Composition and paragenetic types of clinopyroxenes from kimberlites of Peri-Azovian.
Tsymbal S.N. *, Tsymbal Y.S. *, Latsko V.G. **, Bondarenko V. A. **
* Institute of geochemistry, mineralogy and ore formation of N.P.Semenenko, NAS of Ukraine, Kiev, Ukraine; ** Peri-Azovian complex geological party KP "Ukruzhgeology", Volnovaha, Ukraine
Four kimberlite pipes (Petrovsk, Southern, Novolaspinsk, Nadiya) and two dikes (Southern and Novolaspinsk) of Devonian age are known in the southeast part of Ukrainian Shield. Kimberlites are represented by phlogopite varieties of massive and breccial structures. Raised contents of xenogenic minerals of deep paragenesises ‑ pyrope, picroilmenite, chromespinelides, chromous diopside, phlogopite, olivine (usually replaced by serpentine) – are typical for them. The proportion between these minerals essentially varies in different bodies, with chromediopsides making the least fraction among them. Data on the composition of minerals sampled from kimberlite pipe Southern were published by S.N.Tsymbal, et. al (1996) and by B.S. Panov, et. al (1999). Recently new data on composition of chromous diopside from kimberlites of this pipe and associated dike as well as new data on minerals from Novolaspinsk pipe and associated dike are obtained. The Petrovsk and Nadiya kimberlite pipes are charecterised by very rare findings of chromediopside in them. The results on about 400 microprobe analyses plotted on Na2Î - Cr2Î3, Na2O - Al2O3, Ñr2Î3 - Al2O3, Na2O - FeO, ÒiO2 - FeO diagrammes are used for the interpretation of composition of chromediopside sampled from Southern and Novolaspinsk pipes and associated dikes. The comparison of these analyses allows to distinguish several varieties of diopsides that differ between themselves on features of composition and make some separate fields on diagrammes.
Variety I is represented by diopsides with moderate contents of ÑàÎ (21,9-23,1 %) and MgO (15,7-17,5 %), raised FeO (3,4-6,6 %) and Àl2Î3 (1,3-3,1 %), low Ñr2Î3 (0,2-0,5 %) and ÒiO2 (0,10-0,27 %), very low Na2O (<0,15 %). The values of Ca/Ca+Mg ratio is 0,47-0,50. Features of composition allow to consider these diopsides as low-temperature (850-950 °Ñ, temperature here and hereinafter is defined by F.Boyd's method, 1970) and low-baric (AlVI <AlIV) formations originating at crystallization of residual kimberlitic melt. Single findings of this diopside variety are reported only for Southern kimberlite pipe.
The variety II is represented by high-calcic and nearly chromiumless diopsides (ÑàÎ ‑ 23-25 %, MgO ‑ 15,1-17.5 %, Ñr2Î3 <0,05 %) with considerable admixtures of FeO (2,0-5,2 %) and Na2O (0,18-0,57 %). Content of TiO2 in them varies from 0,07-0,15 to 1-2 % and À12Î3 does from 0,01 to 0,7 %. Diopsides of this variety can be subdivided into two groups. Diopsides of the first group show typical lower concentrations of À12O3 (<0,15 %), ÒiO2 (0,1-0,8 %) and FeO (2,1-3,8 %) in comparison with the second group diopsides (À12Î3 ‑ 0,26-0,73 %, ÒiO2 ‑ 0,64-2,0 %, FeO ‑ 3,3-5,2 %). ZrO2 in the range from 0,1 to 0,8 % has been established in diopsides of both groups by microprobe analysis. The described diopsides are diagnosed for the first time by us as part of reactional rims developed on macrocrysts of zircon of "kimberlite" type. The reactional-magmatic nature of these rims does not raise any doubts, and typochemical features of minerals composing them (diopside, amphibole, baddeleyite, phlogopite and calcite) testify that formation of rims on zircons occurred at low-deep conditions as a result of the reaction of zircon with the residual kimberlitic melt enriched in carbonate component and alkalis.
