Rajmahal Basalts Research Articles

Soil-Landform Relationship to Understand Quantitative Pedogenesis under Varying Physiographic Catena of Chhotanagpur Plateau Regions of India

Pedogenic indices were considered to understand maturity of soils in different geomorphic units in quantitative terms. High redness rating and low sand/silt and silt/clay ratio at higher elevations of Rajmahal trap, Hazaribagh plateau and Ranchi plateau indicates rapid pedogenic processes and formation of Paleo-Alfisols (Rhodustalfs and Paleustalfs). High CEC/Clay in lower elevations of Rajmahal trap and Hazaribagh plateau indicates formation of more 2: 1 type of interstratified minerals. Predominant occurrence of amorphous iron (Fe) in soils of Rajmahal basalts was indicated by high ammonium oxalate extractable Fe. Higher proportions of dithionite extractable Fe of clay in gullied lands, uplands of Hazaribagh plateau and hillocks of Ranchi plateau indicate the prevalence of crystalline Fe in Chhotanagpur granite gneissic complex. More crystalline nature of soil clays at higher elevation of Ranchi and Hazaribagh plateaus are noted as indicator of laterization processes. Soils on pediments, uplands and plateaus comprise well matured profiles with formation of paleosols of relict types, whereas that of gullied lands and hillock were least weathered. Undulating plains, old alluvial plains and low-lying plains consists of soils under intermediate stages of weathering with intermediate stages of soil forming processes. These soils (P3, P4, P11 and P12) appear to support agricultural land use comparatively better than other soils despite topographic undulation and soil physicochemical constraints in the region. Presence of considerable amount of interstratified type of 2:1 minerals at lower elevations may attribute to restore better organic carbon sequestering capacity supporting long-term agricultural practices in the region. The present endeavour deals with study of quantitative pedogenesis as a tool to understand soil-landform relationship in varying physiographic catena of Chhotanagpur plateau regions of India.

Paleomagnetism and rock magnetism of early Cretaceous Rajmahal basalts, NE India: Implications for paleogeography of the Indian subcontinent and migration of the Kerguelen hotspot

Abstract We present here new paleomagnetic and rock magnetic results from Rajmahal Traps (RT), India. The results are comprising of 350 standard specimens collected from 18 sites (90 samples). The detailed rock magnetic studies indicate that a Single Domain (SD) magnetite [Fe3O4] is the dominant magnetic carrier with ilmenite, pyrrhotite, and maghemite in the accessory form. The calculated Characteristic Remanent Magnetization (ChRM) directions yield a mean declination of 317.51°; mean inclination of −61.92° (α95 = 3.6°; k = 93.39). The obtained Virtual Geomagnetic Pole (VGP) position of the RT at ~117 Ma is 11.37°N, 297.58°E (62.42°W) with A95 = 3.5°, k = 99.99, dp = 4.33, dm = 5.58, and the calculated paleolatitude is at 43.1°S indistinguishable from that of earlier works. This paleomagnetic pole of RT basalts has minimal effect of secular variation and comply with the Geocentric Axial Dipole (GAD) model. The VGP is plotted on synthetic Apparent Polar Wander Path (APWP) for India, constrained better, and agrees well with the previously reported poles. The postulation of migration of mantle plume and the global mantle circulations are tested, and the present study results are in accord with this, suggesting the southward migration of the Kerguelen hotspot (mantle plume) by at least 6°.

Rift-associated ultramafic lamprophyre (damtjernite) from the middle part of the Lower Cretaceous (125 Ma) succession of Kutch, northwestern India: Tectonomagmatic implications

