Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Santosh Kumar ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi- 221 005, U.P., IN

Abstract

The predominant tholeiitic gabbro-microgranite/granophyre suite has been found in close association with subordinate alkali gabbro-syenite suite at Phenai Mata Igneous Complex (PMIC) (65 Ma). The pyroxene compositions from the selected tholeiitic and alkaline lithotypes of PMIC have been classified and compared to those from alkaline rocks of adjoining complexes. The results indicate that pyroxenes were crystallized in diverse tholeiitic and alkaline parental magmas undergoing differential oxygen fugacity (fO₂) and silica activity (^aSiO₂). The fractionation trend of pyroxenes from tholeiitic rocks of PMIC follows the trend transitional between Ca-poor and Ca-rich augite-ferroaugite trends, though limited iron-enrichment, formed under low fO_2. Whereas pyroxenes in alkaline rocks follow the Ca-rich salite-ferrosalite trend at relatively higher fO₂ and can be related to those of volcanic and subvolcanic alkaline rocks of adjacent alkaline complex probably representing an analogous alkaline pulse. The pyroxenes of alkaline rocks of PMIC and adjoining area can also be distinguished from those of tholeiitic rocks, having higher Ti and Al contents. An early and strong partitioning of Cr in crystallizing diopside, magnetite and probably chromspinel of tholeiitic and alkaline parental magmas impoverished the pyroxenes (Mg/Mg+Fe¹<0.700) of microgranite and syenite in this element during the late stage of crystallization.

Keywords

Igneous Petrology, Clinopyroxene, Mineral Chemistry, Alkaline Rocks, Phenai Mata, Gujarat.

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Anant Shastry ¹, Santosh Kumar ¹

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, U. P., IN

Abstract

The alkaline rocks belonging to ijolite series and carbonatites constitute the Sarnu-Dandali igneous complex (68.5 Ma) associated with Precambrian acid volcanics. Trace elements including REE of ten selected Late Cretaceous rock-types viz. melteigite, melanephelinite, phonolite, carbonatite, basalt and Precambrian rhyolites of SDIC have been examined and discussed. The data of alkaline rocks suggest their derivation from an alkaline (ijolitic) magma formed by the partial melting of mantle source. Ba-Sr-REE enriched carbonatites possibly represent an immiscible split from nephelinite. The basalt (Hawaiite) represents a subalkaline magmatic pulse unrelated to alkaline suite. The associated Precambrian rhyolites have formed by a combination of magmatic processes such as partial melting, fractional crystallization and/or magma-mixing.

Keywords

Geochemistry, Alkaline Rocks, Sarnu-Dandali, Rajasthan.

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Sumer Chopra ¹, K. Madhusudhan Rao ¹, B. Sairam ¹, Santosh Kumar ¹, A. K. Gupta ¹, Hardik Patel ¹, M. S. Gadhavi ¹, B. K. Rastogi ¹

Affiliations
1 Institute of Seismological Research, Sector-18, Gandhinagar - 382 018, IN

Abstract

About 200 shocks were felt and over 50 of M-2 to 4 were located south of Jamnagar in 2006 and same number was felt/located again in 2007 with blast like subterranean sounds soon after heavy rains. Most of the shocks have shallow focal depth of less than 10 km. Some of the tremors have caused cracks in a few houses. During September-October 2006 tremors were confined withm 5 km of Khankotda village which is about 30 km SE of Jamnagar. During August-September 2007 the activity had spread northward up to Jamnagar. Some shocks have occurred about 20-25 km west of this trend faults/lineaments/dykes in the area are studied to identify the causative feature for the tremors. The rainfall data of the district and water level data of nearby observation wells are studied to see any possible relationship with earthquake activity. Data of about hundred cases of swarm type of earthquake sequences in the peninsular India in last several decades are compiled to study their characteristics vis-a-vis monsoons.

Keywords

Swarm-Type of Earthquakes, Subterranean Sounds, Saurashtra Seismicity.