Variety III is represented by calciumferous, low-aluminiferous and low-titaniferous diopsides (ÑàÎ ‑ 20,3-23,0 %, À12Î3 ‑ 0,24-0,75 %, TiO2 ‑ 0,05-0,24 %) with raised alkalinity (Na2O ‑ 0,8-1,87 %) and iron content (FeO ‑ 1,3-5,0 %). Contents of Ñr2Îç in them varies from 0,3 to 2,9 %. On chromium content there are two groups distinguished, with the first one showing Ñr2Îç = 0,3-0,9 % and the second one 1,0-2,9 %. Diopsides of the first group do not have any correlation established between Na2O and À12Î3, Na2O and Ñr2Î3, À12Îç and Ñr2Î3, TiO2 and FeO. But diopsides of the second group show poorly manifested direct dependence between contents of Na2O and Ñr2Îç and, like diopsides of the first group, whole absence of correlation between Na2O and À12Î3, À12Î3 and Ñr2Î3, ÒiO2 and FeO. According to B.S. Panov, et. al (1999) following admixture elements are identified in chromediopsides of the first group with using LA-ICP MS method (in ppm): Ti ‑ 1076-1411; V ‑ 397-426; Ni ‑ 150-174; Co ‑ 20-24; Sc ‑ 67-71; Ga ‑ 4,5-12,7; Sr ‑ 98-136; Zr ‑ 52-72; Hf ‑ 3,2-4,2; Y ‑ 4,1-5,5; Nb ‑ 0,4-4,2; REE ‑ 24,6-35,8. Among REE light lanthanides essentially prevail over heavy ones and among lanthanides themselves Ñå and Nd are predominant. REE distribution patterns of studied chromediopsides normalized on chondrite are similar to that established for chromediopsides from kimberlites of other regions. The presence of significant amount of REE, Zr, Sr, Sc, Ti and other non-coherent elements established in these chromediopsides indicates some possible effect by metasomatic processes on them. In chromediopsides of the second group the contents of admixture elements reach following values (in ppm): Ti ‑ 4273; V ‑ 375; Ni ‑ 418; Ñî ‑ 29; Sc ‑ 42; Ga ‑ 11; Sr ‑ 241; Zr ‑ 81; Hf ‑ 4,5; Y ‑ 5,4; Nb ‑ 6; REE ‑ 46. Among REE Ñå and Nd strongly prevail, which total amount reach nearly 38 gram per ton. These chromediopsides are much more enriched by non-coherent elements, and so more metasomatised in comparison with chromediopsides of the first group. It is not excluded, that the increase in concentration of Na2O and Ñr2Î is also associated with deep metasomatism. Chromediopsides of the variety III are enriched by ureyitic and in smaller amount by jadeitic and ferrosilitic components. Based on the value of Ca/Ca+Mg (0,46-0,48) ratio, the temperature of their formation is interpreted to be 950-1025° C. The diopsides are crystallized at high pressure conditions for which might be indicated the fact that nearly all Al is found in octahedral coordination position.
Variety IV is represented by diopsides with lowered contents of ÑàÎ (19,1-19,4 %) and MgO (16,8-17,0 %), raised À12Î3 (1,4-1,6 %) and Na2O (1,2-1,4 %), high FeO (5,6-5,8 %) and very low Ñr2Îç (0,01-0,08 %). These diopsides show the presence of significant amount of jadeitic and ferrosilitic components. Values of Ca/Ca+Mg ratio varies in range of 0,43-0,45. À1 takes position in octahedral coordination position. These diopsides are crystallized at temperature of 1050-1125°C and high pressure conditions. Single findings of this variety of diopsides are reported for Novolaspinsk kimberlite pipe.
Variety V is represented by diopsides with moderate content of calcium (ÑàÎ ‑ 19,5-22,5 %) and magnesium (MgO ‑ 16,0-17,8 %) and raised contents of À12Î3 (1,7-2,3 %), Ñr2Î3 (0,7-1,4 %), Na2O (1,0-1,5 %) and FeO (1,5-4,0 %). ÒiO2 content values do not exceed 0,3 %. Admixture element are established in following amounts (in ppm): Ti ‑ 1693-2059; V ‑ 282-296; Ni ‑ 265-289; Sc ‑ 35-68; Ga ‑ 4,8-7,3; Sr ‑ 138-188; Nb ‑ 0,9-3,9; Zr ‑ 34-117; Hf ‑ 2,0-3,7; Y ‑ 3,5-8,0; REE ‑ 29-51 (according to B.S. Panov, et.al., 1999). The presence of these elements indicates the fact that these diopsides have been influenced by processes of deep metasomatism. Conditions of their formation are following Ò = 950-1000 ° C, Ð = 38-41 kBar (here and hereinafter evaluation is made by S. Mercier method, 1980). The diopsides of this variety are found in Southern (often) Novolaspinsk (rarely) kimberlite pipes.