Mineralogical, geochemical and isotopic (Sr and Nd) studies on the recently reported ca. 124 Ma ‘anorogenic lamproite’ dyke from the Palanpur area, Kutch seismogenic rift zone, northwestern India, are presented. We propose a new classification for the dyke as a damtjernite (ultramafic lamprophyre; UML) based on its porphyritic-panidiomorphic texture, abundance of phlogopite, presence of nepheline in the groundmass, and the composition of liquidus phases such as olivine, phlogopite, magnetite, and clinopyroxene (diopside). The Palanpur UML is primitive (Mg# = 74–77), silica-undersaturated (SiO2 <40 wt.%), potassic to slightly sodic in nature, and is strikingly similar to the ∼69 Ma UML dykes and sills of the Tethyan Indus suture zone, which are considered as the earliest yet known manifestations of the Deccan Large Igneous Province (LIP). Bulk-rock (87Sr/86Sr)i (0.70460–0.70461) and ɛNd(t) (+2.56 to −0.69) of the Palanpur UML signify derivation from a slightly depleted mantle source similar to that of asthenospheric magmas such as OIB. This is further attested to by the high incompatible trace element ratios (viz., La/Ba, Nb/U, Nb/La and Ta/Yb) that are typical of plume-type magmas. However, the Neoproterozoic TDM depleted mantle Nd model ages (∼655–919 Ma) also necessitate some involvement of a lithospheric mantle component in its genesis. High bulk-rock Fe2O3t and TiO2 contents require the involvement of a fertile peridotitic mantle source, whereas high La/Yb (60–80) implies a control by residual garnet. Higher Rb/Sr and lower Ba/Rb suggest phlogopite as a residual phase and high Nb and lower La/Sm favour carbonatite, rather than silicate melt as metasomatising agent. Low degrees of partial melting of a primitive garnet lherzolite mantle can account for the observed REE patterns in the Palanpur UML. The Palanpur UML shares a temporal similarity to the Kerguelen plume-derived Rajmahal basalts and associated alkaline rocks from the eastern India. The tectonomagmatic significance of its emplacement during the mid-Cretaceous vis-à-vis various models involving the timing of eruption of the Deccan and the Rahmahal Traps and the rifting in the Kutch basin induced by far-field plate reorganization is evaluated.

Mantle source heterogeneity in continental mafic Large Igneous Provinces: insights from the Panjal, Rajmahal and Deccan basalts, India

Abstract In this paper we illustrate the geochemical diversity of the basalts of Panjal ( c. 289 Ma), Rajmahal ( c. 117 Ma) and Deccan ( c. 65 Ma) continental large igneous provinces (continental LIPs) from India as controlled by multiple mantle sources. The Panjal primitive basalts are formed by 5–20%, Rajmahal basalts by 10–20% and Deccan picritic basalts by 1–20% melting of the mantle. The depth of melting beneath Deccan is greater than beneath Rajmahal, which greater than beneath Panjal. We have categorized the Panjal, Rajmahal and Deccan basalts into Type I and Type II. The Type I basalts with lower La/Nb, Th/Nb and Th/Yb ratios and higher ε N d i values ( ε N d i = −3 to +8) are derived from the sublithospheric sources; some of the Type I basalts represent lithosphere-contaminated melts. The Type II basalts with higher La/Nb, Th/Nb and Th/Yb ratios and lower ε N d i values ( ε N d i = −20 to −3) are produced from the subcontinental lithosphere. We estimate that lithosphere and sublithosphere (plume and/or E-MORB patches within asthenosphere) have contributed, respectively, approximately 67 and 33% to the Panjal, 52 and 48% to Rajmahal, and 28 and 72% to Deccan LIPs. Fertility of the Indian subcontinental lithosphere may have increased with decreasing age related to the break-up of Gondwana.

Petrology and geochemistry of the Salma dike, Raniganj coalfield (Lower Gondwana), eastern India: linkage with Rajmahal or Deccan volcanic activity?

The Gondwana (Early Permian to Early Cretaceous) basins of eastern India have been intruded by ultramafic–ultrapotassic (minette, lamproite and orangeite) and mafic (dolerite) rocks. The Salma dike is the most prominent among mafic intrusives in the Raniganj basin. This dike is tholeiitic in composition; MgO varies from 5.4 to 6.3% and the mg number from 54 to 59. In general, the major and trace element abundances are uniform both along and across the strike. There is geochemical and mineralogical evidence for fractional crystallization. The chondrite normalized REE pattern of the Salma dike (La/Yb n =3.5) is similar to that of Deccan dikes of the Son–Narmada rift zone, western India. 87Sr/ 86Sr varies from 0.70552 to 0.70671 suggesting assimilation of crustal material. Some trace element abundances (e.g. Ti, Zr, Y) of the Salma dike are comparable to Group I Rajmahal basalts. The 40Ar– 39Ar whole rock age of 65 Ma for the Salma dike is less than the ca. 114 Ma age for the Rajmahal basalt, but is similar to the generally accepted age for Deccan volcanic rocks. Despite geographical proximity with the Rajmahal basalt, the Salma dike is believed to be related to late phase of Deccan volcanic activity.

40Ar/39Ar Geochronology of the Rajmahal Basalts, India, and their Relationship to the Kerguelen Plateau

During the mid-Cretaceous, extensive magmatism occurred in the Indian Ocean to form volcanic portions of the southern and central Kerguelen Plateau, Elan Bank and Broken Ridge. Basalt was erupted also along the rifted margin of eastern India (Rajmahal). We investigated the ages of these Indian basalts using Ar-40/Ar-39 incremental-heating experiments on whole rocks. Our results are consistent with the hypothesis that the lava pile of similar to230 m thickness in the Rajmahal Hills, Jharkhand, and alkalic basalts in the Bengal Basin were emplaced at similar to118 Ma. Dykes intruded to the SW of the Rajmahal Hills appear to be 2-3 Myr younger than these lavas. Magmatic activity in eastern India therefore was contemporaneous with the final stage of volcanism at Ocean Drilling Program Site 1136 on the Southern Kerguelen Plateau (119-118 Ma), but older than final magmatism at Sites 749 and 750 on the Southern Kerguelen Plateau (112-110 Ma), Site 1137 on Elan Bank (108 Ma) and Site 1138 on the Central Kerguelen Plateau (100 Ma). By combining these age data with plate reconstructions that take into account the motion of hotspots in a convecting mantle, we suggest that eruption of the Rajmahal basalts, formation of the Southern Kerguelen Plateau, and Elan Bank's separation from India are best explained by the presence of the Kerguelen hotspot close to the eastern Indian margin just after 120 Ma.

Petrology of Early Cretaceous flood basalts and dykes along the rifted volcanic margin of eastern India

An approximately 220-m thick sequence of Early Cretaceous flood basalts (the Rajmahal Basalt Group) crop out over some 4300 km 2 in Bihar, eastern India, forming the leading edge of a seaward-dipping reflector sequence emplaced during the break-up of India and Australia/East-Antarctica. Geochemical data support a division of the basalts and associated dykes into high-Ca and low-Ca magma types. High-Ca tholeiites have CaO contents >10.0 wt%, mg# 50.3–59.6 and K 2O 0.11–0.55 wt%. La Nb ranges from 1.29 to 3.62. Rocks of the low-Ca magma type have ≤ 10.5 wt% CaO, mg# 52.1–70.7 and K 2O 0.26–1.1 wt%. La Nb is between 1.6 and 3.29. These element abundances and ratios are similar to those of Cretaceous tholeiites from the central Kerguelen Plateau (ODP Site 120–749). Plate reconstructions indicate that the plateau lay adjacent to the Indian continental margin during Early Cretaceous times. It is shown that certain of the Rajmahal basalts (low-Ca magma type) have been contaminated by Indian upper crust, whilst others (high-Ca lavas) retain the near-flat mantle-normalized trace element patterns of oceanic plateau tholeiites.

Flow stratigraphy of selected sections of the Rajmahal basalts, eastern India

The Rajmahal basalts form a north-south trending belt extending over 4100 km 2 along the eastern margin of the Indian Shield. The basalts form part of a widespread magmatic episode which coincided with continental break-up between India and Australia-Antarctica during the Late Jurassic-Early Cretaceous. In the sections examined, between 2 and 15 flows have been encountered, ranging in composition from tholeiitic basalt to dacite (pitchstone). The flows vary in thickness from < 1 m to > 70 m. Significant volcanic features observed include vent sites, marked by volcanic breccia, welded agglomerate, tuff and lapilli-tuff (including volcanic bombs). The tuffs occur in widely scattered areas, and are more abundant than hitherto recognised. These rocks were probably erupted during the early stages of volcanic activity, following extensive differentiation in sub-volcanic magma chambers. Several characteristics of sediments interbedded with the lavas suggest a humid, sub-tropical lacustrine depositional environment during eruption of the lower part of the volcanic sequence, and a temperate climate during emplacement of the upper flows. Quiescent eruption of the Rajmahal basalts took place largely through fissures sited at the faulted margin of the Indian Shield. This magmatic activity is interpreted as resulting from extension of the lithosphere above a mantle plume.

Palaeomagnetic and geochemical characteristics of the Rajmahal Traps, eastern India

The Rajmahal Traps of Early Cretaceous age (117 ± 1 Ma) are medium/fine-grained rocks of quartz-tholeiitic composition. Plagioclase phenocrysts in the basalts show compositional zoning from An 53 to An 68, whereas groundmass plagioclase compositions range from An 50 to An 58. Clinopyroxene phenocrysts range in composition from En 46–48Wo 35–39Fs 14–19, with groundmass clinopyroxene compositions in the range En 32–46Wo 33–37Fs 18–34. Titanomagnetite occurs as small specks, prismatic grains and elongate needles. Secondary minerals include smectite-chlorite replacing groundmass plagioclase and augite, suggesting that the basalts have undergone hydrothermal alteration subsequent to their emplacement. Major, trace and rare-earth element data suggest two distinct catergories of basalts; Group I, characterised by high MgO, K 2O, Sr, Rb, Cr, Ba and Nb; and Group II, characterised by high Fe 2O 3, TiO 2 and rare-earth element contents. Both basalt groups appear to have been derived by melting of the same mantle source. Rajmahal basalts collected from three sites show very good grouping in their natural remanent magnetic vectors, with normal and reverce polarities. Detailed AF and thermal demagnetisation treatment of these rocks reveals characteristic components at all three sites. Sites A and B exhibit normal magnetisation, and site C exhibits both normal and reverse magnetisations. These results are critically evaluated in relation to models for widespread basaltic magmatism during the break-up of eastern Gondwana.

A study of anhysteretic remanent magnetization with the variation of A. C. and D. C. fields on Rajmahal basalts

Anhysteretic Remanent Magnetization is produced in a rock specimen when an alternating field, sufficient to cause saturation. is applied and reduced to zero in the presence of a constant direct field. The ARM produced increase with the increase of A. C. field range when the D. C. field is kept constant and reaches a saturation value. The saturation value is different for different specimens and increases as the coercive force increases. The ARM developed in various specimens with the variation of D. C. field, when the maximum alternating field intensity is constant, is a linear function of a small D.C. field.

An experiment on the magnetic hysteresis and remanence of Rajmahal basalts

Hysteresis curves of Rajmahal Trap basalts in Bihar are drawn and their coercivities obtained. The experimental arrangements and the procedure are described. The initial remanent magnetisation curves of the specimens are also drawn with the help of the same arrangement. Coercive forces of the rock specimens tested range from 70 to 237 oersteds. The values of saturation magnetisation and remanent magnetisation of two typical specimens are also given.

K/Rb ratios in mesozoic tholeiites from Antarctica, Brazil and India

K and Rb have been determined in a variety of Mesozoic tholeiitic rocks. Seven dolerites and two basaltic dykes from Queen Maud Land, Antarctica, have K/Rb ratios varying between 144 – 267, similar to those reported for Ferrar and Tasmanian dolerites; one basaltic dyke has a high ratio of 615. Six basalts from the Serra Geral formation, Brazil, have K/Rb ratios within the range 345 – 495, similar to those reported for Karroo dolerites. Seven Rajmahal basalts from India have K/Rb ratios varying from 208 – 591 while a dacite sample has a low ratio of 117. The similarity of the K/Rb ratios of the Tasmanian and Antarctic dolerites (both the Ferrar and those from Queen Maud Land) suggests a common history for these dolerites. However, some of these have K/Rb ratios considerably lower than the crustal average (K/Rb= 230). Together with available data for Th and U, these data indicate the possibility of selective crustal contamination of these dolerite magmas. Such contamination is either absent, or present on a much smaller scale, in the Karroo and Serra Geral volcanics. It is not clear whether the large spread of the K/Rb ratios for the Rajmahal basalts is due to selective crustal contamination, or to secondary alteration processes.

Secondary clay in Rajmahal basalts of India and its relation to palagonite-chlorophaeite

SummaryThe mineralogy of the green, yellow, and brown secondary constituents occurring as amygdales, irregular infillings, and replacements in the Rajmahal basalts of eastern India and commonly described under the ill-defined names palagonite and chlorophaeite, has been revised on the basis of optical, X-ray, and thermal studies. The results indicate that these coloured constituents, which have been chiefly derived by the alteration of pyroxenes, iron ores, and primary glass, belong to the clay minerals, particularly smectite and vermiculite. This brings in question the validity of the terms palagonite and chlorophaeite which, though widely current in basaltic literature, have no definite mineralogical identity. It is suggested here that they are but forms of clay.