Mineral chemistry and image analyses of accessory phases present in the mafic dykes from the Bastar craton using SEM & EPMA

Himani Joshi

School of Earth Sciences, Banasthali Vidyapith, Jaipur 304022

Dr. Rajesh Kumar Srivastava

Professor & Head, Centre of Advanced Study in Geology, Institute of Science, Banaras Hindu University, Varanasi 221005, India

Abstract

Dyke Swarm is a large geological structure consisting of a major group of parallel, linear or radially oriented dykes intruded within continental crust. They consist of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event and are magmatic. Such dyke swarms may form Large Igneous Provinces and are the roots of a volcanic province and these dykes form when magma intrudes into a crack then crystallizes as a sheet intrusion, either cutting across layers of rock or through an unlayered mass of rock. Mafic dykes are used to highlight certain structural features that may provide further insight into the origin and geodynamic significance of dyke swarms, particularly those of Precambrian age. For this study, mafic dykes from the Bastar craton have been sampled and their thin sections were prepared and petrographical studies have been done. These samples are usually consists of plagioclases and pyroxenes as essential mineral phases and zircon, rutile, ilmenite and baddeleyite as an accessory phases. Further SEM & EPMA studies have been done for the studies of these accessory phases. Thus, the purpose of this study is to characterize baddeleyite which is a frequently found accessory mineral in silica undersaturated rocks and a rare Zirconium oxide mineral.

Keywords: dyke swarms, large igneous provinces, zircon, baddeleyite

Abbreviations

 INTRODUCTION

Background

A dyke swarm is a large geological structure consisting of a major group of parallel, linear, or radially oriented dikes intruded within continental crust. They consist of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event, and are magmatic. Mafic dyke swarms are groups of vertical dykes with same orientation representing a system of pre-existing tensional crustal fracture swarms along which mafic magmas emplaced (Halls and Fahrin, 1987; Ernst et al., 1995; Hou et al., 2006). Mafic dyke swarms represent conspicuous extensional structures and are widespread in cratons throughout the world, especially in the Archean shields such as the Canadian Shield, the North China Craton and the Indian Cratons (Halls and Fahrin, 1987). Most of giant mafic dyke swarms were developed withtime. The major mafic dyke swarms aredivided into three main types: parallel dyke swarm, small radiating dyke swarm and giant radiating dyke swarm. Each type of the mafic dyke swarm is related to the local stress field. Mafic dyke swarms are excellent time marker and paleo-stress indicators and can be used to reconstruct the paleo-stress fields of cratons. In general, dyke swarms exhibit trends parallel to the contemporaneous regional horizontal maximum compressive stress orientations and perpendicular to the extension direction (Pollard, 1987).

The occurrence of mafic dike swarms in Archean and Paleoproterozoic terrains is often cited as evidence for mantle plumes activity associated with abnormally high mantle potential temperatures. Dyke swarms may extend over 400 km in width and length. The largest dike swarm known on Earth is the Mackenzie dike swarm in the western half of the Canadian Shield in Canada, which are more than 500 Km wide and 3,000 km long. Dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Dolerites sills and dykes are typically shallow intrusive bodies. Dolerites normally has a fine, but visible texture of euhedral lath-shaped plagioclase crystals set in a finer matrix of clinopyroxenes, typically augite, with accessory minerals like Zircon, Rutile, Ilmenite, and Baddeleyite. Accessory mineral, any mineral when it is present in small amounts, as is common, it is called a minor accessory. If the amount is greater or is of special significance, the mineral is called a varietal, or characterizing, accessory and may give a varietal name to the rock. Accessory minerals characteristically are formed during the solidification of the rocks from the magma; in contrast are secondary mineral, which form at a later time through processes such as weathering by hydrothermal alteration. Common minor accessory minerals include topaz, zircon, corundum, fluorite, garnet, monazite, rutile, magnetite, ilmenite, baddeleyite and tourmaline. Typical varietal accessories include biotite, muscovite, amphibole, pyroxene, and olivine.

