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Magneto-mineralogy and Interpretation of AMS Fabrics in the Balugaon Anorthosite Massif, Eastern Ghats Belt, India
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Saurodeep Chatterjee, Debesh Gain, Supriya Mondal
Department of Geological Sciences, Jadavpur University, Kolkata , India
*Presenting Author:
Abstract
Methodology
Massif type anorthosites were emplaced in the granulite facies rocks present in almost all the Proterozoic mobile belts of the world (Ashwal, 1993). Balugaon anorthosite massif of India is under this category. Field evidences suggest that the dominant structural fabric is the S2 foliation, showing an attitude of NE-SW/50°  NW. Petrography suggests that the rock is pyroxene-bearing meta-anorthosite rather than pure anorthosite. Fe-Ti oxides study under reflected light, indicated the presence of both primary (high temp. and low temp.) and secondary Fe-Ti oxides (mainly magnetite and ilmenite). AMS study was carried out to determine the magnetic fabrics of the rocks and its related parameters. Magnetic foliation shows an extremely variable attitude from NE-SW to NW-SE with dip amount of 20°-40 ° towards SE and SW respectively.
Field work was carried out in and around Balugaon anorthosite massif with an aim to collect the attitude of the dominant structural foliation and to collect samples for magneto-mineralogical and AMS studies. For petrography and magneto-mineralogy, chip samples were collected which were further processed to develop thin-polished sections for petrographic and magneto-mineralogical studies. For AMS studies about 15 locations were selected from the Balugaon anorthosite massif (Fig. 1) and cylindrical oriented rock cores with diameter 2.54 cm and height 2.2 cm were collected using a portable drill machine. These core samples were used to measure the AMS parameters using Bartington Susceptibility Meter (MS-2) and AMS-BAR software. Data related to magnetic foliation and lineation were plotted on lower hemisphere circular diagram to obtain their attitudes.
Fig. 2: Photomicrographs of Fe-Ti oxides, showing: (a) and (b) Infiltration of silica along fractures (Stage-3); (c) Extensive veins of silica along cracks (Stage-4); (d) Droplets of magnetite formed within the host silica (Stage-5). High temperature oxidation of magnetite showing (e) 2-3 laths of ilmenite in titano-magnetite (C-3); (f) Thick laths of ilmenite in titano-magnetite (C-5). Secondary occurrence of the ultrafine grains of magnetite (g) Along grain boundary and (h) Along cleavage. ( PPL, 40x).
Results
AMS FOLIATION
Discussion
Magneto-mineralogical studies were done under reflected light microscope which depicted the presence of both the primary and secondary Fe-Ti oxides (mainly magnetite / titano-magnetite and ilmenite). Among the low temperature oxidations (<350°C), stage-3 (Fig. 2a and 2b), stage-4 (Fig. 2c) and stage-5 (Fig. 2d) are present in the studied rocks, according to the classification of low temperature oxidations by Johnson and Hall (1978). Higher degree of low temperature oxidations are identified by the development and migration of the cracks from the periphery of the titano-magnetite grains towards the centre and subsequent filling of these cracks by silicates (Johnson and Hall, 1978). Among the high temperature oxidations (>600° C), C-3 (Fig. 2e) and C-5 (Fig. 2f) stages of Haggerty’s classification (1976) are found here in the studied sections, identified by the variation of the density and thickness of the Ilmenite laths exsolved in the host titano-magnetite . Secondary Fe-Ti oxides are present along grain boundaries (Fig. 2g) and cleavage planes (Fig. 2h) of silicates.
The attitudes of the magnetic foliations showed a varying strike from NE-SW to NW-SE with dip amount of 20° -40° towards SE and SW respectively (Fig. 1).
The high temperature oxidation states are characterized by the development of the ilmenite laths within the host titano-magnetite grain. With increasing temperature higher degree of exolution of titano-magnetite grains occur leading to the increase in the density and thickness of the titanomagnetite laths ( Haggerty, 1976).
Low temperature oxidation states of titano-magnetite grains are identified by development of cracks in the periphery. With the increasing degree of low temperature oxidation the cracks gradually migrates towards the center of the grain. Subsequent filling of the cracks with silicates causes the development of droplet like structure (Johnson and Hall, 1978).
The dominant field foliation is seen to show no parity among themselves. The field foliation has a steady attitude while the magnetic foliation shows extreme variation.
STRUCTURAL FOLIATION
Conclusions
The magneto-mineralogical studies indicates the presence of both the high temperature and low temperature primary Fe-Ti oxides, along with the presence of some amount of secondary ones. The high temperature primary oxidation states were developed during the attaining of the peak metamorphic assemblage. During the post peak assemblage the low temperature primary oxidation states were developed. Further the development of the secondary Fe-Ti oxides along the cleavage planes and the grain boundaries of the silicates suggests that the rock have suffered an event of upliftment during which release of pressure during upliftment.
The magnetic fabrics show no correlation with that of the field fabrics (mainly S2 ). Thus the magnetic fabrics are results of some later stages of deformations other than D2 , the dominant one.
Fig. 1. General Geological map of Balugaon anorthosite massif , Eastern Ghats Belt, India (after Dobmeier and Simmat, 2001)
Introduction
Magneto-mineralogical characteristics are important aspects of study because the nature and the various stages of development of the magnetic minerals are important in obtaining tectonic implications of the host rock from a single sample. In the present study we will be dealing with the magneto-mineralogical characteristics of the rocks from the Balugaon anorthosite massif and will try to draw some tectonic implications from the same.
In addition to this the AMS studies were done which were helpful in determining the nature and the attitudes of the magnetic fabrics. Inter-dependence of the structural and magnetic fabrics are verified.
Contact
References
Saurodeep Chatterjee
Department of Geological Sciences, Jadavpur University, Kolkata
Phone:
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