petrology of the magmatic rocks in nakora area...
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ISSN (Print): 2328-3777, ISSN (Online): 2328-3785, ISSN (CD-ROM): 2328-3793
American International Journal of Research in Formal, Applied & Natural Sciences
AIJRFANS 15- 249; © 2015, AIJRFANS All Rights Reserved Page 56
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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
AIJRFANS 15- 249; © 2015, AIJRFANS All Rights Reserved Page 57
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].
Naresh Kumar, American International Journal of Research in Formal, Applied & Natural Sciences, 10(1), March-May 2015, pp.56-60
AIJRFANS 15- 249; © 2015, AIJRFANS All Rights Reserved Page 58
‘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.
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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.
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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.
Naresh Kumar, American International Journal of Research in Formal, Applied & Natural Sciences, 10(1), March-May 2015, pp.56-60
AIJRFANS 15- 249; © 2015, AIJRFANS All Rights Reserved Page 59
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
Naresh Kumar, American International Journal of Research in Formal, Applied & Natural Sciences, 10(1), March-May 2015, pp.56-60
AIJRFANS 15- 249; © 2015, AIJRFANS All Rights Reserved Page 60
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.