Baddeleyite (ZrO₂) is a common accessory mineral in many silica‐undersaturated plutonic rocks and dykes. U contents of 200–1000 ppm and negligible amounts of initial Pb allow for precise, typically concordant, U‐Pb ages. Baddeleyite transforms into polycrystalline zircon during metamorphism, and xenocrystic baddeleyite is rare (Heaman and LeCheminant, 1993; Schärer et al., 1997). Baddeleyite is a reliable geochronometer for dating the crystallization of mafic and other silica‐undersaturated intrusive rocks. Its high U and negligible initial Pb content enable precise age determinations with statistical errors of a few million years or less. The widespread use of baddeleyite for dating has, however, been limited by the low content of baddeleyite in many samples and by difficulties in isolating baddeleyite.

Statement of the Problems

To find out accessory mineral in dolerite dyke samples, specially to search baddeleyite mineral in mafic dykes using SEM and EPMA.

Objectives

Petrography, Mineral chemistry and Image analyses of accessory phases present in mafic dykes of Bastar craton.

LITERATURE REVIEW

Geological Setting and Stratigraphy

Geology of the Archean Bastar craton is presented elsewhere by a number of workers (Crookshank, 1963; Ramakrishnan, 1990; Naqviand Rogers, 1987; French et al., 2008; Srivastava and Gautam, 2009; Meert et al., 2010; Ramakrishnan and Vaidyanadhan, 2010 and references therein). In general, the Bastar craton is bounded by NW–SE trending Godavari and Mahanadi rifts, ENE–WSW trending Narmada-Son rift and NE–SW trending Eastern Ghats Mobile Belt. It is important to note that ENE–WSW trending Narmada-Son rift (lineament), thought to be existed since the Archean and extends into the mantle (Naqvi et al., 1974; Naqvi and Rogers, 1987), and is an integral part of the Central Indian Tectonic Zone (CITZ). The CITZ has experi-enced polyphase tectonothermal events involving several cycles of volcano sedimentary deposition, deformation, metamorphism and magmatism (Acharyya and Roy, 2000; Acharyya, 2001; Roy and Hanuma Prasad, 2003; Meert et al., 2010). The Bastar craton is mainly covered by a vast tract of granitoids with inliers of supracrustal rocks of the Dongargarh, Sakoli, Sausar, Sukma, Bengpal, and Bailadila Series and these are overlain by many unmetamorphosed Proterozoic sedi-mentary basins (Crookshank, 1963; Ramakrishnan, 1990; Chaudhuri et al., 2002; Ramakrishnan and Vaidyanadhan, 2010).

Fig 1 General stratigraphy of Bastar craton.

