2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Preliminary data about alkaline rocks occurrences from Barmer region,

Rajasthan, India.

Viladkar S.G.¹, Zaitsev V.A.²

1-  Carbonatite Research Centre, Amba Dongar, India   2 Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia

               Chemically contrast alkaline rocks association occur in certain areas of Kutch (Gujarat), and Chhota Udaipur subprovince, Gujarat. Nephelinites also occur along the west coast of India near Murud-Janjira area. In addition, 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 Chawade & Chandrasekaran (1985), and the geochemistry for various alkaline silicate rocks was described by Chandrasekaran et al. (1990). 

               Rhyolitic lava and tuff of the Malani series (ca 745 Ma old) covers a significant portion of the area, and these are overlain by sediments of lower Cretaceous age (ca 120 Ma).   Field relations show that alkaline-ultramafic rocks and associated phonolite intrude both the Precambrian Malani series and the Cretaceous sediments. Alkaline-ultrramafic rocks occur as small plugs and dikes proximal to both Sarnu and Kamthai areas.  In addition, melaphonolite plugs (up to 20 m across) are exposed, while felsic phonolite forms thin dikes within the former.  Both foidite and phonolite bodies contain mafic xenoltihs composed essentially of pyroxene and small amount of olivine. Carbonatite, alvikite and ankeritic carbonatite occur as small, thin dikes (usually from 10 to 30 cm), and vary in length from 5cm to >1 meter.  In addition, they contain a fair number of country rock xenoliths.

Fig 1.

Chemical composition of Badmer rocks.

With the most chemically similar rocks from GEOROC - Geochemical Database

               The chemical data confirm, that there are 3 chemically distinct groups of rocks (fig.1)

Alkaline-ultramafic rocks. in general, fine-grained, microporphyritic and contains strongly zoned phenocrysts of clinopyroxene and rounded olivine grains.  Clinopyroxene may also form glomeroporphyritic clusters.  Olivine phenocrysts (Fo₈₀) commonly exhibit resorption or are mantled by clinopyroxene.  Titaniferous magnetite is abundant in all thin sections examined, either as skeletal grains or surrounding clinopyroxene phenocrysts.  The groundmass consists predominantly of clinopyroxene and interstitial nepheline.  A small amount of brown glass (around 5 modal%) is present in some samples. 

Ultrabasic alkaline rocks, enriched in calcium, alumina, and extremely enriched in titanium. The mostly chemically similar rocks, found in GEOROC – database are volcanic rocs from oceanic islands, mainly named  olivine melilitites, nepheline melilitites, and nephelinites. The most similar rocks from the continental localities are polzenites, melilitites, and olivine-nepheline melilitites from late cretaceous to Paleozoic volcanoes of the Eger rift in Northern Bohemia.

Phonolites. Two types of phonolite have been identified based on textural evidence and colour index.  Type I- Melaphonolite is strongly porphyritic and contains a high content of mafic minerals.  Phenocrysts present in order of decreasing abundance are clinopyroxene, nepheline and sanidine.  Titaniferous magnetite occurs sparingly in some samples.  Clinoyroxene phenocrysts are zoned, containing diopsidic cores and aegirine-rich rims.  The groundmass consists mainly of feldspar with a minor amount of analcime and tiny aegirine needles.

               In contrast, type II- Felsic phonolite is leucocratic, aphyric (glassy), and characterized by a trachytic texture (alignment of feldspar laths enclosed by euhedral nepheline).  Strongly pleochroic aegirine laths vary in proportion.  Nepheline occasionally forms pools of tiny granules surrounded by feldspar.  In some samples, strongly pleochroic biotite (deep-brown to pale-brown) occurs as small flakes.  Sodalite and apatite are common accessory minerals.

Clinopyroxene from melilitites is titaniferous diopside (TiO₂ up to 4.25 wt%) with a considerable amount of ivAl.  The low content of viAl suggests crystallization at low pressure, a common feature of clinopyroxene from basic, alkaline silicate rocks (e.g. Wass, 1979; Meyer and Mitchell, 1988; Simonetti et al., 1996).  In zoned clinopyroxenes from melanephelinite, both Al and Ti contents show an increase from core-to-rim.  Compared to the composition of clinopyroxene from melanephelinite, those from melaphonolite show an increase in Fe and Na contents over Mg and Ca abundances, and a decrease in Al and Ti contents.  The most aegirine-rich compositions are found in the felsic phonolite.  The chemical compositions for clinopyroxene are shown in a Di-Hd-Ac ternary plot, and show a trend of increasing hedenbergite content from melanephelinite to melaphonolite.  There is an abrupt change, however, to highly sodic compositions for clinopyroxene from felsic phonolite with virtually no hedenbergite component.  This abrupt change in composition suggests a sharp increase in the peralkalinity and oxygen fugacity of the magma system at the time of crystallization (Nash and Wilkinson, 1970; Larsen, 1976; Stephenson and Upton, 1982).

Fig. 2

A comparison of compositional trend for alkali pyroxenes in the Na-Mg-(Fe2++Mn) diagram

(atomic per cent). This figure is largely based on Larsen (1976) with additional data published subsequently.

The broken lines indicate pyroxene trends from undersaturated rock suites as follows: (I) Bardiner, East Greenland (Nielsen, 1979); (2) Auvergne, France (Varet, 1969; (3) Oslo (Neuman, 1976); (4) Itapirapua, Brazil (Gomes et aI., 1970); (5) Uganda (Tyler and King, 1967); (6) Norotu, Sakhaline (Yagi, 1966); (7) South Quraq Centre, South Greenland (Stephenson, 1972); (8) Motzfeldt, South Greenland (Jones and Peckett, 1980); (9)Igdlerfigssalik, Greenland (Powell, 1978); and (10) Ilimaussaq, South Greenland (Larsen, 1976). The dotted lines indicate those from oversaturated rock suites as follows: (11) Gough Island (Ferguson, 1978); (12) Pantellerite trend (Nicholls and Carmichael, 1969); (13) Japanese alkali basalts (Aoki, 1964).