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2011 |
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Тезисы международной конференции |
Abstracts of International conference |
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Geochemical peculiarities of multiple–aged kimberlites of the north-east of Siberian platform Rotman A.Y. Geo-Scientific Research Enterprise, ALROSA, Mirny, Russia rotman@cnigri.alrosa-mir.ru, arotman@mail.ru
The geochemical features of the Middle Paleozoic and Mesozoic kimberlites of the northeast part of the Siberian platform which are expressed in the character of distribution and correlation of the number of petrogenic (Al, Ca, Ti, Fe, Mg, K, P) and admixture (Cr, Ni, Co, Mn, V – siderophile; Sc, Y, Th, U, Pb, Zr, Hf, Nb, Ta – high charging, less mobile; Be, Ga, Sr; Ba, Rb, Cs – large ion lithophyl; REE: La, Се, Pr, Nd, Sm, Eu – light, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu – heavy) elements are considered.
The kimberlite appearances (diatrems, more rare veins, dykes and sills) of the Siberian diamondiferous provinces are concentrated in the northeastern part of the Siberian platform and make a kymberlite formation including except actually kymberlites, extended-limited monticellite picrites, picrite porphyrites and alneites. They are connected mainly with two magmatic epochs – the Middle Paleozoic (devon-carbon) and Mesozoic (Trias, Jura), taking various dimensional positions. The rocks of the Middle Paleozoic epoch have an important economic value, including industrially diamondiferous kimberlytes. The petrogeochemical properties of kimberlite and their convergent rocks find out appreciable variations which are defined both endogenous and exogenous factors. According to the chemical compound of kimberlites corresponds to the ultrabasic rocks which are characterized by a high magnesianity, frequently at almost equal quantities SiO2 and MgO (SiO2/MgO=1,1-1,2, sometimes 1,0 and less – the pipes Internationalnaya, Aykhal, Udachnaya-east), a high maintenance of Cr and Ni, high values of relations of Mg/Fe and Ni/Co. The kimberlytes of the region and, first of all, industrially diamondiferous bodies on the parity of oxides of potassium and titan, aluminium and calcium and also on the parity of zirconium and niobium, niobium and lanthan, cerium and strontium correspond to basaltoid kimberlites (the kimberlites of the group I of South Africa). Thus the values MgO/FeOtotal (3,7-5,8) are fixed in the Middle Paleozoic kimberlite fields with industrial diamondiferousness, than in those of the Mesozoic age (1,6-2,8). And the kimberlytes of Chomurdakh field take an intermediate position (2,9-3,1). In a number of pipes of separate northern fields of Western Yakutia (Kuranakh, Chomurdakh, Luchakan, Ary-Mastakh) and in the rocks of intrusive facies (An-21, Lipa, Velikan, Lykhchan-1, Ary-Monchellite, etc.) are observed the increased (more than 2,5-3 mass %) maintenances of TiO2, allowing to carry such appearances to the high titaniferous type at the moderate titaniferous character of industrial diamondiferous bodies. The kimberlites of the pipes of Nakynsky field are stood apart in which, as well as in the separate pipes of Daldyno-Alakit diamondiferous area, it is marked rather low – less than 0,5 mass % – the maintenance of TiO2 at the lower values of an index fractionation (La/Yb) of rare earths. The analysis of correlation of oxides of Al and Ca shows the most sustained structures of kimberlites of Muna field. In other fields with industrial diamondiferousness in many pipes of the enrichment of CaO that is expressed in the high average maintenance of lime in the kimberlites of Mirny and Alakit-Marha fields and in the fields of Anabar group – Al2O3 (Ary-Mastakh and Duken fields) or both oxides simultaneously (Birigindin and Luchakan fields). As a whole the kimberlyte fields of the Mesozoic age concerning those with economically significant efficiency differ in higher concentration of TiO2, FeO and often, P2O5. The behaviour of К2О is ambiguous with appreciable variations of concentration both in the Middle Paleozoic and in the Mesozoic kiberlites at frequent raised of containing potassium in the productive bodies (the pipes Mir, Dachnaya, Internationalnaya, Botuobinskaya, Nurbinskaya, Aykhal, Udachnaya); sometimes the positive correlation of diamoniferrousness and potassium is found out, that was marked by Vasilenko V. B. with his colleagues [Vasilenko et al., 1997]. As the concentrator К2О serves flogopite the quantity of which considerably differs both between pipes and in the pipes between crystallisation products fusions of different phases of introduction. The main feature of chemical composition of kimberlites is the antagonism of oxides of Ca and Mg, reflecting shares of their components – a carbonate (mainly calcite) and olivin/serpentine. Comparing industrially diamondiferous kimberlites with weak diamondiferous and non-diamondiferous appearances according to the distribution of elements-impurities it is possible to ascertain higher maintenances of Ni, Cr, Li and Pt in the first, Co, Mn, V, Ti, Sc, Be, Ga, Ba, Sr, Zr, Nb, Ta, REE, Zn and Sn in the second at approximately equal quantities of Rb, Pb, Mo, Th and U. Also it is necessary to notice that the majority of the listed elements, except for Ni, Co, Cr, Mn and Pt are concentrated in the kimberlites which are in the concentration of many times over exceeding clarke for the ultrabasic rocks. Rather sustained character of distribution of microcells in uneven-age kimberlites is remarkable. The «recoils» of concentration of separate elements are marked in the rocks of Starorechensk field (minimum quantities siderophiles V, Co, Cr, Ni and maximum – rare-earth elements). Under the least maintenances of the majority of rare-earth elements (REE) separate appearances of Kuoyka