PETROLOGY OF THE MAGMATIC ROCKS IN NAKORA AREA …iasir.net/AIJRFANSpapers/AIJRFANS15-249.pdfRAJASTHAN, INDIA Naresh Kumar Department of Geology, Kurukshetra University, Kurukshetra

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PETROLOGY OF THE MAGMATIC ROCKS IN NAKORA AREA OF

MALANI IGNEOUS SUITE, DISTRICT BARMER, WESTERN

RAJASTHAN, INDIA Naresh Kumar

Department of Geology,

Kurukshetra University, Kurukshetra – 136119, Haryana, INDIA

I. INTRODUCTION

The magmatic rocks exposed in Nakora Ring Complex (NRC) are belonging to the Neoproterozoic Malani

Igneous Suite (MIS) in the Trans-Aravalli Block (TAB) of Indian Shield. MIS is the largest (55,000 km2) A-

type anorogenic acid magmatism in the Peninsular India. It owes its origin to hot spot tectonics and controlled

by NE - SW trending lineaments in the TAB [4, 7]. Except Bhushan and Chandrashekaran [2], there is no

published research work available on the field relationships, petrography, petrochemical and petrogenetic

aspects of NRC. The present paper focuses on the study of different rock types with some typical field

relationships as well as petrographical studies in the Nakora area of MIS. Based on the detailed geological

mapping (Fig.1), various magmatic rocks exposed in NRC are grouped into three phases. They are as follows: 1.

Extrusive phase: basalt, trachyte, rhyolite, tuff, ash, perlite, breccia and agglomerate 2. Intrusive phase: gabbro

and granite 3. Dyke phase: basalt, dolerite, rhyolite and microgranite.

II. GEOLOGICAL SETTING

In NRC, mainly the acid volcanic rocks are exposed. The rhyolites show various colours (dark brown, light

brown and purple) and structures viz. volcanic bands (Fig. 2A), volcanic flows, vesicles, amygdales, shreds,

agglomerate and breccia. The volcanic products are volcanic flows, perlite, agglomerate, ash, tuff and explosion

breccia. Tuff has sharp contact with rhyolite and basalt (Fig. 2B, 2C) and thus indicates the bimodal and

anorogenic nature of the suite. The granites show dark and light shades of pink colour. It is medium grained,

massive and compact. Granites show a sharp intrusive contact with basalt and rhyolite (Fig. 2D, 2E) which

reflects subvolcanic setting of the magmatism. Various hydrothermal alteration features viz. encrustation of iron

oxides, sericite, kaolin, vugs, cavities and geodes are observed in the acid volcanoplutonic rocks. The basalt

flows underlie the acid flows and the xenoliths of basalt are observed in the rhyolite (Fig. 2F). Thus the basalt

flows are older to the rhyolites in the study area. The gabbros are associated with the acid rocks. Gabbro is dark

black colour, coarse grained and massive. Numerous dyke rocks of various compositions (mainly dolerite with

minor amount of acid rocks) (Fig. 2G) are cutting across these acid – basic rocks and are trending NE-SW

direction. The dolerite is medium grained, black colour, compact and massive. Flow stratigraphy mapping was

carried out and has been demarked 44 lava flows (34 of rhyolites, 6 of trachytes and 4 of basalts) [12]. Various

primary structures associated with volcanic flows viz. volcanic flows, caves, joints, volcanic vent, vesicles,

amygdales, spheroids, orbicular and rapakivi are studied in detail to elucidate the volcanic cooling history and

their emplacement mechanism [13].

All volcanic flows show sharp contact to each other. Our field session (December, 2006) in NRC observed a

volcanic vent (Fig. 2H) (semicircular to elongated shape and 35 – 40 m wide) (GPS reading: N25o 47

’ 923

’’ E72

o

9’ 627

’’) amidst of acidic volcanic terrain at the top of the hill of Nakora Ji hill (Maini hill) [11]. Storm water is

Abstract: Rocks of Malani Igneous Suite (MIS) exposed in Nakora area comprise predominantly of felsic

volcanic rocks, followed by felsic plutonic rocks and minor amount of mafic rocks. Identification of 44

volcanic flows, volcanic vent and volcano-plutonic associations are the significant features of the present

work. The felsic volcanic rocks include rhyolite and trachyte flows. They are mainly dark brown colour and

show well developed flow structures, flow bands, vesicles and amygdales. Felsic plutonics are granites.

