Rock-forming minerals and their relations in carbonatites of Guli massif.

Merkulova M.V* Pletchov* P.Y, Zaitsev V.A**.

* Geological faculty, MSU, Moscow, Russia

* Vernadsky Institute of geochemistry and analytical chemistry, Moscow, Russia

merkulka-snake@rambler.ru

 

Guli massif - the largest massif of ultrabasic-alkaline rocks, the carbonatites are final phase of its formation. There are two main hypotheses about formation of massif: their metasomatic origin and magmatic-intrusive origin [1,2].

Most of the researchers support the hypothesis of magmatic origin of carbonatites. Immiscibility of carbonate and silicate components of magmas in the crust was shown in the past decade. Experiments were conducted to prove immiscibility of carbonate and silicate melts at pressures below 15 kbar [5]. A number of studies [3, 4, 6] confirms the origin of carbonatites as derivatives of silicate rocks during crystallization differentiation.

Rocks represents the carbonatites (sovites), which consists mainly of calcite (more then 50% of area), olivine, monticellite, phlogopite and other silicates, which are contained in veins of apatite. Veins of apatite and silicates have a clear orientation in the rocks (Fig. 2).

Calcite in the rocks are characterized by a small admixture of strontium (up to 0,65 wt.%). Apatite in the studied carbonatites has the composition of fluoride-apatite with a small admixture of silicon (up to 0,6%) and cerium (up to 0,45 wt.%).

Olivine in rocks represented forsterite, the ratio of magnesium to iron varies between 91.4 and 93.3. Monticellite forms large grains and rims around grains of forsterite (Fig. 1), represented by high-Mg monticellite, which contains iron and manganese (4,7-10 wt.%). Forsterite and monticellite contain inclusions of apatite and calcite. Pyroxene forms several generations - the first – inclusions in monticellite and apatite, the second - the independent diopside (CaNa_0,01Mg_0,98Fe_0,02Mn_0,01Si_1,99O₆) grains. The inclusions can be represented by of wollastonite. Mica in the samples is phlogopite. Sodium, calcium and barium are contained in small quantities. High content of barium (up to 2 wt.%) indicates the presence of barium minal phlogopite – kinositalit (BaO content in which up to 20 wt.%). Garnet forms two varieties: 1. large idiomorphic zoned grain 2. inclusion of garnet in apatite. Garnet has essentially andradite composition with minor isomorphic impurities of manganese and magnesium. Accessory minerals in rocks are magnetite and sulfides.

Installed minerals in rocks may indicate a significant role in the formation of skarn carbonatites. The presence of phlogopite, monticellite in veins of carbonatites indicates the metasomatic process.

 

Fig. 1. Forsterite with monticellite rim.	Fig. 2. Oriented grains of apatite, which is located in the same direction as the grain of phlogopite with high interference color. The width of the field of view - 7 mm.

 

Structural and textural features, mineral associations, direct zoning in minerals, the presence of barium in phlogopite, Sr in calcite, melt and crystallized carbonate and silicate inclusion indicate magmatic origin of carbonatites of Guli massif. Moreover, the formation of these rocks took place in two stages - the formation of calcite carbonatites and injection of foskorite melt. In addition, skarning and recrystallization of fine-grained carbonatites took place in formation of these rocks.

 

References:

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