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

S. F. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay, IN

Abstract

The note incorporates trace element analyses of carbonatites from Amba Dongar and surrounding areas. Mineralogical study indicates that pyrochlore is the only diagnostic mineral present in some of these rocks. Though the rare earth minerals proper are absent, the analyses reveal a fair concentration of the rare earth elements. It is therefore concluded that these elements are mainly contained in calcite and ankerite. Also, some calcareous rocks of dyke-like nature from Bakhatgarh and Dehi, suspected to be carbonatites, have been analysed. However, their trace element contents do not conclusively prove them to be so.

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S. F. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai-400001, IN

Abstract

Field data reveal that the lava flows in Mumbai and surrounding areas erupted after the rifting of the main plateau basalts of the Deccan Volcanic Province. Majority of the lava flows occur as subaqueous lavas forming hyaloclastites, spilitic pillow lavas and partially subaqueous and subaerial flows. The basic lavas are associated with rhyolitic flows and contemporaneous intrusions of basic and trachytic magmas forming hybrid rocks of monzonitic and dioritic variety. It has been shown that petrochemically the basic magma of the area is very different from the magma types associated with the mainland horizontal flood basalts ofthe Deccan Volcanic Province. The basic rocks of the Mumbai-Salsette area show lower concentrations of Cr, Ni, Y, V and Ti; and higber concentrations of K, Rb, Ba, Zr and Nb. Majority of these samples contain similar concentrations of phosphorus except for three Salsette samples which show unusual high values. The volcanics of the Mumbai-Salsette area are classified as forming the Salsette Subgroup with three Formations viz., Mumbai Island, Madh-Utan and Manori.

Keywords

Deccan Trap, Bombay (Mumbai) Island, Spilites, Hyaloclastites, Stratigraphy.

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S. F. Sethna ¹, Priya Javeri ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai-400001, IN

Abstract

Major, trace and rare earth element analyses of spilitic basalts and some dolerite dykes and microprobe mineral analyses of a spilitic basalt are reported. It is postulated that these rocks represent subaqueous flows erupted under fairly shallow water which was insufficient for complete spilitisation of the basalt. The process of metasomatism under subaqueous conditions is considered to have been responsible for the alteration of tholeiite to spilite.

Keywords

Petrology, Geochemistry, Spilites, Deccan Traps, Daman.

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S. F. Sethna ¹, Ateeq Khateeb ¹, D. R. K. Rao ², P. D. Saraf ²

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai - 400 001, IN
2 Indian Institute of Geomagnetism, Colaba, Mumbai - 400 005, IN

Abstract

The area around Nandurbar consists of compound lava flows (basalts) of Deccan Trap which are intruded by numerous prominent dykes. These dykes are mainly dolerites and some are quite massive and extensive. One major dyke was traced for over 50 km, from north of Dondaicha, westwards as far as Umaj, near the Gujarat - Maharashtra border. Petrochemical studies indicate tholeiitic nature of the magma. There is also an indication of two distinct magma types represented by high Ti and low Ti varieties. These two magma types have intruded as distinct phases with the high Ti dykes being cut across by the low Ti dykes.

The palaeomagnetic work centers around these dykes. In addition a few lava flows were studied for comparison. The older set of dykes show normal magnetization whilst the younger set shows reverse directions. The lava flows are also reversely magnetized. It is thus evident that both the dyke phases are younger than the surrounding lava flows. The older phase of dykes could have been feeders to the younger lava flows of the Deccan Volcanic Province but the younger phase is distinctly post Deccan Trap.

Keywords

Palaeomagnetism, Deccan Trap, Dykes, Tapti Valley, Maharashtra.

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R. N. Sukheswala ¹, S. F. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay, IN

Abstract

The area of Bassein lies 30 miles north of Bombay. Within a compass of about 150 square miles the rocks encountered here are basalt flows and dykes, dolerite dykes, ·diorites, thin veins of aplite, a single dyke of camptonite and pyroclasts.

An explanation is offered for the origin of felsic rocks of differing composition. It is suggested that the initial rhyolitic magma is not a product of differentiation of Deccan basalt but originated as a result of partial fusion of the sialic crust during long continued ascent and outpouring of vast volume of basalt lavas.

Rocks of dioritic composition are described, and it has been shown that the diorite and its associates like meladiorite. leucodiorite, granophyre, andesite. etc., are the result of contamination of rhyolite magma with the country rock dolerite. In support of this, chemical, microscopic and field evidences are cited. Thin stringers of aplite are considered to be the residual liquid product of the contaminated rhyolite magma.

