrain. The magnetism is typically resident in one or more phases of magnetite growth and has recorded the rock magnetism over protracted periods of geologic time. This magnetic record will provide the only evidence for the ancient tectonic history of the Saltora Anorthosite, India and is the essential information required for correlating the crustal rocks of Saltora, India with that of elsewhere. 2. Secondly, the oriental anisotropy of minerals is an important character, which can be measured in form of magnetic anisotropy (MA). Fabric in crustal rocks is mainly controlled by two factors. a. Effect of pressure due to metamorphism, and b. Effect of geomagnetic field over the existing magnetic particles during metamorphism and post-metamorphic episodes. Considering the principal textural relationship in rocks from different sampling sites, at least three generations of ferromagnetic mineral assemblage are distinguishable During rapid uplift of the high-grade granulite terrain and at this stage temperatures fell from peak value in the ranges 850-750C (Zhang and Piper 1994). This last generation of ferromagnetic mineral growth is also identified as microcrystalline iron oxides. This is due to release of pressure during tectonic uplift of the high-grade terrain. The overall nature of the magnetic fabric is very interesting from the studied area. From the Km-Pj plot it is seen that the relationship of the bulk susceptibility and the degree of anisotropy is quite complex, and the controlling factor of the magnetic fabrics are not revealed. However, from the stereo-plot it is evident that the maximum and intermediate susceptibility axes plots towards the periphery and the minimum plots at the center, with few exceptions where the plot is reverse. Considering the dominant plot, it can be said that the magnetic foliation in that case is near horizontal and thus explains for the oblate fabric in the Flinn and Jelinek plot. Now as these rocks are high grade rocks they contain a high amount of magnetite developed during metamorphism (Zhang and Piper, 1994) and as these contributes towards high susceptibility of the rocks, it can be considered that the magnetic foliation represents the plain containing the ferromagnetic grains. This explains the magnetic fabric in the high grade metamorphic rocks. Now in the reverse case, i.e. minimum towards the periphery and maximum and intermediate towards the center have significances from the anorthosite intrusion. As the Anorthosite intrusion event occurred through favorable places, the orientation of the susceptibility axes remained horizontal. Thus, in the present study the magnetic fabric is mainly mineralogically controlled by ferro-magnetic minerals.