Variety VI is represented by diopsides with lowered calñium content (ÑàÎ ‑ 18,0-20,7 %, MgO ‑ 16,0-17,4 %) high contents of Ñr2Î3 (1,7-2,7 %) and Na2O (1,4-2,4 %), moderate À12Î3 (0,9-1,6 %, rarely to 2,6 %), FeO (1,8-3,9 %) and ÒiO2 (0,21-0,45 %). These diopsides are considerably enriched in ureyitic and jadeitic components. Value of Ca/Ca+Mg = 0,43-0,45 ratio allows to assume crystallisation of diopsides at temperature of 1050-1125 °Ñ and pressure 39-44 kBar. These chromediopsides are commonly found in kimberlites of both studied kimberlitic pipes, but generally in small amount.
Variety VII is represented by low-calcic diopsides (ÑàÎ ‑ 17,0-20,5 %, MgO ‑ 16-18 %) with rather high contents of Ñr2Î3 (1,5-2,5 %, rarely to 2,8-3,4 %), Na2O (from 1,4-1,5 to 2,5-2,8 %) and À12Î3 (from 1,7-2,0 to 2,5-2,8 %), low FeO (1,7-3,2 %) and TiO2 (0,1-0,5 %). Aluminium occupies mainly or mostly octahedral coordination position. These diopsides constantly contain raised amounts of jadeitic and ureyitic components. Following elements are established by LA-ICP MS analysis (in ppm): Ti ‑ 1393-2439; V ‑ 306-429; Ni ‑ 351-394; Co ‑ 17-23; Sc ‑ 24-38; Ga ‑ 3,5-7,5; Sr ‑ 91-182; Nb ‑ 0,3-0,4; Zr ‑ 19-41; Hf ‑ 0,9-2,5; Y ‑ 2,1-3,3; REE ‑ 18,5-31,1 (B.S. Panov, et.al . 1999). In comparison with other varieties these chromediopsides are mostly enriched in Ni and depleted in incompatible elements Zr, Sr, REE, Nb. Among REE light lanthanides - La, Ñå and Nd - prevail. The value of Ca/Ca+Mg ratio in diopsides varies within range of 0,42-0,46. The temperatures of formation of the most diopsides are 1100-1150° Ñ and rarely reach more low values (to 1050°Ñ) with pressure values of 37-43 kBar. Diopsides of this variety show the presence of significant amount of ureyitic and jadeitic components. These diopsides comprize 30 % of total amount of all studied mineral grains found in kimberlites from Southern and Novolaspinsk pipes. Diopsides plotted also show field overlapping on compositional diagrams.
The variety VIII is represented by typical subcalcic chromediopsides in which MgO prevails over ÑàÎ (17,7-19,2 % over 15,5-17,6 %), and Ñr2Î3 reaches 1,2-2,2 %. They are characterized by raised contents of À12Î3 (2,1-2,8 %), Na2O (1,1-1,8 %), FeO (2,5-3,6 %) and ÒiO2 (0,25-0,50 %). For diopside with the least chromium content the value of Ca/Ca+Mg = 0,36 is established, that indicates its crystallisation at temperature of about 1300°Ñ, with pressure values reaching approximately 48 kBar.
Subcalcic chromediopsides of similar composition are known to be found in kimberlites from many provinces of the world and are considered to be products of disintegration of very deep broken-down lherzolites lying at the basis of lithospheric mantle (Sobolev, 1974). Such chromediopsides are rarely found in Southern and Novolaspinsk kimberlite pipes.
Thus, diopsides of different composition and origin are established and studied in kimberlites from pipes Southern and Novolaspinsk and dikes associated with them. Most diopsides are interpreted to be the products of disintegration of granular lherzolites from middle and bottom deep horizons of lithospheric mantle. They are formed at temperature range from 900 to 1150 ° C and pressure range from 30 to 43 kBar. Almost all of them have been effected by mantle metasomtism and in different degrees enriched by incompatible admixture elements (Zr, Sr, REE, Y, etc.). Subcalcic diopsides are also rarely found, which Ð-Ò parametres of crystallization is 1300 °C and 48 kBar. Broken-down lherzolites are interpreted to be primary source for diopsides. In addition, varieties of low-temperature and low-baric diopsides formed at the stage of crystallization of residual kimberlitic melt. The reactionary interaction of this melt with macrocrysts of silicate minerals, in particular zircon, is identified. Kimberlites of East Peri-Azovian region show the presence of chromediopsides which composition corresponds to chromediopsides generally associated with diamond.
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