Detailed optical investigations on the pyroxenes and feldspars of basalts and dolerites are carried out. While pyroxene is mainly augite, pigeonite and/or orthopyroxene are conspicuous by their absence. Important minerals of various other rock types are also described.

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S. F. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay, IN

Abstract

The carbonatites in India are closely associated with alkaline rocks, and are restricted to the stable continental area along major deep-seated faults. The detailed structural settings of all the carbonatite-alkaline complexes are yet to be worked out. Alkali metasomatism (fenitisation) that commonly accompanies carbonatite eruption also remains to be studied for most of the carbonatites so far discovered. Very little geochemical work has been done to prove or disprove the concept of associational major and minor element characteristics of the alkaline rocks and carbonatites. Whereas highly diagnostic isotope ratios have been reported for the igneous carbonate rocks from various parts of the world, only very few determinations have been done on the Indian carbonatites. Since experimental data have proved the magmatic origin of carbonatites and it is presumed that carbonatites are secondary magmas out of either kimberlitic or alkali-basaltic magmas, the ultrabasic rocks associated with the alkaline rocks and carbonatites of India need to be studied from the point of view of establishing the parent magma. Carbonatites have proved to be hosts for numerous economic mineral deposits. It is, therefore, necessary to undertake extensive geochemical surveys of the known outcrops as well as of the soils overlying and surrounding the outcrops.

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S. F. Sethna ¹, S. G. Viladkar ¹

Affiliations
1 Geology Department, St. Xavier's College, Bombay 400001, IN

Abstract

The temperatures of crystallization of 23 calcitic carbonatites have been estimated on the basis of the amount of MgCO₃ going into solid solution with calcite. Majority of the carbonatites have a temperature of crystallization between 350° to 475° C., while the one at Sevattur shows a rather high temperature of crystallization (about 600° C). There is a distinct relation between grain size and temperature of crystallization for the Amba Dongar carbonatites.

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S. F. Sethna ¹, H. K. Batnwala ¹

Affiliations
1 Geology Department, St. Xavier's College, Bombay 400001, IN

Abstract

The intermediate and acid rocks of Salsette Island, Bombay, that were collectively identified as rhyolites, rhyodacites and trachytes by previous workers, have been remapped to locate the outcrops of acid and intermediate rocks separately. Eighty-one samples, collected systematically, have been analysed for Na₂O and K₂O contents and thirty-five of these for major elements. On the basis of geochemistry and mineralogy these rocks have been classified into five groups, viz., (I) potassic rhyolites, (2) sodipotassic rhyolites, (3) potassi-sodic rhyolites, (4) quartz-rich trachytes, and (5) quartzbearing trachytes.

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S. F. Sethna ¹, S. M. Borges ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay 400001, IN

Abstract

The upper portions of the Bagh sandstones of Siriwasan have been penetrated and soaked by a sill-like intrusive of carbonatite which has also penetrated the overlying Deccan Trap basalts at a few places. The carbonatite is intimately associated with tinguaite which shows the same form and occurs as discrete dykes as well. The carbonatite mainly forms the matrix of the brecciated sandstone but in places, where it collected as pools, large pockets of sovite are found. Pools of sovite are also found within tinguaite which shows brecciation at places. Thirty-four samples have been analysed for Sr, Rb, Zr, Nb, Zn, Mn, P, Ce, La, Y and Th. Thirteen of these have also been analysed for U. Analytical data show a consanguinity of the carbonatites and the tinguaites. Immiscible liquid fractions of a carbonatite-alkalic magma are considered to have produced the different rock types.

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S. F. Sethna ¹, W. Czygan ², B. S. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay 400 001, IN
2 Mineralogisch Institut, Albert-Ludwigs Universitat, D-7800 Freiburg i BR, DE

Abstract

The oxygen fugacity and temperature of equilibration of the iron-titanium oxides for thirty samples of basaltic rocks, taken from different parts of the Tholeiitic Flood Basalt Province of Deccan Trap (Tertiary); are given. Temperatures range from 950° to 1200°C and oxygen fugacity from 9 to 12 (as -log₁₀). Picritic basalts from Igatpuri along the Western Ghats, that form compound lava fiows, give lower temperatures of equilibration of the iron-titanium oxides (considered to represent the liquidus of the host rock) as compared to the simple thin lava flows of normal tholeiitic basalt from the eastern parts of the Deccan trap outcrop.

