Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Yuzhnosakunskii (Sakun)

stripes

Occurrence number: 
136-01-006
Country: 
Russia
Region: 
Aldan
Location: 
Longitude: 119.65, Latitude: 56.78
Carbonatite: 
No

Many of the intrusions of this 10 km2 complex take a somewhat arcuate form and cut through Precambrian metamorphosed sedimentary rocks and granite-gneisses and partly through upper Palaeozoic granitoids. In the north potassic alkaline rocks are in contact with Palaeozoic quartz syenites. Within the complex layering is well developed and two series have been distinguished. The lower series is represented by potassic mesocratic rocks and the upper series by highly potassic leucocratic rocks. The mesocratic series rocks, which extend over 6 km2, are rather more abundant than the leucocratic series (3 km2). There is a rhythmic repetition of rock types. The mesocratic series consists of pulaskite (70%), lusitanite (10%), leucocratic alkaline syenite, pseudoleucite and nepheline syenites (10%) and micaceous pyroxenite (5-7%). Shonkinite, fergusite, malignite and synnyrite are subordinate. Within the repeated units a distinct rock type generally prevails. For instance, at the bottom of the mesocratic series lusitanites are usually found which are succeeded by micaceous pyroxenites and then by pulaskites. Another feature which is observed in the structure of the mesocratic layers is that the mafic minerals, both within individual layers and through the sequence as a whole, decrease upwards. In addition to microlayering the upper salic series has a stratification, which is not clearly seen macroscopically, due to the non-uniform distribution of feldspathoids which are finely intergrown with K-feldspar. The upper series is formed predominantly by four rock types: (a) nepheline-bearing syenites, often with a pseudoleucitic texture, (b) alkaline syenites, which are essentially trachytic textured pulaskites, and massive tonsbergites, (c) kalsilite-bearing syenites (synnyrites), and (d) feldspathoidal syenites or micaceous microcline metasomatic rocks. The complex alternation of these rocks can be traced in a section having a thickness of about one kilometre. This diverse group of rocks comprises essentially six minerals: K-feldspar (orthoclase and microcline), nepheline, kalsilite, clinopyroxene (aegirine-bearing diopside), lepidomelane and garnet (andradite-melanite with a small admixture of grossular component). Many rocks contain porphyrocrysts and ovoids of pseudoleucite up to 5-10 cm in diameter. They are of a variable composition and comprise kalsilite, nepheline and K-feldspar. Details of the chemistry of these rocks can be found in Zhidkov (1990). Pokrovskii and Zhidkov (1993) obtained an initial 87Sr/86Sr ratio of 0.70428, δ18O values of 3.8-5.9 for the dominant igneous rock types and δD of -75 to -87 for pyroxenite and pseudoleucite syenite but notably lower values of -100 to -122 for synnyrite and feldspathoidal syenite .

Age: 
K-Ar determinations on biotite from synnyrite gave 330±18 Ma and from biotite pyroxenite 321±8 Ma (Zhidkov, 1990). Pokrovskii and Zhidkov (1993) obtained 288±5 Ma from a Rb-Sr whole rock isochron.
References: 

POKROVSKII, B.G. and ZHIDKOV, A.Ya. 1993. Origin of the ultrapotassic rocks of the Synnyr and Southern Sakun massifs (Transbaikal area) from isotopic data evidence. Petrology, 1: 195-204.
ZHIDKOV, A.Ya. 1990. The Palaeozoic stage of magmatism. In G.V. Polyakov and V.V. Kepezinskas (eds) Potassic alkaline magmatism of the Baikal-Stanovoy rift system. 32-64. Nauka, Siberian Division of the USSR Academy of Sciences, Novosibirsk.

Map: 
Fig. 2_222. Yuzhnosakunskii (after Zidkov, 1990, Fig. 20).
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith