2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Isotope minerals Rb-Sr dating pegmatites of Khibina massif, Kola region.

Bayanova T.B., Elizarov D.V., Ikorsky S.V.

Geological Institute Kola Science Centre RAS, Apatity, Russia;

tamara@geoksc.apatity.ru

Three pegmatites of Khibina massif have been dated using by Rb-Sr method with rock-forming minerals. New isotope Rb-Sr isochron ages are presented for pegmatites.

Interval of origin for rocks of Khibina massif from pyroxenites up to iolite lies from 377 to 365 Ma and were dated by Rb-Sr method (Kogarko et all., 1981, Kramm et all., 1993). Rock-forming minerals from Kukisvumchorr apatite deposit of the massif were dated by Rb-Sr method due to methodical target. There were separated titanite, nepheline, apatite and pyroxene and Rb-Sr isochron age equals 371±6Ma (Bayanova, 2004).

The aim of new Rb-Sr investigations was dating rock-forming minerals from three pegmatites of the Khibina massif: range of Taxtarvumchorr, comb of eudialyte and river of Golzovka.

Pegmatites of range Taxtarvumchorr are situated on western slope of one of its circus in 1.6-1.7 km from west of Malyi Vudiavr lake. Country rocks of pegmatites are trakhitoid khibinites, which lie in zones of intensive albitization. To Rb-Sr dating there were separated minerals of Taxtarvumchorr pegmatites eudialyte, needle aegirine, eunigmatite, feldspar and nepheline. New isotope Rb-Sr isochron age of the minerals is 371±4Ma (Fig.1,a) which is interpreted as age of pegmatites forming.

The second group of samples was collected on the crest of the eudialyte bridge. Body of pegmatites represents vein with thickness up to 0.5 m and length up to 5-6 m. Host rocks are represented by massive risschorrites, pegmatites have geological tortuous contact. Main rock-forming minerals of pegmatites are presented by feldspar, nepheline and eudialyte, secondary minerals were aegirine and arfvedsonite. For Rb-Sr isotope dating eudialite, amphibole (arfvedsonite) and nepheline were selected. New Rb-Sr minerals isochron dating yielded 364±4 Ma, initial value of Sr (Isr) equal 0.7038 and all data corresponded of pegmatites origin time (Fig.1,b).

The third sample of pegmatites was taken on north-western part of Khibina massif near Golzovka river. Vein of pegmatites with latutides of 5 m has lenticular form and gently sloping bedding. Vein mainly consists of nepheline crystals and feldspar with numerous inclusions of aegirine. Also few single grains of amphibole and eudialyte mark which are widespread among feldspar. For Rb-Sr isotope dating there were picked five main minerals eudialyte, amphibole, feldspar and two nepheline syenite of different colors. Using five minerals Rb-Sr age are 380±56 Ma with initial Sr ratio 0.7024 which reflect slightly crust contamination (Fig.1,c).

Therefore new methodically Rb-Sr isotope dating on rock-forming minerals from three pegmatites from Khibina massif reflect its coeval origin age (380±54Ma) for initial alkaline-ultra basic with 388±6 Ma and (364±4Ma) for final magmatic activites with 362±11 Ma according to data (Arzamaszev et all., 2007) Age of Taxtarvumchorr range pegmatites equals 371±4Ma are very coeval with data by (Kogarko et all.,1981, Kogarko and Kramm, 2001) for main phase of agpaitic syenite of Khibina massif and also iolite with Rb-Sr data 371±6 Ma (Bayanova, 2004).

All investigations are supported by Division of Earth Science RAS, program 2.

Referenses

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Bayanova T.B. Age of key geological complexes of Kola region and duration of magmatic processes.// Nauka. Saint-Petersburg. 2004. 176 P.

KogarkoL.N., Kramm Y, Blaksland A. et all. Age and genesis of Khibina massif alkaline rocks. // Doclady RAS 1981. T.260. N.4. P.1001-1004.

Kogarko L.N. and Khain V.E. Alkaline magmatism in the history of the Earth: experience of geodynamics interpretation.//Doclady of RAS 2001. T.377. N.5. P.677-679.

Kramm U., Kogarko L.N., Kononova V.A., Vartiainen H. The Kola Alkaline Province of the CIS and Finland: Precise Rb-Sr ages define 380-360 Ma age range for all magmatism // Lithos. 1993. V.30. P.33-44.