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Summer Research Fellowship Programme of India's Science Academies

Distinct cretaceous mafic dyke swarms in chhotanagpur gneissic terrain

Srinivasan Abinash

MSc. Geology (First Year), School of Earth Ocean and Climate Science, IIT Bhubaneswar, Argul, Bhubaneswar, Odisha 752050

Dr. Rajesh K Srivastava

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

Abstract

Chhotanagpur Gneissic terrain hosts the early cretaceous Rajmahal and late cretaceous Deccan igneous activities. A number of Cretaceous mafic dykes intrude the Gondwana sedimentary formations. One set of these strikes NNE to ENE, are very fresh and mainly exposed within the Jharia, Bokaro and Karanpura basins. The other set trends NW to NNW, intrude the Raniganj basin and show hydrothermal alteration. Most of the samples from both these groups display ophitic to sub-ophitic textures and are mainly composed of pyroxenes and plagioclase. On the basis of petrographic and geochemical characteristics the NNE to ENE dykes are identified as high-Ti dolerite (HTD) dykes and the NW to NNW dykes as low-Ti dolerite (LTD) dykes. The HTD dykes contain relatively high values of iron, and high-field strength elements than those from the LTD dykes with an overlapping MgO contents. Paleomagnetic and limited geochronological data for the studied dykes suggests their emplacement during early Cretaceous period (110–115 Ma); the Salma dyke, dated 65 Ma, is an exception. Geochemically all the studied samples show plume-derived character but their unambiguous affinity to either the early Cretaceous Kerguelen (Rajmahal) or the late-Cretaceous Reunion (Deccan) plume is not straightforward as they share bulk-rock characteristics of rocks derived from both these plumes. Robust geochronological and paleomagnetic constraints are required to understand the relative contributions of the two Cretaceous mantle plumes in the genesis of the mafic igneous activity in this domain.

Keywords: chhotanagpur gneissic terrain, high-Ti dolerite, low-Ti dolerite, kerguelen plume, rèunion plume, EPMA

Abbreviations

Abbreviations
LIPs Large Igneous Provinces
CGT Chhotanagpur Gneissic Terrain
CITZ Central Indian Tectonic Zone
EPMA Electron probe microanalysis
WDS Wavelength Dispersive spectroscopy
EDS Energy Dispersive Spectroscopy
CL Cathodoaluminescence
BSEs Backscattered electrons
SEM Scanning Electron Microscope
Plag Plagioclase
Pyx Pyroxene
Olv Olivine
Srp Serpentine
Ilm Ilmenite
Chl Chlorite
HTD High-Ti Dolerite
LTD Low-Ti Dolerite

INTRODUCTION

Background

Large Igneous Provinces (LIPs) are defined as magmatic provinces with areal extents >1 Mkm2, igneous volumes >0.1 Mkm3 and maximum lifespan of ̴50 Ma, having intraplate tectonic settings or geochemical affinities, and are characterized by igneous pulse(s) of short duration (1–5 Ma), during which a large proportion (>75%) of the total igneous volume has been emplaced (​O. Eldholm, 2005​; ​Richard E. Ernst, 2008​). Continental and oceanic flood basalts, volcanic rifted margins, giant dolerite dyke swarms, sill provinces and layered mafic–ultramafic intrusions can be an integral part of any LIP. Amongst these, mafic dyke swarms in shield areas, on account of their vertical depth and lateral extent provide the most complete record of short-lived, mantle-generated magmatic (LIPs) events through time and space (​Halls, 1982​ ; ​Fahrig. 1987​; ​Halls, et al., 1987​; ​Parker, et al., 1990​; ​Baer and Heimann, 1995​; ​Ernst and Buchan, 2001​; ​Hanski, et al, 2006​; ​Bleeker, et al., 2006​; ​Srivastava, et al 2010​). Record of these mafic dyke swarms provides information on ‘‘the pulse of the Earth’’ beyond that related to mid-oceanic ridge oceanic crust generation and record the rhythm of intraplate mantle melting events (​Ernst and Buchan, 1997​; ​Bleeker, 2004​; ​Ernst, et al., 2010​). Mantle plumes are also believed to be the source for many or all of the LIPs.

The entire Indian shield consists of mafic dykes and dyke swarms of variable compositions, structural orientations, and emplacement periods that are profoundly discernible. Although mafic dykes are more common in Archaean cratons, early- and late-Cretaceous mafic and alkaline igneous activity is also well-recorded from the Indian Shield (​Athavale and Verma, 1970​; ​Karkare and Srivastava, 1990​;  Storey et al., 1992​; ​Kent, 1996​; ​Kent et al., 1997​; ​Kent, 1998​; ​Kent, et al, 2002​; ​Srivastava and Sinha, 2004a, ​Srivastava and Sinha, 2004b, ​Srivastava and Sinha, 2007​; ​Paul, 2005​; ​Srivastava, et al, 2005​; ​Srivastava et al., 2008a​; ​Srivastava et al., 2008b​; ​Srivastava et al., 2009​; ​Ray et al., 2007​; ​Misra, 2008​; ​Patil and Arora, 2008​; and references therein). These early- and late-Cretaceous igneous events have been widely attributed to plume tectonics ​(Foulger, 2010 and references therein)​ involving either the Réunion plume (​Courtillot et al., 1988​; ​White and McKenzie, 1989​; ​Chalapathi Rao and Lehmann, 2011​; ​Chalapathi Rao, et al, 2011​) in case of late-Cretaceous Deccan Traps or the Kerguelen plume in case of early-Cretaceous Rajmahal–Sylhet Traps (​Kent, 1991​; ​Baksi, 1995​; ​Coffin, et al, 2002​; ​Kent et al., 1997​; ​Kent, et al, 2002​; ​Srivastava and Sinha, 2007​). Damodar valley may be the only region that has witnessed both the early Cretaceous Rajmahal as well as the late Cretaceous Deccan igneous activities. ​

Statement of the Problems

This report presents new mineralogical, petrological and geochemical data on wide-spread Cretaceous mafic dykes intruding the Gondwana sedimentary basins in the Damodar valley within the Chhotanagpur Gneissic Terrain (CGT) at the northern-most margin of the Singhbhum craton. The availability of petrological and geochemical information available on these dykes compared to their Precambrian counterparts is exiguous. Palaeomagnetic data on these dykes is also limited.The genetic association of these Cretaceous mafic dykes with the Deccan or Rajmahal–Sylhet Traps is vague since both of these events have been described from the Damodar valley and the CGT domain.