They are hypersolvus-subsolvus, peralkaline and occur in anorogenic setting. Mafic rocks include basalt,

gabbro and dolerite. Occurrence of xenoliths of basalt in rhyolite and trachyte indicate that basalt flows are

the earliest. Volcanic vent and Luni rift mechanism in NRC suggests that the Nakora Ring Complex is the

result of a central volcanic eruption controlled by Luni lineament.

Keywords: Petrology, Magmatism, Nakora, Malani Igneous Suite, Rajasthan

Naresh Kumar, American International Journal of Research in Formal, Applied & Natural Sciences, 10(1), March-May 2015, pp.56-60


filled in the vent and percolates down as springs through the lava tubes / fractures like network (dimension: 30

m length, 5 m width and 2 m height).

III. PETROGRAPHY

Under microscope, rhyolite shows flow structures (Fig. 3A) with porphyritic, aphyritic, spherulitic (radiating

growth of feldspar and quartz from a common centre), orbicular, rapakivi and perlitic textures. They consist

mainly of quartz, orthoclase feldspar and arfvedsonite as phenocrysts and embedded in the quartzofeldspathic

groundmass. The microcrystalline aggregates of quartz, alkali feldspar, blue colour amphibole (riebeckite),

pyroxene (light green aegirine), blood red aenigmatite, magnetite and hematite are present in groundmass. Thin

veins consist of fine quartz crystals with various forms viz. drop like, embayed and fractured pattern cut the

orthoclase as well as the groundmass. Phenocrysts of alkali feldspar show Carlsbad twinning whereas altered

and fractured orthoclase is observed. The spheroidal rhyolite shows mafic (dark/light brown) and felsic (light

grey) layers which represents the flow texture. The mafic layer consists of hematite, magnetite and arfvedsonite

and they are fine grained. The felsic layer consists of quartz, orthoclase, perthite and are medium grained. The

rapakivi rhyolite shows light brown and dark brown layers. The light brown mainly consists of quartz and

orthoclase but dark brown consists of hematite and magnetite. Phenocrysts of quartz are embedded in the dark

brown layer and orthoclase is enclosed in light brown layer and both are observed in the fine grained

groundmass also. The dark brown layer consists of more black (magnetite) and brown (hematite) colour opaques

as compared to light brown layer. The tuff is very fine grained and shows flow bands. The tuffs are micro to

cryptocrystalline in nature with flow bands and consist of quartz and alkali feldspar as essential minerals. It

contains fine grained angular crystals of quartz and alkali feldspar in the groundmass.

Trachyte shows porphyritic texture but sometimes directive flow and parallelism of alongated crystals represent

trachytic texture. Trachyte consists essentially of alkali feldspar, quartz, arfvedsonite and riebeckite as

phenocrysts and embedded in the quartzofeldspathic groundmass. It contains less amount of quartz but shows

same textural and mineralogical features as of rhyolites. Alkali feldspars are mainly of orthoclase and are

showing Carlsbad twinning (Fig. 3B). Riebeckite is also observed in groundmass. It is fine grained, needle

shape, blue colour, pleochroic (X – light blue, Y- blue, Z- dark blue) and shows extinction angle X ۸ C 3o-5

o.

The tuffs are micro to cryptocrystalline in nature with flow bands and consist of quartz, alkali feldspar (Fig. 3C).

The basalts consist of labradorite and augite as essential minerals and show ophitic and subophitic textures (Fig.

3D). Quartz, hematite and magnetite occur as accessory minerals. Sometimes large vesicles (4–6 mm) in basalt

are filled by secondary minerals like quartz and calcite. Granites display hypidiomorphic, granophyric,

equigranular and microgranitic textures. They consist essentially of quartz, orthoclase, perthite (hypersolvus

type) (Fig. 3E), albite (subsolvus type) and arfvedsonite as essential minerals. The accessory minerals are

hematite, magnetite, apatite, zircon, and sphene. Orthoclase displays Carlsbad twinning and albite shows

lamellar twinning. Perthite is coarse grained, vein & cloudy types and contains quartz, orthoclase and hematite

as inclusions. Also, it is altered to kaolin and sericite which indicate the subsolidus modification processes.

