- Email: [email protected]
Rock magnetic and paleomagnetic investigations of Sylhet traps, Shillong Plateau, NE India
Journal of Geodynamics 127 (2019) 31–41
Contents lists available at ScienceDirect
Journal of Geodynamics journal homepage: www.elsevier.com/locate/jog
Rock magnetic and paleomagnetic investigations of Sylhet traps, Shillong Plateau, NE India M.R. Kapawar, Venkateshwarlu M.
T
⁎
CSIR-National Geophysical Research Institute, Hyderabad, 500007, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Rock magnetism AMS Paleomagnetism ST basalts Shillong Plateau
Magnetic signals in basalts from the Sylhet traps (ST), Shillong Plateau, India is investigated using rock magnetism as a proxy to explain the potentiality of the ST basalts as an indicator of magnetic mineralogy. A paleomagnetic study was carried out in order to unravel the Paleopole. In detail, rock magnetic and Anisotropy of Magnetic susceptibility (AMS) results have been reported for the first time on the ST basalts. A total of 38 oriented block samples were collected from 8 sites along Cherrapunji-Shella bazaar road section, Meghalaya, India. Samples were subjected to rock magnetic measurements such as magnetic susceptibility (K), Isothermal Remanent Magnetization (IRM), hysteresis loops, backfield (coercivity remanence) and k-T (Susceptibility vs. Temperature) analysis to identify the magnetic properties of basalts. Advanced Variable Field Translation Balance (AVFTB) was used for some rock magnetic studies. A few samples were also subjected to Anisotropy of Magnetic susceptibility (AMS) study to understand the petrofabric orientations and shape anisotropy in the ST basalts. The Remanence ratio (Mrs/Ms) and the Coercivity ratio (Bcr/Bc) ranging from 0.105 to 0.333 and from 1.25 to 1.87 respectively indicate sizes of magnetic grains are mainly in the range of Pseudo Single Domain (PSD) limits with minority of them in the ranges of Single Domain (SD) limits. The results of k-T study in combination with other rock magnetic results shows that magnetite (Fe3O4) [Curie Temperature = 565-590 °C] as the dominant magnetic carrier with a subordinate content of titanomagnetite Fe2+(Fe3+Ti)2O4 [Curie Temperature = 380–450 °C]. From the AMS study, parameters like lineation, foliation are calculated, and results define all the three shape anisotropies present in the ST basalts. The ST basalts display paleomagnetic directions with mean declination = 255°, mean inclination = -57° (α95 = 3.44°; k = 16.74). The pole position of the ST at 117 Ma is -26.79 °N, 330.75 °E (dp = 3.6; dm = 5) with a paleolatitude 37.6 °S. This infers that the ST is a later volcanic event than Rajmahal traps (RT) which is considered contemporary to the ST.
1. Introduction After breaking-up from Gondwana supercontinent the Indian plate has started its northward drift which is affirmed by the numerous paleomagnetic studies carried out on the Deccan Traps (Kono, 1972; Radhakrishnamurthy, 1990; Vandamme, 1991), Rajmahal Traps (McDougall, 1970; Sherwood, 1996; Poornachandra Rao, 1996a, 1996b) and Sylhet Traps (Athavale, 1963; Poornachandra Rao, 1993). The Rajmahal-Bengal-Sylhet igneous region form a part of Large Igneous Province (LIP) which experienced a mantle plume activity attributed to the Kerguelen hotspot at around 118 Ma (Kent et al., 2002). The present study is the first attempt to carry out detailed rock magnetic and AMS studies of the ST basalts to evaluate magnetic mineralogy, their domain states, and the shape anisotropy parameters. AMS study was focused to evaluate the lava flow trends and possible fissure
⁎
location. In regard to paleomagnetic study, the Sylhet Traps lava flows are least studied (Athavale, 1963; Poornachandra Rao, 1993) which could not provide enough information due to the paucity of reliable paleomagnetic data. No clear expression has been reported by them whether the secondary component was completely removed to achieve the primary magnetization component. Also no comments about, which statistical analysis method was followed to get primary ChRM directions. Eventually no firm statement was made about Indian subcontinent paleogeographic position at around 117 Ma ago from these studies. In an effort to contribute to global Gondwana paleomagnetic dataset we have attempted the paleomagnetic study of Sylhet Traps basalts using high sensitive instruments. Here we are reporting the paleopole for India during the Early Cretaceous period (˜117 Ma) from Sylhet Traps, Shillong Plateau, India.
Corresponding author. E-mail address: mamila_v@rediffmail.com (V. M.).
https://doi.org/10.1016/j.jog.2019.05.003 Received 2 November 2018; Received in revised form 5 April 2019; Accepted 5 May 2019 Available online 06 May 2019 0264-3707/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
2. Geology and sampling
samples (2 samples/site) were selected for rock magnetic study. IRM acquisition curves and Backfield demagnetization analysis was performed for getting saturation field and coercivity spectra respectively by using the Pulse magnetizer (MMPM-10). For IRM acquisition and backfield demagnetization standard specimens were used. Advanced Variable Field Transition Balance (AVFTB) was used for Hysteresis Loop and thermomagnetic (k-T) study. A hysteresis loop is generated by applying alternating magnetic fields to samples until they reach saturation. The hysteresis loop analysis was performed to understand the grain size and domain states, while the k-T study is for knowing the Curie point and thermal alterations in magnetic carriers during heating and cooling of samples. For these measurements to perform on AVFTB, powdered samples were used (all weighing in the range of 300–500 mg).
The Sylhet lavas were first documented by Palmer (1923) and subsequently their geological setting, petrochemistry and tectonic history was reported by Talukdar (1967), Talukdar and Murthy (1971). The mafic volcanic rocks of the Shillong Plateau are represented by the Sylhet Traps (Baksi, 1995) and exposed ˜450 km east of the Rajmahal Traps. The Sylhet Traps (ST) constitutes an integral part in Shillong Plateau and is a typical plateau basalt volcanism and spread over an area of ˜700 Km2 located at the southern extremity of state of Meghalaya, NE India. The extent of the ST is small, and is exposed in a narrow strip of ˜80 km long and 5-6 km wide E-W belt, at the southern margin of the Shillong Plateau. The ST is sandwiched between Dawki fault from the south and Raibah fault or Brahmaputra fault from the north, and is overlain by Cretaceous Sandstones (Athavale, 1963) in Meghalaya. The effusion through linear fissures is characteristic of the ST flood basalts (Talukdar and Murthy, 1971) as neither feeder dikes nor volcanic vents have been reported within the ST formation. On the basis of plate reconstructions as well as geochronological and geochemical data, the early episode of Kerguelen volcanism is believed to be related to a flood basalt province in eastern India comprising Rajmahal- Bengal- Sylhet traps of 116 ± 3.5 Ma (e.g. Pantulu et al., 1992; Baksi, 1995; Basu et al., 2001; Ghatak and Basu, 2006). Twenty continuous lava flows and three tuff beds with a total thickness of ˜259 m are reported by (Ray, 2005), and also quoted that, the Sylhet Traps are contemporaneous with the Kerguelen plume generated Rajmahal and Bengal Traps evidenced from 40Ar-39Ar geochronology with age 116.0 ± 3.5 Ma. The ST is situated ˜450 km east of RT and the portion between the RT and ST has been overlain by alluvium deposited by Brahmaputra and Ganga Rivers. The ST overlies the eroded Precambrian basement complex, and is in turn overlain by Cretaceous sandstone of Shillong Plateau (Talukdar and Murthy, 1971). The ST of Shillong Plateau is exposed to the north of Dawki fault and to the south of Raibah fault and these boundaries would have been the controlling factor for the effusion of the ST basalts (Talukdar and Murthy, 1971). The exposures exhibit dark grey to faint black appearance and finegrained texture with mostly north-easterly dips. Structures like 'flow breccia', 'layering' and 'flow folds’ (Talukdar and Murthy, 1971), rhyolites with lense shaped amygdals associated with volcanic ash and tuff (Talukdar, 1967) were reported. Field observations during sample collection suggests the presence of varieties of basalts from compact basalts, partly jointed/ fractured basalts to amygdaloidal basalts suggests about the degree of crystallization and viscosity of lava flows. These ST basalts are exposed along the valley slopes and deep gorges and for sampling in present study the exposures were obtained near quarry sections at Kynrem falls. Few samples contained calcitic veins and amygdals of diameter 1-5 mm filled with zeolites of greenish color. Standard sample collection technique was followed for collection of oriented samples. Fresh, unaltered samples were collected and avoided deformed outcrops. A minimum of 5 oriented rock samples were collected from each site. The sites were located along the CherrapunjeeShella bazaar road section and quarries near Kynrem falls (Fig. 1) whereas other exposure locations were disturbed by constructions. The samples were oriented using a Brunton compass. Each oriented rock sample was drilled and cut into 176 standard cylindrical specimens (2.5 cm in diameter and 2.2 cm in length). These 176 specimens (on an average 5-6 specimens/sample; 5 samples/ site) were obtained from 8 sites.
3.2. AMS methodology Anisotropy of Magnetic Susceptibility (AMS) study was carried out on 114 (∼14 from each site) fresh specimens from 8 sites using AGICO MFK1-FA Susceptibility apparatus to investigate the nature of petrofabrics and shape anisotropy. The MFK1-FA has three operating frequencies and field intensities; we kept the measuring range, i.e., 976 Hz of frequency and 2–700 A/m field intensity. Practically, the AMS study was first performed by Voight and Kinoshita (1907). Statistical data processing by Jelinek (1978) was performed for mean tensors and by using software supplied by AGICO named ANISOFT 4.2 (Chadima and Jelinek, 2009). AMS can be described as an ellipsoid of magnetic susceptibilities with three principle susceptibility axes designated as the maximum (Kmax), intermediate (Kint), and minimum (Kmin). These quantities can be combined in various ways to describe different features of the ellipsoid and of the corresponding magnetic fabrics (Hrouda, 1982). The anisotropy parameters like, bulk susceptibility Km= (Kmax + Kint + Kmin)/3, magnetic foliation F (Kint + Kmin)/Kmax and magnetic lineation L = (Kmax + Kint)/Km (Khan, 1962) are defined. The shape parameter ‘T’ is defined as (2 η2-η1- η3) / (η1- η3) where, η1= ln(Kmax), η2= ln(Kint) and η3= ln(Kmin). The magnitude of susceptibility is measured in 15 independent directions which enables to comment about the AMS ellipsoid, which is a standard least square fit method for the reliability of measurements (Cox and Doell, 1967). 3.3. Paleomagnetism procedures Paleomagnetic studies were executed using AGICO JR-6 Spinner magnetometer, Molspin Alternating Field (AF) demagnetizer and MMTD-80A Thermal demagnetizer. These specimens were subjected to laboratory demagnetization techniques to isolate the Characteristic Remanent magnetization (ChRM) directions for the ST basalts. Standard alternating field demagnetization (AFD) and thermal demagnetization (ThD) procedures were adopted for demagnetization. AFD was performed on 95 specimens in successive 17 demagnetization step intervals 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 30, 40, 60, 80, 100, 120, 140 and 150 m T. Whereas stepwise ThD was performed on 48 specimens in 12 progressive demagnetization steps from 100, 200, 300, 350, 400, 450, 500, 520, 540, 560, 580 up to 600 °C. There was no considerable decay observed in NRM intensity in progressive thermal and AF demagnetization up to ∼400 °C and 80 m T respectively for most of the specimens. Nevertheless, a high-temperature component (HTC) and high field component (HFC) which decays towards the origin from ∼400 °C and 80 m T respectively to total demagnetization can be fitted using Principal Component Analysis (PCA) method (Kirschvink, 1980) of a best-fit line on Remasoft-3.0 software. For determination of both HFC and HTC at least six consecutive demagnetization intervals were chosen that decay towards the origin of orthogonal projections which yields ChRM directions. The Zijderveld plot (Zijderveld, 1967) is used to display and evaluate the demagnetization behaviors for all the specimens. All the measurements were carried out at the
3. Laboratory experiments 3.1. Rock magnetic properties To identify the main magnetic carriers of ST basalts an array of rock magnetic measurements was carried out. Sixteen representative 32
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 1. Simplified Geological map of the Shillong plateau showing the location of sampling sites of ST basalts (red dot) and the two fault trends the Dawki fault and the Raibah fault (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Paleomagnetism laboratory, CSIR-National Geophysical Research Institute, Hyderabad, India.
