A B C D E F G H I J K L M N O P Q R S T U V W X Y Z All
Dessai, A. G.
- Thermal Structure of the Lithosphere Beneath the Deccan Trap along the Western Indian Continental Margin: Evidence from Xenolith Data
Authors
1 Department of Geology, Goa University, Taleigao Plateau, Goa 403 206, IN
2 Danish Lithosphere Center, Oster Volgade 10, DK - 1350, Copenhagen, DK
3 Department of Earth Sciences, University of Florence, 50121 via G. la Pira, IT
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 54, No 6 (1999), Pagination: 585-598Abstract
Late alkaline lamprophyre-intrusives from the Deccan Trap along the western Indian continental margin entrain rare granulite and pyroxenite xenoliths some of which contain kaersutite ± phlogopite ± apatite ± scapolite ± sulphides providing evidence orfluid induced metasomatism.P-Testimates on carefully selected mafic granulite and pyroxenite xenoliths define a palaeogeotherm from 550°C, 5 kb to -830°C, 8.5 kb for the Mumbai region along the western continental margin. This elevated geotherm suggests advective heat transfer from magmas ponded near the crust mantle boundary.
Xenolith petrology coupled with seismic data suggests that the lower crust beneath Mumbai consists of mafic granulites under- and intra-plated by pyroxenites. Spinel peridotites interlayered with pyroxenites predominate below -20 km. The seismic Moho is located within the layered pile.
Keywords
Petrology, Granulite, Pyroxenite, Crustal Xenolith, Lamprophyre, Deccan Trap, Maharashtra.- Lithomarges from Goa - A Study Based on Infrared Spectroscopy and Differential Thermal Analysis
Authors
1 Department of Geology, University of Poona, Pune 411 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 25, No 9 (1984), Pagination: 598-603Abstract
The DTA and IR studies of the lithomarges (saprolites) from Goa have shown that they are predominantly made up of kaolinite, chlorite group minerals, goethite and quartz, with minor amounts of gibbsite and talc.
The lithomarges have resulted from weathering of rocks of varying lithologies under subtropical climatic conditions. Depending upon the microenvironments different alteration products may be derived from the same starting material. Absolute and relative accumulation of iron gives rise to the ferricretes while relative enrichment of aluminium as silicates leads to the formation of lithomarges at depth.
- Geochemistry and Petrology of Xenolith-Bearing Lamprophyres from Murud-Janjira, Raigarh Dist., Maharashtra, India
Authors
1 Department of Geology, University of Poona, Pune 411007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 30, No 1 (1987), Pagination: 61-71Abstract
The N-S trending lamprophyre dykes from Murud-Janjira contain felsic and ultramafic xenoliths alongside unusually zoned clinopyroxene phenocrysts and xenocrysts that are found in an analcime base. On the basis of mineralogy and chemistry, the lamprophyres have been classified as monchiquites.The lamprophyres are probably emplaced along a system ef chasmic faults that are parallel to the West Coast arm of the Khambat triple junction. They may represent flank activity of the mid-ocean ridge anomaly and are the products of tlte waning stages of Deccan magmatism.
- Mineralogy and Geochemistry of the Calcretes in Alluvial Sediments from Pune, India
Authors
1 Department of Geology, University of Poona, Pune 411007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 29, No 6 (1987), Pagination: 584-593Abstract
The calcretes in the alluvial sediments from Pune district, Maharashtra occur as hardpan, powdery, laminated, concretionary and pedotubule horizons. The calcretes are associated with clay minerals belonging to the smectite, chlorite and kaolin groups. The chemical analysis of the calcretes indicates that they are predominantly composed of calcium-carbonate and silica, (Cao: 33 to 37%, SiO2 : 25 to 31%) with magnesium in very subordinate amounts. Trace elements present include Sr, Zr, V, Nb, Y, and Cs. On the basis of textural, mineralogical and chemical data, it is shown that the calcretes have resulted from two different processes, viz., by accumulation in the zone of fluctuation of groundwater tabie and by precipitation from percolating water during pedogenesis.- Podiform Chrolnites in Mantle Peridotites from Indus Ophiolite Belt, Ladakh Himalaya
Authors
1 Department of Geology, Goa University, Bambolim, Goa 403 202, IN
2 Department of Geology, University of Poona, Pune 411 007, IN
3 C.N.R.S., University of Science and Technology, 34060 Montpellier, FR
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 39, No 1 (1992), Pagination: 29-38Abstract
Peridotites in ophiolites from Ladakh Himalaya occur as disconnected slices within an ophiolite melange exhibiting faulted, sheared and tectonic contacts. Chromite occurs as pods, schlieren banded as accessory ore within dunites. The pods are generally concordant to semiconcordant to the foliation of the host dunites. In places, however, they are discordant to the foliation. The chrome spinels (sensu lato) exhibit bimodal distribution. Those in harzburgite conform to Type I alpine type peridotites. Whereas the ones from dunite are akin to Type III peridotites as classified by Dick and Bullen (1984). The spinels from wehrlites are compositionally similar to those from ophiolite cumulates.Keywords
Chromite, Indus Ophiolite Belt, Ladakh Himalaya.- Magma Fractionation and Mixing in Nephelinite Plug Associated with Deccan Magmatism at Murud-Janjira, South of Bombay, India
Authors
1 Department of Geology, Goa University, Taleigao Plateau, Goa 403 205, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 43, No 5 (1994), Pagination: 493-509Abstract
The nephelinite occurs as a N-S trending plug (500 × 200 m.) intrusive into basalts. The rock is porphyritic and principally composed of complexly zoned phenocrysts of clinopyroxene, Fe-Ti oxide and olivine <5%) in a fine grained groundmass dominated by clinopyroxene, nepheline and Fe-Ti oxides. Mineralogically and chemically the rocks vary between pyroxene melanephelinites and nephelinites.
