Silicate-carbonate inclusions in clinopyroxenes of shonkinites, Inagli massif (Aldan Shield, Russia)

Rokosova E. Y.^1,2, Vasil’ev Yu. R. ²

1Novosibirsk State University, Novosibirsk, Russia, 2v.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

rokosovae@gmail.com

The Inagli massif belongs to the alkaline ultramafic complexes of potassic series. The massif is situated in the northwestern margin of the Aldan Shield (Yakutia, Russia). It is about 20 km² in area and is topographically manifested as a cupola structure with a central caldera. It is nearly isometric in shape and has a concentrically zoned structure. The central part of the massif is a stock, 16 km² in area, made up of dunite. The stock is surrounded by alkali gabbroids (shonkinites), melanocratic alkali syenites and pulaskites. A narrow (50 m) zone of peridotites is situated between the dunites and the alkali gabbroids. Sills of syenite porphyry occur at the periphery of the massif within the Cambrian carbonate sequence. The central dunite is threaded by numerous pegmatitic veins and veinlets of diverse mineral assemblages which include phlogopite, potassium feldspar, chrome diopside, richterite and Mg-arfvedsonite [1,2].

The genesis of rocks of alkaline ultramafic massifs is still a matter of wide debate. This concerns the identification of the parental magmas, the characteristics of mantle source and P-T parameters, as well as processes of evolution in the melts during crystallization.

Silicate-carbonate inclusions are found in clinopyroxenes of shonkinites of Inagli massif. Shonkinites consist of (vol.%) 50-55 clinopyroxene, 10-15 olivine, 5-10 serpentine, 10-20 potassium feldspar + pseudo-leucite, 3-7 biotite, 5 magnetite, 3 apatite. Clinopyroxene is prismatic, rarely irregular in the shape and has slightly greenish-yellow color. Clinopyroxene is often cracked and contains chadacrysts of olivine, titanomagnetite, apatite, potassium feldspar, phlogopite. Clinopyroxene is represented by diopside (wt.%: 49.7-55.2 SiO_2, 0.26-1.6 TiO_2, 1.5-3.1 Al₂O₃, 4.7-6.5 FeO, 0.14-0.26 MnO, 13.6-15.9 MgO, 21-21.98 CaO, 0.6-0.9 Na₂O, 0.05-0.1 P₂O₅; Mg# = 0.8-0.86).

Considered primary melt inclusions are arranged singly, in different parts of the grains of clinopyroxene. Inclusions are irregular or rounded in the shape and varies from 7 to 30 µm in size.

Daughter phases of inclusions are represented by pale brown laths of phlogopite (wt.%: 40 SiO₂, 13.72 Al₂O₃, 19.45 MgO, 9.3 FeO, 10.2 K₂O, 2.8 TiO₂, 2.1 F), colorless grains of potassium feldspar and/or albite (wt.%: 60.5 SiO₂, 18.6 Al₂O₃, 15.9 K₂O, 0.9 Na₂O and/or 63.8 SiO₂, 19.4 Al₂O₃, 2.67 СаO, 11.2 Na₂O), prismatic grains of apatite, grains of magnetite (wt.%: 86.1-88.9 FeO, 1.4-1.7 TiO₂, 1.7-5.1 Gr₂O₃, 1.3-1.95 Al₂O₃, 0.76-0.78 CaO), and fine-grained carbonate-salt phases. The latter are observed in the interstices between other daughter phases (Fig. a).

I nclusions were heated on a stage with a silite heater. At 800°C the carbonate-salt part of the inclusions started to melt and one or two oval gas bubbles appeared between the daughter phases. At 1000-1100°C gas bubbles became more rounded and disappeared in carbonate-salt melt. Melting of silicate glass occurred at the same temperatures. At 1230°C the carbonate-salt melt was transformed in a globule and then gradually decreased in size. The homogenization of carbonate-salt melt globule in a silicate melt occurred at 1280-1300°C. With a small decrease in temperature to 1270-1280°C the carbonate-salt globule appeared again and gradually increased in size during the cooling. After heating the inclusions consist of silicate glass, gas bubble and carbonate-salt globule at room temperature. The latter occupies up to 1/5 of the volume of the inclusion (Fig. b).

