2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

THE COMPOSITION OF THE LITHOSPHERIC MANTLE UNDER SIBERIAN CRATON BASED ON GARNETS DISTRIBUTION IN KIMBERLITE PIPES

Spetsius Z.V., Tarskhix O.V.

Scientific Investigation Geology Enterprise, ALROSA Co Ltd, Mirny, Russia (spetsius@cnigri.alrosa-mir.ru)

It is discussed the composition of the lithospheric mantle of the Siberian craton on the base of the garnet megacrysts distribution from mantle xenoliths in different kimberlite fields of the Yakutian kimberlite province. There have been used the original data on relationship and distribution of different mantle paragenesis mafic and ultramafic garnets from more than 70 kimberlite pipes. The lateral mantle heterogeneity is discussed on the base of these data.

It is well known that kimberlite magma is a carrier of diamonds and indicator minerals first of all garnet from the mantle levels to the surface. This process leads to the disintegration of deep mantle diamondiferous rocks and release of diamonds and indicator minerals that are incipient the mantle metasomatose under fluid introducing and aggressive  influence  of kimberlitic magmas on mantle rocks and minerals that usually are replaced and affected by kimberlitic magmas. These influences lead to more wide variation of garnet megacrysts composition in comparison with rockforrming garnet from mantle xenoliths. It should be pointed that in kimberlites are present garnets and other minerals that have been taken from different mantle levels.

A large number of data shows that there exists lateral petrographic heterogeneity of the upper mantle that it is fixed as on the mantle xenoliths distribution and relationship of different garnet types in kimberlites of different pipes and fields of Yakutian kimberlite province and other regions (Sobolev, 1977; Dawson, 1980; Spetsius, Serenko, 1990; Kharkiv et al., 1998; Spetsius et al., 2008). We have used the distribution of garnet xenocrysts in kimberlitic pipes from different fields in the Yakutian kimberlite province to estimate the SCLM composition of the main regions of the Siberian craton. The major-element compositions of pyrope garnets from kimberlite pipes provide information on the composition of the lithospheric mantle.

 We have used the distribution of garnet xenocrysts in kimberlitic pipes from different fields in the Yakutian kimberlite province to estimate the SCLM composition of the main regions of the Siberian craton. The major-element compositions of pyrope garnets from kimberlite pipes provide information on the composition of the lithospheric mantle. The garnet population from kimberlites was divided on to 4 groups that correspond to xenoliths representing the main petrographic types of mantle rocks: garnet lherzolites, dunite-harzburgites, wehrlite-websterites and eclogites. The studies of a representative population (250-300 garnet grains) from 70 kimberlite pipes in different fields show that the lherzolitic paragenesis predominates in the SCLM of all kimberlitic fields of the Siberian craton.

Our data confirm that for the upper mantle under Malo-Botuobinsky kimberlite field is typical the prevalence of ultramafic mantle rocks and amount of eclogitic garnets in investigated parcels is usually less than10%. It should be pointed that in kimberlites of south-west part of this field (pipes International and XXIII congress of Communists Party) eclogitic garnets are present in a less number than in kimberlites of north-east part.

Kimberlites of the Alakit-Marxinsky field are formed some space-independent connivent groups of pipes (Kharkiv et al., 1998). For the Alakit-Marxinsky field kimberlites are characteristic a wide variations in the distribution of different eclogitic types garnets. In kimberlites of the number of pipes: Komsomolskaya, Morkoka,  Krasnopresnrnskaya and Radiogeodesicheskaya the number of eclogitic garnets is less than 5% from the whole population and in three kimberlites body  (Iskorka, Kollectivnaya, Svetlaya) is fixed the predominance of eclogitic garnets (> 60%).

For some pipes with industrial grade is remarkable the following tendency – the kimberlites with high grade have as usual low content of eclogitic type garnets. Exclusion from this rule is the Morkoka pipe that is situated on the west-south of the field. Content of eclogitic substance in kimberlites of this pipe is the lowest in described field and diamonds are not discovered. This fact could be explained from that point of view that kimberlitic magmas were more aggressive and intruding kimberlites probably have more high temperature that is suggested from wide presence of kylephitic rims on many garnets grains. Probably this leads to the destroying of diamonds as well.

