The peculiarities of sodalitization of pegmatoid mariupolites in the Oktyabrsky massif (Ukraine)

N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

Sodalite is the typical mineral of agpaitic feldspathoid syenites. However in some alkaline massifs sodalite can crystallize of post or late magmatic stages.

As an example of this process can be Oktyabrsky massif (Ukraine, Azov area) where nepheline from mariupolites is replaced by sodalite and cancrinite. As is well known magority of nepheline and the alkaline syenites are presented by miaskite varieties in this massif and peralkaline rocks (some mariupolites, aegirine foyaites, phonolites) only appearance in the late stage of forming.

The investigated rocks has are coarse grained (pegmatoid) texture and consist of coarse grains (up to 10 cm) of grey nepheline, white albite, light or dark blue sodalite, yellow cancrinite, reddish zeolites (pockets) and impregnation of black plates biotite, sometime dark green aegirine. Accessoric minerals are presented by zircon, fluorite and pyrochlore (Tabl.). Such substitution consecution is observed: nepheline ® cancrinite ® sodalite (Fig. ). In some areas development of zeolites (natrolite) is observed in a spaces between nepheline and cancrinite. As can be seen (Fig. ) the border between nepheline and cancrinite is more smooth but between sodalite and cancrinite is strongly. Probably this can explain by highly reactionary and enriched in sodium and chlorine solutions that sodalite-forming assisted.

Tabl. The microprobe analysis of minerals from sodalitization of pegmatoid mariupolites

	1	2	3	4	5	6	7	8	9	10	11	12	13
SiO₂	38,37	37,84	64,83	65,05	44,75	38,91	48,69	35,38	65,02	53,18	64,80	37,11	3,32
TiO₂	–	–	–	0,03	–	0,02	–	2,73	0,03	0,03	–	1,97	5,65
UO₂	–	–	–	–	–	–	–	–	–	–	–	–	1,63
ThO₂	–	–	–	–	–	–	–	–	–	–	–	–	0,33
Al₂O₃	32,65	32,66	18,44	18,34	32,67	31,27	29,49	13,59	18,13	1,81	17,54	11,28	0,26
Ce₂O₃	–	–	–	–	–	–	–	–	–	–	–	–	2,56
Nb₂O₅	–	–	–	–	–	–	–	–	–	–	–	–	55,64
Ta₂O₅	–	–	–	–	–	–	–	–	–	–	–	–	2,89
FeO	0,06	0,03	0,05	0,02	0,53	0,08	–	30,72	0,30	27,64	0,15	30,33	1,20
MnO	–	0,02	–	–	–	0,15	–	0,34	–	0,24	–	0,29	0,65
MgO	–	0,01	–	–	–	0,01	–	3,63	–	0,18	0,01	4,57	0,16
CaO	0,01	0,03	0,02	–	–	3,43	0,65	0,05	0,01	1,07	–	0,01	9,65
SrO	–	–	–	–	0,03	0,02	0,05	–	–	–	0,01	0,02	0,48
BaO	–	0,05	0,47	0,44	–	–	–	–	0,02	–	0,05	–	–
Na₂O	24,03	23,30	0,40	0,48	16,16	16,43	11,14	0,07	0,43	13,21	0,32	0,68	1,56
K₂O	0,01	0,05	17,17	16,65	6,50	0,04	0,13	9,88	17,39	0,03	17,00	9,74	0,25
SO₃	0,04	0,01	0,02	–	–	0,01	0,01	–	–	–	–	0,02	–
Cl	5,12	5,43	0,03	0,02	–	0,01	–	–	0,01	–	0,01	0,03	0,01
F	–	–	–	–	–	0,42	–	0,63	0,03	–	–	0,69	0,85
*Total*	*100,29*	*99,43*	*101,42*	*101,04*	*100,63*	*90,80*	*90,18*	*97,01*	*101,37*	*97,39*	*99,88*	*96,72*	*87,08*

1, 2 – sodalite, 3, 4 – potassic feldshpar inclusions in sodalite, 5 – nepheline, 6 – cancrinite, 7 – natrolite, 8 – coarse-grained biotite, 9 – large-scale potassic feldshpar inclusions between nepheline and sodalite, 10 – fine-grained aegirine inclusions in feldshpar inclusions, 11 – potassic feldshpar, 12 – biotite inclusions in potassic feldshpar, 13 – fine-grained pyrochlore inclusions in nepheline.