ST-2 showing 50 m T of Bcr value which is the indication of presence of comparatively higher coercivity magnetic mineral than other samples.
4. Results
4.1.2. Hysteresis loop From the Hysteresis curves (Fig. 3) for the ST basalts, hysteresis parameters saturation magnetization (Ms), saturation remanence (Mrs), coercivity force (Bc) and coercivity remanence (Bcr) were determined. The hysteresis ratios known separately as Remanence ratio (Mrs/Ms) and Coercivity ratio (Bcr/Bc) can be obtained (Supplementary Table 1), which are helpful in identifying the domain states and grain sizes of magnetic minerals. The maximum field applied is 1000 m T. The inverse relationship between the ratios is similar to Day plots for ferrimagnetic magnetite and titanomagnetite (Day et al., 1977; Dunlop, 2002a, 2002b). The hysteresis curve lines seem to be closer (thin) and such behaviour of curves is an indication of fine to medium magnetic grain size of the ST basalts. Most of the samples analysed are showing Remanence ratio (Mrs/ Ms) ranging from 0.105 to 0.333 with an average value of 0.218 and values of Coercivity ratio (Bcr/Bc) range between 1.25 to 1.87 with an average 1.593. Overall the values obtained are indicating, the ST basalts reside in Single Domain (SD) to Pseudo Single Domain (PSD) state of magnetic grains. Day plot (Fig. 4) which gives an idea of limiting values for SD, PSD and MD magnetic grains and useful tool for magnetic granulometry indicator. Usually, the values of most of the magnetitebearing rocks fall in MD state, where SD and PSD plays a role of paleomagnetic recorders. But in the case of these samples most of the samples fall in PSD state of magnetic grain size which may be due to combined effect of SD and MD grains.
4.1. Rock magnetic study The rock magnetic parameters are useful in understanding the magnetic mineralogy, their grain size, domain states and Curie temperatures. This is the first attempt of detailed rock magnetic study and implications associated with it for the ST basalts. The magnetic minerals in basalts generally belong to the solid solution series known as titanomagnetites solid solution series (Radhakrishnamurthy, 1990). 4.1.1. Isothermal remanent magnetization (IRM) acquisition curves and backfield demagnetization A pulse magnetizer was used for IRM and back-field DC demagnetization measurements. The significant fall in IRM during thermal demagnetization allows for estimation of curie temperatures because maximum blocking temperatures are always slightly less than the Curie temperature. The majority of the samples show the continuous rise in IRM curve (Fig. 2a) up to 200 m T and saturate. Sample ST_1, ST_2, and ST_6 show a steep rise up to 200 m T, and this rises gently up to 1000 m T and saturation is noted which is indication of the presence of magnetic minerals of titanomagnetite solid solution series. These IRM acquisition curves are due to the magnetocrystalline anisotropy in samples, those samples which saturated near 200 m T are due to magnetization acquired in easy direction and others are due to acquisition of magnetization in hard direction. The back-field DC demagnetization curves (Fig. 2b) indicates the remanence coercivity (Bcr) values of the samples range between 24–35 m T suggests low coercive magnetic mineral present in samples.
4.1.3. Thermomagnetic (k–T) curves The thermomagnetic (k–T) curve indicate the Curie temperature (Tc) and the degree of alteration that occurs during heating. Tc of pure 33
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 2. Representative curves of (a) isothermal remanent magnetization (IRM) acquisition and (b) back-field DC demagnetization shows generalized coercivity distribution in samples of ST basalts.
Fig. 3. Hysteresis curves for the ST basalt samples of Shillong Plateau. Curves show presence of soft (low Bc) and fine grained magnetic carrier.
magnetite is ˜577 °C and a similar behaviour is noted in the basalt samples of present study. Most of the samples analysed have shown the Tc ˜580 °C. The Curie temperature data for all the analysed ST basalt samples is given (Supplementary Table 2) along with the rock samples description. The analysed samples for Thermomagnetic (k–T) curves (Fig. 5) were heated from room temperature up to 700 °C and again cooled back to room temperature in the presence of 100 m T magnetic field. The k–T curves show a small decrease in the susceptibility at temperatures around 380–450 °C which is considered as Tc1 and afterwards falling to
zero at around 580 °C which is Tc2. This is the observation for most of the analysed samples. As per Vahle et al. (2007) the degree of reversibility of heating and cooling run allows an estimate of phase changes, which can be interpreted in terms of stability of the original magnetic phases. Within these variations, the heating cycle displays less thermal stability and lower intensity than cooling cycle. The cooling and heating curves are appearing irreversible due to the intermediate magnetic phase but similar for all the samples where heating curves are always higher than cooling curves. The magnetic phases in samples are interpreted as not stable since they are irreversible. This intermediate 34
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
stereoplots are depicted in Fig. 6 suggests presence of different shape anisotropies due to the effect of the magnetic carrier for resultant AMS ellipsoids. Normally, low degree of anisotropy values i.e., less than 1.03 depicts variation in the resultant shape anisotropy of AMS ellipsoids, i.e., from prolate to oblate. Out of eight sites examined four sites i.e., ST_4, ST_5, ST_6 and ST_8 are showing a triaxial ellipsoidal shape anisotropy (where all the principle susceptibility axes K1, K2, and K3 form distinct and individual groups with same orientations). Three sites are showing Prolate ellipsoidal shape (maximum axes were showing well grouping and directed perpendicular to the bedding plane, whereas the minimum axes lie in the bedding plane), i.e., ST_1, ST_2, and ST_7 here the K1 is well grouped, K2 and K3 arced within 90° around it. Site ST_3 is showing oblate ellipsoidal shape anisotropy (where K3 is well grouped; K1 and K2 are scattered through the stereonet and within the 90° arc around K3). From the calculated mean susceptibility (Km) and anisotropy degree (P) of the ST basalts, it is helpful in identifying the occurrence and contribution of mineral towards susceptibility (if any) and its control over anisotropy. To understand this, Km vs. P plot (Fig. 7 A) prepared for the whole sampling unit and the samples analysed show the major ferrimagnetic contribution to the anisotropy of magnetic susceptibility which understood from even scattering. The Jelinek plot (1978) in Fig. 7B, shows both prolate as well as oblate nature of the fabric for the ST basalts and no clear majority can be identified. This could be because of eruption of lava in a very small period of time with slow rate of successions and can be confirmed as there was no intertrappean beds are found in study area. The Pj vs. T plot (Fig. 7C) is for representing both shape of susceptibility ellipsoid and its magnitude. For the ST basalts it shows a transitional prolate to oblate nature of the ellipsoid. The scattering through the plot may indicate presence of dominant unstable inverse magnetic fabric, and can be confirmed by the presence of all three (prolate, oblate and triaxial) anisotropies in samples. The Pj values in the ST basalts range between 1.018 to 1. 031. The T and Km values ranging from -0.155 to 0.322 and 1.11 × 10−1 to 8.31 × 10−2 SI units respectively (Supplementary Table 3). It suggests that the direction of remanence will not significantly alter if the ratio between the maximum and minimum susceptibilities (degree of anisotropy) is lower than 5% (Hrouda, 1982). The orientations of AMS ellipsoids in these samples proposes no solid evidence about the magma flow directions and the fissure location through which the lava could have erupted and the ST came into existence. Due to this we restrict our conclusions from AMS measurements up to Shape anisotropy only.
Fig. 4. Day plot of Mrs/Ms and Bcr/Bc after analysis method by Day et al. (1977) to understand the domain state of magnetic grains for the ST basalts. SD = Single Domain, PSD = Pseudo Single Domain, and MD = Multi-Domain. Red dots depict analysed samples that are falling in respective domain states (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
magnetic phase can be inferred as titanomagnetite. The transformation of titanomagnetite to magnetite was reported to occur between 350 and 700 °C (Dunlop and Ozdemir, 1997), depending on the presence of impurities and grain size of titanomagnetite. The behaviour of the k-T curves in the Fig. 5 shows two curie temperature points, i.e., TC1 and TC2 at around 380–450 °C and ˜580 °C, respectively which indicates the presence of two ferrimagnetic carriers with one dominant, the magnetite (˜580 °C) and other as an accessory, the titanomagnetite (around 380–450 °C). We can also speculate that, the appearance of titanomagnetite in heating curves and the absence in cooling curves indicate the complete transformation of titanomagnetite to magnetite with progressive temperature. The combination of all these rock magnetic measurements suggest that the main magnetic remanence carrier resides in magnetite (Fe3O4) of PSD/SD state with subordinate titanomagnetite [Fe2+(Fe3+, Ti)2O4].