Granitoids vary from 3.5 to 3.6 Ga TTG basement gneisses and granites to relatively un-deformed and unmetamorphosed ~2.5 Ga granites (Sarkar et al., 1993; Rajesh et al., 2009). Mafic dykes of different ages and compositions are conspicuous in the Bastar craton and mainly intrude Archean granitoids and metamorphites (Srivastava and Gautam, 2009 and references therein). Most of these dykes trend in NW–SE to WNW–ESE. On the basis of field setting, geochemistry, available ages of granitoids and U–Pb geo-chronology of a set of mafic dykes, three sets of mafic dyke swarms are identified in the southern parts of the Bastar craton (Bandyopadhyay et al., 1990; Ramakrishnan, 1990; Sarkar et al., 1990, 1993, 1994; Srivastava et al., 1996; Srivastava and Singh, 2004; Srivastava, 2006a,b; Mondal et al., 2007; French et al., 2008; Srivastava and Gautam, 2008, 2009; Srivastava et al., 2011); these include ~2.7 Ga sub-alkaline mafic dykes (BD1) (Srivastava et al., 2009), ~2.4–2.5 Ga boninite-norite dykes (BN) (Srivastava, 2008), and 1.88–1.89 Ga sub-alkaline mafic dykes (BD2) (French et al., 2008). A dense concentration of mafic dyke swarm is also well exposed in central part of the Bastar craton, particularly around Bhanupratappur-Keshkal region (Ramachandra et al., 1995; Gautam and Srivastava, 2011), however these are thought to be equivalent to BD1 and BN swarms (Srivastava and Gautam, 2012). Mafic dykes of present study exposed in northern parts of the Bastar craton (NBC), particularly south of the Proterozoic Chhattisgarh sedimentary basin. Most of these mafic dykes emplaced within Archean gneisses, 2.55–2.48 Ga granitoids and Paleoproterozoic supracrustal rocks (Subba Rao et al., 2003, 2004, 2008; Hussain et al., 2008; Srivastava and Gautam, 2009), however a couple of mafic dykes are also reported to cut Proterozoic Chhattisgarh sedimentary basins (Tripathi and Murti, 1981; Das et al., 2011; Sinha et al., 2011). Predominantly they trend in NW–SE (to NNW–SSE), however a number of dykes also trend in N–S and ENE–WSW to nearly E–W. NW–SE trending mafic dykes are mainly distributed around southern periphery of the Chhattisgarh basin; around Kusumkasa, north of Keshkal, Gariaband, Chhura, and Patewa. A mafic dyke trends NW–SE, not reported earlier, intrude sandstone of the Chandrapur Group of the Chhattisgarh basin near Dongargaon. N–S trending mafic dykes are concentrated around Lakhna and one of such dykes is encountered between Sohela-Padmapur roads. ENE–WSW trending mafic dykes mainly exposed around Dongargarh, Chhura, Lakhna and Bandalimal. At Chhura, an ENE–WSW mafic dyke is encountered to cut a NW–SE mafic dyke suggesting younger age for the former. A couple of ENE–WSW mafic dykes is encountered from Bandalimal area, which is emplaced within the Chhattisgarh sedimentary basin; mainly Singhora Group of rocks (Das et al., 2011; Sinha et al., 2011). A couple of nearly E–W trending Mafic dykes are also reported from Lakhna area, which are supposed to be younger to the N–S dolerite, rhyolite and trachyte dykes (Ratre et al., 2010; Pisarevsky et al., 2013). It is important to point out that there are a number of geological and geochemical evidences, that have been reported in the literature (Naqvi et al., 1974; Naqvi and Rogers, 1987; Rajurkar et al., 1990; Ramakrishnan, 1990; Kale, 1991; Neogi et al., 1996; Srivastavaet al., 1996; Chaudhuri et al., 2002; Srivastava, 2008; Srivastava and Gautam, 2009), evidently support the emplacement of early Precambrian mafic dykes of the Bastar craton in an intracratonic rift setting.Limited geochronological data are available on mafic dykes of the Bastar craton. Few mafic dykes emplaced in southern parts of the Bastar craton have been dated. These includes (i) a couple of samples from the BD2 dykes placed at 1883 ± 1.4 Ma and 1891.1 ± 0.9 Mausing the U–Pb isotope system in zircon and baddeleyite (French et al., 2008) and (ii) one sample from the metamorphosed the BN dyke placed at 2118 ± 2 Ma using U–Pb isotope system in metamorphic rutile (Srivastava et al., 2011). The later age is interpreted to indicate the time of exsolution of retrograde rutile from Ti-rich actinolite. This represents a robust minimum age constraint for the timing of emplacement of the BN, and by inference the BD1 dyke swarm (Srivastava et al., 2011). Srivastava et al. (2009) indicated that the BD1 dykes were emplaced ~2.7 Ga. Few mafic samples from northern parts of the Bastar craton are also radiometrically dated. Das et al. (2011) have presented Sm–Nd mineral and whole rock age for a sample from ENE–WSW trending mafic intrusive body from Bandalimal area, which yield an isochron age of 1421 ± 23 Ma; they considered it as the emplacement age of the intrusive. Rhyolite, trachyte and alkali gab-bro samples from N–S trending Lakhna dykes are dated by U–Pb zircon method; zircons from rhyolite dyke gave age of 1450 ± 22 Ma, whereas trachyte dykes placed at 1453 ± 19 Ma. A sample from alkali gabbro of same area corresponds to age of 1442 ± 30 Ma, which represents the best age estimate for the emplacement of the dyke (Ratre et al., 2010). Pisarevsky et al. (2013) also obtained a similar U–Pb zircon age (1466.4 ± 2.6 Ma) for one of the N–S trending rhyolitic dykes of N–S Lakhna area.

Fig 2 (a) Major cratons and structural features of India (after Naqvi and Rogers, 1987). Major structural features are: 1.Small thrusts in western Dharwar craton; 2.Eastern Ghats front; 3.Sukinda; 4. Singhbhum; 5. Son Valley; and 6. Great Boundary Fault. EGB; Eastern Ghat Belt. (b) Proterozoic basins of the Bastar Craton (after Chaudhuri et al, 2002). (c)Geological map of the northern part of the Bastar craton (based on Ramachandra et al., 1995; French et al, 2008).   