Inasmuch as sodalite and cancrinite practically don’t contain potassium and less SiO₂ than in the replaceable nepheline, these elements form a such new minerals as potassic feldshpar and partly biotite. Commonly potassic feldshpar crystallizes in the shape of the finest (from 1-2 to 50 mm) inclusions which forms a dense impregnation in sodalite. They have isometrical or elongated shapes. The last usually are cross orientation to the border of nepheline and cancrinite (Fig. ). There are also larger (up to 100 mm) potassic feldshpar inclusions band and sometime isometrical shapes with weak revealed crystallographic contours. The last are observed in nepheline, cancrinite and sodalite and maybe they represent the initial potassic feldshpar crystals. Some enlarged potassic feldshpar inclusions in sodalite could appear from more fine ones. The larger and irregular-shaped potassic feldshpar grains crystallize jointly with biotite segregations.

Simultaneously with nepheline alteration occurs formation of low magnesity biotite (Tabl. ) which maybe crystallizes in the result of aegirine replacement. Small (<50 mm) aegirine relicts are fixed as inclusions in the potassic feldshpar (Fig. , Tabl. ). It looks as if biotite rises as accumulations jointly with the potassic feldshpar. Individual fine biotite inclusions are fixed in the potassic feldshpar. By microprobe data (Tabl.) the coarse-scaly biotite differs from the fine-scaly inclusion biotite. The first biotite has more alumina content with ratio (Na+K)/Al = 0.80 (К_а) and in the second one this ratio 1.03. The last biotite variaty (annite) is the initial mineral of mariupolite and reflects a high obviously alkalinity of this rock. Probably the large-flake biotite arisen at late- or postmagmatic stage of rock alteration when alkali and partly aluminum redistribution takes place in the process of nepheline substitution by sodalite. Potassium content in nepheline is about 6.5% and it was redistributed between potassic feldshpar and biotite. Sodium enrichment of sodalite have been partially compensated by alkali deficiency in the biotite (К_а=0.80).

Fig. Pegmatoid mariupolite: a) general outlook of rock, b) fragment of this rock. Sodalite and cancrinite substitute nepheline, in sodalite fine regular inclusions of microcline. Nambers on the figure are the same as the ones of analyses in the Table.

It should be noted that is weakly exhibited in alkaline rocks of Oktyabrsky massif. Probably this process is more evolved in the enriched by volatile components melt-solutions from which crystallized these pegmatoid mariupolites. At the some time cancrinitization of nepheline that observed in the fine-grained aegirine foyaites and agpaitic phonolites was widely displayed in this massif. Commonly cancrinitization and ceolitization (shpreushtein) are completed late magmatic substitution of nepheline from these rocks. So far as cancrinite contains CO₂ (about 5% according to previous investigation) it should be supposed that late- and postmagmatic solutions have been enriched in this component. Perhaps impregnation of fluorite and calcite are related to this stage of rock alteration and also local carbonatization and a veins of calcite rocks that were considered as carbonatites in opinion of some investigators. These essentially carbonate rocks forming small veins and nests in the hosted silicate alkaline rocks differ from a typical carbonatites by such peculiarities: absence or very low contents of apatite and high Y contents relatively to REE (like in silicate alkaline rocks of Oktyabrsky massif).

Probably later solutions were enriched in alkali and chlorine that was a reason of local sodalitization of nepheline rocks (their pegmatite varieties).

This study was financially supported by the project of Ukrainian NAS №1-F/2011 in collaboration with RFBR.