4.3. Paleomagnetism 4.2. AMS study The results of paleomagnetic study from 8 sites are presented in Table 1 and plotted in Fig. 8. Natural Remanent Magnetization (NRM) intensities of all specimens vary between 0.15–4.68 A/m. The samples were subjected to demagnetization techniques i.e., Alternating field demagnetization (AFD) and Thermal demagnetization (ThD). The remanent magnetization after each demagnetization step was measured on JR6- spinner magnetometer having signal to noise ratio of 2.4 μA/ m6. The Orthogonal projections of demagnetization curves (Zijderveld plot) of representative samples are provided for both AFD (Fig. 8A) and ThD (Fig. 8B). During AF demagnetizations the NRM’s of the samples have decreased effectively between 20–120 mT steps where it is observed that the viscous component is fully removed and ChRM directions are attained between steps 20–120 mT. Similarly, in the thermal demagnetization most of the samples are showing either the decrease in intensities beyond 520 °C or remaining nearly constant and immediate fall is recognised at the unblocking temperatures around 580 °C. Magnetic Susceptibility was measured after each thermal demagnetization interval to take review of chemical changes occurred during heating. In these samples no considerable changes in susceptibility have been recognised, revels that the magnetic phases are stable
To check the reliability of data for lava flow directions, the AMS study was carried out for the ST basalt samples of Shillong Plateau. Many studies of dykes, lava flows and ignimbrites (Staudigel et al., 1992; Canon-Tapia et al., 1997; Varga et al., 1998; Zhu et al., 2003; Caballero-Miranda et al., 2009) have been carried out to deduce the magma emplacement dynamics and source recognition. AMS ellipsoids depends on the mineral abundances and different mineral AMS (Borradaile and Jackson, 2010). The common AMS parameters like, mean susceptibility (Km), common anisotropy factors lineation (L) and foliation (F), anisotropy degree (P), corrected anisotropy degree (Pj) and shape parameters T and U were evaluated (Supplementary Table 3) from the three principle susceptibility axes (K1, K2 and K3). The magnetic susceptibility (K) values were measured by using MFK1-FA Susceptibility apparatus and range between 0.14 and 8.81 × 10-4 SI units which suggests samples suitability to perform AMS study and as a substitute indication for chemical composition variation during formation of rocks. The K values are high enough and suggests significant contribution of ferrimagnetic carrier for the resultant AMS fabric (Tarling and Hrouda, 1993). AMS 35
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 5. Representative thermomagnetic (k–T) curves for the ST basalts of Shillong Plateau. Heating (red) and cooling (blue) cycles are shown with respective arrows (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
detected in studied samples. These rock magnetic studies combined suggest the presence of magnetite (Fe3O4) as the main magnetic carrier of PSD/SD state with subordinate content of titanomagnetite [Fe2+(Fe3+,Ti)2O4]. This magnetic carrier is in agreement with the results obtained from thermal demagnetization on the very same samples, whose intensity decays completely at ˜580 °C.
with increasing temperatures. The data acceptance limits for ChRM calculations of individual specimens are constrained within MAD ≤ 50 and α95 ≤ 100, as per the standard fisherian statistics analysis proposed by Fisher (1953) and which are further considered for mean Virtual Geomagnetic Pole (VGP) position calculations. An unblocking temperature of ˜580 °C is consistent with the combined rock magnetic results, supporting our initial conclusions that the primary remanence carrier is magnetite. Mean ChRM directions for the ST basalts were calculated, displays a mean direction of Dm = 255°, Im = -57°, α95 = 3.44° and k = 16.74. Calculations for each site have been made to get Virtual Geomagnetic Pole (VGP) using present day latitude: 25.21 °N and longitude: 91.71 °E of the location.
5.2. AMS study and associated implications The AMS fabrics in the samples depict the arrangement of magnetic grains and appearance of all three AMS ellipsoids reveals a weak degree of anisotropy. Particularly for magnetite grains, the domain size is controlled by grain shape. Though the presence of titanomagnetite minerals is confirmed in rock magnetic measurements they have a minor contribution to the AMS. Therefore, it is presumed that AMS of the ST basalts is dominated by a low coercive PSD/SD magnetic grains of magnetite. Anisotropy of the magnetic susceptibility caused by different phenomena such as magma flow, tectonism or other biasing factors (Tarling and Hrouda, 1993; Dragoni et al., 1997; Elming and Mattsson, 2001). In most of the stereonets of the AMS ellipsoids are scattered, specifies less alignment of fabrics due to higher viscosity and slower velocity of magma flow. The horizontal movement of the eastern block (Shillong Plaetau) along the Dawki fault might have affected the AMS in these samples. It can be concluded that ST marks the tale of the magma emplacement activity in Rajmahal- Bengal- Sylhet Traps
5. Discussion 5.1. Significance of rock magnetic tests The rock magnetic properties of the ST basalts show less variation, which is purely controlled by cooling history of lava. The IRM acquisition curves and coercivity spectrum suggests the presence of predominantly low coercivity component and few are showing intermediate coercivity component. Day plot drawn from the values of hysteresis parameters depicts, the presence of PSD and SD state dominated magnetic mineral. Also the thin nature of hysteresis loops suggests the presence of fine to medium size of magnetic grains. Curie point behavior of samples around 380–450 °C and 580 °C have been 36
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 6. Equal area projection demonstrates the distribution of the principal susceptibility axes in the analysed ST basalt samples. Principal susceptibility axes K1 as squares (blue), K2 as triangles (green) and K3 as circles (pink). Comparatively larger arcs are with 95% confidence circles (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
ChRM’s at the field intervals of 20–120 mT and at the temperature interval of 400–580 °C. The comparison of mean directions of various rock formations from India (Table 2) with the data obtained for the ST basalts from present study.
province. 5.3. ChRM directions comparisons The Paleomagnetic study on the ST basalts was initiated by Athavale (1963) and followed by Poornachandra Rao (1993) where they have reported the very first paleomagnetic directions from the ST basalts. They have reported two directions one is explained by referring it with the contemporary Rajmahal traps while the other was not explained. In the present study, the paleomagnetic directions obtained are similar to the one which was unexplained previously. The paleomagnetic measurements done previously was by using classical instruments which might not have cautiously removed the secondary (viscous) magnetization components to meet primary magnetization component. They have not reported (Athavale, 1963) the intervals of demagnetizations to achieve ChRM’s by AF demagnetization and thermal demagnetization techniques. In the present study to get accurate ChRM directions, samples were demagnetized up to 150 m T in AFD and up to 600 °C in ThD measurements which completely removed the secondary component to obtain the isolated ChRM directions. Here we have reported our
5.4. Larger Paleomagnetic story of India Though there is a small extent of ST is present at Shillong Plateau, it is suggested that during its formation process it might have behaved like a different tectonic unit (or maybe two magma emplacement events occurred for the existence of ST). The paleomagnetic directions revealed by these exposures were severely affected by the horizontal movement along dawki fault which has shifted around 250 Km eastward (Evans, 1964) from RT exposures. Since the obtained directions from the present study has varying azimuths by almost 100° with stable dip as compared to previously reported directions, is suggestive of horizontal adjustments within the Sylhet Traps exposures. This we concluded by the effect of horizontal movement of ST and surrounding block along the Dawki tear fault for about 250 Km’s. This final explanation agrees with the study of Evans 37
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 7. A) is the specimen-wise bivariate scatter plot of susceptibility mean vs. degree of anisotropy. B) Jelinek plot of lineation vs foliation (L vs. F) for the ST basalts. C) Flinn diagram of corrected anisotropy degree vs shape parameter (Pj versus T; Tarling and Hrouda, 1993).
6. Conclusions
(1964), where it is concluded that, the north-eastern part of Indian subcontinent has moved eastward by about 250 Km. This study yields a mean Inclination value −57° and from this, we calculated the paleolatitude for India during the Early Cretaceous period (˜117 Ma) which is around 37.6°S. This is the first time we are reporting the paleolatitude of India from Sylhet Traps. One previous study by Poornachandra Rao (1996b) from Rajmahal Traps has reported the paleolatitude of India at 43 °S. From this, it is clearly understood that there may be more than one lava eruption events in a shorter interval of time at Kerguelen hotspot while the Indian plate was drifting northwards. Fig. 9 depicts the relative paleolatitude of India calculated from the data of the present study on Sylhet Traps basalts.
We are presenting here for the first time the detailed rock magnetic, anisotropy of magnetic susceptibility study results carried out on basalts of the Sylhet Traps and drawn following conclusions: 1 The combined rock magnetic results indicate that these basalt contains magnetite as the primary magnetic carrier with PSD and SD state nature of magnetic grains and with low titanomagnetite mineral content. 2 All the three (triaxial, prolate and oblate ellipsoids) possible shape anisotropies are present in the ST basalt samples which will not allow to give concrete statement about lava flow dynamics in the
Table 1 Concise data of Paleomagnetic results of ST basalts from this study. Site Name
Rock Type
Lat (0)
Long (0)
N
n
Dm (0)
Im (0)
α95 (0)
k
MAD
Plat (0)
Plong (0)
dp
dm
ST-1 ST-2 ST-3 ST-4 ST-6 ST-7 ST-8
Basalt Basalt Basalt Basalt Basalt Basalt Basalt Grand Mean
25.21 25.22 25.22 25.22 25.22 25.23 25.22
91.71 91.71 91.71 91.71 91.72 91.63 91.70
2 3 2 3 4 5 5
8 6 8 9 16 13 24
213 260 237 266 270 234 260 255
−53 −49 −44 −52 −42 −67 −61 −57
16.6 18 11.4 11.75 7.9 5.4 5.5 3.44
12 14.76 24.58 23.19 23.89 60.01 29.66 16.74
4.23 4.10 4.51 4.01 3.12 3.45 3.01 3.77
−60.1 −20.57 −39.18 −16.82 −10.39 −42.2 −23.95 −26.79
337.22 336.22 347.76 333.16 339.84 315.63 324.91 330.75
15.9 15.7 8.9 11 5.9 7.4 6.5 3.6
23 23.8 14.3 16.1 9.7 8.9 8.4 5
Lat and Long are site latitude and site longitude in degrees, N = no. of samples, n = no. of specimens, Dm = Mean Declination, Im = Mean Inclination, α95 = circle of confidence with 95% probability level, k = Precision parameter, MAD = maximum angular deviation, Plat and Plong = paleoco-ordinates of the Pole, dp and dm = oval of confidence with 95% probability within which the true pole lies. 38
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 8. Zijderveld’s plot showing orthogonal projections with solid (open) data points representing vector endpoints projected onto the horizontal (vertical) plane. Table 2 The comparison of mean directions of various formations from India with the data obtained for the ST basalts from present study. Sr. No.