The mineral chemistry implies that the magma has undergone varying degrees of polybaric crystallisation in a subcrustal magma chamber with mixing of more fractionated magma with primitive pulses in which are incorporated disaggregated mantle xenolith phases.
Keywords
Nephelinite, Igneous Petrology, Deccan Traps, Bombay, Maharashtra.- Petrogenesis of the Bondla Layered Mafic-Ultramafic Complex, Usgaon, Goa
Authors
1 Department of Earth Science, Goa University, Taleigao Plateau, Goa 403 206, IN
2 Energy Technology, CSIRO, New Illawarra Road, Lucas Heights, New South Wales, AU
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 73, No 5 (2009), Pagination: 697-714Abstract
The Bondla mafic-ultramafic complex is a layered intrusion that consists predominantly of peridotites and gabbronorites. A chromitite-pyroxenite-troctolite horizon serves as a marker to subdivide the intrusion into two zones. The Lower Zone displays gravity stratified layers of chromite that alternate with those of olivine, which up-section are followed by olivine+pyroxene-chromite cumulates. The Upper Zone comprises gabbroic rocks that exhibit uniform layering. On the basis of modal and cryptic variation exhibited by the minerals this zone can be subdivided in to several lithohorizons starting from the troctolites at the base to gabbronorites and leucogabbros at the top. The junction between the two zones is marked by the distinct reversal in cryptic variation exhibited by the chromites and pyroxenes.The peridotite chromites contain higher Al2O3 and lower Cr2O3 than those from the chromitite above. Similarly clinopyroxenes from pyroxenite and troctolites are more magnesian that those from the peridotites stratigraphically below them. The complex in general is characterized by a gabbroic mineral assemblage in which both Ca-rich and Capoor pyroxenes coexist and displays a Fe-enrichment trend providing evidence of evolution from a contaminated tholeiitic magma. The rocks are characterized by low-TiO2; Ni, Cr and V, show negative correlation with Zr whereas the large ion lithophile elements (LILE) are positively correlated and the Nb/La ratio varies from 0.4-0.6. These characteristics are consistent with a low-TiO2 sub-alkaline tholeiitic magma that may have been modified by fractional crystallization and successive injections of more primitive melts in the magma chamber. The complex evolved in a periodically replenished magma chamber that consisted of two separate but interconnected sub-chambers.
Keywords
Layered Complex, Peridotites, Gabbronorite, Chromitite, Cumulate, Goa.References
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- The Geology of Goa Group: Revisited
Authors
1 Department of Earth Science, Goa University, Taleigao Plateau, Goa - 403 206, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 78, No 3 (2011), Pagination: 233-242Abstract
The supracrustals that constitute the Goa Group of Gokul et al. (1985) can be divided into two lithostratigraphic sequences namely the Barcem Group and the Ponda Group. The former comprises predominantly greenstones (metabasalts) and rests on a basement of the 3300-3400 Ma Anmode Ghat trodhjemite gneiss with a crudely developed quartz-pebble conglomerate at the base, and shows lithological similarities with the lower part of the Bababudan Group. The younger sequence is dominated by clastics, and is assigned to a new stratigraphic group formally termed the Ponda Group which is equivalent to the Chitradurga Group of the Dharwar Supergroup. This group rests on a basement of the 2700-2900 Ma Chandranath granite gneiss with a distinct unconformity marked by a polymict, granite-clast metaconglomerate. The conglomerate displays many similarities with the Talya conglomerate that occurs at the base of the Chitradurga Group. It is overlain by a psamolitic sequence which is followed in ascending order by the chemogenic sediments that host the BIF and by the deep water turbidite sequence (argillite-graywacke association) with intercalations of mafic volcanics. The supracrustal sequence is intruded by the Bondla layered mafic-ultramafic complex along a major shear zone (NW-SE) that largely controls the course of the northwesterly flowing tributary of River Mandovi. The late intrusive, Canacona potassic granite marks the culmination of the sedimentation in the Shimoga-Goa basin.Keywords
Goa Group, Supracrustals, Anmode Ghat Trondhjemite Gneiss.References
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