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Petrological problems of Badmer
nephelinite-phonolite association, Rajasthan, India
Viladkar S.G.
Carbonatite Research Centre, Amba Dongar, India
Nephelinite-phonolite rocks are
exposed in the northwestern part of India around Barmer at Sarnu,
Dandali and Kamthai (Rajasthan). The occurrence of alkaline rocks at
Sarnu-Dandali were first reported by Udas et al. (1974) and later
described by Narayan Das et al. (1978). Study of the area was followed
by Chandrasekaran et al. (1990) and
Occurrences of nephelinite and
phonolite from Rajasthan are situated within the Luni Graben. In
general, nephelinite outcrops as small plugs without sharp contacts with
the country rock. Felsic and melanocratic phonolite (melaphonolite)
occur mainly in the form of dikes that contain xenoliths of metamorphic
basement. Thin veins consisting mainly of feldspar invade the felsic
phonolites. The contact between melaphonolite and felsic phonolite is
usually sharp; the latter intrude the former in the form of thin dikes.
Compared to the carbonatite occurrences from the Sarnu-Dandali area,
carbonatite dikes from the Kamthai area are more numerous and larger in
size.
Unpublished work of Viladkar suggest
that Mg numbers (63 to 70), Cr (146 to 484 ppm) and Ni (54 to 123 ppm) )
contents for melanephelinites from Barmer are similar to those from
olivine-bearing nephelinites from Napak (Simonetti and Bell, 1994a).
Such low abundances of Ni and Cr may be attributed to olivine
fractionation, and are not indicative of primitive magmas. The
composition of melanephelinite is also similar to the average
composition of olivine-poor nephelinite as determined by Le Bas (1987).
Compared to the major element composition from melanephelinite, analyses
for felsic phonolites show higher Si, Al, Na and K contents, and lower
Ca, Fe, Mg, P and Ti abundances.
Among all of the silicate rocks
analyzed from Barmer, melanephelinite contains the lowest abundance of
REEs, and their chondrite-normalized patterns are steeply-sloping, LREE-enriched.
Two melaphonolites contain an extremely high content of total REEs.
Among the felsic phonolites, sample 20BAR shows both the highest
abundance of total REEs and LREEs indicating that it is likely to be the
most differentiated from the series. In addition, two samples (one
melaphonolite and felsic phonolite) show a prominent, negative Eu
anomaly, which may be attributed to the fractionation of alkali
feldspar.
References
Udas, G.R., 1974. Geochemistry of
Pre-cambrian carbonatite complexes of India: Present status of and gaps
in our knowledge. Journal Geological Society of India 7, 457-462.
Narayan Das, G.R., Bagchi, A.K.,
Chaube, D.N., Sharma, C.V. & Navaneetham, K.V., 1978. Rare metal
content, geology and tectonic setting of the alkaline complexes across
the Trans Aravalli region, Rajasthan. Recent Researches in Geology 7,
201-217.
Le Bas, M.J. 1987. Nephelinites and
carbonatites. In Fitton J.G. and Upton, B.G.J (eds) Alkaline Igneous
Rocks. Geol. Soc. London.Spec. Publ. 30, 53-83
Simonetti, A and Bell, K., (1994) Nd,
Pb and Sr isotopic data from the Napak carbonatite-nephelinite centre
eastern Uganda: an example of open system crystal fractionation. Contri.
Mineral. Petrol. 115, 356-366.
Chandrasekaran, V., Srivastava, R.K.
and Chawade, M.P. (1990) Geochemistry of the alkaline rocks of
Sarnu-Dandali Area, District Barmer, Rajasthan, India. J. Geol. Soc.
India. 36, 365-382.
Viladkar S.G., Zaitsev V.A.Preliminary
data about alkaline rocks occurrences from Barmer region, Rajasthan,
India//Abstracts of
International
conference
Ore potential of
alkaline, kimberlite and carbonatite magmatism, Minsk 2011 pp
45-47. |