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Ahmad, Talat
- Precambrian Mafic Magmatism in the Indian Shield: An Introduction
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Authors
Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN
2 Department of Geology, Delhi University, Delhi - 110 007, IN
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN
2 Department of Geology, Delhi University, Delhi - 110 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 72, No Spl Iss 1 (2008), Pagination: 9-13Abstract
No Abstract.
- Precambrian Mafic Magmatism in the Himalayan Mountain Range
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Authors
Affiliations
1 Department of Geology, Delhi University, Delhi - 110 007, IN
1 Department of Geology, Delhi University, Delhi - 110 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 72, No Spl Iss 1 (2008), Pagination: 85-92Abstract
Precambrian mafic magmatic rocks are restricted to the western Himalayas from Himachal in the west through Garhwal to Kumaun in the east. They occur as low-grade volcano-sedimentary sequences of the Lesser Himalaya in the form of volcanic flows intercalated with continentally derived sediments and also as dykes occurring as feeders and some traversing through these sequences. Thus, all the Precambrian mafic magmatic rocks are not contemporary but may represent more than one magmatic episode. However, it is not very clear how much is the gap between the individual magmatic phases. In addition to the low-grade sequences, mafic magmatic rocks are also recorded from the Chail, Jutogh and Vaikrita Groups and their equivalents in Garhwal and Kumaun regions. In these three sequences the mafic magmatic rocks are represented by amphibolites which occur as shapeless bodies, boudins, disrupted dykes/sills varying in size from 1-2 m in thickness and 3-5 m in width. The basic rocks of all the units consist of clinopyroxene (augite and ferro-augite) and plagioclases as the dominant phases with minor olivine (now serpentine). Commonly observed secondary phases include amphiboles, chlorite, opaques, epidote, quarts, biotite and titanites.
Geochemical data indicates that these rocks are compositionally basalt and basaltic - andesites. They have typical tholeiitic lineage with Fe-enrichment and are classified as low-Ti tholeiites. Rare earth elements (REE) and incompatible trace elements data indicate enriched light rare earth element (LREE) - large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE) characteristics for the mafic rocks of the lower grade Lesser Himalayan sequences, Chail and Jutogh Groups. The mafic rocks of Vaikrita Group, on the other hand show nearly flat REE and incompatible trace elements patterns with distinct positive Sr-anomaly, which is very different from the other groups. These characteristics indicate derivation of Vaikrita mafic rocks from different sources compared to the other samples. Lithological association and geochemical characteristics indicate derivation of the mafic magmatic rocks from enriched lithospheric mantle sources by varying degrees of partial melting followed by clinopyroxene ± minor olivine ± plagioclases ± opaques in a rift tectonic environment.
Keywords
Magmatism, Precambrian, Himalaya.- Geochemical and Petrogenetic Study of Proterozoic Sewariya and Govindgarh Granitoids from South Delhi Fold Belt
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Authors
Affiliations
1 Department of Geology, University of Delhi, Delhi 110 007, IN
1 Department of Geology, University of Delhi, Delhi 110 007, IN
Source
Current Science, Vol 109, No 8 (2015), Pagination: 1458-1465Abstract
The present study reports the geochemical composition of Sewariya two-mica granites (SG) and Govindgarh granites (GG) intruding rocks of Delhi Supergroup along the western margin of South Delhi Fold Belt in Rajasthan, India. Both granite varieties are highly evolved possessing high SiO2, Al2O3, low MgO, CaO, Fe2O3, Ni, Cr and V and are calc-alkaline in nature. In chondrite normalized REE diagrams, SG are characterized by highly fractionated REE patterns (avg LaN/ YbN = 21.45) and sharp negative Eu* anomaly (Eu/Eu* = 0.25), whereas GG do not show significant REE fractionated patterns (avg LaN/YbN = 3.31) and have variable Eu anomalies. From primitive mantle normalized multi-element diagrams, crustal signatures (low Nb, Ti and high Pb, U, Th) can be inferred for both the granitoid varieties. Also, strong peraluminous nature, high A/CNK, normative corundum and abundant mica content point towards a (meta)sedimentary protolith for them. An arc setting is indicated by their calc-alkaline nature; volcanic arc or syncollisional affinities in tectonic discriminant diagrams (Nb versus Y; Rb versus Nb + Y). They are peraluminous leucogranites derived from crustal melts with little mantle contribution as is evident from their mineralogy and geochemical characteristics. The anatectic conditions prevalent during the formation of these granites differed with SG being formed under dehydration conditions, while the formation of GG involved fluid-present melting reactions.Keywords
Geochemistry, Granitoid Magmatism, Petrography, Peraluminous Leucogranites.Full Text
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- Geochemistry and Petrogenesis of Amphibolites from the Southern Part of Gadag Greenstone Belt, Karnataka