Fig. Inclusion in clinopyroxene of shonkinites of Inagli massif.

a) Inclusion before heating. The composition of daughter phases in inclusion was determined by the scanning microscope LEO1430VP.

b) Inclusion after heating. The composition of salt globule and silicate glass was determined by microprobe “Camebax-micro”.

The composition of homogenized silicate glasses of inclusions corresponds to trachyandesitobasalts (wt.%): 51-55.5 SiO_2, 0.6-0.8 TiO_2, 13.4-14.8 Al₂O₃, 4.3-6.7 FeO, 0.1-0.25 MnO, 12.5-6 MgO, 9.2-4.5 CaO, 2.2-2.8 Na₂O, 4.2-6.9 K₂O, 0.5-1 P₂O₅, 0-0.08 BaO, 0.45-0.5 Cl, 0.02-0.11 SO₃). The composition of carbonate-salt globules of inclusion is as follows (wt.%): 17.2-23.2 SiO_2, 0.4-1 TiO_2, 1.7-6.1 Al₂O₃, 2.6-4.9 FeO, 0.1-0.13 MnO, 3.1-5.1 MgO, 13.3-22.8 CaO, 1.8-4 Na₂O, 1.9-3 K₂O, 0.9-1.66 P₂O₅, 0-0.14 BaO, 0.06-0.5 SrO, 0.33-1.28 Cl, 0.19-1.3 SO₃. The composition of carbonate-salt globules is close to the carbonatite lavas of Fort Portal (Uganda) [3]. Normative composition of carbonate-salt globules includes diopside, phlogopite, potassium feldspar, albite, anorthite, apatite, calcite, barium and calcium chlorides, calcium and strontium sulfates.

Thus, the clinopyroxenes of shonkinites crystallized from homogeneous silicate carbonate-salt melt at 1280-1300°C. The melt separates into the silicate and carbonate-salt components under decreasing of temperature. It was shown in a review article about liquid immiscibility in deep-seated magmas [4] that the carbonate-salt melts spatially separated from silicate parental magma are enriched in Ca, alkalis, CO₂, S, F, Cl, P, H₂O and represent the original carbonatite melts. The letter separates into immiscible fractions of carbonate, alkaline-chloride, alkaline-sulfate, and alkaline-phosphate compositions under decreasing of temperature and pressure and nonequilibrium conditions. It should be noted that earlier Naumov et al [5] have concluded based on the study of polyphase inclusions in chrome diopsides of Inagli massif that the chrome diopsides crystallized from a silicate melt, which contained the emulsion salt globule of sulfate-dominated compositions.

References:

1. Korchagin A.M. (1996) Inagli pluton and its natural resources. Moscow: Nedra, 156 pp.

2. Kostyuk V.P., Panina L.I., Zhidkov A.Ya., Orlova M.P., Bazarova T.Yu. (1990) Potassium alkaline magmatism of Baikal-Stanovoy rifting system. Novosibirsk: Nauka. Siberian Branch, 239 pp.

3. Beloussov V V, Gerasimovsky V I, Goryatchev A V, Dobrovolsky V V, Kapitsa AP, Logatchev N A, Milanovsky E E, Polyakov A I, Rykunov L N, Sedov V V (1974) East – African rift system, 3 – Geochemistry, seismology: main results. Nauka, Moscow, 287 pp.

4. Panina L. I., Motorina I.V. (2008). Liquid immiscibility in deep-seated magmas and the origin of carbonatite melts. Geochemistry International 5, 487-504.

5. Naumov V.B., Kamenetsky V.S., Thomas R, Kononkova N.N., Ryzhenko B.N. (2008) Inclusions of silicate and sulfate melts in chrome diopside from the Inagli deposit, Yakutia, Russia. Geochemistry International, 46, 554-564.