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Nita Tolia ¹, S. F. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay 400001, IN

Abstract

At Amboli Hill, in Jogeshwari, is exposed a spectacular columnar jointed basalt outcrop which has been inferred to represent a sill-like body into the intertrappean beds. The structural deformation of the intertrappeans gives a distinct saucer-shaped bottom giving the impression of a lopolith. Petrographic and chemical studies of the basalt indicate that the rock is tholeiitic in composition and does not show any significant variation in mineral content and chemical composition across the exposed thickness of the body. The intertrappean beds are predominantly made of volcanic breccias with thin intercalations of sedimentary beds. Detailed study of the petrography and chemistry of the breccia fragments indicate that they are of spilitic character which leads to tbe conclusion tbat the rock is a hyaloc1astite formed by subaqueous eruption of the basaltic lava.

Keywords

Amboli, Basalt, Bombay, Deccan Trap, Intertrappeans.

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S. F. Sethna ¹, B. S. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay 400 001, IN

Abstract

Major and trace element analyses of sixteen samples of basaltic rocks. representing a vertical section of 1600m (from Bhiwandi to Jgatpuri and beyond into Kalsubai Hill) of the Western Ghats, are presented. The analyses show a very wide variation in Mg number, and indicate very distinct differentiation trends. The MgO contents vary between 4.55% and 12.43% showing a continuous variation from tholeiitic basalts to picrite basalts that are enriched in phenocrystal olivine. It is shown that the MgO-rich picritic basalts have differentiated to form the normal low-MgO basalts. It is also shown that the very high contents of MgO is due to accumulation of phenocrysta] olivine.

Keywords

Crustal Contamination, Deccan Traps, Petrocbemistry, Petrography, Picrite, Western Ghats, Maharashtra.

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S. F. Sethna ¹, Christopher P. R. D'sa ¹

Affiliations
1 Department of Geology. St. Xavier's College, Bombay 400001, IN

Abstract

Petrographic and petrochemical studies indicate the Ijolite to be somewhat magnesium-rich. The agpaitic index is very close to one and the magma is considered to be of an intermediate type (between miaskitic and agpaitic). The structural setting of the area is of the rift type, as the Panvel flexure occurs just a little to the west of the pluton.

Keywords

Geochemistry, Petrology, Ijolite, Deccan Trap. Carbonatite, Janjira, Maharashtra.

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S. F. Sethna

Abstract

No Abstract.

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S. F. Sethna ¹, Priya Javeri ¹

Affiliations
1 Department of Geology St. Xavier's College Bombay - 400001, IN

Abstract

The occurrence of a trachyte intrusive into the Deccan Trap basalts at Sutrakar near Talasari is being reported for the first time. Major, trace and rare earth element analyses are given along with the petrography. It has been speculated that the origin of the trachytic magma is similar to that at Bombay-Salsette area where it has been postulated earlier by Lightfoot et al. (1987) that the trachytic magma could be generated as a partial melt from a gabbroic sill complex in the lower part of the crust due the crustal thinning and sagging during the formation of the Panvel Flexture.

Keywords

Trachyte, Petrology, Deccan Trap, Maharashtra.

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R. N. Sukheswala ¹, S. F. Sethna ¹

Affiliations
1 Geology Department, St. Xavier's College, Bombay, IN

Abstract

Phenai Mata hill (Lat. 22°8'N., Long. 73°55'E.), believed to be a composite plug, is successively filled by basalt, gabbro, granophyre and nepheline syenite. The paper focusses attention on the layered character of the gabbro. Rhythmic layering is due to the repetition of thick anorthositic and gabbroic layers. Cryptic layering is marked by extreme variation in anorthite content of plagioclase. The differentiation trend is established by the chemical and petrographic studies of samples collected at regular intervals. Up to 600 feet the enrichment is towards both iron and alkali, after which the gabbro (tholeiitic) differentiates to a granitic residuum.

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S. F. Sethna ¹, Prajakta Kothare ¹, Priya Javeri ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai - 400 001, IN

Abstract

Distinct vertical rhythmic layering is seen in one of the dykes 68 km southeast of Rajkot. The dyke extends for almost 50 km in length but has a limited width of 10 to 15 metres. The vertical layering is due to alternating layers rich in plagioclase and pyroxene, which is encountered mainly in the central part of the dyke. The layering is formed due to molecuIar diffusion at the front of cooling and crystallization with the temperature gradient catching up gradually to form the next layer. Alternatively it may be possible that layering is due to multiple injections of melt into the fracture that was responsible for disturbing the equilibrium from time to time.

Keywords

Petrology, Deccan Trap, Olivine Dolerite Dyke, Layering, Saurashtra.

