Mineralogy of great bulk of kimberlites of the Kirovograd megablock of the Ukrainian Shield
Tsymbal S.N., Tsymbal Y.S.
Institute of geochemistry, mineralogy and ore formation of N.P.Semenenko, NAS of Ukraine, Kiev, Ukraine
Lelekovsk and Shchorsovsk manifestations of thin thickness dikes of phlogopite kimberlites of hypabyssal facies are established in the central part of the Kirovograd megablock of paleo-proterozoic age. According to Rb-Sr isochronous method their age is 1770±9,4 million years (Yutkina, et.al., 2005). By present time the petrography, mineralogy and geochemistry of these kimberlites and xenoliths of deep rocks containing in them (Tsymbal, et. al., 1999) is completely enough studied. But the data on mineralogy of matrix of it is not enough.
EMS JSM-6700F with energy dispersional system JED-2300 for the microanalysis as well as microprobe JXA-8200 by JEOL firm (Japan) have been used at investigation of the Kirovograd kimberlites. As a result it was established, that kimberlite matrix is represented mainly by microphenocrysts of olivine, phlogopite and calcite, the relation between which varies in small sites. titanomagnetite, perovskite, sphene, apatite, clinopyroxene, amphibole, ilmenite and rutile are much less abundant.
Olivine is a predominant mineral of most kimberlites. Its crystals are completely replaced by serpentine which composition allows to assume, that initially it was high-magnesian forsterite.
Phlogopite is the second as to the abundance mineral in rock mass. It forms lathes which "flow round" olivine crystals and xenoliths of deep rocks. There are three varieties of it established.
The phlogopite of the first variety shows high magnesian content (MgO ‑ 26,3-27,3 %, FeO ‑ 5,0-6,4 %), low contents of K2O (7,4-9,0 %) and Al2O3 (10,5-11,7 %) and high BaO (4,1-4,8 %). The contents of TiO2 admixture makes 0,13-0,16 %. It is crystallised almost simultaneously with olivine.
The phlogopite of the second variety is widely abundant in matrix. Lath like crystals of it were formed after olivine, but before calcite, apatite, titanomagnetite and perovskite. The phlogopite of the second variety is widely abundant in matrix. Lath like crystals of it were formed after olivine, but before calcite, apatite, titanomagnetite and perovskite. Their central parts differ from phlogopite of the first variety by lower magnesian content (MgO ‑ 20,8-21,3 %) and high alumina (Al2O3 ‑ 14,7-16,2 %) and titanium (TiO2 ‑ 2,6-3,0 %). In addition it contains more K2O (9,2-10,0 %) and less BaO (1,5-2,7 %) than the first variety.
The phlogopite of the third variety comprizes tetraferriphlogopite rims around phlogopites of the second variety. If phlogopites of the second variety is charecterized by FeO contents of 6,5-7,2 %, tetraferriphlogopite rism its content reaches 7,8-8,4 % at almost the same contents of MgO and K2O, but lower Al2O3 (12,3-13,4 %), TiO2 (1,9-2,6 %) and BaO (0,59-0,78 %). According to the papers on phlogopite synthesis it is shown, that at temperature 800î C and pressure less than 2 kBar phlogopite can contain up to 3 % of BaO. Based on this fact we would like to make a conclusion about possible formation of phlogopite of the first variety at temperature more than 8000 C, and phlogopite of the third variety at much lower temperatures.
Titanomagnetite is widely enough distributed mineral foun in rock mass of kimberlites. It forms microcrystals or their intergrowths and rarely segregations in phlogopite and calcite. Sometimes it contains inclusions of sphene and perovskite. There are two varieties distinguished based on its chemical composition.
The titanomagnetite of the first variety shows high contents of TiO2 (11,9-14,8 %) and FeO+Fe2O3 (74-97 %) and also the presence of Cr2O3 (usually less than 0,3 %, rarely 1-1,6 %), Al2O3 (up to 3,5 %), MnO (0,3-1,4 %) admixtures.
Titanomagnetite of the second variety is different from the first by low contents of FeO (59-69 %) and high Cr2O3 (6-15 %), Al2O3 (3,2-4,7 %) and MnO (1,5-5,5 %). Magnetite component reaches 40 mol % a pier in its composition. And significant amounts of chromite and ulvospinel are also established.
Compositional features and interrelations of titanomagnetites with other minerals of matrix testify that the varieties enriched Cr and Al represent earlier generation in comparison with low-chromium and high-alumina titanomagnetites. Both varieties are high-temperature formations.
Perovskite is a characteristic mineral of kimberlite matrix. It is represented by idiomorphic crystals of octahedral habit, their intergrowths and group segregation of homogeneous or zonal structure. Its individual inclusions were observed in low-chromium and low-aluminina titanomagnetites. Sometimes inclusions of Mn-ilmenite and rutile are found. Two varieties of perovskite are established.