Objectives of the Research

Petrographic and mineralogical studies of dolerite dykes from Chhotanagpur Gneissic Terrain located at Koderma, Raniganj, and Jharia.

GEOLOGICAL SETTING AND STRATIGRAPHY

Mafic dykes of different ages (Mesoproterozoic to Cretaceous) are well exposed in the CGT. A number of ENE–WSW to E–W trending Mesoproterozoic mafic dykes intrude the basement gneisses of the CGT. There is much debate over the relationship between Singhbhum craton and CGT. Some workers consider CGT as a mobile belt and distinct terrain ( ​Ghose, et al., 1983​ ; ​Mukhopadhyay, et al., 1988​; ​Mahadevan, 2002​; ​​Ghose, et al​​ and references therein), others view it as a cratonised mobile belt ( ​Naqvi, et al., 1987​; ​Kumar, et al​; ​Sharma, et al., 2009​; ​Srivastava, et al, 2009​; ​Srivastava, et al., 2012​). ​Sharma, et al., 2009​ discussed this issue at length and stated many geological reasons that questioned its mobile belt nature. It is believed that Singhbhum craton consists of Singhbhum nucleus, Chhotanagpur Gneissic Terrain and Singhbhum mobile belt; latter separates the former two blocks (Fig. 1; cf. ​Naqvi, et al., 1987​; ​Srivastava et al., 2014​).

geologic map 3.jpg
    (A) Geological map of the Chhotanagpur Gneissic Terrain (CGT) and distribution of Gondwana basins in the Damodar Valley, Eastern India (modified after Acharyya, 2003; Srivastava et al., 2012). VB: Vindhyan basin; CHB: Chhattisharh basin; EGB: Eastern Ghats Belt. (B) Simplified geological map of part of CGT showing Damodar valley Gondwana sedimentary basins and Rajmahal basalts (modified after Mahadevan, 2002). BMB: Bihar mica belt; Jh: Jharia; Rg: Raniganj; Bk: Bokaro; Kp-N: North Karanpura; Kp-S: South Karanpura.

    The Chotanagpur Gneissic Terrain is an ENE-WSW trending high grade terrain in the eastern part of Central Indian Tectonic Zone (CITZ), covering an area about 80,000 km2. It is bordered by Singhbhum mobile belt in the south, the Quaternary Gangetic alluvium in the north and Rajmahal basalts in the northeast. CGT is separated from the CITZ in the west by younger Gondwana sediments. The CGT mainly consists of gneisses and migmatites with enclaves of metasedimentary rocks of varying grade. These are intruded by metabasic, anorthositic and granitic plutons alongwith major shear zones in the E-W direction with strike extension of about 500 km and a width of 200 km. Younger mafic, ultramafic and alkaline intrusives of early tertiary age are present. Rocks are deformed and metamorphosed ranging from greenschist to granulite facies of metamorphism but mostly exhibiting ambhibolite facies (​Sharma, et al., 2009​). Detailed structural studies have identified three stages of deformation (​Ghose, et al., 1983​) which gave rise to the distinct fold patterns and related linear fabric.

    In the absence of reliable geochronological data, the stratigraphic relationships of the different lithologies of the CGT may be understood from field observations. Granitoids such as syenite, nepheline syenite and alkali granite intruded the massif anorthosite. Metamorphosed mafic and ultramafic rocks are mostly concordant and are co-folded with the gneisses and the metasediments. Metabasic xenoliths occur both in the massif anorthosite and the granitoids. These relationships indicate that the metabasic rocks are younger than the basement gneisses ​Ernst and Buchan, 1997​ and metasediments, but are older than the massif anorthosites and the granite plutons, the latter being younger than the massif anorthosite (1550±12 Ma, ​Chatterjee et al, , , 2007a​). The magmatism, however, continued in Phanerozoic time and are represented by volcanic eruptions in the Late Triassic rhyodacite at Mahuadanr in the Netarhat plateau, south of Daltonganj Gondwana basin, within the granite gneiss basement (217-214 Ma, ​Sarkar et al, 1974​), and Lower Cretaceous Rajmahal flood basalts (118 Ma) and contemporary tholeiitic dolerite and lamprophyric dyke swarms (113-115 Ma) in the Gondwana basins (​Baksi, 1995​; ​Kent et al, 1998​; ​Kent et al, 2002​). The Salma dolerite (65 Ma) in the Raniganj coalfield close to the eastern India shield margin, equivalent to Deccan volcanism, marks the cessation of magmatic activities in the CGC.

    ​Kent, et al, 1997​ have presented petrological, geochemical and isotopic data for mafic dykes from the Raniganj (including the 65 Ma Salma dyke), Jharia and Koderma areas, whereas ​Paul, 2005​ has presented geochemical data for a 45–55 km long and 50–100 m wide, NNW trending 65 Ma Salma dyke intruding the Raniganj coal field as well the Precambrian basement of the CGT. These Cretaceous mafic dykes, mostly very fresh and doleritic in nature, intrude the Gondwana sedimentary basins of the Damodar valley . There are other mafic dykes with similar trend (NW to NNW) to that of the Salma are also reported from the Raniganj basin. Their length varies between 100 and 600 m and width ranges from 10 to 50m (​Srivastava et al., 2014​). Another set of mafic dykes having an absolutely different trend (NNE to ENE) are reported to cut across Jharia basin but not the adjacent Precambrian basement. The biggest dyke of this group is 6 km in length and occurs near Madhuban. Others vary in length from 50 m to 500 m and width of 10 m to 20 m. A number of 110–115 Ma NE trending mafic dykes are also reported to cut the Precambrian rocks around Koderma (see Fig. 1B) ( ​Agarwal and Rama, 1976​; ​Kent, et al, 1997​). Thus, the field observations bring out two sets of Cretaceous mafic dykes; one trending in NW to NNW (also includes the Salma dyke) and other in NNE to ENE direction.