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Santosh Kumar ¹, Brajesh Singh ¹, Charu C. Josh ¹, Abhishek Pandey ¹

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, Uttranchal, IN

Abstract

Felsic magmatism in Amripur and adjoining localities of Kumaun Lesser Himalaya is represented by Palaeoproterozoic (ca 1890 Ma) Bt-Ms granitoids and quartz feldspar porphyries, referred to as Amritpur granitoids (AG) and Amripur porphyry (AP) respectively Magnetic susceptibility( MS) values (range of average ϰ=0062 to 0 522x10³ SI) measured for various types of AG correspond to ilmenite-series (reduced type) granites, which are consistent with their peralumnous nature. The MS of AP ranges from 0 399 to 0 912x10³ SI with an average MS of 0 528x10³ SI, which are typical to ilmenite series granite, although AP represents both metaluminous and peraluminous types Compositions of biotite from Medium-Grained, leucocratic equigranular variety of AG, referred to as Amritpur leucogranite (ALG), are primary in nature, and can be classified as Fe²⁺ (ferri)biotite enriched in siderophyllite component FeO^t/MgO of biotites from AtG vanes from 2 93 to 4 94 which typically crystallized into crustally derived, peraluminous (S-type) felsic magma, with probable 3Fe ⇋ 2AI substitution in biotites during ALG melt evolution but 3Mg⇋2AI substitution cannot be inferred Bulk Fe₂O₃,/FeO ratio of most AG corresponds to llmenite series (Fe₂O₃/FeO<O 5) consistent with the observed MS values whereas AP samples equally represent both llmemate and magnetite (Fe₂O₃/FeO > 0 5) senes. The results strongly favour that the AG magma evolved under reducing environment.

Keywords

Palaeoproterozolic Granitoids, llmenite Series, Biotlite Composition, Amritpur, Kumaun Lesser Himalaya.

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Santosh Kumar ¹, Thepfuvilie Pieru ¹, Vikoleno Rino ¹, B. C. Lyngdoh ²

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, Uttranchal, IN
2 Directorate of Mineral Resources, RISA Colony, Shillong, Meghalaya, IN

Abstract

No Abstract.

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Santosh Kumar ¹

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, Uttranchal, IN

Abstract

Pearce element ratio (PER) diagrams were constructed for tholentic gabbro, alkali gabbro, dolerite and tholentic basalt of Phenal Mata Igneous complex (PMIC) Identifying potassium as a conserved element particularly during the course of basic magma(s) differentiation senes The observed variations among the tholentic basalts, dolentes, tholentic and alkali gabbros of PMIC on PER diagrams appear to be commonly controlled by the fractional crystallhzation of plagioclase, chnopyroxene and olivine in variable proportion Tholentic basalts and tholentic gabbros of PMIC could be derived from the same parental magma but proportion of crystallizing phases and even assemblages differ as reflected by their different slopes on PER diagrams The different intercepts with a common slope for tholentic gabbros and dolerites however strongly suggest their similarly involved differentiation mechanism but d1fferent parentage probably formed by slight contamiation of episodic tholentic magma by an alkaline one, which subsequently experienced the same differentation mechanism.

Keywords

Pearce Element Ratios, Petrogenesis, Basic rocks, Phenal Mata, Baroda district, Gujarat.

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Santosh Kumar ¹

Affiliations
1 Dept. of Geology, Kumaun University, Nainital, IN

Abstract

No Abstract.

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Santosh Kumar ¹, R. K. Ganjoo ²

Affiliations
1 Kumaun University, Nainital, IN
2 University of Jammu, Jammu, IN

Abstract

No Abstract.

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Santosh Kumar ¹, S. Raju ², Manjari Pathak ¹, Abhishek Pandey ³

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, IN
2 Geological Survey of India, Dimapur - 797 112, IN
3 Rolta Academy, MIDC, Marol, Andheri East, Mumbai - 400 093, IN

Abstract

Late Archaean to Palaeoproterozoic felsic magmatic lithounits exposed in the central part of the Bundelkhand massif have been mapped and their redox series (magnetite vs ilmenite series) evaluated based on magnetic susceptibility (MS) data. The central part of Bundelkhand massif comprises of multiple felsic magmatic pulses (∼2600-2200 Ma), commonly represented by coarse grained granite (CGG-grey granite, CPG-pink granite), medium grained pink granite (MPG), fine grained pink granite (FPG) , grey and pink rhyolites and granite porphyry (GP). However, the pink colour of these felsic rocks is the result of hydrothermal fluid-flushing leading to potassic alteration of grey granites. MS values of CGG vary from 0.058 to 14.75×10^-3 SI with an average of 6.35×10^-3 SI, which mostly represent oxidized type, magnetite series (73%) granites involving infracrustal (igneous) source materials. CPG (av. MS=3.95×10^-3 SI) is indeed a pink variety of CGG, the original oxidizing nature of which must have been similar to the bulk of CGG, but has been moderately to strongly reduced because of distinctly more porphyritic nature together with partial assimilation of metapelitic (supracrustal) materials, surmicaceous enclaves, carbonaceous material included in the source materials, and to some extent, induced by hydrothermal and later deformational processes. MPG (av. MS= 1.15×10^-3 SI) as lensoidal stock-like bodies intrudes the CPG and represent both magnetite series (18%) and ilmenite series (82%) granites, which are probably formed by heterogeneous (mixed) source rocks. GP (av. MS=6.26×10^-3 SI) occur as dykes (mostly trending NE-SW) intrudes the MPG, CPG and migmatites and bears the nature similar to oxidized type, magnetite series granite. FPG (av. MS= 0.666×10^-3 SI) trending NE-SW occur as lensoid bodies including a large outcrop, is intrusive into both CPG and MPG, and is moderately to very strongly reduced type, ilmenite series granites, which may be derived by the melting of metapelitic crustal sources. FPG hosting microgranular (mafic magmatic) enclaves commonly exhibit high MS values (7.31-10.22×10^-3 SI), which appear induced by the mixing and mingling of interacting felsic and mafic magmas prevailed in an open system. Grey (av. MS=10.30×10^-3 SI) and pink (av. MS=6.72×10^-3 SI) rhyolites represent oxidized type, magnetite series granites, which may have been derived from infracrustal (magmatic) protoliths. Granite series evaluation of felsic magmatic rocks of central part of Bundelkhand massif strongly suggests their varied redox conditions (differential oxygen fugacity) mostly intrinsic to magma source regions and partially modified by hydrothermal and tectonic processes acting upon them.