METHODOLOGY

Thin Section Preparation

A Thin section is a thin, flat piece of material prepared for examination with a microscope, in particular, a piece of rock about 0.03mm thick.

Thin section is prepared by cementing a thin slide of rock to glass and carefully grinding using Carborundum powder to produce a paper-thin layer of rock.

The standard thickness is 0.03 mm.

• Procedure for preparation of Thin-section:

• CUTTING THE SPECIMEN Firstly, rock samples are observed in light for their grain, size, color and texture. Second, each sample is then sliced into small rectangular chips for preparing slides using diamond saw blade.
• GRINDING Now grind these chips on 850rpm rotating lap using 120 mesh size silicon carbide powder (SiC) and few drops of water to get even surface for 20 minutes.Repeat this step with 220 mesh size powder for 20 minutes so that a well-defined mineral grain boundary is visible with smooth surface.
• MOUNTING & FINAL CUTTING The sample is then mounted to a numbered glass slide with Araldite; press the chip at an oblique angle against glass slide to eliminate bubbles. Resection the specimen to reduce the thickness of the chip and minimize grinding time and isdone by using Petro thin machine and then ground the slides using 600, 800 and 1000 mesh size silicon carbide powder on a glass plate with water until a thickness is reduced to 0. 03mm. The hand grinding step provides excellent control over final thickness.
• POLISHING The specimen is placed on the lap and moved in a circular direction opposite to the rotation of the lap. Using moderate pressure on the specimen, polish the specimen for few minutes, then rotate the specimen 180° and polished for an additional 15 minutes by using 1µm Alumina gel.

a) Petrocut

b) Grinder

c) Araldite

d) Heating plate

e) Petrothin

f) Polisher

Fig 3
Different types of sample preparation tools

 Scanning Electron Microscope

The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interaction reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample. In most applications, data are collected over a selected area of the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties. Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50 to 100 nm). The SEM is also capable of performing analyses of selected point locations on the sample; this approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions (using EDS), crystalline structure, and crystal orientations (using EBSD). The design and function of the SEM is very similar to the  EPMA and considerable overlap in capabilities exists between the two instruments.

Principles

Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interaction when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images), backscattered electrons (BSE) diffracted backscattered electrons (EBSD) that are used to determine crystal structures and orientations of minerals, photons (Characteristics X-rays that are used for elemental analysis and continuum X-rays), visible light (Cathodoluminescence CT), and heat. Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples (i.e. for rapid phase discrimination). X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete orbitals (shells) of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a in fixed wavelength (that is related to the difference energy levels of electrons in different shells for a given element). Thus, characteristic X-rays are produced for each element in a mineral that is "excited" by the electron beam. SEM analysis is considered to be "non-destructive"; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyse the same materials repeatedly.

Fig 4 Scanning electron microscope (Carl Zeiss).

Instrumentation

Essential components of all SEMs include the following:

a.       Electron Source ("Gun")

b.      Electron Lenses

c.       Sample Stage

d.      Detectors for all signals of interest

e.      Display / Data output devices

f.       Infrastructure Requirements:

                                                              i.      Power Supply

                                                            ii.      Vacuum System

                                                          iii.      Cooling system

                                                          iv.      Vibration-free floor

                                                            v.      Room free of ambient magnetic and electric fields

Applications

The SEM is routinely used to generate high-resolution images of shapes of objects (SEI) and to show spatial variations in chemical compositions: 1) acquiring elemental maps or spot chemical analyses using EDS, 2 discrimination of phases based on mean atomic number ((commonly related to relative density) using BSE, and 3) compositional maps based on differences in trace element "activitors" (typically transition metal and Rare Earth elements) using CL. The SEM is also widely used to identify phases based on qualitative chemical analysis and/or crystalline structure. Precise measurement of very small features and objects down to 50 nm in size is also accomplished using the SEM. Backscattered electron images (BSE) can be used for rapid discrimination of phases in multiphase samples. SEMs equipped with diffracted backscattered electron detectors (EBSD) can be used to examine microfabric and crystallographic orientation in many materials.