    One of these mafic dykes, named Salma dyke, has been dated by Ar–Ar method to be of 65.4 ± 0.3 Ma ( ​Kent, et al, 2002​) which is close to the age of the main eruption of the Deccan volcanism (​Chenet, et al, 2007​). On the other hand, a number of NE trending mafic dykes have K–Ar ages 10 Ma (​Agarwal and Rama, 1976​; ​Kent, et al, 1997​; ​Kent R. W., 2002​). Subsequently ​Kent, et al, 1997​, ​Kent R. W., 2002​ , have dated one of these Koderma dolerite dykes by Ar–Ar method which yielded a plateau age at 115.3 ± 0.4 Ma. Another dolerite dyke exposed around Kalidaspur area of the Raniganj basin gave Ar–Ar plateau age at 112.5 ± 0.5 Ma (​Kent et al, 2002​). These ages are closer to Kerguelen plume-related early Cretaceous igneous activities

    METHODOLOGY

    The order in which the petrography and mineralogy is carried out is as follows:

    • Thin section preparation,
    • Petrography of dykes samples using polarizing microscope
    • EPMA analysis of mineral phases in thin sections

    Thin Section Preparation

    • The samples were cut in proper size using petrocut machine.
    • One of its sides was grinded using carborundum powder having a mesh value of 120 and 220.
    • Further that side was polished using powder having values 400,600 and 800 respectively.
    • The rock was then mounted into glass slide using Araldite as adhesive (40 gm of hardener + 50 gm of resin) at a temperature of 50ºC which is obtained by keeping it over a hot plate machine.
    • To avoid air bubbles, the glass slide is kept under bonding fixer and left for 20-24 hours for drying.
    • A thickness of 30 microns was acquired by placing the sample in petrothin machine.
    • The section was then polished over a nylon sheet using 1μm; 0.3μm; 0.05μm alumina gel.

    Electron Probe Microbe Analysis

    Electron probe microanalysis (EPMA) is an analytical technique that is widely used for determining the elemental composition of solid specimens. Also EPMA is able to produce maps showing the distribution of elements over the surface of a specimen while also accurately measuring their concentrations. It is also a non-destructive technique.

    Principle of the technique

    EPMA makes use of the x-ray spectrum emitted by a solid sample when it is bombarded with a focused beam of electrons. All elements from atomic number 4 (Be) to 92 (U) can be detected. When these beams hit the atoms at the surface of a sample, it ejects the electrons from inner higher energy shells (K, L & M), creating a vacancy that is filled by electrons from outer low energy shells. The higher-shell electron sheds energy in the form of an x-ray, which is characteristic of the particular element. Hence, these are called characteristic x-rays. By comparing their intensities with those emitted from standard samples, it is also possible to determine the concentration of the elements quantitatively

    The qualitative analysis is simple as in this only the lines in the spectrum need to be identified. On the other hand, quantitative analysis involves measuring line intensities for each element in the sample and for the same elements in calibration standards of known composition.

    .

    PANO_20190703_124749.jpg
      CAMECA SXFive (EPMA)

      Techniques

      EPMA consists of 2 analytical techniques:

      • Wavelength Dispersive spectroscopy (WDS)
      • Energy Dispersive Spectroscopy (EDS)

      In WDS emitted x-rays are separated based on their wavelengths; on the other hand, in EDS the emitted x-rays are separated according to their energies. WDS, has better spatial resolution than EDS; however it is a more time-consuming technique. EDS, may be used to conduct quick initial analysis to identify the major elements, while in WDS can perform a more detailed analysis to identify the trace elements and the concentrations with more accuracy.

      EPMA instruments also hold an optical microscope for pinpointing area of interest and three detectors

      • Cathodoaluminescence (CL) related with crystal structure, trace impurities, lattice defects and crystal distortion
      • Secondary electrons, similar to SEM, providing topographical information by using low energies to escape from the sample surface with few nm.
      • Back scattered images (BSE) decipher atomic number differences based on fact that no. of BSE increases with atomic number (brighter area indicate greater mean atomic number).

      Accuracy and sensitivity

      It has high analytical sensitivity (<0.5% for major elements) and detection limits (100 ppm for trace elements).

      Spatial resolution

      The spatial resolution is governed by the penetration and spreading of the electron beam in the specimen. Since the electrons penetrate an approximately constant mass, spatial resolution is a function of density. In case of silicate minerals (like pyroxene, quartz, feldspar etc.), the density of which is mostly about 3 g/cc, the nominal resolution is about of 2 m under typical conditions although it can be upto 1m.

      Sample preparation

      Since EPMA only deals up to a shallow depth of sample, specimens should be well polished so that surface roughness does not affect the results. As many geological samples are electrically non-conducting, the surface of the polished sample is coated with conducting material. Coating must be applied to provide a path for the incident electrons to flow to ground. The usual coating material is carbon (20 nm thick carbon coating), which has a minimum influence on x-ray intensities on account of its low atomic number.

      The basic output of EPMA x-ray map is a spectrum of peaks that represents x-rays with EDS or WDS, where each peak corresponds to a specific element. The size and proportions of phases identified in the analyzed material can be determined by combining several x ray maps. Raw data produced by EDS, WDS and other detectors are automatically acquired, processed and analyzed by software supplied with EPMA, displaying the data as quantitative tables or x-ray maps. Mineral chemistry of collected samples was determined by CAMECA SXFive instrument at DST-SERB National Facility, Department of Geology (Center of Advanced Study), Institute of Science, Banaras Hindu University. Polished thin section were coated with 20 nm thin layer of carbon for electron probe micro analyses using LEICA EM ACE200 instrument. The CAMECA SXFive instrument was operated by SXFive Software at a voltage of 15 kV and current 10 nA with a LaB6 source in the electron gun for generation of electron beam.