field in which limits are noted as the Middle Paleozoic and Mesozoic kimberlites stand apart. The microelement structure of kimberlites of various age and various efficiency shows a considerable disorder of spectra of behaviour of easy rare-earth elements (LREE) and the reduction of scope of concentration of heavy rare-earth elements (HREE) in industrially diamondiferous kimberlite bodies. Among the last the raised maintenances of REE are fixed in the kimberlite pipes of Mirny and Alakit-Markha fields and the lowest – in the rocks of Nakyn field at the intermediate position of kimberlites of Daldyn field. The Mesozoic appearances of Anabar fields are differed on the decrease in the concentration of Rb, Th, U, Nb, Ta, Pb and some other elements of rock for which as a whole it is possible to note more flat spectra of distribution of elements and raised maintenance of HREE in comparison to kimberlites of industrially diamondiferous fields. They are stood apart from all kimberlite appearances by the character of distribution of rare elements of rock of the pipes of Nakyn field: they are enriched by Cs, Rb, Ba, K, they are impoverished by Ti, LREE and they have distinct negative anomalies of Th, U, Nb. In indicator minerals the distribution of rare and rare-earth elements finds out various tendencies: in monoclinic piroxen distribution has a flat character of the spectra and the spectra for garnet and other minerals are characterized by an appreciable scope of concentration of elements. V.Griffin with co-authors [Griffin et al., 1999] showed that the increase in values of Y/Ga and the reduction of Zr/Y in peridotite garnet are directly connected with the increase in the volume of rocks containing them and is controlled by the clinopiroxen/garnet relations and a total quantity of both minerals. Thus the highest values of Zr/Y are peculiar to the Middle Paleozoic kimberlites of industrially diamondiferous fields and the lowest values of this relation and the greatest of Y/Ga – to the Mesozoic kimberlites in Kuoyka field. The mantle sources influenced on the noted geochemical features of uneven-age of kimberlites. On the available data of Sr-Nd isotope [Agashev et al., 2000; Bogatikov et al., 2004; Kostrovitsky et al., 2007 and others] the kimberlytes of industrially diamondiferous fields and non-diamondiferous appearances of Olenek group were generated at the expense of the isotope of the most primitive asthenosphere homogeneous mantle (tank BSE) and the geochemical standing apart kimberlytes of Nakyn and Muna fields are formed at the expense of sources ЕМ1 and HIMU accordingly. As a whole, considering the initial mantle substrata of rock of lithosphere mantle (lherzolites, harzburgites, dunites at the subordinate share of clinopiroxenites and echlogites) which fragments are fixed in a kind ksenolites in kimberlites, it is possible to ascertain the tendency of their exhaustion of the basalt-like component. This line is more expressed in geochemical features of kimberlites diamondiferous fields of the Middle Paleozoic age. In Mesozoic appearances of the kimberlite formations the share of this component (TiO2, FeO, Al2O3, Na2O) increases. The spectrum of kimberlite microcells is indicative by the enrichment of (concerning the ultrabasic rocks) not coherent Rb, Sr, Ba, Zr, Nb, Hf, Ta, Nd, Sm, La, Ce, Th and U, that is explained by the influence on the formation of kimberlite fusions of hot plums. The last are formed on the border of a kernel-mantle [Haggerty, 1994, and others] The maintenances of the rare and rare-earth elements show a plum of the origin of kimberlite provinces which are normalized at the structure of a primitive mantle [McDonough, Sun, 1995] which show negative (Рb, To, Zr, Sr) and positive (Nb, Th, Nd, Sm) anomalies [Аgashev et al., 2000]. The high scope of maintenances both between pipes and in separately taken pipes find out siderophile and lithophyl elements but radioactive elements are characterized by the most sustained distributions in kimberlites of separate bodies and, as consequence, the distinction of pipes on this indicator are peculiar. The noted geochemical distinctions of the rocks of kimberlite formations, along with the qualitative-quantitative variations of the spectrum of mantle minerals (diamond and its paragenetic associated rocks) reflect the lateral fluctuations of the top mantle of the province, the capacity of lithosphere and the features of uneven-age magmatic appearances.
References: Vasilenko V. B, Zinchuk N.N., Kuznetsova L.G. The petrochemical models of diamond deposits of Yakutia. Novosibirsk: Science: The enterprise of the Russian Academy of Sciences, 1997, 574 p (in Russian). Griffin W.L., Ryan C.G., Kaminsky F.V. et al. 1999. The Siberian lithosphere traverse: mantle terranes and asembly of the Siberian Cratons //Tectonophysics. V. 310. P. 1-35. Agashev A.M., Orikhashi Y., Vatanabe Т, Pokhilenko N.P., Serenko V.P. The izotope-geochemical characteristic of kimberlites of the Siberian platform in connection with a problem of their origin// Geology and geophysics. 2000. V. 41 (1). P. 90-99. Bogatikov O. A, Kononova V. A, Golubeva J.J., Zinchuk N.N., Ilupin I.P., Rotman A.Y., Levsky L.K., Ovchinikova G. V., Kondrashov I.A. The petrogeochemical and isotope variations of structure of kimberlites of Yakutia and their reasons // Geochemistry. 2004. № 9. P. 915-939. Kostrovitsky S.I., Мorikio Т., Serov I.V., Yakovlev D.A., Amirzhanov A.A. The isotope systematization of kimberlites of the Siberian platform // Geology and geophysics. 2007. V. 48. № 3. P. 350-371. Haggerty S.E. Superkimberlites: a geodinamic window to the Earth, s core // Earth and Planetary science letters. 1994. V. 122. P. 57-69 McDonough, Sun S.S. The composition of the Earth // Chemical Geology. 1995. V. 120. P. 223-253. |
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