Formation
Age
Locality
Site Coordinates Lat (N)
D (°)
I (°)
α95 (°)
Reference
338 339 164 157 327 320.6 332 & 243 235 255
−32 −57 +48 +51 −64 −65.2 −59 & −60 −62 −57
5 – 2
Deutsch, 1959 Sahastrabudhe, 1963 Sahastrabudhe, 1963 Verma, 1967 Clegg, 1958 Poornachandra Rao, 1996a Athavale, 1963
Long (E)
1 2 3 4 5 6 7
Upper Deccan Traps Upper Deccan Traps Lower Deccan Traps Tirupati Sandstone Rajmahal traps Rajmahal traps Sylhet traps
Eocene Eocene Upper Cretaceous Middle Cretaceous Jurassic Early Cretaceous Jurassic
Nipani Mahabaleshwar Linga Janampeta Rajmahal hills Rajmahal hills –
16° 17° 21° 16° 25° – 25°
26’ 55’ 38’ 46’
8 9
Sylhet traps Sylhet traps
– Early Cretaceous
– Cherrapunjee
– 25° 13’
39
74° 22’ 73° 38’ 78° 55’ 81 ° 9’ 87° 51’ – 91° – 91° 43’
2 (α50) 6 7 16 5 3.44
Poornachandra Rao, 1993 Present Study
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 9. The red square indicates the paleolatitude of India obtained from the Sylhet trap basalts (37.6° S) of the present study. (Redrawn after C. T. Klootwijk, 1976) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
study area. Therefore we limit our AMS conclusions up to shape anisotropy only. 3 The ST basalts revealed a paleomagnetic direction with a mean declination = 255° and mean inclination = −57° (α95 = 3.44°; k = 16.74) and the paleopole position of -26.79 °N, 330.75 °E (dp = 3.6 and dm = 5). The obtained paleomagnetic directions can also give idea about the post-collisional migration of eastern block of Indian subcontinent (Shillong Plateau) from the Rajmahal Traps along the Dawki fault for about 250 Km and during this the block might have rotated a little. 4 A paleolatitude for the ST basalts obtained at 37.6 °S clearly indicates that the Sylhet Traps volcanic event is the later episode than the Rajmahal Traps.
of magnetic susceptibility analysis of the Cantera Ignimbrite, San Luis Potosi, Mexico: flow source recognition. Earth Planets Space 61, 173–182. Canon-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1997. The internal structure of lava flows-insights from AMS measurements II: Hawaiian pahoehoe, toothpaste lava and a’a. J. Volcanol. Geotherm. Res. 76, 19–46. Chadima, M., Jelinek, V., 2009. Anisoft 4.2: Anisotropy Data Browser for Windows. Agico, Inc. Clegg, J.A., Radhakrishnamurthy, C., Sahastrabudhe, P.W., 1958. The Remanent magnetism of the Rajmahal traps of north-eastern India. Nature 181, 830–831. Cox, A., Doell, R.R., 1967. Measurement of high coercivity magnetic anisotropy. In: Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Paleomagnetism. Elsevier, Amsterdam, pp. 477–482. Day, R., Fuller, M.D., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: Grain size and composition dependence. Phys. Earth Planet. Inter. 13, 260–267. Deutsch, E.R., Radhakrishnamurthy, C., Sahastrabudhe, P.W., 1959. Paleomagnetism of the Deccan traps. Ann. Geophys. 16, 39. Dragoni, M., Lanza, R., Tallarico, A., 1997. Magnetic anisotropy produced by magma flow: theoretical model and experimental data from Ferrar dolerite sills (Antarctica). Geophys. J. Int. 128, 230–240. Dunlop, D.J., 2002a. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data. J. Geophys. Res. 107 (B3), 2056. Dunlop, D.J., 2002b. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 2. Application to data for rocks, sediments, and soils. J. Geophys. Res. 107 (B3), 2057. Dunlop, D.J., Özdemir, Ö., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge Univ. Press, New York, pp. 573. Elming, S.A., Mattsson, H., 2001. Post jotnian basic intrusions in the fennoscandian shield, and the breakup of baltica from Laurentia: a palaeomagnetic and AMS study. Precambrian Res. 108 (3–4), 215–236. Evans, P., 1964. The tectonic framework of Assam. J. Geo. Soc. of India 5, 80–96. Fisher, R.A., 1953. Dispersion on a sphere. Proc. R. Soc. Lond. Ser. A 217, 295–305. Ghatak, A., Basu, A.R., 2006. Remnants of the Kerguelen Plume in Eastern India: evidence from Nd-Sr-Pb isotopic and trace element geochemistry of Sylhet traps, Shillong Plateau. AGU Fall Meeting. Hrouda, F., 1982. Magnetic anisotropy of rocks and its application in geology and geophysics. Geophys. Surv. 5 (1), 37–82. Jelinek, V., 1978. Statistical processing of anisotropy of magnetic susceptibility measured on groups of specimens. Studia Geophysica et Geodaetica 22, 50–62. Kent, R.W., Pringle, M.S., Muller, R.D., Saunders, A.D., Ghose, N.C., 2002. 40Ar/39Ar geochronology of the Rajmahal basalts, India, and their relationship to the Kerguelen Plateau. J. Petrol. 43, 1141–1153. Khan, M.A., 1962. Anisotropy of magnetic susceptibility of some igneous and metamorphic rocks. J. Geophys. Res. 67, 2873–2885. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data: examples from Siberia and Morocco. Geophys. J. R. Astro. Soc. 62, 699–718. Klootwijk, C.T., 1976. The drift of the Indian subcontinent: an interpretation of recent paleomagnetic data. Geologische Rundschau. 65 (1), 885–909. Kono, M., Kinoshita, H., Aoki, Y., 1972. Paleomagnetism of the Deccan Trap Basalts in India. J. Geomag. Geoelectr. 24, 49–67. McDougall, I., McElhinny, M.W., 1970. The Rajmahal Traps of India: K-Ar ages and Paleomagnetism. Earth Planet. Sci. Lett. 9, 371–378.
Acknowledgments We thank the Director, CSIR-NGRI for the permission to publish this work (NGRI/Lib/2018/Pub-50). MK thank J. Mallikharjuna Rao and K. P. Sarma for sharing their knowledge. MK is grateful to S. J. Sangode, Ravi Shankar, Raj Kumar, Ramesh Babu, L Srikanth and Sujit Pradhan for their timely help. MK further extend thanks to CSIR, Delhi for the JRF (P-81-101). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jog.2019.05.003. References Athavale, R.N., Radhakrishnamurthy, C., Sahastrabudhe, P.W., 1963. Paleomagnetism of some Indian rocks. Geophys. J. R. Astron. Soc. 7, 304–313. Bakshi, A.K., 1995. Petrogenesis and timing of volcanism in the Rajmahal flood basalt province, northeast India. Chem. Geol. 121, 73–90. Basu, A.R., Weaver, K.L., Sengupta, S., 2001. A Plume Head and Tail in the Bengal Basin and Bay of Bengal: Rajmahal and Sylhet Trap With Surrounding Alkalic Volcanism and the Ninety East Ridge. EOS Transactions. American Geophysical Union v p. V12A–0950. Borradaile, G.J., Jackson, M., 2010. Structural geology, petrofabrics and magnetic fabrics (AMS, AARM, AIRM). J. Struct. Geol. 32, 1519–1551. Caballero-Miranda, C.I., Torres-Hern´andez, J.R., Alva-Valdivia, L.M., 2009. Anisotropy
40
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
structural data. Geology 20, 841–845. Talukdar, S.C., 1967. Rhyolite and alkali basalt from the Sylhet Traps. Khasi Hills, Assam. Curr. Sci. 9, 238–239. Talukdar, S.C., Murthy, M.V.N., 1971. The Sylhet Traps, their tectonic history, and their bearing on problems of Indian flood basalt provinces. Bull. Volcanol. 35, 602–618. Tarling, D., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall, London, pp. 217. Vahle, C., Kontny, A., Gunnlaugsson, H.P., Kristjánsson, L., 2007. The stardalur magnetic anomaly revisited - new insights into a complex cooling and alteration history. Phys. Earth Planet. Inter. 164, 119–141. Vandamme, D., Courtillot, V., Besse, J., 1991. Paleomagnetism and age determinations of the Deccan Traps (India): results of a Nagpur- Bombay traverse and review of earlier work. Rev. Geophys. 29, 159–190. Varga, R.J., Gee, J.S., Staudigel, H., Tauxe, L., 1998. Dykes surfaces lineations as magma flow indicators within the sheeted dyke complex of the Troodos ophiolite, Cyprus. J. Geophys. Res. 103, 5241–5256. Verma, R.K., Pullaiah, G., 1967. Paleomagnetism of Tirupati Sandstones from Godavari Valley, India. Earth Planet. Sci. Lett. 2, 310–316. Voight, W., Kinoshita, S., 1907. Bestimmung absoluter Werte von Magnetiserungszahlen, inbesondere fur Kristalle. Annale der Phys 24, 492–514. Zhu, R., Shi, C., Qingsong, L., 2003. Anisotropy of magnetic susceptibility of Hannuoba basalts, northern China: constraints on the vent position of the lava sequences. Geophys. Res. Lett. 30 (2) 38(1–4). Zijderveld, J.D.A., 1967. In: Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Palaeomagnetism and Rock Magnetism. Elsevier, Amsterdam, pp. 254–286.
Palmer, R.W., 1923. Geology of part of the Khasi and Jaintia Hills. Assam: Rec. Geol. Survey India 55, 143–168. Pantulu, G., McDougall, J.D., Gopalan, K., Krishnamurthy, P., 1992. Isotopic and Chemical Compositions of Sylhet Traps Basalts: Links to the Rajmahal Traps and Kerguelen Hotspot. American Geophysical Union, San Francisco, pp. 328 72. Poornachandra Rao, G.V.S., Mallikharjuna Rao, J., 1996a. Paleomagnetism of the Rajmahal Traps of India: implication to the reversal in the Cretaceous Normal superchron. J. Geomag. Geoelectr. 48, 993–1000. Poornachandra Rao, G.V.S., Subba Rao, M.V., Mallikharjuna Rao, J., 1993. Geochemical and paleomagnetic signatures in Sylhet Trap flood basalt volcanism: preliminary results. Geol. Mag. 130, 163–184. Poornachandra Rao, G.V.S., Mallikharjuna Rao, J., Subba Rao, M.V., 1996b. Palaeomagnetic and geochemical characteristics of the Rajmahal Traps, eastern India. J. Southeast Asian Earth Sci. 13 (2), 113–122. Radhakrishnamurthy, C., Subbarao, K.V., 1990. Palaeomagnetism and rock magnetism of the Deccan traps. Proc. Indian Acad. Sci. Earth Planet Sci. 99 (4), 669–680. Ray, J.S., Pattanayak, S.K., Pande, K., 2005. Rapid emplacement of the Kerguelen plume–related Sylhet Traps, eastern India: evidence from 40Ar-39Ar geochronology. Geophys. Res. Lett. 32, L10303. Sahastrabudhe, P.W., 1963. Paleomagnetism and geology of the Deccan traps. Paper Presented at Seminar on Geophysical Investigation in the Peninsular Shield. Sherwood, G.J., Basu, M.S., 1996. A paleomagnetic and rock magnetic study of the northern Rajmahal Volcanics, Bihar, India. Jr. of Southeast Asian Earth Sci. 13 (2), 123–131. Staudigel, H.G., Gee, J.S., Tauxe, L., Varga, R.J., 1992. Shallow intrusive direction of sheeted dikes in the Troodos ophiolite: anisotropy of magnetic susceptibility and
41
Contents lists available at ScienceDirect
Journal of Geodynamics journal homepage: www.elsevier.com/locate/jog
Rock magnetic and paleomagnetic investigations of Sylhet traps, Shillong Plateau, NE India M.R. Kapawar, Venkateshwarlu M.