Abstract Views :172 |
PDF Views:2
Authors
M. Ram Mohan
1,
D. Srinivasa Sarma
1,
S. N. Charan
1,
V. Balaram
1,
V. B. Rajasekhar
1,
Talat Ahmad
2
Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN
2 Department of Geology, University of Delhi, Delhi-110007, IN
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN
2 Department of Geology, University of Delhi, Delhi-110007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 72, No 4 (2008), Pagination: 484-494Abstract
Gadag Greenstone Belt (GGB) is the northern continuation of Chitradurga Greenstone Belt (CGB). It consists of a variety of metavolcanic and metasedimentary rocks. Two types of metavolcanic assemblages are found in this terrane (l) the tholeute-calc-alkaline island arc bimodal assemblage and (u) the tholeute-high-Mg basalt assemblage The tholente-calc-alkaline assemblage is exposed in the central and northern parts, whereas the tholentic-high-Mg basaltic assemblage is found in the southwestern part of the belt. Tholente-high-Mg basalts are represented by the coarse-grained amphibolites formed under lower amphibolite facies conditions REE and HFSE data along with major element compositions confirm that these coarse-grained amphibolites are tholentic basalts derived from an intraoceanic island arc setting. The REE patterns are coherent, flat to slightly LREE depleted (La/Ybn =0.79 to 1.20, La/Smn =0.84-0.97, Gd/Ybn=1.07-1.50) with no Eu anomaly. Relationship between compatible and incompatible elements suggests least effects of alteration and no crustal contamination or fractional crystallization. The mixing calculations indicate that these rocks are derived by partial melting of a depleted mantle source, with source composition in between that of the N-MORB and high-Mg basalts.Keywords
Geochemistry, Petrogenesis, Amphibolites, Gadag Greenstone Belt.- Brine-Rich Hydrothermal Fluid Circulation in the Upper Level of Nidar Ophiolite Sequence, Ladakh:Evidences from Fluid Inclusions
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Authors
Affiliations
1 Wadia Institute of Himalayan Geology, DehraDun - 248 001, IN
2 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, JP
3 Department of Geology, Delhi University , Delhi 110 007, IN
1 Wadia Institute of Himalayan Geology, DehraDun - 248 001, IN
2 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, JP
3 Department of Geology, Delhi University , Delhi 110 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 70, No 5 (2007), Pagination: 780-786Abstract
Microthermometric study of fluid inclusions in gabbro from the Nidar ophiolitic sequence revealed that brine rich fluid (34-45 wt% NaC1) at the temperature of more than 400°C were predominant throughout the gabbro High saline inclusion homogenized by halite dissolution at the temperature of 276-400°C. All the moderate to high temperature(l30-390°C) inclusions are liquid and vapour dominated, commonly secondary in origin, majority having seawater-Like salinity( 17- 8 wt% NaC1).The highly saline fluid was probably derived by phase separation of hydrothermal seawater or an exsolved magmatic aqueous phase at the temperature of 250-412°C. The migration of the Phase-Separated along fractures resulted in segregation of the vapour and brines and preferential entrapment of High-Saline inclusions at depth. These brines may have originated when seawater approached the top of the magma chamber. The subsequent fracturing of the gabbro at the temperature of 130-390°C facilitated the entry of seawater which pervasively altered the gabbro. This hydration processes resulted into the formation of secondary minerals or by mixing with Phase-Separated fluid, resulting in salinities Two-Times higher than that of seawater.
Keywords
Brine, Fluld Inclusions, Nidar Ophiolite, Ladakh.- Geochemistry of Pegmatites from South Delhi Fold Belt: A Case Study from Rajgarh, Ajmer District, Rajasthan
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Authors
Affiliations
1 Department of Geology, University of Delhi, Delhi 110 007, IN
1 Department of Geology, University of Delhi, Delhi 110 007, IN
Source
Current Science, Vol 106, No 12 (2014), Pagination: 1725-1730Abstract
On the basis of geochemical studies, pegmatites emplaced in the Rajgarh Group of Delhi Supergroup in the South Delhi Fold Belt have been classified into three groups. They show a variety of rare earth element enrichment patterns, LREE/HREE values and Eu anomalies. The geochemical affinities of these pegmatites suggest their calc-alkaline nature, volcanic arc granite signature in tectonic discrimination diagrams (Nb vs Y and Rb vs Nb + Y) and a probable S-type parentage as inferred from their high A/CNK value, peraluminous character, presence of high normative corundum and abundance of garnet and muscovite. These features have been related to subductionrelated processes which might have generated the parent granitic melt forming these pegmatites.Keywords
Geochemical Studies, Granitic Melt, Pegmatites, Subduction.Full Text
- Precambrian Mafic Magmatism in the Indian Shield: Retrospect and Prospect
Abstract Views :154 |
PDF Views:0
Authors
Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN
2 Department of Geology, Delhi University, Delhi - 110 007, IN
1 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN
2 Department of Geology, Delhi University, Delhi - 110 007, IN