Electron Probe Micro-Analysis

An electron probe micro-analyser is a microbeam instrument used primarily for the insitu non-destructive chemical analysis of minute solid samples. EPMA is also informally called an electron microprobe. It is fundamentally the same as an SEM, with the added capability of chemical analysis. The primary importance of an EPMA is the ability to acquire precise, quantitative elemental analyses at very small "spot" sizes (as little as 1-2 microns), primarily by wavelength-dispersive spectroscopy (WDS). The spatial scale of analysis, combined with the ability to create detailed images of the sample, makes it possible to analyse geological materials in situ and to resolve complex chemical variation within single phases (in geology, mostly glasses and minerals). The electron optics of an SEM or EPMA allow much higher resolution images to be obtained than can be seen using visible-light optics, so features that are irresolvable under a light microscope can be readily imaged to study detailed microtextures or provide the fine-scale context of an individual spot analysis. A variety of detectors can be used for:

• imaging modes such as secondary-electron imaging (SEI), back-scattered electron imaging (BSE), and cathodoluminescence imaging (CLI),

• acquiring 2D elemental maps,

• acquiring compositional information by energy-dispersive spectroscopy (EDS) and wavelength-dispersive spectroscopy (WDS),

• Analysing crystal-lattice preferred orientations (EBSD).

Fundamental principles of EPMA

An electron microprobe operates under the principle that if a solid material is bombarded by an accelerated and focused electron beam, the incident electron beam has sufficient energy to liberate both matter and energy from the sample. These electron-sample interactions mainly liberate heat, but they also yield both derivative electrons and x-rays. Of most common interest in the analysis of geological materials are secondary and back-scattered electrons, which are useful for imaging a surface or obtaining an average composition of the material. X-rays generation is produced by inelastic collisions of the incident electrons with electrons in the inner shells of atomin the sample; when an inner-shell electron is ejected from its orbit, leaving a vacancy, a higher-shell electron falls into this vacancy and must shed some energy (as an X-ray) to do so. These quantized x-rays are characteristic of the element. EPMA analysis is considered to be "non-destructive"; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to re-analyse the same materials more than one time.

Fig 5 CAMECA SX Five.

Instrumentation

EPMA consists of four major components:

1.      An electron source, commonly a W-filament cathode referred to as a "gun."

2.      A series of electromagnetic lenses located in the column of the instrument, used to condense and focus the electron beam emanating from the source; this comprises the electron optics and operates in an analogous way to light optics.

3.      A sample chamber, with movable sample stage (X-Y-Z), that is under a vacuum to prevent gas and vapor molecules from interfering with the electron beam on its way to the sample; a light microscope allows for direct optical observation of the sample.

4.      A variety of detectors arranged around the sample chamber that are used to collect x-rays and electrons emitted from the sample. 

Applications

        i. Quantitative EPMA analysis is the most commonly used method for chemical analysis of geological materials at small scales.

      ii.  In most cases, EPMA is chosen in cases where individual phases need to be analysed

     iii. EPMA is also widely used for analysis of synthetic materials such as optical wafers, thin films, microcircuits, semi-conductors, and superconducting ceramics. igneous and metamorphic minerals, or where the material is of small size or valuable for other reasons (e.g., experimental run product, sedimentary cement, volcanic glass, matrix of a meteorite, archaeological artifacts such as ceramic glazes and tools).

    iv. In some cases, it is possible to determine a U-Th age of a mineral such as monazite without measuring isotopic ratios.

 RESULTS AND DISCUSSION

Petrography

Petrographic studies of all the samples (NB10/2, NB10/3, NB10/9, NB11/3, NB10/35, NB10/35 and WDC 17/55) were carried out using LEICA DM750P petrographical microscope under transmitted light, and microphotographs were captured by using software. eica Application Suite V4/Framework.exe” “LAS’ (LAS V4.10).

Petrographic studies of all the sample reveals that the major mineral constituents are plagioclases and pyroxenes. Plagioclase exhibits subhedral crystals, colourless, low to moderate relief in PPL and in XPL it shows first order grey interference color, lamellar twining is clearly visible in the laths of plagioclases whereas pyroxenes exhibits subhedral crystals, perfect two sets of cleavage, high relief, pale green to brownish green pleochroism in PPL. In XPL it shows second order bluish pink interference color, lamellar twinning and straight extinction in orthopyroxenes and inclined extinction in clinopyroxenes. Some grains show alteration which may commonly alter to uralite and zoning is also clearly visible in some pyroxenes.

a) Ophitic texture shown by plagioclases and pyroxenes.

b) . Lamellar twining shown by Pyroxene crystal .

c) ) zoning is clearly visible in pyroxenes surrounded by plagioclase laths.

d) Ilmenite showing skeletal texture.