Keywords

Magnetic Susceptibility, Magnetite Series, Ilmenite Series, Granitoids, Bundelkhand Massif, Central India.

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Santosh Kumar ¹, Manjari Pathak ¹

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, IN

Abstract

Felsic magmatic rocks in Kameng corridor of western Arunachal Himalaya are represented by extensively exposed Palaeoproterozoic porphyritic muscovite-biotite granite (GGn) of the Bomdila Group and small stock-like Mesoproterozoic hornblende-biotite granite (HBG) of the Salari Group. Mineralogy and chemical composition of biotites from GGn and HBG have been utilized to understand the nature and tectonic environment of their parental felsic melts. Biotites in GGn (FeO^t/MgO=3.1-4.6) are Fe-biotites and have shown affinity with primary biotites co-precipitating with muscovite in a peraluminous (S-type) felsic melt of syn-collisional tectonic environment. Biotites in HBG (FeO^t/MgO=1.3-2.2) are transitional between Fe and Mg biotites evolved from Fayalite-Magnetite-Quartz (FMQ) to Nickel-Nickel Oxide (NNO) buffers and are related to primary biotites co-existing with amphibole and other ferromagnesian minerals in a calc-alkaline metaluminous (I-type) felsic melt mostly formed in a subduction setting. Both GGn and HBG biotites exhibit Mg⇌Fe substitution, which is more pronounced in HBG biotites. GGn biotites exhibit 2Al⇌3Fe²⁺ substitution as expected in peraluminous melt, whereas 3Mg⇌2Al substitution normally expected to operate in metaluminous melt is less pronounced in HBG biotites. GGn biotites are markedly enriched in siderophyllite, and depleted in phlogopite components as compared to HBG biotites, which point to diverse genetic conditions. HBG biotites indicate oxidizing environment of the felsic melt unlike the reducing nature of the porphyritic granite (GGn).

Keywords

Proterozoic Granitoids, Biotite Chemistry, Tectonomagmatism, Western Arunachal, Himalaya.

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Santosh Kumar ¹, Thepfuvilie Pieru ²

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, IN
2 Department of Geology, Patkai Christian College, Chumukedima, Patkai - 797 103, IN

Abstract

Neoproterozoic (690±19 Ma) felsic magmatism in the south Khasi region of Precambrian northeast Indian shield, referred to as south Khasi granitoids (SKG), contains country-rock xenoliths and microgranular enclaves (ME). The mineral assemblages (pl-hbl-bt-kf-qtz-mag) of the ME and SKG are the same but differ in proportions and grain size. Modal composition of ME corresponds to quartz monzodiorite whereas SKG are quartz monzodiorite, quartz monzonite and monzogranite. The presence of acicular apatite, fine grains of mafic-felsic minerals, resorbed maficfelsic xenocrysts and ocellar quartz in ME strongly suggest magma-mixed and undercooled origin for ME. Molar Al₂O₃/CaO+Na₂O+K₂O (A/CNK) ratio of ME (0.68-0.94) and SKG (0.81-1.00) suggests their metaluminous (I-type) character. Linear to sub-linear variations of major elements (MgO, Fe₂O₃^t, P₂O₅, TiO₂, MnO and CaO against SiO₂) of ME and SKG and two-component mixing model constrain the origin of ME by mixing of mafic and felsic magmas in various proportions, which later mingled and undercooled as hybrid globules into cooler felsic (SKG) magma. However, rapid diffusion of mobile elements from felsic to mafic melt during mixing and mingling events has elevated the alkali contents of some ME.