T
⁎
CSIR-National Geophysical Research Institute, Hyderabad, 500007, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Rock magnetism AMS Paleomagnetism ST basalts Shillong Plateau
Magnetic signals in basalts from the Sylhet traps (ST), Shillong Plateau, India is investigated using rock magnetism as a proxy to explain the potentiality of the ST basalts as an indicator of magnetic mineralogy. A paleomagnetic study was carried out in order to unravel the Paleopole. In detail, rock magnetic and Anisotropy of Magnetic susceptibility (AMS) results have been reported for the first time on the ST basalts. A total of 38 oriented block samples were collected from 8 sites along Cherrapunji-Shella bazaar road section, Meghalaya, India. Samples were subjected to rock magnetic measurements such as magnetic susceptibility (K), Isothermal Remanent Magnetization (IRM), hysteresis loops, backfield (coercivity remanence) and k-T (Susceptibility vs. Temperature) analysis to identify the magnetic properties of basalts. Advanced Variable Field Translation Balance (AVFTB) was used for some rock magnetic studies. A few samples were also subjected to Anisotropy of Magnetic susceptibility (AMS) study to understand the petrofabric orientations and shape anisotropy in the ST basalts. The Remanence ratio (Mrs/Ms) and the Coercivity ratio (Bcr/Bc) ranging from 0.105 to 0.333 and from 1.25 to 1.87 respectively indicate sizes of magnetic grains are mainly in the range of Pseudo Single Domain (PSD) limits with minority of them in the ranges of Single Domain (SD) limits. The results of k-T study in combination with other rock magnetic results shows that magnetite (Fe3O4) [Curie Temperature = 565-590 °C] as the dominant magnetic carrier with a subordinate content of titanomagnetite Fe2+(Fe3+Ti)2O4 [Curie Temperature = 380–450 °C]. From the AMS study, parameters like lineation, foliation are calculated, and results define all the three shape anisotropies present in the ST basalts. The ST basalts display paleomagnetic directions with mean declination = 255°, mean inclination = -57° (α95 = 3.44°; k = 16.74). The pole position of the ST at 117 Ma is -26.79 °N, 330.75 °E (dp = 3.6; dm = 5) with a paleolatitude 37.6 °S. This infers that the ST is a later volcanic event than Rajmahal traps (RT) which is considered contemporary to the ST.
1. Introduction After breaking-up from Gondwana supercontinent the Indian plate has started its northward drift which is affirmed by the numerous paleomagnetic studies carried out on the Deccan Traps (Kono, 1972; Radhakrishnamurthy, 1990; Vandamme, 1991), Rajmahal Traps (McDougall, 1970; Sherwood, 1996; Poornachandra Rao, 1996a, 1996b) and Sylhet Traps (Athavale, 1963; Poornachandra Rao, 1993). The Rajmahal-Bengal-Sylhet igneous region form a part of Large Igneous Province (LIP) which experienced a mantle plume activity attributed to the Kerguelen hotspot at around 118 Ma (Kent et al., 2002). The present study is the first attempt to carry out detailed rock magnetic and AMS studies of the ST basalts to evaluate magnetic mineralogy, their domain states, and the shape anisotropy parameters. AMS study was focused to evaluate the lava flow trends and possible fissure
⁎
location. In regard to paleomagnetic study, the Sylhet Traps lava flows are least studied (Athavale, 1963; Poornachandra Rao, 1993) which could not provide enough information due to the paucity of reliable paleomagnetic data. No clear expression has been reported by them whether the secondary component was completely removed to achieve the primary magnetization component. Also no comments about, which statistical analysis method was followed to get primary ChRM directions. Eventually no firm statement was made about Indian subcontinent paleogeographic position at around 117 Ma ago from these studies. In an effort to contribute to global Gondwana paleomagnetic dataset we have attempted the paleomagnetic study of Sylhet Traps basalts using high sensitive instruments. Here we are reporting the paleopole for India during the Early Cretaceous period (˜117 Ma) from Sylhet Traps, Shillong Plateau, India.
Corresponding author. E-mail address: mamila_v@rediffmail.com (V. M.).
https://doi.org/10.1016/j.jog.2019.05.003 Received 2 November 2018; Received in revised form 5 April 2019; Accepted 5 May 2019 Available online 06 May 2019 0264-3707/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
2. Geology and sampling
samples (2 samples/site) were selected for rock magnetic study. IRM acquisition curves and Backfield demagnetization analysis was performed for getting saturation field and coercivity spectra respectively by using the Pulse magnetizer (MMPM-10). For IRM acquisition and backfield demagnetization standard specimens were used. Advanced Variable Field Transition Balance (AVFTB) was used for Hysteresis Loop and thermomagnetic (k-T) study. A hysteresis loop is generated by applying alternating magnetic fields to samples until they reach saturation. The hysteresis loop analysis was performed to understand the grain size and domain states, while the k-T study is for knowing the Curie point and thermal alterations in magnetic carriers during heating and cooling of samples. For these measurements to perform on AVFTB, powdered samples were used (all weighing in the range of 300–500 mg).
The Sylhet lavas were first documented by Palmer (1923) and subsequently their geological setting, petrochemistry and tectonic history was reported by Talukdar (1967), Talukdar and Murthy (1971). The mafic volcanic rocks of the Shillong Plateau are represented by the Sylhet Traps (Baksi, 1995) and exposed ˜450 km east of the Rajmahal Traps. The Sylhet Traps (ST) constitutes an integral part in Shillong Plateau and is a typical plateau basalt volcanism and spread over an area of ˜700 Km2 located at the southern extremity of state of Meghalaya, NE India. The extent of the ST is small, and is exposed in a narrow strip of ˜80 km long and 5-6 km wide E-W belt, at the southern margin of the Shillong Plateau. The ST is sandwiched between Dawki fault from the south and Raibah fault or Brahmaputra fault from the north, and is overlain by Cretaceous Sandstones (Athavale, 1963) in Meghalaya. The effusion through linear fissures is characteristic of the ST flood basalts (Talukdar and Murthy, 1971) as neither feeder dikes nor volcanic vents have been reported within the ST formation. On the basis of plate reconstructions as well as geochronological and geochemical data, the early episode of Kerguelen volcanism is believed to be related to a flood basalt province in eastern India comprising Rajmahal- Bengal- Sylhet traps of 116 ± 3.5 Ma (e.g. Pantulu et al., 1992; Baksi, 1995; Basu et al., 2001; Ghatak and Basu, 2006). Twenty continuous lava flows and three tuff beds with a total thickness of ˜259 m are reported by (Ray, 2005), and also quoted that, the Sylhet Traps are contemporaneous with the Kerguelen plume generated Rajmahal and Bengal Traps evidenced from 40Ar-39Ar geochronology with age 116.0 ± 3.5 Ma. The ST is situated ˜450 km east of RT and the portion between the RT and ST has been overlain by alluvium deposited by Brahmaputra and Ganga Rivers. The ST overlies the eroded Precambrian basement complex, and is in turn overlain by Cretaceous sandstone of Shillong Plateau (Talukdar and Murthy, 1971). The ST of Shillong Plateau is exposed to the north of Dawki fault and to the south of Raibah fault and these boundaries would have been the controlling factor for the effusion of the ST basalts (Talukdar and Murthy, 1971). The exposures exhibit dark grey to faint black appearance and finegrained texture with mostly north-easterly dips. Structures like 'flow breccia', 'layering' and 'flow folds’ (Talukdar and Murthy, 1971), rhyolites with lense shaped amygdals associated with volcanic ash and tuff (Talukdar, 1967) were reported. Field observations during sample collection suggests the presence of varieties of basalts from compact basalts, partly jointed/ fractured basalts to amygdaloidal basalts suggests about the degree of crystallization and viscosity of lava flows. These ST basalts are exposed along the valley slopes and deep gorges and for sampling in present study the exposures were obtained near quarry sections at Kynrem falls. Few samples contained calcitic veins and amygdals of diameter 1-5 mm filled with zeolites of greenish color. Standard sample collection technique was followed for collection of oriented samples. Fresh, unaltered samples were collected and avoided deformed outcrops. A minimum of 5 oriented rock samples were collected from each site. The sites were located along the CherrapunjeeShella bazaar road section and quarries near Kynrem falls (Fig. 1) whereas other exposure locations were disturbed by constructions. The samples were oriented using a Brunton compass. Each oriented rock sample was drilled and cut into 176 standard cylindrical specimens (2.5 cm in diameter and 2.2 cm in length). These 176 specimens (on an average 5-6 specimens/sample; 5 samples/ site) were obtained from 8 sites.
3.2. AMS methodology Anisotropy of Magnetic Susceptibility (AMS) study was carried out on 114 (∼14 from each site) fresh specimens from 8 sites using AGICO MFK1-FA Susceptibility apparatus to investigate the nature of petrofabrics and shape anisotropy. The MFK1-FA has three operating frequencies and field intensities; we kept the measuring range, i.e., 976 Hz of frequency and 2–700 A/m field intensity. Practically, the AMS study was first performed by Voight and Kinoshita (1907). Statistical data processing by Jelinek (1978) was performed for mean tensors and by using software supplied by AGICO named ANISOFT 4.2 (Chadima and Jelinek, 2009). AMS can be described as an ellipsoid of magnetic susceptibilities with three principle susceptibility axes designated as the maximum (Kmax), intermediate (Kint), and minimum (Kmin). These quantities can be combined in various ways to describe different features of the ellipsoid and of the corresponding magnetic fabrics (Hrouda, 1982). The anisotropy parameters like, bulk susceptibility Km= (Kmax + Kint + Kmin)/3, magnetic foliation F (Kint + Kmin)/Kmax and magnetic lineation L = (Kmax + Kint)/Km (Khan, 1962) are defined. The shape parameter ‘T’ is defined as (2 η2-η1- η3) / (η1- η3) where, η1= ln(Kmax), η2= ln(Kint) and η3= ln(Kmin). The magnitude of susceptibility is measured in 15 independent directions which enables to comment about the AMS ellipsoid, which is a standard least square fit method for the reliability of measurements (Cox and Doell, 1967). 3.3. Paleomagnetism procedures Paleomagnetic studies were executed using AGICO JR-6 Spinner magnetometer, Molspin Alternating Field (AF) demagnetizer and MMTD-80A Thermal demagnetizer. These specimens were subjected to laboratory demagnetization techniques to isolate the Characteristic Remanent magnetization (ChRM) directions for the ST basalts. Standard alternating field demagnetization (AFD) and thermal demagnetization (ThD) procedures were adopted for demagnetization. AFD was performed on 95 specimens in successive 17 demagnetization step intervals 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 30, 40, 60, 80, 100, 120, 140 and 150 m T. Whereas stepwise ThD was performed on 48 specimens in 12 progressive demagnetization steps from 100, 200, 300, 350, 400, 450, 500, 520, 540, 560, 580 up to 600 °C. There was no considerable decay observed in NRM intensity in progressive thermal and AF demagnetization up to ∼400 °C and 80 m T respectively for most of the specimens. Nevertheless, a high-temperature component (HTC) and high field component (HFC) which decays towards the origin from ∼400 °C and 80 m T respectively to total demagnetization can be fitted using Principal Component Analysis (PCA) method (Kirschvink, 1980) of a best-fit line on Remasoft-3.0 software. For determination of both HFC and HTC at least six consecutive demagnetization intervals were chosen that decay towards the origin of orthogonal projections which yields ChRM directions. The Zijderveld plot (Zijderveld, 1967) is used to display and evaluate the demagnetization behaviors for all the specimens. All the measurements were carried out at the
3. Laboratory experiments 3.1. Rock magnetic properties To identify the main magnetic carriers of ST basalts an array of rock magnetic measurements was carried out. Sixteen representative 32
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 1. Simplified Geological map of the Shillong plateau showing the location of sampling sites of ST basalts (red dot) and the two fault trends the Dawki fault and the Raibah fault (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Paleomagnetism laboratory, CSIR-National Geophysical Research Institute, Hyderabad, India.
