Mineralogy and geochemistry of alkaline and subalkaline rocks of the Tatarka complex,

Yenisey Ridge

Romanova I.V.*, Matushkin N.Yu.**, Romanov M.I.***

*Institute of Geology and Mineralogy, Novosibirsk, Russia; **Institute of Petroleum Geology and Geophysics, Novosibirsk, Russia; *** SNIIGGiMS, Novosibirsk, Russia

vernikovskayaiv@gmail.com

 

Various in composition intrusive and volcanic rocks with elevated alkalinity, including nepheline and alkaline syenites as well as carbonatites and A-type granites, were formed within the Tatarka–Ishimba suture zone of the Yenisey Ridge during the Neoproterozoic time. They form both small massifs of oval or rounded shapes and dykes, bedded and lens-shaped bodies that lay along the whole suture extent. We present new mineralogical, geochemical and geochronological (U-Pb) data for the Srednetatarka and Yagodka alkaline and associated rocks belonging to this zone.

The Srednetatarka alkaline massif is situated in the Southern part of the Yenisey Ridge Central-Angara terrane, in the middle reaches of the Tatarka river, which is a right tributary of the Angara river. The massif occupies approximately 15 km² of area, has isometric form, and features a stock of similar rock composition (~ 5 km²) located farther to the south-east. The Srednetatarka massif rocks are represented by nepheline syenite (foyaites), feldspar bearing ijolites and alkaline syenites.

Among foyaites there are middle- and coarse-grained varieties, up to pegmatites. Foyaites mainly consist of alkali feldspar, usually microcline (~40-65 vol. %), nepheline (~30-40 vol. %) and aegirine (up to 15 vol. %). These rocks may contain single grains of arfvedsonite and biotite as well as accessory titanite, fluorite, astrophyllite, eudialyte, zircon, apatite, analcime. Ijolites exteriorly appear middle- and coarse-grained dark gray rocks with taxitic structure. They are built up of nepheline (~45-50 vol. %), microcline (~20-25 vol. %) and aegirine-augite (~25-30 vol. %). Accessory minerals are represented by fluorite and titanite. In the nepheline bearing rocks albite occurs as perthitic intergrowths or replaces alkali feldspar. Alkaline syenites are leucocratic middle-grained rocks, that are composed of orthoclase (~70 vol. %), albite (~15 vol. %), aegirine (up to 15 vol. %), biotite, fluorite, and zircon. Pyrochlore, columbite, lavenite, euxenite, monazite, ilmenite and other minerals were discovered in the massif rocks (Sveshnikova et al, 1976).

A research study performed on the X-ray spectrum microprobe analyzer “Camebax” shows that in nepheline from foyaites and ijolites K₂O and Na₂O concentrations vary from 4.9 to 6.1 wt.% and from 14.2 to 16.1 wt.%, respectively. The mineral is characterized by low-level impurity content of FeO_tot, TiO₂, MnO, Cr₂O₃, MgO, CaO (in total < 0.7 wt.%), most of which is accounted for FeO_tot. Aegirines from foyaites and foyaite pegmatites comprise 80-97 % of aegirine minal, 2-20 % of hedenbergite minal and up to 1 % of diopside minal. They are characterized by elevated concentrations of TiO₂ (0.5-2.9 wt.%), Al₂O₃ (1.1-3.3 wt.%) and MnO (0.3-1.2 wt.%). A slight zoning is found in aegirines, attributed to Ti and Fe distribution. The aegirine minal content (27-36 %) in aegirine-augites from ijolites decreases, while diopside (19-29 %) and hedenbergite (41-48 %) minal contents increase. TiO₂ concentrations decline to 0.3-0.9 wt.%.