      BSE images interpretation

      Backscattered electrons (BSEs) are generated by elastic scattering events. When the electrons in the primary beam travel close to the atom’s nuclei in the specimen, their trajectory is deviated due to the force they feel with the positive charges in the nuclei. Depending on the size of the atom nuclei, the number of backscattered electrons differs. In order to form an image with BSE (backscattered electrons), a detector is placed in their path. When they hit the detector a signal is produced which is used to form the image. All the elements have different sized nuclei. As the size of the atom nucleus increases, the number of BSE increases. Thus, BSE can be used to get an image that showed the different elements present in a sample. Larger atoms (with a greater atomic number, Z) have a higher probability of producing an elastic collision because of their greater cross-sectional area. Consequently, the number of backscattered electrons (BSE) reaching a BSE detector is proportional to the mean atomic number of the sample. Thus, a "brighter" BSE intensity correlates with greater average atomic number in the sample, and "dark" areas have lower average atomic number.

      RESULTS AND DISCUSSION

      Petrography

      Most of the mafic dykes samples studied were medium to coarse grained and doleritic in nature. Some fine grained samples were also observed which were collected close to the margins of the dykes intrusion in CGT. Mafic dykes trending NNE-ENE are medium to coarse grained with ophitic-sub ophitic texture (Fig. 3.1 to 3.4). Some coarse varieties exhibit hypidiomorphic texture (Fig. 5.3-5.4). Clinopyroxenes and plagioclase are major minerals. Ilmenite and titanomagnetite are present in sufficient amount. Due to the presence of Ilmenite (FeTiO3) and Ti-rich magnetite (Fig. 3.1 to 3.4), both of which contain titanium, this variety is classified as High Ti Dolerites. Accessory phases include biotite, titanite etc.

      mineral plate 3.jpg
        Photomicrographs of the studied dolerite samples from CGT. Ophitic texture with lath shaped plagioclase embedded in large pyroxene grains can be seen.
        mineral plate 1.jpg
          Photomicrographs of the studied dolerite samples from CGT. 4.1,4.2 represents a Ti rich Cpx grain at the center surrounded by plagioclase laths and other Cpx grains in plane polarized and cross polarized light respectively. 4.3, 4.4 represents a serpentine grain formed by the alteration of olivine in PPL and XPL respectively
          mineral plate 2.jpg
            Photomicrographs of the studied dolerite samples from CGT. 5.1, 5.2 Serpentine with relicts of olivine in PPL and XPL respectively. 5.3, 5.4 Hypidiomorphic texture in PPL and XPL respectively
            low Ti- BSE.jpg
              BSE images of dolerite dykes from CGT.  A- Subhedral grain of ilmenite surrounded by plagioclase and  pyroxene. B- Anhedral pyroxene crystal with plagioclase and specs of ilmenite. C, D- Ilmneite grains with pyroxene and plagioclase.
              High Ti (KD-14-11).jpg
                BSE images of dolerite dykes from CGT. A, C- Subhedral ilmenite grain with pyroxene and plagioclase. B, D- Olivine grains along with plagioclase and ilmenite. Variation in brightness within olivine may be due to compositional variation or alteration into serpentine.
                High Ti- RJ-13-1.jpg
                  BSE images of dolerite dykes from CGT. A, B- Ilmenite grains with pyroxene and plagioclase. C, D- Chlorite grain with plagioclase and pyroxene.

                  NW-NNW trending Salma dykes and other similar dykes, were relatively coarser, Showed ophitic- sub ophitic or hypidiomorphic texture (Fig. 5.3-5.4). Clinopyroxenes and plagioclase are major constituents. These samples are classified as Low Ti Dolerites as these constituted ilmenite and magnetite with low modal proportion of Ti. Also, clouding of plagioclase grain is seen which may be due to alteration

                  Olivine and Orthopyroxene were also seen in some samples (Fig. 5.1-5.2, Fig. 7.B-7.D). Alteration of olivine into serpentine was frequent (Fig.4.3-4.4, Fig. 5.1-5.2). Also the presence of chlorite in some samples (Fig. 8.C-8.D) indicated the evidences of alterations. Fine grained samples of both these varieties display porphyritic texture.

                  Mineralogy

                  During petrography, minerals such as pyroxene, feldspar, ilmenite and titanomagnetite were observed as major phases in the dyke samples respectively. The geochemical data has been obtained through EPMA technique which is represented in table 1-3 below

                  Pyroxene

                  Three varieties of pyroxenes were determined from the dyke samples i.e. Augite, Pigeonite and Orthopyroxene having compositional range Wo4.4-34.4En32.5-74.1Fs11.9-45.5. TiO2 ranges from 0.36-1.12 wt. %. Major oxides include SiO2 (48.72- 54.41 wt. %), FeO (6.94-27.33 wt. %), MgO (10.72-27.31 wt. %), CaO (2.25-18.64 wt. %). Using the triangular classification plot of pyroxene (Fig. 9), it was inferred that the composition of majority of the grains correspond to augite, followed by pigeonite and orthopyroxene.

                  pyroxene.jpg
                    Ternary classification plot of Wollastonite-Enstatite-Ferrosilite (after Deer et al. 2013)

                    Feldspar

                    The Plagioclase grains displayed a complete solid solution series ranging from pure Anorthite to pure Albite (Fig. 10). The composition of majority of the grains were equivalent to that of Labradorite. The end member variation is An2-97.2Ab21.2-58.5Or0.4-52.2. SiO2 variation ranges from 47.35-73.40 wt. %. Other major oxides include Al2O3 (1.83-31.58 wt. %), CaO (0.32- 17.98 wt. %), K2O (0.07-8.55 wt. %), Na2O (0.24-4.98 wt. %). K-Feldspar grains (probably sanidine) were also determined.