ST-2 showing 50 m T of Bcr value which is the indication of presence of comparatively higher coercivity magnetic mineral than other samples.
4. Results
4.1.2. Hysteresis loop From the Hysteresis curves (Fig. 3) for the ST basalts, hysteresis parameters saturation magnetization (Ms), saturation remanence (Mrs), coercivity force (Bc) and coercivity remanence (Bcr) were determined. The hysteresis ratios known separately as Remanence ratio (Mrs/Ms) and Coercivity ratio (Bcr/Bc) can be obtained (Supplementary Table 1), which are helpful in identifying the domain states and grain sizes of magnetic minerals. The maximum field applied is 1000 m T. The inverse relationship between the ratios is similar to Day plots for ferrimagnetic magnetite and titanomagnetite (Day et al., 1977; Dunlop, 2002a, 2002b). The hysteresis curve lines seem to be closer (thin) and such behaviour of curves is an indication of fine to medium magnetic grain size of the ST basalts. Most of the samples analysed are showing Remanence ratio (Mrs/ Ms) ranging from 0.105 to 0.333 with an average value of 0.218 and values of Coercivity ratio (Bcr/Bc) range between 1.25 to 1.87 with an average 1.593. Overall the values obtained are indicating, the ST basalts reside in Single Domain (SD) to Pseudo Single Domain (PSD) state of magnetic grains. Day plot (Fig. 4) which gives an idea of limiting values for SD, PSD and MD magnetic grains and useful tool for magnetic granulometry indicator. Usually, the values of most of the magnetitebearing rocks fall in MD state, where SD and PSD plays a role of paleomagnetic recorders. But in the case of these samples most of the samples fall in PSD state of magnetic grain size which may be due to combined effect of SD and MD grains.
4.1. Rock magnetic study The rock magnetic parameters are useful in understanding the magnetic mineralogy, their grain size, domain states and Curie temperatures. This is the first attempt of detailed rock magnetic study and implications associated with it for the ST basalts. The magnetic minerals in basalts generally belong to the solid solution series known as titanomagnetites solid solution series (Radhakrishnamurthy, 1990). 4.1.1. Isothermal remanent magnetization (IRM) acquisition curves and backfield demagnetization A pulse magnetizer was used for IRM and back-field DC demagnetization measurements. The significant fall in IRM during thermal demagnetization allows for estimation of curie temperatures because maximum blocking temperatures are always slightly less than the Curie temperature. The majority of the samples show the continuous rise in IRM curve (Fig. 2a) up to 200 m T and saturate. Sample ST_1, ST_2, and ST_6 show a steep rise up to 200 m T, and this rises gently up to 1000 m T and saturation is noted which is indication of the presence of magnetic minerals of titanomagnetite solid solution series. These IRM acquisition curves are due to the magnetocrystalline anisotropy in samples, those samples which saturated near 200 m T are due to magnetization acquired in easy direction and others are due to acquisition of magnetization in hard direction. The back-field DC demagnetization curves (Fig. 2b) indicates the remanence coercivity (Bcr) values of the samples range between 24–35 m T suggests low coercive magnetic mineral present in samples.
4.1.3. Thermomagnetic (k–T) curves The thermomagnetic (k–T) curve indicate the Curie temperature (Tc) and the degree of alteration that occurs during heating. Tc of pure 33
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 2. Representative curves of (a) isothermal remanent magnetization (IRM) acquisition and (b) back-field DC demagnetization shows generalized coercivity distribution in samples of ST basalts.
Fig. 3. Hysteresis curves for the ST basalt samples of Shillong Plateau. Curves show presence of soft (low Bc) and fine grained magnetic carrier.
magnetite is ˜577 °C and a similar behaviour is noted in the basalt samples of present study. Most of the samples analysed have shown the Tc ˜580 °C. The Curie temperature data for all the analysed ST basalt samples is given (Supplementary Table 2) along with the rock samples description. The analysed samples for Thermomagnetic (k–T) curves (Fig. 5) were heated from room temperature up to 700 °C and again cooled back to room temperature in the presence of 100 m T magnetic field. The k–T curves show a small decrease in the susceptibility at temperatures around 380–450 °C which is considered as Tc1 and afterwards falling to
zero at around 580 °C which is Tc2. This is the observation for most of the analysed samples. As per Vahle et al. (2007) the degree of reversibility of heating and cooling run allows an estimate of phase changes, which can be interpreted in terms of stability of the original magnetic phases. Within these variations, the heating cycle displays less thermal stability and lower intensity than cooling cycle. The cooling and heating curves are appearing irreversible due to the intermediate magnetic phase but similar for all the samples where heating curves are always higher than cooling curves. The magnetic phases in samples are interpreted as not stable since they are irreversible. This intermediate 34
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
stereoplots are depicted in Fig. 6 suggests presence of different shape anisotropies due to the effect of the magnetic carrier for resultant AMS ellipsoids. Normally, low degree of anisotropy values i.e., less than 1.03 depicts variation in the resultant shape anisotropy of AMS ellipsoids, i.e., from prolate to oblate. Out of eight sites examined four sites i.e., ST_4, ST_5, ST_6 and ST_8 are showing a triaxial ellipsoidal shape anisotropy (where all the principle susceptibility axes K1, K2, and K3 form distinct and individual groups with same orientations). Three sites are showing Prolate ellipsoidal shape (maximum axes were showing well grouping and directed perpendicular to the bedding plane, whereas the minimum axes lie in the bedding plane), i.e., ST_1, ST_2, and ST_7 here the K1 is well grouped, K2 and K3 arced within 90° around it. Site ST_3 is showing oblate ellipsoidal shape anisotropy (where K3 is well grouped; K1 and K2 are scattered through the stereonet and within the 90° arc around K3). From the calculated mean susceptibility (Km) and anisotropy degree (P) of the ST basalts, it is helpful in identifying the occurrence and contribution of mineral towards susceptibility (if any) and its control over anisotropy. To understand this, Km vs. P plot (Fig. 7 A) prepared for the whole sampling unit and the samples analysed show the major ferrimagnetic contribution to the anisotropy of magnetic susceptibility which understood from even scattering. The Jelinek plot (1978) in Fig. 7B, shows both prolate as well as oblate nature of the fabric for the ST basalts and no clear majority can be identified. This could be because of eruption of lava in a very small period of time with slow rate of successions and can be confirmed as there was no intertrappean beds are found in study area. The Pj vs. T plot (Fig. 7C) is for representing both shape of susceptibility ellipsoid and its magnitude. For the ST basalts it shows a transitional prolate to oblate nature of the ellipsoid. The scattering through the plot may indicate presence of dominant unstable inverse magnetic fabric, and can be confirmed by the presence of all three (prolate, oblate and triaxial) anisotropies in samples. The Pj values in the ST basalts range between 1.018 to 1. 031. The T and Km values ranging from -0.155 to 0.322 and 1.11 × 10−1 to 8.31 × 10−2 SI units respectively (Supplementary Table 3). It suggests that the direction of remanence will not significantly alter if the ratio between the maximum and minimum susceptibilities (degree of anisotropy) is lower than 5% (Hrouda, 1982). The orientations of AMS ellipsoids in these samples proposes no solid evidence about the magma flow directions and the fissure location through which the lava could have erupted and the ST came into existence. Due to this we restrict our conclusions from AMS measurements up to Shape anisotropy only.
Fig. 4. Day plot of Mrs/Ms and Bcr/Bc after analysis method by Day et al. (1977) to understand the domain state of magnetic grains for the ST basalts. SD = Single Domain, PSD = Pseudo Single Domain, and MD = Multi-Domain. Red dots depict analysed samples that are falling in respective domain states (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
magnetic phase can be inferred as titanomagnetite. The transformation of titanomagnetite to magnetite was reported to occur between 350 and 700 °C (Dunlop and Ozdemir, 1997), depending on the presence of impurities and grain size of titanomagnetite. The behaviour of the k-T curves in the Fig. 5 shows two curie temperature points, i.e., TC1 and TC2 at around 380–450 °C and ˜580 °C, respectively which indicates the presence of two ferrimagnetic carriers with one dominant, the magnetite (˜580 °C) and other as an accessory, the titanomagnetite (around 380–450 °C). We can also speculate that, the appearance of titanomagnetite in heating curves and the absence in cooling curves indicate the complete transformation of titanomagnetite to magnetite with progressive temperature. The combination of all these rock magnetic measurements suggest that the main magnetic remanence carrier resides in magnetite (Fe3O4) of PSD/SD state with subordinate titanomagnetite [Fe2+(Fe3+, Ti)2O4].