In ijolites, foyaites and alkaline syenites the SiO₂ content varies from 51.21 to 64.53 wt.%, total alkali amount К₂О+Na₂O amounts to 12.37-14.03 wt.%, whereas the Na₂O/К₂О ratio value changes from 1.16 to 2.37. Values for the sum К₂О+Na₂O, being moderate for nepheline bearing rocks, and at the same time the elevated content of Al₂O₃ (21.23-23.72 wt.%) affect mineralogical features of the rocks, which are high amounts of alkali feldspar and nepheline. Low MgO concentrations (< 0.05 wt.% and 0.40 wt.% respectively) and slight CaO contents (0.74-1.81 wt.% and 4.74 wt.% respectively) are detected in foyaites and ijolites. The concentrations of these components are significantly lower in aegirines than in aegirine-augites.

Yagodka massif is situated in the Northern part of the Yenisey Ridge Angara-Kan terrane, in the Yagodka river basin. The Yagodka massif rocks form stock-like bodies (area 0.5-1.2 km²), represented by alkaline, quartz and alkaline feldspar (AF) syenites, associated with trachybasalts. Alkaline syenites are represented by coarse-grained rocks with taxitic structure, which mainly consist of alkali feldspar (90-95 %) and a moderate amount of aegirine-augite, hornblende, riebeckite, biotite, titanite, zircon and ilmenite (Kuznetsov, 1988).

The AF syenites and quartz syenites are grey massive rocks of middle- and coarse-grained texture. Aggregates of mafic minerals, frequently associated with the accessory ones, are found in taxitic insulations among alkali feldspar coarse grains. Alkali feldspar occupies about ~85 vol. % in the AF syenites, whereas biotite contents amount to ~10 vol. %. These rocks may contain quartz as well as zircon, apatite and ore minerals. In the quartz syenites the share of alkali feldspar decreases (~55-70 vol. %), while that of quartz and plagioclase increases up to 5-15 and 10-15 vol. % respectively. In these rocks augite, hornblende, grunerite, biotite (up to 10 vol. %) and accessory minerals such as zircon, titanite, apatite, fluorite, ilmenite, hematite are also present.

Biotites from AF and quartz syenites are characterized by high FeO_tot concentrations (34.6-37.5 wt.%), moderate Al₂O₃ contents (10.9-13.1 wt.%) and low MgO contents (0.2-1.7 wt.%). Their concentration of TiO₂ varies from 1.3 to 3.7 wt.%, Cl and MnO contents reach 1.4 and 0.8 wt.% respectively. Impurities of Ca, Na, Nb have also been detected. Pyroxenes from the quartz syenites correspond to augites with transition to hedenbergites, according to the classification (Morimoto et al., 1988). They comprise up to 81-90 % of hedenbergite minal, 7-13 % of diopside minal and 2-8 % of aegirine minal. In comparison with the Srednetatarka massif alkaline pyroxenes these pyroxenes contain lower concentrations of Al₂O₃ (0.01-0.7 wt.%), TiO₂ (0.03-0.4 wt.%) and, on the contrary, higher MnO amounts (0.9-2.0 wt.%). Grunerites are defined by low values (0.02-0.04) of the Mg / (Mg + Fe²⁺) ratio as well as by a sufficiently high MnO content (2.7-3.1 wt.%) and minor CaO and Nb₂O₅ contents (each 0.2-0.8 wt.%), Na₂O, Al₂O₃, TiO₂, K₂O, Cl (in total < 0.8 wt.%). Hornblendes compared with the grunerites comprise lower amounts of MnO (0.6-2.1 wt.%) and greater amounts of CaO and Al₂O₃ (each up to 10.7 wt.%), K₂O and Na₂O (each 1.1-2.7 wt.%), MgO and TiO₂ (each 0.1-1.6 wt.%), Cl (up to 1.4 мас.%), Nb₂O₅ (up to 0.5 wt.%). According to Mg / (Mg + Fe²⁺) value (0.01-0.09) and Si content (6.18-6.90 a.p.f.u.) the calcic amphiboles relate to ferro-edenite and hastingsite (^VIAl < Fe³⁺) (Leake et al, 1997). Ilmenites contain MnO (3.1-7.0 wt.%) and Nb₂O₅ (0.9-3.5 wt.%).

