2011

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Peculiarities of alkaline rocks genesis

Donskoy A.N.

N.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NAS of Ukraine, Kiev, Ukraine; donick_gg@mail.ru

Alkaline series of the world are confined to zones of tectonic and petrogenic structures. They were formed at final stage of activization cycle. The genesis is caused by energetic flows and geodynamics of the certain consolidated structures.

The Siilinjärvi massif aged 2.5 bn years is one of the most ancient one on the Baltic Shield.

Further wide expansion of alkaline massifs of syenite series took place in Proterozoic era. They are related with ancient platforms been formed at same time, such as the Ukrainian and Baltic Shields, Southern Africa and the Canadian Shield. That series are confined to long periods of tectonic and magmatic activity, periods of global reconstruction of Earth’s crust. On the platforms are related with huge deep faults transporting fluids. Because the all nepheline rock massifs were formed after ending of Archean, Early Proterozoic orogenesis and restricted to raised blocks the Sakharjok and Eletozersk massifs were forming simultaneously with Proterozoic alkaline massifs of the Ukrainian Shield, Gremyakha-Vyrmes (the Baltic Shield); Yellowknife, Kaminak, Haliburton-Bancroft, Blue-Mountain (the Canadian Shield); Palabora, Spitzkoppe (Southern Africa). The edge parts of the Precambrian shields are characterized by relation with long-lived deep faults (use to be intersecting) which reach bottom of Earth’s crust and mantle.

Middle Paleozoic era is characterized by alkaline series’ expansion and their multiformity. Miaskite massifs were formed among Uralian Hercynides. On the Baltic Shield in paleorift zone the Lovozero and Khibiny plutons were formed. In Kazakhstan, Central Asia, Altai-Sayans area, Northern Mongolia area, Trans-Baikal, Northern Baikal area these rocks’ series are related with rift formation of Middle Paleozoic era. In Mezozoic and Kainozoic eras the effusive rocks’ domination of alkaline branch is noted (Andreeva et al., 1984; Borodin, 1974).

Alkaline series are confined to consolidated tectonic structures formed at final stage of tectonic cycle. Alkaline melts’ generation, physical and chemical activity of polygenic alkaline fluids, their migrations in sial zone of Earth, material’s correlation with host rocks are caused by energetic flows, tectonic and petrogenic activization of the certain consolidated structures.

There is no well defined difference between magmatism products and metasomatism ones in alkaline rocks of syenite series; effusive and extrusive phases are absent. Pegmatite formation is widespread. The rocks of sodium and potassium branches occur preferably. The carbonatites occur often. The syenites dominate among the rocks of the series. In the alkaline rocks of Phanerozoic series the effusive and extrusive rocks are widespread with plutonic ones (Donskoy et al., 2004).

Geochemical singularity of alkaline processes are often determined by not only different physical and chemical formation conditions but geochemical features of origin magmas, i.e. main differences are related with deep Earth’s processes caused magmas’ and fluids’ generation.

References:

Andreeva E.D., Kononova V.A., Sveshnikova E.V., Yashina R.M. Magmatic rocks. Alkaline rocks. Moscow: Nauka, 1984. Vol. 2. 416 p (in Russian).

Main provinces and rock assotiations of alkaline rocks / Under the editorship of Borodin L.S. Moscow: Nauka, 1974. 376 p (in Russian).

Donskoy A.N., Kulish E.A., Donskoy N.A. Nepheline rocks of Ukraine: complex aluminum-alumina and rare metal ores. Kiev: Logos, 2004. 222 p (in Russian).