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

                      lmenite

                      Ilmenite grains from this study have high concentration of titanium (TiO2: 48.41-50.70 wt. %) and iron (FeO: 42.70-46.69 wt. %). Presence of other oxides are almost negligible (table 3). The ternary classification diagram of grains with high modal concentrations of TiO2 (Fig. 11) clearly indicate that these grains correspond to ilmenite

                      .

                      spinel.jpg
                        : Ternary classification plot of TiO2-FeO-Fe2O3 (after Deer et. al 2013) for ilmenite and iitanomagnetite

                        Titanomagnetite

                        Titanomagnetite grains are found to be rich in iron oxides (FeO: 0.00-64.95 wt. %; Fe2O3: 0.00-74.96 wt. %) and titanium (TiO2: 21.07-32.06 wt. %). They are deficit in terms of Aluminium (Al2O3: 1.09-4.38 wt. %), magnesium (MgO: 0.00-1.36 wt. %), manganese (MnO: 0.00-1.92 wt. %), calcium (CaO: 0.00-0.21 wt. %) and other oxides. From figure 11, we can infer that the titanonomagnetite grains with similar concentrations of TiO2 shows two types of variations w.r.t. FeO and Fe2O3 content. Some of the samples plot in the Pseudobrookite- Magnetite division while the others plot near the Ulvospinel region. The former have a greater concentration of FeO and the later are rich in Fe2O3 but poor in FeO.