Arfvedsonite shows medium grained, pleochroic (X dark bluish green, Y Bluish green, Z yellowish green) and

the extinction angle X ^ C of 12° to 15°. Hypersolvus granites of Nakora area fall in the field of alkali granites

(except 2 subsolvus samples which fall in the granite field) on QAP diagram (Streckeisen, 1973) (Fig. 4)

reflecting the alkaline and peralkaline nature on Lameyre and Bowden (1982) fields which are superimposed on

Streckeisen (1973) to distinguish the different plutonic series and their fractionates. Gabbro shows ophitic

texture and consists of albite with minor amount of labradorite, augite, olivine and magnetite (Fig. 3F). The

original calcic plagioclase feldspar is modified into sodic plagioclase feldspar during subsolidus stage [10].

Interstitial glass is observed between plagioclase feldspar and augite. Dolerite shows ophitic and subophitic

textural features but the minerals are finer than gabbro. It consists of untwinned / twinned plagioclase feldspar

(labradorite), augite with / without olivine (Fig. 3G). The accessory minerals comprise hematite and magnetite.

IV. CONCLUSION

On the basis of detailed geological observations during field works viz. identification of the colour, form and

structure of rocks and minerals, presence/absence of pyroclasts, volcanic assemblages and pyroclasts etc., 44

volcanic flows are identified in the Nakora rocks. The total thickness of 44 flows is 1776 m out of which 40

flows (1743 m total thickness) are of felsic composition (> 60% SiO2) and 4 flows are of basaltic composition

(total thickness 33 m). The different types of rhyolites (rapakivi, orbicular and spheroidal) show flow bands,

porphyritic, aphyritic, spherulitic and perlitic textures whereas granite shows hypidiomorphic, granophyric,

equigranular and microgranitic textures. The eruption started with less viscous basic magma with large volatile

contents. But at few places pyroclasts mark the beginning of eruption as the sudden and violent type. The nature

of NRC is vulcanian type with cyclic characters which depends on the mode of eruption, type of

physicochemical compositions. The field and petromineralogical data suggest that the Nakora magmatic rocks

are formed by cogenetic process from a comagmatic suite of rocks. The observed hydrothermal and subsolidus

processes point towards the rare metals and rare earth mineralization potential of the rocks of Nakora area [4,6].



‘U’ turn of Luni river is investigated during the field investigations in NRC. It takes sudden ‘U’ turn from West

to South direction which indicates the relationship between Nakora volcanic vent and rift dynamics [1]. The

fractures and cracks along the Luni lineament may be served as channels for magma rising upto the surface in

the study area which advocates a relationship between tectonism and volcanism [3, 5, 8, 9]. It also suggests that

the Luni rift served as channel way for the magma rising to the surface as flow at Nakora.

ACKNOWLEDGMENTS

Authors express their gratitude to University Grants Commission, New Delhi for the grant in the form of Major

Research Project (no: F.31-193/2005 SR, dated 31st March 2006) to GV and Project Fellowship to NK.

REFERENCES [1] Bailey, D.K., (1974). Continental drifting and alkaline magmatism. In: T.S.Sorensan (Ed.), Alkaline rocks. John Wiley and

Sons, pp. 148-159. [2] Bhushan, S.K. and Chandrasekaran, V., (2002). Geology and geochemistry of the magmatic rocks of the Malani Igneous Suite

and Tertiary Alkaline Province of Western Rajasthan. Mem. Geol. Sury. Ind., v. 126, pp. 1-129.

[3] Dhont, D., Chorowicz, I., Yurur, T., Froger, L.L., Kose, O. and Gundogdn, N., (1998). Emplacement of volcanic vent and geodynamics of Central Anatolia, Turkey. J. Volcanol. Geotherm. Res., v. 85 (1-4), pp. 33-54.

[4] Kochhar, N., (2000). Atttributes and significance of the A-type Malani magmatism, NW Peninsular India. In: M. Deb (Ed.)