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S. F. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, 10A/l4, Tata Colony, Tardeo, Mumbai - 400 034, IN

Abstract

The occurrence of intermediate and acid rocks such as trachyte and rhyolite are known to occur mainly along the west coast around Mumbai and in the Saurashtra area, such as in Girnar, Barda, Osham and Alech hills. There are also other isolated occurrences in Phenai Mata region and Pavagadh hill (Sethna, 1989). It is interesting to note that both along the west coast near Mumbai and in the Saurashtra area there are occurrences of relatively high positive gravity anomalies of up to +70 m Gal Bouguer anomaly (Negi et al. 1992). It seems unlikely that such a close spatial relationship of these anomalies, and the occurrence of more evolved rock types could be entirely fortuitous. Whilst, intuitively, negative gravity anomalies might be expected in areas where less dense, evolved rocks occur, thought should be given as to the petrogenesis and possible origin of these evolved rocks.

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S. F. Sethna ¹, Prajakta Kothare ¹, B. S. Sethna ¹, Priya Javeri ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai - 400 001, IN

Abstract

Forty-five major dykes intruding the Deccan Trap were studied in the central and south-eastern parts of Saurashtra. There are some distinct structural differences seen between the area north of Amreli and that south of Amreli. The lava flows north of Amreli are practically horizontal, while south of Amreli they seem to be forming a major southerly-plunging antiform. In the area north of Amreli, dolerite dykes are invariably enriched in fresh olivine and are associated with one ankaramite dyke, a hybrid monzodolerite and a stock-like body of essexite (foid monzogabbro). In the area south of Armeli, dolerites are generally olivine free but in a few samples altered olivine is present. These dolerites are associated with rhyolitic dykes, which invariably contain microphenocrysts of quartz and are thus described as quartz porphyries.

Keywords

Deccan Trap, Intrusives, Petrography, Structure, Saurashtra.

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S. F. Sethna ¹, Prajakta Kothare ¹, B. S. Sethna ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai - 400 001, IN

Abstract

The monzodolerite dyke with a width of 25 to 30 meters, extends nearly east-west for almost 30 kms. The dyke intrudes horizontal lava flows of the Deccan Volcanic Province. Major, trace and rare earth element analyses for three samples of the monzodolerite have been given. SEM-EDXA analyses for feldspars and pyroxenes have also been given. On the basis of field evidence (in the form of unassimilated blebs of more basic magma surrounded by the monzodolerite), mineralogical variation and whole rock geochemistry, the rock is presumed to have formed by the mixing of basic and intermediate magmas rather than by a process of pure differentiation. The intermediate magma is presumed to have formed by differentiation of basaltic magma at some depth.

Keywords

Petrology, Deccan Traps, Monzodolerite, Magma Mixing, Saurashtra.

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Jalil Iranmanesh ¹, S. F. Sethna ²

Affiliations
1 Atomic Energy Organization of Iran, PO Box, 11365-8486, Tehran, I.R, IR
2 Department of Geology, St. Xavier's College, Mumbai, 400001, IN

Abstract

A petrographic and geochemical study of the Mashhad granite in the northeastern part of I.R. Iran reveals that it is of two main varieties - the major body of S-Type continental collisionlsyncollision biotite/muscovite granite and pegmatites; the other of I-Type, Mthin plate/post collision uplift - homblende-hedenbergite/green biotite granite with the latter intrusive into the former. The homblendehedenbergite-compared to the biotite-granite, shows appreciable enrichment in the high field strength (HFS) and rare earth elements. One peculiar character in these granites is the negative correlation of Si0₂ and distinct positive correlation of CaO against the HFS and RE elements, and the latter character is attributed to the presence of large amounts of sphene and apatite in the homblende-hedenbergite granite.

Keywords

Geochemistry, Granite, Petrography, Iran.

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S. F. Sethna ¹, Hemant Merchant ¹

Affiliations
1 Department of Geology, St. Xavier's College, Bombay, IN

Abstract

The basalt flow in the Malabar, Cumbala, Worli and Bandra Hills is the youngest lava flow in Bombay Island. It has been riddled by a network of thin veinlets which were in the past thought to be of secondary origin. A detailed examination has revealed that these veins are microaplitic and of an early hydrothermal phase being composed of alkali felspar, quartz and some hydrothermal crystallisation of quartz, zeolite, adularia and fluorite. The formation of these veins has been attributed to late stage differentiation of basaltic lava. This is perhaps a unique example of differentiation of basaltic magma after extrusion.

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