                        Mineral chemistry (oxide wt. %) of the studied pyroxene from dolerite dykes of CGT
                        Oxide KD-13/2 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 SM-13/2 SM-13/2 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1
                        SiO2 49.85 51.16 52.18 52.49 51.27 51.22 50.69 52.80 53.47 53.19 49.87 48.72 50.23 53.05 54.41 51.50 52.31 52.15
                        TiO2 0.83 0.93 0.50 0.51 0.95 0.93 1.12 0.46 0.42 0.36 0.56 1.08 1.23 0.54 0.37 0.65 0.60 0.61
                        Al2O3 2.48 1.40 1.80 1.71 1.28 1.37 1.57 0.84 0.90 0.91 0.77 1.31 2.36 0.95 0.84 1.79 1.92 1.69
                        Cr2O3 0.00 0.02 0.37 0.43 0.02 0.03 0.01 0.01 0.03 0.00 0.06 0.19 0.30 0.16 0.23 0.49 0.72 0.49
                        FeO 12.50 11.95 7.42 7.44 13.41 13.88 12.94 16.48 15.83 14.52 27.33 20.02 14.84 15.00 13.64 7.13 7.74 6.94
                        MnO 0.13 0.29 0.10 0.24 0.44 0.34 0.41 0.26 0.38 0.28 0.23 0.58 0.61 0.55 0.47 0.32 0.32 0.35
                        MgO 14.38 15.09 17.56 17.71 15.43 15.13 14.66 22.31 23.80 23.63 14.89 10.72 16.06 22.31 27.31 17.75 17.36 17.80
                        CaO 17.57 17.38 18.64 18.23 17.00 16.91 18.15 4.96 4.90 5.73 5.01 14.99 14.05 6.05 2.25 17.87 18.19 17.51
                        Na2O 0.29 0.30 0.33 0.26 0.25 0.25 0.36 0.08 0.09 0.21 0.15 0.19 0.24 0.08 0.04 0.23 0.26 0.26
                        K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.08 0.05 0.05 0.07 0.05 0.05
                        TOTAL 98.02670000000001 98.50380000000001 98.8986 99.01549999999997 100.0519 100.0555 99.90809999999999 98.20 99.84 98.84 98.86 97.85 100.00 98.72 99.64 97.79 99.46 97.88
                        Cations based on 6 oxygen
                        Si 1.912 1.945 1.939 1.945 1.931 1.932 1.917 1.974 1.961 1.964 1.959 1.935 1.895 1.969 1.965 1.933 1.936 1.950
                        Al 0.088 0.055 0.061 0.055 0.057 0.061 0.070 0.026 0.039 0.036 0.036 0.061 0.105 0.031 0.035 0.067 0.064 0.050
                        0.024 0.007 0.017 0.020 0.000 0.000 0.000
                        Al 0.057 0.024 0.043 0.018 0.067 0.061 0.088 0.012 0.000 0.004 0.000 0.000 0.000 0.011 0.001 0.013 0.019 0.024
                        Fe(iii) 0.000 0.001 0.011 0.013 0.001 0.001 0.000 0.000 0.032 0.040 0.035 0.023 0.070 0.000 0.020 0.034 0.018 0.000
                        Cr 0.024 0.027 0.014 0.014 0.027 0.026 0.032 0.000 0.001 0.000 0.002 0.006 0.009 0.005 0.007 0.015 0.021 0.015
                        Ti 0.342 0.355 0.187 0.212 0.353 0.375 0.318 0.013 0.012 0.010 0.016 0.032 0.035 0.015 0.010 0.018 0.017 0.017
                        Fe(ii) 0.004 0.009 0.003 0.008 0.014 0.011 0.013 0.516 0.452 0.407 0.860 0.640 0.395 0.466 0.391 0.189 0.221 0.217
                        Mn 0.822 0.855 0.973 0.979 0.866 0.851 0.826 0.008 0.012 0.009 0.008 0.019 0.019 0.017 0.014 0.010 0.010 0.011
                        Mg 0.722 0.708 0.742 0.724 0.686 0.683 0.735 1.244 1.302 1.301 0.872 0.635 0.903 1.235 1.470 0.993 0.958 0.992
                        Ca 0.022 0.022 0.024 0.018 0.018 0.018 0.027 0.199 0.193 0.227 0.211 0.638 0.568 0.240 0.087 0.719 0.721 0.701
                        Na 0.000 -0.000 0.000 0.000 0.001 0.000 0.000 0.006 0.007 0.015 0.011 0.014 0.017 0.006 0.003 0.017 0.018 0.019
                        K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.004 0.002 0.002 0.003 0.002 0.003
                        TOTAL 4.017 4.007 4.013 4.006 4.020 4.018 4.026 3.997 4.009 4.012 4.009 4.006 4.021 3.997 4.006 4.011 4.006 3.999
                        End Members
                        Wo 1.4 0.9 0.8 2.0 0.8 1.4 1.4 1.0 1.9 1.4 2.0 2.5 1.5 1.4 1.1 1.5 1.6 1.1
                        En 42 44 50 50 44 43 42 63 65 66 44 32 46 63 74 51 50 52
                        Fs 21 20 12 12 22 23 21 27 25 23 45 35 25 25 21 12 13 12
                        Mineral chemistry (oxide wt. %) of the studied Feldspar from dolerite dykes of CGT
                        Oxide KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14-/1 KD-14/11 SM-13/2 SM-13/2 SM-13/2 SM-13/2 SM-13/2 SM-13/2 SM-13/2 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1 RJ-13/1
                        SiO2 53.26 52.77 52.63 55.48 51.25 51.02 53.72 51.20 55.89 52.66 55.11 57.94 53.24 52.57 52.26 54.07 55.20 53.81 47.35 51.93 52.40 51.19 50.43 52.14 51.84 52.56 54.65
                        TiO2 0.00 0.00 0.00 0.00 0.02 0.00 0.04 0.01 0.00 0.03 0.23 0.19 0.21 0.23 0.21 0.22 0.27 0.22 0.18 0.70 0.19 0.19 0.20 0.20 0.20 0.18 0.20
                        Al2O3 27.82 28.36 28.30 26.10 29.63 29.36 28.20 29.80 26.80 28.79 26.64 25.46 28.17 28.20 28.75 27.97 27.32 28.07 31.58 1.83 28.67 29.27 29.85 28.91 29.61 29.13 27.43
                        FeO 0.14 0.15 0.17 0.50 0.47 0.37 0.44 0.54 0.62 0.54 0.72 0.74 0.82 0.77 0.61 0.65 0.95 1.11 0.40 8.35 0.63 0.64 0.81 0.82 0.82 0.53 0.77
                        CaO 11.97 12.11 12.11 9.64 13.38 13.37 11.45 13.13 10.24 12.03 9.61 8.24 11.71 12.06 12.27 11.57 10.77 11.14 15.75 17.98 12.41 13.05 13.47 12.43 12.77 12.54 10.51
                        Na2O 4.66 4.60 4.49 5.97 4.18 3.92 4.75 4.09 5.80 4.61 6.00 6.84 4.87 4.72 4.49 4.87 5.39 5.29 2.37 0.24 4.63 4.07 3.89 4.33 4.24 4.42 5.73
                        K2O 0.18 0.16 0.15 0.36 0.18 0.13 0.24 0.18 0.34 0.25 0.35 0.44 0.25 0.25 0.19 0.26 0.28 0.19 0.13 0.07 0.18 0.16 0.16 0.25 0.21 0.23 0.31
                        TOTAL 98.02 98.15 97.85 98.05 99.11 98.18 98.84 98.94 99.69 98.91 98.66 99.86 99.27 98.79 98.80 99.61 100.17 99.83 97.75 81.10 99.10 98.57 98.81 99.07 99.69 99.59 99.60
                        Cations based on 32 oxygen
                        Si 9.836 9.742 9.744 10.206 9.467 9.467 9.838 9.433 10.134 9.672 10.101 10.443 9.752 9.689 9.622 9.845 9.989 9.801 8.893 12.340 9.627 9.471 9.333 9.587 9.483 9.600 9.950
                        Ti 0.000 0.000 0.000 0.007 0.000 0.000 0.005 0.001 0.000 0.004 0.032 0.026 0.029 0.031 0.029 0.030 0.037 0.030 0.025 0.124 0.026 0.026 0.028 0.027 0.027 0.025 0.028
                        Al 6.055 6.171 6.174 5.659 6.420 6.420 6.085 6.470 5.727 6.232 5.754 5.409 6.081 6.126 6.239 6.002 5.827 6.025 6.989 0.513 6.207 6.382 6.511 6.264 6.383 6.271 5.886
                        Fe 0.022 0.024 0.026 0.077 0.058 0.058 0.067 0.083 0.093 0.083 0.111 0.111 0.126 0.118 0.094 0.100 0.143 0.170 0.063 1.659 0.096 0.099 0.125 0.126 0.126 0.081 0.117
                        Ca 2.368 2.396 2.403 1.901 2.658 2.658 2.246 2.591 1.990 2.368 1.888 1.591 2.298 2.380 2.420 2.257 2.087 2.174 3.169 4.578 2.442 2.586 2.671 2.448 2.503 2.453 2.049
                        Na 1.668 1.645 1.613 2.130 1.409 1.409 1.687 1.462 2.038 1.641 2.131 2.390 1.728 1.687 1.604 1.720 1.892 1.867 0.862 0.111 1.648 1.458 1.395 1.542 1.504 1.566 2.023