SiO₂ contents in AF and quartz syenites alter from 61.85 to 66.82 wt.%, Na₂O+K₂O values change from 10.23 to 12.02 wt.%, while Na₂O/К₂О values range in 0.68-1.18. The rocks are characterized by high Fe₂O_3tot contents (3.84-6.29 wt.%), elevated Al₂O₃ contents (14.72-17.35 wt.%), moderate MgO contents (up to 0.36 wt.%) and modest CaO contents (1.18-1.94 wt. %). It was established that Fe₂O_3tot is concentrated in mafic and ore minerals such as augite, grunerite, hornblende, biotite, ilmenite, hematite.

Magmatic rocks of the massifs under study have a similar distribution patterns of rare elements, including REE. The rocks are enriched in light REE and have flat heavy REE patterns. They are characterized by negative anomalies of Eu (Eu/Eu* = 0.22-0.79), being lower for the Yagodka massif rocks. On the spider diagrams, with elements concentrations normalized to primitive mantle, the rocks are characterized by negative anomalies of Ba, Sr, P, Ti and enriched in Rb, Th, U, Nb, Ta, Hf, Zr, Tb and Y. (La/Yb)_N values change from 4.5 to 16.6, with the greatest ones related to foyaites and AF syenites. The presence of negative anomalies of Ba, Sr, Eu, P and Ti is indicative of the widespread process of fractional crystallization during the alkaline magma evolution stages, generally related to fractionation of alkali feldspar (Ba), plagioclase (Sr, Eu), apatite (P), titanite or ilmenite (Ti).

We obtained U-Pb ages of titanite and zircon from foyaite of the Srednetatarka massif (instruments – Finnigan MAT-261 and SHRIMP-II), which give 700 ± 2 Ma и 711 ± 3 Ma respectively (Vernikovsky et al, 2008). The U-Pb age of zircon from quartz syenite of the Yagodka massif (instrument – SHRIMP-II) corresponds to 681 ± 7 Ma.

The Srednetatarka and Yagodka massifs rocks form a gentle spectrum from alkaline basic to subalkaline acid rocks. Among them, the foyaites and ijolites of the former massif differ chiefly in the presence of nepheline and alkaline minerals, whereas the subalkaline rocks of the latter massif contain FeO-enriched minerals. Pyroxenes, close to hedenbergite in composition, and grunerite are quite rare in quartz syenites. More typical representatives are usually their magnesium-rich analogues. Close geochemical particularities are ages established for these rocks testify that the Srednetatarka and Yagodka massifs were formed in the same tectonic Tatarka-Ishimba zone.

 

References:

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Kuznetsov Yu.A. Selectas. Novosibirsk: Nauka. 1988. Vol. 1. p. 221. (in Russian)

Sveshnikova E.V., Semenov E.I., Khomyakov A.P. The Transangara alkaline pluton: its rocks and minerals. Moscow: Nauka. 1976. p. 80. (in Russian)

Leake B.E., Woolley A.R., Arps C.E.S., Birch W.D., Gilbert M.C., Grice J.D., Hawthorne C., Kato A., Kisch H.J., Krivovichev V.G., Linthout K., Laird J., Mandarino J.A., Maresch W.V., Nickel E.H., Rock N.M.S., Schumacher J.C., Smith D.C., Stephenson C.N., Ungaretti L., Whittaker E.J.W., Youzhi G. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International Mineralogical Association, commission on new minerals and mineral names // Canadian Mineralogist. 1997. Vol. 35. p. 219-246.

Morimoto N., Fabries J., Ferguson A.K., Ginzburg I.V., Ross M., Seifert F.A., Zussman J., Aoki K., Gottardi G. Nomenclature of pyroxenes // American Mineralogist. 1988. Vol. 73. p. 1123-1133.