Fig 6 ) Photomicrographs of studied dolerite samples from Bastar craton.

These samples are coarse to medium grained with ophitic to sub-ophitic texture. Accessory phases are ilmenite, rutile, zircon and baddeleyite where zircon and baddeleyite are not distinguishable under the microscope but can be identified by doing SEM analysis. Ilmenite is an opaque and Iron black color mineral. Unlike (NB 10/3 and NB 10/35) contains olivine which is highly fractured and quartz is colourless, cleavage is absent, low relief in PPL and represents first order grey interference color in XPL as Accessory phases.

Mineral Chemistry

Electron microprobe analyses by CAMECA SX of the minerals present were carried out at Department of Geology, Institute of Science, Banaras Hindu University.

a) Anhedral crystal of Plagioclase with pyroxene.

b) Represents Ilmenite along with plagioclases and pyroxenes where zoning is clearly visible in Pyroxene.

Fig 7 BSE of sample number NB 10/15 from Bastar area

Pyroxene

Measured and recalculated pyroxenes based on 6 oxygen atoms are shown in table 1. The primary classification of these pyroxene classify the pyroxene as Augite (Morimoto et al.,1988) having compositional range between Wo_14.56-36.01 En_30.87-57.71Fs_20.19-36.81 and major oxides were SiO2(48.22-53.93wt%),MgO(10.56-17.05wt%),FeO(11.95-19.05wt%) and CaO(6.31-17.3wt%) respectively.

Fig 8 . Classification of Pyroxene (from Morimoto et al., 1988).

Feldspar

Measured and recalculated feldspars based on 32 oxygens atoms are shown in table 2. Three types of Feldspars were found in analysis. The classification of these feldspars classifies the feldspars as labradorite, oligoclase and bytownite. A complete solid solution series is seen from albite to anorthite. Composition ranges from Or_0.13-1.77 Ab _28.84-72.85An _26.65-70.23 and major oxides were Al ₂O₃(22.44-31.55wt%),Na ₂O(3.20-8.11wt%) and CaO(5.26-13.47wt%) respectively.

Fig 9 Ternary classification plot of Anorthite-Albite-K Feldspar (after Deer et al. 2013)

Amphibole

Measured and recalculated amphibole based on 23 oxygen atoms are shown in Table3. Two types of Amphiboles were identified as Ednite and Magnesiohastingsite. Composition range of major oxides were CaO(9.88-18.25wt%), FeO(14.33-22.12wt%) and MgO (8.68-13.84) respectively.

Fig 10 Classification of Amphiboles (from Leake et.al.,2008).

Ilmenites

Measured and recalculated ilmenite based on 6 oxygen atoms shown in Table 4. Compositional range of major oxides were TiO2(51.98-52.55 wt%),FeO(44.38-45.07 wt%).

Fig 11 Ternary plot diagram between TiO2-FeO-Fe2O3.

SEM Analysis

Scanning Electron Microscopy of the accessory minerals present were carried at Department of Geology, Institute of science, Banaras Hindu University.

a) Image analysis of Baddeleyite

b)   Baddeleyite sample which is more than 30 microns.

Fig 12 BSE images of Accessory mineral Baddeleyite of sample number  NB 10-15.

NB10-15 Area 8

NB10-35 Area 13

Baddeleyite have monoclinic platy habit as less than 100µm grain of baddeleyite can’t be identified under microscope. Electron Imaging of thin sections to identify Baddeleyite, determine its host and this is the best way to determine the baddeleyite content of the rock and its potential for U-Pb geochronology.

The baddeleyite analysed in this study is among the purest natural ZrO2 is sample number NB10-15 Area 8 which elongated grain and more than 30µm in size. In the above figures bright color is characterising baddeleyite and darker color is showing zircon as the process zirconation is taking place.

CONCLUSION AND RECOMMENDATIONS

• Petrographical studies of these samples(include NB10/2, NB10/3 from Bandalimal; NB10/9, NB11/3 from Lakhana; NB10/15 from Chhura; NB10/35 from Dongargarh area and WDC17/55 from Dharwar area) have been done and plagioclases and pyroxenes were found as essential minerals along with ilmenite as an accessory phase. The rocks were coarse to medium grained showing ophitic texture.