                        K 0.041 0.038 0.034 0.084 0.032 0.032 0.057 0.042 0.080 0.058 0.081 0.101 0.059 0.060 0.045 0.060 0.065 0.045 0.030 0.021 0.042 0.038 0.037 0.058 0.049 0.053 0.072
                        TOTAL 19.990 20.014 19.993 20.064 20.044 20.044 19.986 20.082 20.061 20.058 20.096 20.072 20.072 20.090 20.054 20.014 20.040 20.112 20.033 19.345 20.089 20.060 20.100 20.053 20.075 20.049 20.126
                        End Members
                        An 58.2 58.7 59.3 46.2 64.8 58.2 56.3 63.3 48.4 58.2 46.0 39.0 56.3 57.7 59.5 55.9 51.6 53.2 78.0 97.2 59.1 63.3 65.1 60.5 61.7 60.2 49.4
                        Ab 40.3 40.3 39.8 51.8 34.4 40.3 42.3 35.7 49.6 40.3 52.0 58.5 42.3 40.9 39.4 42.6 46.8 45.7 21.2 2.4 39.9 35.7 34.0 38.1 37.1 38.5 48.8
                        Or 1.4 0.9 0.8 2.0 0.8 1.4 1.4 1.0 1.9 1.4 2.0 2.5 1.5 1.4 1.1 1.5 1.6 1.1 0.7 0.4 1.0 0.9 0.9 1.4 1.2 1.3 1.7
                        Mineral chemistry (oxide wt. %) of the studied Ilmenite & titanomagnetite from dolerite dykes of CGT
                        OXIDES KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-14/11 SM-13/2 SM-13/2 SM-13/2 SM-13/2 SM-13/2 SM-13/2 SM-13/2 RJ-13/1 RJ-13/1 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-13/2 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 KD-14/11 SM-13/2 SM-13/2
                        SiO2 0.07 0.10 0.10 0.09 0.06 0.05 0.05 0.00 0.00 0.00 0.00 0.00 0.23 3.11 0.10 0.09 2.78 0.12 0.12 1.44 0.06 0.03 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.01
                        TiO2 25.72 25.23 24.65 32.06 24.11 25.47 21.07 25.97 25.72 25.50 25.04 27.87 24.22 25.05 24.03 24.74 21.95 24.52 23.58 21.57 21.36 50.14 49.99 49.78 49.67 49.67 48.96 48.98 49.47 48.41 50.17 49.74 49.90 50.15 50.22 50.13 50.58 50.70
                        Al2O3 1.32 1.22 1.09 1.22 1.37 1.11 3.68 1.19 1.09 1.45 1.32 1.61 1.32 2.22 1.25 1.61 4.38 1.52 1.14 1.32 1.62 0.06 0.02 0.00 0.05 0.05 0.06 0.09 0.13 0.03 0.19 0.19 0.18 0.18 0.15 0.15 0.12 0.09
                        Cr2O3 0.04 0.04 0.05 -0.00 0.02 0.02 0.66 0.18 0.20 0.23 0.20 0.21 0.34 0.31 0.30 0.31 0.33 0.36 0.32 0.34 0.39 0.00 0.00 0.00 0.16 0.16 0.16 0.12 0.10 0.15 0.15 0.21 0.17 0.18 0.18 0.15 0.27 0.27
                        V2O3 0.60 0.56 0.41 0.38 0.41 0.43 1.21 0.71 0.62 0.91 1.01 0.83 0.66 0.49 0.67 0.80 0.75 0.76 0.90 1.03 1.03 0.35 0.25 0.16 0.42 0.42 0.44 0.53 0.39 0.42 0.76 0.70 0.68 0.55 0.78 0.79 0.54 0.48
                        Fe2O3 15.20 72.40 73.91 0.00 17.87 16.06 71.15 70.59 70.79 69.46 73.10 67.41 70.44 7.78 74.36 70.34 69.03 71.59 73.30 69.37 74.96 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
                        FeO 53.49 0.00 0.00 64.95 52.12 53.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 56.99 0.00 0.00 0.00 0.00 0.00 0.00 0.00 43.92 45.44 46.69 44.07 44.07 43.31 43.59 44.48 44.25 44.90 45.61 46.64 44.73 45.45 45.08 44.48 42.70
                        MnO 0.63 0.35 0.41 0.41 0.42 0.64 0.43 0.04 0.37 0.00 0.62 0.54 0.68 0.68 0.72 0.57 0.44 0.48 0.72 0.74 1.92 0.64 0.64 0.53 0.36 0.36 0.54 0.38 0.63 0.81 0.62 0.62 0.59 0.69 0.55 0.59 0.64 0.49
                        MgO 0.21 0.07 0.04 0.36 0.09 0.26 1.36 0.08 0.00 0.26 0.00 0.81 0.06 0.08 0.00 0.27 0.10 0.08 0.09 0.55 0.00 0.28 0.27 0.31 0.00 0.00 0.00 0.13 0.61 0.09 0.90 0.19 1.28 0.63 0.86 0.75 0.20 0.16
                        CaO 0.01 0.04 0.16 0.02 0.01 0.05 0.21 0.00 0.06 0.02 0.07 0.02 0.17 0.09 0.11 0.16 0.12 0.12 0.06 0.18 0.08 0.60 0.03 0.04 0.11 0.11 0.13 0.16 0.05 0.13 0.05 0.06 0.05 0.09 0.00 0.05 0.15 0.14
                        ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
                        TOTAL 97.29 100.00 100.82 99.48 96.49 97.08 99.83 98.76 98.86 97.84 101.37 99.31 98.14 96.80 101.54 98.89 99.89 99.53 100.21 96.54 101.41 96.03 96.63 97.52 94.83 94.83 93.60 94.01 95.86 94.28 97.73 97.31 99.49 97.19 98.19 97.70 97.03 95.04
                        Cations based on 32 oxygen
                        Si 0.020 0.031 0.030 0.025 0.018 0.016 0.016 0.000 0.000 0.000 0.000 0.000 0.075 0.938 0.032 0.027 0.856 0.040 0.037 0.468 0.018 0.008 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
                        Ti 5.915 5.999 5.817 7.131 5.602 5.875 4.884 6.228 6.196 6.142 5.874 6.611 5.855 5.687 5.626 5.918 5.086 5.848 5.598 5.256 5.008 11.843 11.752 11.589 11.931 12.193 11.914 11.847 11.681 11.675 11.590 11.605 11.275 11.675 11.552 11.600 11.845 12.144
                        Al 0.477 0.454 0.405 0.426 0.500 0.400 1.337 0.446 0.411 0.548 0.486 0.600 0.501 0.791 0.457 0.602 1.589 0.567 0.423 0.503 0.594 0.023 0.008 0.000 0.018 0.010 0.024 0.033 0.049 0.011 0.070 0.068 0.065 0.066 0.054 0.055 0.045 0.034
                        Cr 0.011 0.011 0.012 0.000 0.006 0.005 0.162 0.044 0.050 0.059 0.050 0.053 0.086 0.073 0.073 0.079 0.080 0.090 0.079 0.088 0.095 0.000 0.000 0.000 0.040 0.040 0.040 0.030 0.026 0.037 0.035 0.052 0.040 0.044 0.044 0.038 0.066 0.068
                        V 0.146 0.142 0.103 0.089 0.101 0.107 0.299 0.182 0.160 0.234 0.252 0.210 0.171 0.119 0.168 0.204 0.186 0.193 0.229 0.267 0.258 0.089 0.062 0.039 0.107 0.160 0.115 0.138 0.098 0.107 0.186 0.175 0.163 0.136 0.192 0.196 0.134 0.123
                        Fe(iii) 3.497 0.000 0.000 1.172 4.153 3.706 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.767 17.417 16.834 16.001 17.058 17.382 16.885 17.556 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
                        Fe(ii) 13.674 0.000 0.000 14.889 13.462 13.593 0.000 0.000 0.000 0.000 0.000 0.000 0.000 14.384 0.000 0.000 0.000 0.000 0.000 0.000 0.000 11.533 11.876 12.085 11.769 11.272 11.715 11.723 11.676 11.863 11.530 11.832 11.717 11.579 11.624 11.598 11.581 11.373
                        Mn 0.163 0.094 0.110 0.102 0.111 0.166 0.111 0.115 0.101 0.145 0.163 0.144 0.186 0.173 0.189 0.153 0.116 0.128 0.192 0.204 0.506 0.170 0.169 0.138 0.099 0.162 0.148 0.102 0.167 0.219 0.161 0.162 0.149 0.180 0.143 0.153 0.168 0.133
                        Mg 0.095 0.034 0.018 0.160 0.043 0.117 0.625 0.021 0.000 0.126 0.000 0.380 0.031 0.038 0.002 0.129 0.047 0.037 0.041 0.267 0.000 0.132 0.124 0.145 0.000 0.124 0.000 0.062 0.285 0.042 0.412 0.087 0.573 0.290 0.391 0.343 0.092 0.077
                        Ca 0.003 0.013 0.053 0.005 0.004 0.015 0.069 0.026 0.020 0.008 0.024 0.008 0.059 0.030 0.036 0.053 0.039 0.041 0.019 0.063 0.027 0.202 0.009 0.013 0.036 0.040 0.044 0.056 0.018 0.046 0.016 0.019 0.018 0.029 0.000 0.018 0.050 0.046
                        Zn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
                        TOTAL 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 23.992 24.000 24.000 24.000 24.000 24.000 24.000 24.000 24.000 23.982 23.998