Keywords

Neoproterozoic Granitoids, Microgranular Enclaves, Magma-Mixing, South Khasi, Meghalaya.

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N. Surdas Singh ¹, Santosh Kumar ¹

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, IN

Abstract

Mafic dykes of Almora region intrude the Precambrian crystalline rocks of Kumaun Lesser Himalaya. Mafic dykes exhibit fine grained margin and medium to coarse grained core, melanocratic, low to highly ferromagnetic (MS=0.85-38.58×10^-3SI) in nature commonly showing subophitic to ophitic textures with ol-pl-cpx-hbl-bt-mt-ap-sp assemblage, and modally correspond to leucogabbro and olivinegabbro (sensu stricto). Olivine (Fo₆₁-Fo₃₃), clinopyroxene (Wo₄₆-En₄₂-Fs₂₂ to Wo₄₀-En₃₆-Fs₁₅) and plagioclase (An₅₈-An₁₂) have crystallized in the temperature range of ca 1400-980°C at pressure <2 kbar in an olivine tholeiitic basalt parent. Low acmite (Na_pfu=0.033-0.025), (Mg^#=0.64-0.82), Ti-Al contents of clinopyroxenes and their evolution along enstatite-ferrosilite join (i.e. Mg⇀Fe substitution) strongly suggest tholeiitic nature of mafic dyke melt with changing activities of alumina and silica. Clinopyroxene compositions of mafic dykes differ markedly as compared to those observed for adjoining Bhimtal volcanics but closely resemble to that crystallized in tholeiitic melts of Deccan province. Observed Cr vs. Mg^# variation, enriched LILE (Sr, Ba)-LREE and positive Eu-anomaly of the studied mafic dykes are indicative of fractional crystallization of olivine-clinopyroxene-plagioclase from a crustally-contaminated tholeiitic basalt magma derived from enriched mantle source. The mafic dykes of Almora are geochemically identical to mafic dykes of Nainital, but are unrelated to Precambrian mafic volcanic flow and dykes of NW Himalaya and dykes of Salma and Rajmahal regions.

Keywords

Petrology, Geochemistry, Mafic Dykes, Almora, Kumaun Lesser Himalaya.

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Santosh Kumar ¹, Vikoleno Rino ¹, A. B. Pal ²

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, IN
2 Hindustan Copper Limited, Malanjkhand, Balaghat - 481 116, IN

Abstract

The Cu(±Mo±Au) hosting Malanjkhand granitoids (MG, -2400 Ma) represent Palaeoproterozoic felsic magmatism in Central India, which constitute part of the Bundelkhand Protocontinent. The enclaves hosted in MG can be classified broadly into two categories (1) Microgranular enclaves (2) Country-rock xenoliths. The microgranular enclaves (ME) are mesocratic to melanocratic, fine to coarse grained, porphyritic and commonly showing typical igneous textures. Field and microstructural evidences of ME and MG suggest the interaction between coeval enclave (mafic) and granite magmas. Modal compositions of ME are mostly granodiorite, quartz monzodiorite and monzogranite, whereas MG belong to quartz monzodiorite and monzogranite both being related to the calc-alkaline granodiorite (medium-K) series but evolve into the field of granitoids generated by crustal fusion. The primary enclave magma generating various ME can be represented by a ME (SiO₂=48.58, MgO=8,59 and Fe₂O₃-15.15 wt%), geochemically similar to calc- alkaline basalt. On various bi-variate geochemical plots MG and ME show a wide compositional variation suggesting mixing of enclave and granite magmas. On a multicationic R₁-R₂ plot the tie lines joining the ME and respective host MG suggest that sufficient chemical disequilibria exist between them. Using experimental melt compositions in terms of Si02 and K20 contents, the MG melt is likely to be derived from a tonalitic source, which subsequently experienced fractional crystallization, evolving within a high-K calc-alkaline field, whereas the ME plot in various fields suggesting the involvement of magma-mixing phenomena and to a certain extent potassium migration leading to a different degree of chemical disequilibria with their host MG, rather than crystal-liquid separation. Contents of selected trace elements (Ni, Rb, Sr, Y, Zr) of few pairs of ME-MG suggest partial to complete equilibration between them. However, Ti and Y contents, closer to primitive mantle, are indicative of mantle contribution to the source rocks from which ME and MG are derived.

Keywords

Palaeoproterozoic Granitoids, Microgranular Enclaves, Typology, Geochemistry, Magma-Mixing, Malanjkhand, Madhya Pradesh.

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