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L. N. Gupta ¹

Affiliations
1 Department of Geology, Panjab University, Chandigarh, IN

Abstract

Zircons from the Central Gneisses of the Dhauladhar range, north of Dharmsala have been separated and studied to establish the stability of zircons during metamorphism and migmatization of the sediments; field and petrological studies indicate that the rocks have formed by migmatization of the metasediments. Crystal habit, colour, clouding, corrosion, overgrowth and outgrowth, length and breadth, elongation ratio and the (reduced major axis of zircons separated from several rock types have been studied in detail. The number of euhedral zircons increases during the migmatization apparently due to the recrystallization of the zircons. The zircon data corroborates with the structural and petrographic observations. The zircon study is useful as a criterion in deciphering the petrogenesis of the granitic rocks.

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R. Elsdon ¹, L. N. Gupta ²

Affiliations
1 Department of Geology, University College, Belfield, Dublin 4, IE
2 Centre of Advanced Study in Geology, Panjab University, Chandigarh 160014, IN

Abstract

Petrochemical studies on fourteen representative samples of the Lansdowne Granite Gneiss indicate close compositional uniformity. Most of the analyses fall within the field of granodiorite, and lie close to the low-pressure ternary minimum in the system Qz-Or-Ab-H₂O. Inter-element relationships indicate an initial magmatic origin, with pronounced gneissosity due to later deformation.

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D. B. Guha ¹, L. N. Gupta ²

Affiliations
1 Geological Survey of India, Jaipur, IN
2 Centre of Advanced Study in Geology, Panjab University, Chandigarh, IN

Abstract

Patchy, vein, film, and crypto types of perthites, rapakivi structure, and myrmekite texture are commonly developed in the potash-feldspar augen of the granitic gneisses of the Doda area, Jammu and Kashmir State.

The scanned perthites show an interlamination of microc1ine and albite resulting in a criss-cross pattern. Islands of soda-feldspar in potash-feldspar show embayed margins. These interrelationships appear to be the result of exsolution phenomenon.

Segregated sodic-calcic components replace the host potash-feldspar releasing blebs and globules of quartz which are present in the form of myrmekite texture.

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L. N. Gupta ¹

Affiliations
1 Central of Advanced Study in Geology, Panjab University, Chandigarh, IN

Abstract

Secondary growths which include overgrowths, outgrowths, and multiple growths are formed during metamorphism and partial melting. Primary growths include growth twins and parallel growths. Twinning may occur during the early crystallization of zircon in magmatic rocks. The process of synneusis may result in parallel growths or attachment of zircons that have initially undergone separate growth histories.

The occurrence of primary growths in paragneisses and migmatites is due to their derivation from magmatic sources and their survival during mechanical transportation and crushing of the samples at the time of separation.

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L. N. Gupta ¹, Naveen Chaudhri ¹, A. K. Prasad ¹

Affiliations
1 Centre of Advanced Study in Geology, Panjab University, Chandigarh, IN

Abstract

Basic and metabasic rocks occur in the form of an oval-shaped, plug-like body. intrusive into the granitic gneisses and metasediments belonging to the Chail Formation. The core is made of fresh olivine gabbro metamorphosed to varying degrees at the margins. Chemical data suggest that the basic and metabasic rocks are co-magmatic and the parent magma was of alkali olivine basalt composition. The observed diversity in physical and textural parameters exhibited by different rocks are considered to be the expressions of varying rates of cooling and effects of autometamorphism. These are more pronounced at the margins than in the central parts of the body.

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L. N. Gupta ¹, S. Marwaha ¹

Affiliations
1 Centre of Advanced Study in Geology, Punjab University, Chandigarh, IN

Abstract

Growth of plagioclase crystals accompanied by rapid cooling and contemporaneous crystallisation with falling temperature is considered to cause disequilibrium due to incomplete reaction between the growing crystal and the residual sodic melt. This causes the formation of normal zoning. Regular oscillatory zoning has been explained mainly due to the process of diffusion of An-molecules towards the crystal face of growing plagiolcase in a magma. Growth of zoned plagioclase was accompanied by fractional crystallisation and accumulation of magma of trondhjernitic composition. During this process, zoned plagioclases came in contact with one another and resulted in synneusis. Partial resorption resulted in truncated zones followed by crystallisation of a more calcic rim.

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Naresh Kochhar ¹, G. Vallinayagam ¹, L. N. Gupta ¹

Affiliations
1 Department of Geology, Panjab University, Chandigarh, IN

Abstract

Zircons are hydrothennal or late-magmatic. The high UO₂ contents of the Tosham zircons reflect the high UO₂ of the host rocks. The Siwana granites, though high in Zr values, have very poor zircon yield. This is probably due to the pcralkalinity of the Si wana magma.

Keywords

Malani Igneous Suite, Zircon, Siwana Granite, Geochemistry, Rajasthan.