                        CONCLUSION AND RECOMMENDATIONS

                        Petrographic study of these dykes indicate the presence of plagioclase, pyroxene and ilmenite as the major mineral phases. Ophitic-subophitic texture is observed in almost all dyke samples. Some coarse grained dyke samples also exhibit hypidiomorphic texture. Two varieties of dykes is reported based on the concentration of titanium (Srivastava et al., 2014). One with higher concentration of titanium and the other with relatively lower values. The former varieties were termed as High-Ti Dolerites whereas the later were classified as Low-Ti Dolerite dykes. There is presence of serpentinized olivine, represented by yellowish green color under plane polarized light in microscope which was not reported in earlier studies. Mineralogical study indicate the presence of pyroxenes classified as augite, pigeonite and orthopyroxenes. Plagioclase composition shows a complete solid solution series between albite and anorthite component along with occurrences of some grains of K-Feldspar. Ilmenite and titanomagnetite is identified as mineral phases having high Ti concentrations. Two varieties of titanomagnetite has been observed which belongs to ulvospinel-magnetite and Pseudobrookite-magnetite series. Besides that, some pyroxene also contains significant amount of Ti.

                        Although the presence of Ti rich phases were identified from the dolerite dykes, the primary processes responsible for their formation are not identified. Further analysis of these dolerite dyke samples is required to to address these processes. Also, the petrographic and mineralogical study were carried out in a limited number of samples; robust studies can be useful in determining various other aspects of crystallization evolution of Ti-rich dolerite dykes.

                        ACKNOWLEDGEMENTS

                        Foremost, I would like to express my sincere gratitude to my guide Prof. Rajesh K. Srivastava for the continuous support and enlightening me with his immense knowledge and motivation during my summer research. His guidance helped me in all the time of research and writing of this report.

                        I express my sincere gratitude to Dr. Dinesh Pandit and Dr. Amiya Kumar Samal for their encouragement, insightful comments, and constant help during the course of the period of my summer research and helping me out while preparing this report.

                        My sincere thanks also goes to Dr. Raj Kumar Singh for motivating and recommending me to opt for summer research opportunities through Indian Academy Of Sciences.

                        I thank my fellow labmates, seniors and research scholars in BHU for helping me out in lab works and guiding me throughout the preparation of this report.

                        I also want to thank Indian Academy of Sciences for providing me a great platform to work with some of the finest Professors in one of the oldest and prestigious universities in India.

                         I would also like to thank AuthorCafe for providing a great platform to write our reports, offering help in every step.

                         Lastly, I would like to thank my parents for their constant support and encouragement throughout my life.

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                        Source

                        • Fig 1: Srivastava et al. Petrology and geochemistry of high-titanium and low-titanium mafic dykes from the Damodar valley, Chhotanagpur Gneissic Terrain, eastern India and their relation to Cretaceous mantle plume(s). Journal of Asian Earth Sciences, Elsevier, pg 36)
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