4.3. Paleomagnetism 4.2. AMS study The results of paleomagnetic study from 8 sites are presented in Table 1 and plotted in Fig. 8. Natural Remanent Magnetization (NRM) intensities of all specimens vary between 0.15–4.68 A/m. The samples were subjected to demagnetization techniques i.e., Alternating field demagnetization (AFD) and Thermal demagnetization (ThD). The remanent magnetization after each demagnetization step was measured on JR6- spinner magnetometer having signal to noise ratio of 2.4 μA/ m6. The Orthogonal projections of demagnetization curves (Zijderveld plot) of representative samples are provided for both AFD (Fig. 8A) and ThD (Fig. 8B). During AF demagnetizations the NRM’s of the samples have decreased effectively between 20–120 mT steps where it is observed that the viscous component is fully removed and ChRM directions are attained between steps 20–120 mT. Similarly, in the thermal demagnetization most of the samples are showing either the decrease in intensities beyond 520 °C or remaining nearly constant and immediate fall is recognised at the unblocking temperatures around 580 °C. Magnetic Susceptibility was measured after each thermal demagnetization interval to take review of chemical changes occurred during heating. In these samples no considerable changes in susceptibility have been recognised, revels that the magnetic phases are stable
To check the reliability of data for lava flow directions, the AMS study was carried out for the ST basalt samples of Shillong Plateau. Many studies of dykes, lava flows and ignimbrites (Staudigel et al., 1992; Canon-Tapia et al., 1997; Varga et al., 1998; Zhu et al., 2003; Caballero-Miranda et al., 2009) have been carried out to deduce the magma emplacement dynamics and source recognition. AMS ellipsoids depends on the mineral abundances and different mineral AMS (Borradaile and Jackson, 2010). The common AMS parameters like, mean susceptibility (Km), common anisotropy factors lineation (L) and foliation (F), anisotropy degree (P), corrected anisotropy degree (Pj) and shape parameters T and U were evaluated (Supplementary Table 3) from the three principle susceptibility axes (K1, K2 and K3). The magnetic susceptibility (K) values were measured by using MFK1-FA Susceptibility apparatus and range between 0.14 and 8.81 × 10-4 SI units which suggests samples suitability to perform AMS study and as a substitute indication for chemical composition variation during formation of rocks. The K values are high enough and suggests significant contribution of ferrimagnetic carrier for the resultant AMS fabric (Tarling and Hrouda, 1993). AMS 35
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 5. Representative thermomagnetic (k–T) curves for the ST basalts of Shillong Plateau. Heating (red) and cooling (blue) cycles are shown with respective arrows (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
detected in studied samples. These rock magnetic studies combined suggest the presence of magnetite (Fe3O4) as the main magnetic carrier of PSD/SD state with subordinate content of titanomagnetite [Fe2+(Fe3+,Ti)2O4]. This magnetic carrier is in agreement with the results obtained from thermal demagnetization on the very same samples, whose intensity decays completely at ˜580 °C.
with increasing temperatures. The data acceptance limits for ChRM calculations of individual specimens are constrained within MAD ≤ 50 and α95 ≤ 100, as per the standard fisherian statistics analysis proposed by Fisher (1953) and which are further considered for mean Virtual Geomagnetic Pole (VGP) position calculations. An unblocking temperature of ˜580 °C is consistent with the combined rock magnetic results, supporting our initial conclusions that the primary remanence carrier is magnetite. Mean ChRM directions for the ST basalts were calculated, displays a mean direction of Dm = 255°, Im = -57°, α95 = 3.44° and k = 16.74. Calculations for each site have been made to get Virtual Geomagnetic Pole (VGP) using present day latitude: 25.21 °N and longitude: 91.71 °E of the location.
5.2. AMS study and associated implications The AMS fabrics in the samples depict the arrangement of magnetic grains and appearance of all three AMS ellipsoids reveals a weak degree of anisotropy. Particularly for magnetite grains, the domain size is controlled by grain shape. Though the presence of titanomagnetite minerals is confirmed in rock magnetic measurements they have a minor contribution to the AMS. Therefore, it is presumed that AMS of the ST basalts is dominated by a low coercive PSD/SD magnetic grains of magnetite. Anisotropy of the magnetic susceptibility caused by different phenomena such as magma flow, tectonism or other biasing factors (Tarling and Hrouda, 1993; Dragoni et al., 1997; Elming and Mattsson, 2001). In most of the stereonets of the AMS ellipsoids are scattered, specifies less alignment of fabrics due to higher viscosity and slower velocity of magma flow. The horizontal movement of the eastern block (Shillong Plaetau) along the Dawki fault might have affected the AMS in these samples. It can be concluded that ST marks the tale of the magma emplacement activity in Rajmahal- Bengal- Sylhet Traps
5. Discussion 5.1. Significance of rock magnetic tests The rock magnetic properties of the ST basalts show less variation, which is purely controlled by cooling history of lava. The IRM acquisition curves and coercivity spectrum suggests the presence of predominantly low coercivity component and few are showing intermediate coercivity component. Day plot drawn from the values of hysteresis parameters depicts, the presence of PSD and SD state dominated magnetic mineral. Also the thin nature of hysteresis loops suggests the presence of fine to medium size of magnetic grains. Curie point behavior of samples around 380–450 °C and 580 °C have been 36
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 6. Equal area projection demonstrates the distribution of the principal susceptibility axes in the analysed ST basalt samples. Principal susceptibility axes K1 as squares (blue), K2 as triangles (green) and K3 as circles (pink). Comparatively larger arcs are with 95% confidence circles (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
ChRM’s at the field intervals of 20–120 mT and at the temperature interval of 400–580 °C. The comparison of mean directions of various rock formations from India (Table 2) with the data obtained for the ST basalts from present study.
province. 5.3. ChRM directions comparisons The Paleomagnetic study on the ST basalts was initiated by Athavale (1963) and followed by Poornachandra Rao (1993) where they have reported the very first paleomagnetic directions from the ST basalts. They have reported two directions one is explained by referring it with the contemporary Rajmahal traps while the other was not explained. In the present study, the paleomagnetic directions obtained are similar to the one which was unexplained previously. The paleomagnetic measurements done previously was by using classical instruments which might not have cautiously removed the secondary (viscous) magnetization components to meet primary magnetization component. They have not reported (Athavale, 1963) the intervals of demagnetizations to achieve ChRM’s by AF demagnetization and thermal demagnetization techniques. In the present study to get accurate ChRM directions, samples were demagnetized up to 150 m T in AFD and up to 600 °C in ThD measurements which completely removed the secondary component to obtain the isolated ChRM directions. Here we have reported our
5.4. Larger Paleomagnetic story of India Though there is a small extent of ST is present at Shillong Plateau, it is suggested that during its formation process it might have behaved like a different tectonic unit (or maybe two magma emplacement events occurred for the existence of ST). The paleomagnetic directions revealed by these exposures were severely affected by the horizontal movement along dawki fault which has shifted around 250 Km eastward (Evans, 1964) from RT exposures. Since the obtained directions from the present study has varying azimuths by almost 100° with stable dip as compared to previously reported directions, is suggestive of horizontal adjustments within the Sylhet Traps exposures. This we concluded by the effect of horizontal movement of ST and surrounding block along the Dawki tear fault for about 250 Km’s. This final explanation agrees with the study of Evans 37
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 7. A) is the specimen-wise bivariate scatter plot of susceptibility mean vs. degree of anisotropy. B) Jelinek plot of lineation vs foliation (L vs. F) for the ST basalts. C) Flinn diagram of corrected anisotropy degree vs shape parameter (Pj versus T; Tarling and Hrouda, 1993).
6. Conclusions
(1964), where it is concluded that, the north-eastern part of Indian subcontinent has moved eastward by about 250 Km. This study yields a mean Inclination value −57° and from this, we calculated the paleolatitude for India during the Early Cretaceous period (˜117 Ma) which is around 37.6°S. This is the first time we are reporting the paleolatitude of India from Sylhet Traps. One previous study by Poornachandra Rao (1996b) from Rajmahal Traps has reported the paleolatitude of India at 43 °S. From this, it is clearly understood that there may be more than one lava eruption events in a shorter interval of time at Kerguelen hotspot while the Indian plate was drifting northwards. Fig. 9 depicts the relative paleolatitude of India calculated from the data of the present study on Sylhet Traps basalts.
We are presenting here for the first time the detailed rock magnetic, anisotropy of magnetic susceptibility study results carried out on basalts of the Sylhet Traps and drawn following conclusions: 1 The combined rock magnetic results indicate that these basalt contains magnetite as the primary magnetic carrier with PSD and SD state nature of magnetic grains and with low titanomagnetite mineral content. 2 All the three (triaxial, prolate and oblate ellipsoids) possible shape anisotropies are present in the ST basalt samples which will not allow to give concrete statement about lava flow dynamics in the
Table 1 Concise data of Paleomagnetic results of ST basalts from this study. Site Name
Rock Type
Lat (0)
Long (0)
N
n
Dm (0)
Im (0)
α95 (0)
k
MAD
Plat (0)
Plong (0)
dp
dm
ST-1 ST-2 ST-3 ST-4 ST-6 ST-7 ST-8
Basalt Basalt Basalt Basalt Basalt Basalt Basalt Grand Mean
25.21 25.22 25.22 25.22 25.22 25.23 25.22
91.71 91.71 91.71 91.71 91.72 91.63 91.70
2 3 2 3 4 5 5
8 6 8 9 16 13 24
213 260 237 266 270 234 260 255
−53 −49 −44 −52 −42 −67 −61 −57
16.6 18 11.4 11.75 7.9 5.4 5.5 3.44
12 14.76 24.58 23.19 23.89 60.01 29.66 16.74
4.23 4.10 4.51 4.01 3.12 3.45 3.01 3.77
−60.1 −20.57 −39.18 −16.82 −10.39 −42.2 −23.95 −26.79
337.22 336.22 347.76 333.16 339.84 315.63 324.91 330.75
15.9 15.7 8.9 11 5.9 7.4 6.5 3.6
23 23.8 14.3 16.1 9.7 8.9 8.4 5
Lat and Long are site latitude and site longitude in degrees, N = no. of samples, n = no. of specimens, Dm = Mean Declination, Im = Mean Inclination, α95 = circle of confidence with 95% probability level, k = Precision parameter, MAD = maximum angular deviation, Plat and Plong = paleoco-ordinates of the Pole, dp and dm = oval of confidence with 95% probability within which the true pole lies. 38
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 8. Zijderveld’s plot showing orthogonal projections with solid (open) data points representing vector endpoints projected onto the horizontal (vertical) plane. Table 2 The comparison of mean directions of various formations from India with the data obtained for the ST basalts from present study. Sr. No.