• Baddeleyite was found as accessory mineral phase in Scanning electron microscope (SEM) analysis which is of 10µm in size and showing elongated shape. Mineralogical studies suggest that the pyroxenes were of two types: Augite and Pigeonite, Feldspar were identified as Labradorite, Oligoclase and Bytownite; Amphiboles were identified as Ednite and Magnesiohastingsite.So, we may conclude that the rocks in the given thin sections is dolerite.

• Further dating of this Baddeleyite can be done as baddeleyite is a key mineral in geochronology of mafic rocks as it crystallizes in silica-undersaturated systems that do not grow zircon. Baddeleyite is a major carrier of Hf, Ti, Fe and U and strongly fractionates Hf from Zr as it can have very high Zr/Hf (> 50). The baddeleyite rims on mantle zircon xenocrysts have a unique chemical composition with significantly higher TiO₂ and lower FeO contents. Baddeleyite is an ideal mineral for U-Pb dating because it has abundant U (up to 3000 ppm), negligible initial common Pb.

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ACKNOWLEDGEMENTS

I would like to give my special thanks to Prof. Rakesh Bhatnagar, Vice Chancellor, Banaras Hindu University , Varanasi for providing me adequate facilities during the period of training and also to Dr. Rashmi Sharma, Dean of School of Earth Sciences, Banasthali Vidyapith, Rajasthan for granting me permission to undergo training at Banaras Hindu University, Varanasi.

I would like to express my deepest sense of gratitude to my guide Dr. Rajesh Kumar Srivastava, FASc, FNASc. Professor & Head Centre of Advanced Study in Geology , Institute of Science , Banaras Hindu University, Varanasi , for his immense contribution, scholarly guidance, kind support and consistent encouragement during my present study. He is a constant source of inspiration to create a sense of achievements, completion and passion for this work.

I am also very thankful to Dr. Amiya Kumar Samal, Assistant Professor and Dr. Dinesh Pandit, Assistant Professor for their valuable suggestions and encouragement that help me to a very great extent to accomplish this task.

I would like to give my deepest whole hearted thanks to Ms. Sneha Raghuvanshi, Research Scholar, whose guidance made this work easy for me. I will always grateful for her guidance and constant help in all steps of this internship program and Mr. Akshay Kelkar, Research Scholar, for his constant help and guidance throughout this internship.

I express my heartful thanks to my Professors, who offered their continuous advice and encouragement throughout the course of this internship. I am thankful to Mr. Vivek Deep, Dr. Mamta Chauhan, Mr. Amit K. Mishra, Dr. Mamata Devi, Dr. Reshmi MR and Dr. Subhashree Mishra and all the faculty members of School of Earth Sciences, Banasthali Vidyapith, Rajasthan.

I am really grateful to the Summer Research Fellowship Program (SRPF) for providing me with such golden opportunity. It would not have been possible without this program. Also, the Authorcafe platform is extremely user friendly and helped in compiling my report.

Again, it is my parents and friends whose support and care have always helped and motivated me in accomplishing any task.

LIST OF FIGURES

• General stratigraphy of Bastar craton.
• (a) Major cratons and structural features of India (after Naqvi and Rogers, 1987). Major structural features are: 1.Small thrusts in western Dharwar craton; 2.Eastern Ghats front; 3.Sukinda;
  4. Singhbhum; 5. Son Valley; and 6. Great Boundary Fault. EGB; Eastern Ghat Belt.
  (b) Proterozoic basins of the Bastar Craton (after Chaudhuri et al, 2002). (c)Geological
  map of the northern part of the Bastar craton (based on Ramachandra et al.,1995; French et al, 2008).   
• Different types of sample preparation tools
• Scanning electron microscope (Carl Zeiss).
• CAMECA SX Five
• Photomicrographs of studied dolerite samples from Bastar craton.
• BSE of sample number NB 10/15 from Bastar area.
• Classification of Pyroxene (from Morimoto et al., 1988).
• Ternary classification plot of Anorthite-Albite-K Feldspar (after Deer et al. 2013).
• Classification of Amphiboles (from Leake et.al.,2008).
• Ternary plot diagram between TiO2-FeO-Fe2O3.
• BSE images of Accessory mineral Baddeleyite of sample number NB10/1