The maximum of the eclogitic garnets presence is fixed in the north part of this field (pipes Iskorka,  Kollektivnaya and Svetlaya) but these data are based on not a really representative number of analyses (parcels on 100 grains) and should be confirmed. Not excluded that in this case anomaly garnet composition is explained by the selective capture of the mantle substance. It should be stressed the predominantly ultramafic composition of upper mantle of the Alakit-Marxinsky field with the rather high presence of eclogites in the central and north parts of the field.

In the most kimberlite bodies of the Daldynsky field is observed the predominance of the ultramafic substance but on the south and on the north of this field are presented pipes with the wide distribution of eclogitic garnets such as Zagadochnaya, Nu pogodi, Aeromagnitnaya, Zimnaya, Polunochnaya and Udachnaya (till 30% and more). In the centrsl part of this field are situated kimberlite pipes in which the quantity of eclogitic substance usually less than 20% (pipe Leningradskaya). The similarity in the distribution of mantle substance in kimberlites of Alakit-Marxinsky and Daldynsky fields is it suits with their geological setting along the unique zone of the deep fractures.  The character of the distribution of the mantle substance in kimberlites of this field suggests on prevalence in the lithospheric mantle of these fields of ultramafic rocks with lenses of eclogites.  

From these data it is possible to state that upper mantle under the Daldynsky field is more heterogenic and probably more dislocated in comparison with the Malo-Botuobinsky and Alakit-Marxinsky fields and that eclogites are there more wide distributed that is matching with the data on the distribution of different types of mantle xenoliths (Spetsius, Serenko, 1990). The character of the distribution of the kimberlite bodies with the industrial grade on the territory of this region allowed concluding that the predominant part of diamondiferous rocks is situated on the east of the Alakit-Marxinsky field.

For kimberlites of the Verxne-Munsky field is characteristic the predominance of ultramafic garnets parageneses among mantle associations (Fig. 1). In kimberlites of this field are wide distributed pseudomorphoses of full replacement of garnets and pyroxenes that suggest about strong metasomatic influence on the mantle rocks. Eclogites in kimberlites of these pipes are rather seldom as it is evidenced from the distribution of different types of garnets in the concentrate of these kimberlites.

In kimberlites of Nakynsky field eclogites and ultramafites are presented in approximately equal quantity according to the distribution of different types of garnets (see Fig. 1). It should be pointed that for the pipe Nyurbinskaya and Botuobinskaya is characteristic the wide presence of diamondiferous eclogite xenoliths (Spetsius et al., 2008) and high content of eclogitic type garnets of diamond association that allowed to proposed the wide distribution of eclogitic substance in the lithospheric mantle of this field. 

Fig. 1. Distribution of paragenetic associations of garnet megacrysts in kimberlite pipes of the Verxne-Munsky and Nakynsky field.

Our results allowed concluding that the substrate of the lithospheric upper mantle under investigated kimberlite fields is presented predominantly by ultramafic rocks by the subordinate presence of eclogitic substance and that the lithospheric mantle has as vertical and lateral heterogeneity. However, in the central and eastern parts of the province, occupied by the Daldynsky and Nakynsky kimberlitic fields, eclogites have assumed an important role in composition of lithospheric mantle and may consist up to 30% of the mantle volume sampled by the kimberlites. It should be stressed that in every kimberlite fields are observed separate pipes with the high contents of the eclogitic substance (excluding of the Verxne-Munsky field). Not excluded that in some cases the anomaly lithospheric mantle composition is the result of the selective capturing of the mantle substance.

References

Sobolev, N.V. (1977). Deep-seated Inclusions in Kimberlites and the Problem of Upper Mantle Composition, Novosibirsk: Nauka, (in Russian). Engl. Translation by Brown, D.A. & Boyd, F.R., (ed.), American Geophysics Union, Washington, D.C., 279 p.

Spetsius, Z.V. & Serenko, V.P. (1990). Composition of continental upper mantle and lower crust beneath the Siberian platform. Moscow: Nauka,  272 p. (in Russian).

Kharkiv A.D., Zinchuk N.N., Kruychkov A.I. Primary diamond deposits of the world. М.: Nedra,1998, 555 p. (in Russian).

Dawson, J.B., 1980. Kimberlites and their xenoliths. Berlin: Springer-Verlag, 252 p.

Spetsius, Z.V., Taylor, L.A., Valley, J.W., DeAngelis, M.T., Spicuzza, M., Ivanov, A.S., and Banzeruk. V. I., 2008. Diamondiferous xenoliths from crustal subduction: garnet oxygen isotopes from the Nyurbinskaya pipe, Yakutia. Eur. Jour. Mineral., 20, 375-385.