Crustal evolution and metallogeny in the Northwestern Indian Shield. Narosa Publishing House, New Delhi, Chapter 9, pp. 158-188.

[5] Korme, T., Chorowicz, I. Collet, B. and Bonavia, F.F., (1997). Volcanic vents rooted on extension fractures and their

geodynamic implications in the Ethiopian rift. J. Volcanol. Geotherm. Res., v. 79 (3-4), pp. 205-222. [6] Narayan Das, G.R., Bagchi, A.K., Chaube, D.N., Sharma, C.V. and Navaneetham, K.V., (1978). Rare metal content, geology and

tectonic setting of the alkaline complexes across the trans-Aravalli region, Rajasthan. In: V.K.Verma and P.K.Verma (Eds.)

Recent Res. Geology, Hindustan Publishing Corporation Ltd, Delhi. v. 7, pp. 201-217. [7] Pareek, H.S., (1981). Perochemistry and petrogenesis of Malani Igneous Suite, India. Geol. Soc. Am. Bull., v. 92, pp. 206-273.

[8] Polacci, M. and Papale, P., (1999). The development of compound lava fields at Mount Etna. Phy. Chem. Earth, v. 24, pp. 949-

952. [9] Toprak, V., (1998). Vent distribution and its relation to regional tectonics, Cappadocian volcanics, Turkey. J. Volcanol.

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[10] Vallinayagam, G., (1997). Minerals chemistry of Siwana Ring Complex, W. Rajasthan, India. Ind. Mineral., v. 31, pp. 37-47. [11] Vallinayagam, G. and Kumar, N., (2007). Volcanic vent in Nakora Ring Complex of Malani Igneous Suite, Northwestern India.

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[12] Vallinayagam, G. and Kumar, N., (2008). Flow Stratigraphy of Nakora Ring Complex, Malani Igneous Suite, Rajasthan, NW Peninsular India. Geol. Sury. India. Special Publication, v. 91, pp. 127-135,

[13] Kumar, N. and Vallinayagam, G., (2009). Primary Volcanic Structures from Nakora area of Malani Igneous Suite, Western

Rajasthan : Implications for Cooling and Emplacement of Volcanic Flows. Current Science, v. 98(4), pp. 550-557, 2010.

FIGURES CAPTION

Fig. 1 Geological map of Nakora area.



Fig. 2 Field Photographs

Fig. 2A. Flow bands in non porphyritic rhyolite; 2B. Sharp contact between rhyolite and tuff; 2C. Sharp contact between basalt and

tuff; 2D. Sharp contact between basalt and granite; 2E. Sharp contact between rhyolite and granite; 2F. Xenolith of basalt in

rhyolite; 2G. Dolerite dykes; 2H. Panoramic view of volcanic vent.

Fig. 3 Photomicrographs



Fig. 3A. Flow structures in the rhyolite and phenocrysts of quartz and orthoclase feldspar are are showing parallelism with flow

direction, (25x, PPL); 3B. Trachyte is showing porphyritic texture, orthoclase is showing Carlsbad twinning, (40x, XPL); 3C. Tuff is

showing quartz and orthoclase as a phenocrysts in fine grained groundmass, (40x, XPL); 3D. Ophitic texture is showing by basalt;

labradorite, augite and magnetite are present as a assential minerals in groundmass, (40x, XPL); 3E. Granite is showing

microgranitic texture with quartz, perthite, hematite and magnetite, (40x, PPL); 3F. Gabbro is showing ophitic texture with

labradorite, augite and magnetite as assential minerals, (100x, XPL); 3G. Dolerite is showing subophitic texture with untwinned

labradorite, augite, hematite and magnetite, (40x, XPL).

Fig. 4 Quartz – Alkali feldspar – Plagioclase (QAP diagram of Streckeisen, 1973) diagram with the fields of Lameyre and Bowden

(1982) showing the model composition of Nakora granites.

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PETROLOGY OF THE MAGMATIC ROCKS IN NAKORA AREA …iasir.net/AIJRFANSpapers/AIJRFANS15-249.pdfRAJASTHAN, INDIA Naresh Kumar Department of Geology, Kurukshetra University, Kurukshetra