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Naveen Chaudhri ¹, L. N. Gupta ¹

Affiliations
1 Centre of Advanced Study in Geology, Panjab University, Chandigarh - 160 014, IN

Abstract

The gabbroic rocks of Mandi area occur as an oval-shaped, 500 m thick intrusive body within the Mandi granite. The central zone of the body is made up of fresh gabbro and it grades into metagabbro and epidiorite towards the margins. The relatively high abundances of Ti, AI, Ca and Mg in clinopyroxenes of the central part as compared to the clinopyroxenes of the marginal part suggest different rates of cooling within the Mandi gabbroic body. The zoned plagioclases have calcic-rich cores and calcic-poor rims. The Al - (Fe+Ti) - Mg values and incompatible trace element ratios of the rocks indicate tholeiitic nature of the body. The discrimination diagrams based on major elements as well as immobile minor and trace elements suggest that these rocks are of MORB type. However, the chemical characteristics like negative Nb, Ti and P anomalies in the primordial mantle-normalised spiderdiagram together with low TiN and ZrN ratios testify that the Mandi gabbroic rocks are akin to low-Ti continental flood basalts. [Mg]-[Fe] modelling, incompatible versus compatible trace element data and lower Ni abundances for the given Mg contents suggest 10-15 % fractional crystallisation of olivine after the rocks were generated by partial melting under pressure conditions ranging between 25 and 40 kb. The intrusive appears to have formed by low degree of partial melting of a non-pyrolitic metasomatised mantle source that was heterogeneous with respect of Zr and Fe/Mg ratios.

Keywords

Geochemistry, Petrology, Mandi Gabbro, Himalaya.

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Ravindra Kumar ¹, G. S. Gill ¹, L. N. Gupta ¹

Affiliations
1 Centre of Advanced Study in Geology, Panjab University, Chandigarh - 160 014, IN

Abstract

Soft sediment deformational structures have been observed in three sections of the basal Pinjor Formation exposed in the frontal Himalayan foothills in northern Haryana. The Nadah Choe section (A) shows contortion, Thinning-Thickening and faulting within 5 m thick sequence of sandstone, sandy shale and calcareous siltstone. The section (B) in a stream in the southwest of Nadah shows autoclasts, contortions around clay ball and faulting about 2 m thick Fluvio-Lacustrine sequence. The third section © is exposed in the Ghaggar river shows diapiric, Flame-Like intrusion of mud into the overlying beds of siltstone. The three sections are about 1.5 km strike length of gently dipping beds of basal Pinjor Formation. These structures were formed by liquefaction attending shear stress and can be related to major seismic events in this region during the deposition of the basal Pinjor Formation.

Keywords

Earthquakes, Pinjor Formation, Nadah, Haryana.

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L. N. Gupta ¹, Naresh Kochhar ¹

Affiliations
1 Centre of Advanced Study in Geology, Panjab University, Chandigarh - 160014, IN

Abstract

No Abstract.

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L. N. Gupta ¹, D. B. Guha ¹

Affiliations
1 Centre of Advanced Study in Geology, Panjab University, Chandigarh, IN

Abstract

Muscovites separated from pelitic schists of the Dada area show a limited range of composition between ideal muscovite and ferrimuscovite. The paragonite content in the muscovites ranges between 14 and 21 %. The tetrahedral Al^iv increases with the increase in the grade of metamorphism though this increase is more for the muscovite of the aureole rock, i.e., in the andalusite to staurolite zone than in the kyanite-staurolite zone. Similar results are also shown by the (FeO+Fe₂O₃) and Na/(Na+K) with respect to AI₂O₃ and total Al respectively.

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L. N. Gupta ¹

Affiliations
1 Centre of Advanced Study in Geology, Punjab University, Chandigarh, IN

Abstract

Lansdowne granite gneiss shows a tectonic contact with the underlying Lansdowne metamorphites. The metamorphites show repeated and superposed progressive regional metamorphism. Three phases of deformation and folding are noticed. The rare assemblage, chloritoid, staurolite, kyanite, andalusite and quartz is present in the area.

Granite is metamorphosed into augen gneiss, mylonites and phyllonites. Muscovitisation, tourmalinisation and greisening indicate chemical alteration in the granitic rocks. Textural features are correlated with deformation and folding.

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Sunil Dhar ¹, Naresh Kochhar ², Rajni Sharma ³, L. N. Gupta ²

Affiliations
1 PG Department of Geology, H.P. University, Regional Centre, Dharamsala, (H.P.) - 176 2 15, IN
2 Department of Geology, Panjab University, Chandigarh, 160 014, IN
3 HCC Ltd., C-1 Works, Tala Hydroelectric Project, PB# 390, Phuentsholing, BT

Abstract

This paper describes mineral chemistry of biotites from Jalor, Tosham and Jhunjhunu ring complexes of Malani igneous suite, India. The chemistry reveals iron enrichment trend in all the biotite samples of the three complexes. FeO*/MgO ratio is the highest (6.72) in Jalor samples whereas lower values of (4.08) and (3.72) have been observed in Tosham and Jhunjhunu samples respectively. Higher MgO has not affected the enhanced proportions of FeO in the biotites of Tosham and Jhunjhunu. Mg ⇌ Fe and 2A1 ⇌ 3Fe²⁺ substitution is dominant in Jalor samples whereas, influence of 3M ⇌ Al is deciphered in the Tosham and Jhunjhunu samples. Most of the biotites fall in the anorogenic (A-type) suites, representing extensional tectonic environment for their host rocks.

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