Formation
Age
Locality
Site Coordinates Lat (N)
D (°)
I (°)
α95 (°)
Reference
338 339 164 157 327 320.6 332 & 243 235 255
−32 −57 +48 +51 −64 −65.2 −59 & −60 −62 −57
5 – 2
Deutsch, 1959 Sahastrabudhe, 1963 Sahastrabudhe, 1963 Verma, 1967 Clegg, 1958 Poornachandra Rao, 1996a Athavale, 1963
Long (E)
1 2 3 4 5 6 7
Upper Deccan Traps Upper Deccan Traps Lower Deccan Traps Tirupati Sandstone Rajmahal traps Rajmahal traps Sylhet traps
Eocene Eocene Upper Cretaceous Middle Cretaceous Jurassic Early Cretaceous Jurassic
Nipani Mahabaleshwar Linga Janampeta Rajmahal hills Rajmahal hills –
16° 17° 21° 16° 25° – 25°
26’ 55’ 38’ 46’
8 9
Sylhet traps Sylhet traps
– Early Cretaceous
– Cherrapunjee
– 25° 13’
39
74° 22’ 73° 38’ 78° 55’ 81 ° 9’ 87° 51’ – 91° – 91° 43’
2 (α50) 6 7 16 5 3.44
Poornachandra Rao, 1993 Present Study
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
Fig. 9. The red square indicates the paleolatitude of India obtained from the Sylhet trap basalts (37.6° S) of the present study. (Redrawn after C. T. Klootwijk, 1976) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
study area. Therefore we limit our AMS conclusions up to shape anisotropy only. 3 The ST basalts revealed a paleomagnetic direction with a mean declination = 255° and mean inclination = −57° (α95 = 3.44°; k = 16.74) and the paleopole position of -26.79 °N, 330.75 °E (dp = 3.6 and dm = 5). The obtained paleomagnetic directions can also give idea about the post-collisional migration of eastern block of Indian subcontinent (Shillong Plateau) from the Rajmahal Traps along the Dawki fault for about 250 Km and during this the block might have rotated a little. 4 A paleolatitude for the ST basalts obtained at 37.6 °S clearly indicates that the Sylhet Traps volcanic event is the later episode than the Rajmahal Traps.
of magnetic susceptibility analysis of the Cantera Ignimbrite, San Luis Potosi, Mexico: flow source recognition. Earth Planets Space 61, 173–182. Canon-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1997. The internal structure of lava flows-insights from AMS measurements II: Hawaiian pahoehoe, toothpaste lava and a’a. J. Volcanol. Geotherm. Res. 76, 19–46. Chadima, M., Jelinek, V., 2009. Anisoft 4.2: Anisotropy Data Browser for Windows. Agico, Inc. Clegg, J.A., Radhakrishnamurthy, C., Sahastrabudhe, P.W., 1958. The Remanent magnetism of the Rajmahal traps of north-eastern India. Nature 181, 830–831. Cox, A., Doell, R.R., 1967. Measurement of high coercivity magnetic anisotropy. In: Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Paleomagnetism. Elsevier, Amsterdam, pp. 477–482. Day, R., Fuller, M.D., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: Grain size and composition dependence. Phys. Earth Planet. Inter. 13, 260–267. Deutsch, E.R., Radhakrishnamurthy, C., Sahastrabudhe, P.W., 1959. Paleomagnetism of the Deccan traps. Ann. Geophys. 16, 39. Dragoni, M., Lanza, R., Tallarico, A., 1997. Magnetic anisotropy produced by magma flow: theoretical model and experimental data from Ferrar dolerite sills (Antarctica). Geophys. J. Int. 128, 230–240. Dunlop, D.J., 2002a. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data. J. Geophys. Res. 107 (B3), 2056. Dunlop, D.J., 2002b. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 2. Application to data for rocks, sediments, and soils. J. Geophys. Res. 107 (B3), 2057. Dunlop, D.J., Özdemir, Ö., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge Univ. Press, New York, pp. 573. Elming, S.A., Mattsson, H., 2001. Post jotnian basic intrusions in the fennoscandian shield, and the breakup of baltica from Laurentia: a palaeomagnetic and AMS study. Precambrian Res. 108 (3–4), 215–236. Evans, P., 1964. The tectonic framework of Assam. J. Geo. Soc. of India 5, 80–96. Fisher, R.A., 1953. Dispersion on a sphere. Proc. R. Soc. Lond. Ser. A 217, 295–305. Ghatak, A., Basu, A.R., 2006. Remnants of the Kerguelen Plume in Eastern India: evidence from Nd-Sr-Pb isotopic and trace element geochemistry of Sylhet traps, Shillong Plateau. AGU Fall Meeting. Hrouda, F., 1982. Magnetic anisotropy of rocks and its application in geology and geophysics. Geophys. Surv. 5 (1), 37–82. Jelinek, V., 1978. Statistical processing of anisotropy of magnetic susceptibility measured on groups of specimens. Studia Geophysica et Geodaetica 22, 50–62. Kent, R.W., Pringle, M.S., Muller, R.D., Saunders, A.D., Ghose, N.C., 2002. 40Ar/39Ar geochronology of the Rajmahal basalts, India, and their relationship to the Kerguelen Plateau. J. Petrol. 43, 1141–1153. Khan, M.A., 1962. Anisotropy of magnetic susceptibility of some igneous and metamorphic rocks. J. Geophys. Res. 67, 2873–2885. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data: examples from Siberia and Morocco. Geophys. J. R. Astro. Soc. 62, 699–718. Klootwijk, C.T., 1976. The drift of the Indian subcontinent: an interpretation of recent paleomagnetic data. Geologische Rundschau. 65 (1), 885–909. Kono, M., Kinoshita, H., Aoki, Y., 1972. Paleomagnetism of the Deccan Trap Basalts in India. J. Geomag. Geoelectr. 24, 49–67. McDougall, I., McElhinny, M.W., 1970. The Rajmahal Traps of India: K-Ar ages and Paleomagnetism. Earth Planet. Sci. Lett. 9, 371–378.
Acknowledgments We thank the Director, CSIR-NGRI for the permission to publish this work (NGRI/Lib/2018/Pub-50). MK thank J. Mallikharjuna Rao and K. P. Sarma for sharing their knowledge. MK is grateful to S. J. Sangode, Ravi Shankar, Raj Kumar, Ramesh Babu, L Srikanth and Sujit Pradhan for their timely help. MK further extend thanks to CSIR, Delhi for the JRF (P-81-101). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jog.2019.05.003. References Athavale, R.N., Radhakrishnamurthy, C., Sahastrabudhe, P.W., 1963. Paleomagnetism of some Indian rocks. Geophys. J. R. Astron. Soc. 7, 304–313. Bakshi, A.K., 1995. Petrogenesis and timing of volcanism in the Rajmahal flood basalt province, northeast India. Chem. Geol. 121, 73–90. Basu, A.R., Weaver, K.L., Sengupta, S., 2001. A Plume Head and Tail in the Bengal Basin and Bay of Bengal: Rajmahal and Sylhet Trap With Surrounding Alkalic Volcanism and the Ninety East Ridge. EOS Transactions. American Geophysical Union v p. V12A–0950. Borradaile, G.J., Jackson, M., 2010. Structural geology, petrofabrics and magnetic fabrics (AMS, AARM, AIRM). J. Struct. Geol. 32, 1519–1551. Caballero-Miranda, C.I., Torres-Hern´andez, J.R., Alva-Valdivia, L.M., 2009. Anisotropy
40
Journal of Geodynamics 127 (2019) 31–41
M.R. Kapawar and V. M.
structural data. Geology 20, 841–845. Talukdar, S.C., 1967. Rhyolite and alkali basalt from the Sylhet Traps. Khasi Hills, Assam. Curr. Sci. 9, 238–239. Talukdar, S.C., Murthy, M.V.N., 1971. The Sylhet Traps, their tectonic history, and their bearing on problems of Indian flood basalt provinces. Bull. Volcanol. 35, 602–618. Tarling, D., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall, London, pp. 217. Vahle, C., Kontny, A., Gunnlaugsson, H.P., Kristjánsson, L., 2007. The stardalur magnetic anomaly revisited - new insights into a complex cooling and alteration history. Phys. Earth Planet. Inter. 164, 119–141. Vandamme, D., Courtillot, V., Besse, J., 1991. Paleomagnetism and age determinations of the Deccan Traps (India): results of a Nagpur- Bombay traverse and review of earlier work. Rev. Geophys. 29, 159–190. Varga, R.J., Gee, J.S., Staudigel, H., Tauxe, L., 1998. Dykes surfaces lineations as magma flow indicators within the sheeted dyke complex of the Troodos ophiolite, Cyprus. J. Geophys. Res. 103, 5241–5256. Verma, R.K., Pullaiah, G., 1967. Paleomagnetism of Tirupati Sandstones from Godavari Valley, India. Earth Planet. Sci. Lett. 2, 310–316. Voight, W., Kinoshita, S., 1907. Bestimmung absoluter Werte von Magnetiserungszahlen, inbesondere fur Kristalle. Annale der Phys 24, 492–514. Zhu, R., Shi, C., Qingsong, L., 2003. Anisotropy of magnetic susceptibility of Hannuoba basalts, northern China: constraints on the vent position of the lava sequences. Geophys. Res. Lett. 30 (2) 38(1–4). Zijderveld, J.D.A., 1967. In: Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Palaeomagnetism and Rock Magnetism. Elsevier, Amsterdam, pp. 254–286.
Palmer, R.W., 1923. Geology of part of the Khasi and Jaintia Hills. Assam: Rec. Geol. Survey India 55, 143–168. Pantulu, G., McDougall, J.D., Gopalan, K., Krishnamurthy, P., 1992. Isotopic and Chemical Compositions of Sylhet Traps Basalts: Links to the Rajmahal Traps and Kerguelen Hotspot. American Geophysical Union, San Francisco, pp. 328 72. Poornachandra Rao, G.V.S., Mallikharjuna Rao, J., 1996a. Paleomagnetism of the Rajmahal Traps of India: implication to the reversal in the Cretaceous Normal superchron. J. Geomag. Geoelectr. 48, 993–1000. Poornachandra Rao, G.V.S., Subba Rao, M.V., Mallikharjuna Rao, J., 1993. Geochemical and paleomagnetic signatures in Sylhet Trap flood basalt volcanism: preliminary results. Geol. Mag. 130, 163–184. Poornachandra Rao, G.V.S., Mallikharjuna Rao, J., Subba Rao, M.V., 1996b. Palaeomagnetic and geochemical characteristics of the Rajmahal Traps, eastern India. J. Southeast Asian Earth Sci. 13 (2), 113–122. Radhakrishnamurthy, C., Subbarao, K.V., 1990. Palaeomagnetism and rock magnetism of the Deccan traps. Proc. Indian Acad. Sci. Earth Planet Sci. 99 (4), 669–680. Ray, J.S., Pattanayak, S.K., Pande, K., 2005. Rapid emplacement of the Kerguelen plume–related Sylhet Traps, eastern India: evidence from 40Ar-39Ar geochronology. Geophys. Res. Lett. 32, L10303. Sahastrabudhe, P.W., 1963. Paleomagnetism and geology of the Deccan traps. Paper Presented at Seminar on Geophysical Investigation in the Peninsular Shield. Sherwood, G.J., Basu, M.S., 1996. A paleomagnetic and rock magnetic study of the northern Rajmahal Volcanics, Bihar, India. Jr. of Southeast Asian Earth Sci. 13 (2), 123–131. Staudigel, H.G., Gee, J.S., Tauxe, L., Varga, R.J., 1992. Shallow intrusive direction of sheeted dikes in the Troodos ophiolite: anisotropy of magnetic susceptibility and
41