The first variety shows 34,2-37,3 % CaO and 51,5-54,1 % TiO2. Raised contents of FeO (2,8-5,6 %), Na2O (0,47-0,80 %) and Nb2O5 (0,5-2,3 %) are established. Such elements as ThO2 (0,13-0,26 %), Ta2O5 (to 0,1-0,2 %), ZrO2 (to 0,1-0,2 %), MnO (<0,1 %), Y2O3 (<0,07 %) and UO2 (<0,06 %) are found as admixtures. Rare-earth elements reach 3,3-3,9 %. Among them prevail Ce2O3 (1,7-2,1 %), La2O3 (0,7-1,0 %) and Nd2O3 (0,7-1,0 %). The perovskite of this composition does not differ from perovskites of kimberlites from Yakutia.
Perovskite of the second variety is rarely developed. Its inclusions are found in titanomagnetite which almost does not contain Cr2O3 and Al2O3. This perovskite shows high iron content (FeO - 16,7 %) and sligtly lower contents of TiO2 (46,3 %), CaO (27,8 %), TR2O3 (2,3 %) and ThO2 (0,02 %) in comparizon with the first variety. It shows the presense of significant amount Fe3 + and accordingly latrapitic component.
Based on relations observed between perovskite and other minerals found in kimberlite matrix it is possible to make a conclusion, that perovskite was crystallised after olivine and phlogopite but almost simultaneously with titanomagnetite of late generation. It is one of the main concentrators of the rare earths and niobium in the Kirovograd kimberlites.
Chromespinelides are rarely found in a matrix of the investigated kimberlites, as a rule, in the form of inclusions in titanomagnetite enriched by Cr and Al. One of such microinclusions has following composition (%): Cr2O3 ‑ 30,54; Al2O3 ‑ 36,85; MgO ‑ 16,78; FeO ‑ 13,51; Fe2O3 ‑ 2,44; TiO2 ‑ 0,56; MnO ‑ 0,15; ZnO ‑ 0,11; NiO ‑ 0,16. It is alumochrompicotite in which spsinel component makes 70 mol %, and chromite does 30 mol %.
Rutile forms thin rims around sphene or individual crystals. Among admixtures FeO (0,45-1,05 %), CaO (0,75-1,25 %) and Nb2O5 (0,35-0,85 %) are defined.
Manganous ilmenite is mineral that is rarely established in bulk rock mass of kimberlites. Its single inclusions are found in titanomagnetite and perovskite in association with rutile. In titanomagnetite MnO reaches makes 4,4 % and in perovskite does 9,5 %, with FeO reaching 6,5 % and 3,5 % accordingly. For this ilmenite raised contents of CaO (0,75 and 0,35 %), low MgO (<0,2 %) and Al2O3 (<0,3 %) is typical. For more manganous ilmenite Cr2O3 (1,3 %) admixture is established. It was formed earlier than titanomagnetite and perovskite.
Sphene is rarely distributed. Usually it forms inclusions in titanomagnetite and sometimes itself contains titanomagnetite and rutile as inclusions. Its composition varies within (%): SiO2 ‑ 30,5-31,7; TiO2 ‑ 35,0-38,0; CaO ‑ 26,4-29,0. Admixtures are represented by FeO (1,1-3,7 %), V2O5 (0,16-0,33 %) and Nb2O5 (0,1-0,3 %).
Clinopyroxene is found in matrix of kimberlites in the form of idiomorphic crystals, their intergrowths and rosette-like segregations in "taxitic" calcite. Locally its contents reaches 15-20 %. Sometimes it is found in associatiation with apatite and amphibole of richterite type. Its crystallisation occurred before formation of apatite and calcite. For the first time it has been found by petrographic investigations (Tsymbal, et. al, 1999) and currently confirmed by microprobe analyses. On composition clinopyroxene relates to high-calcic diopside (CaO ‑ 24,3-25,1 %, MgO ‑ 14,4-15,3 %, FeO ‑ 5,2-5,9 %) with insignificant admixture of Cr2O3 (<0,05 %). It is characterised by raised contents of TiO2 (0,4-1,1 %), Al2O3 (0,4-1,1 %) and Na2O (0,35-0,50 %). All the Al is found in tetrahedron coordination. There is an admixture of jadeitic component found. Based on the value of Ca / (Ca+Mg) ratio the estimated temperature of equillibrium of diopside did not exceed 800î C. Estimations according to method of J.Mersie (1980) have shown, that it crystallised at temperature about 750î C and pressure less than 20 kBar. On composition the diopsides investigated by us are similar to diopsides from most kimberlites of other regions and are essentially different from xenocrysts of clinopyroxene.
Amphiboles are rarely established in kimberlite matrix and, as a rule, found in the form of idiomorphic crystals in taxitic segregations of calcite. The compositional values vary within (%): SiO2 ‑ 56,0-58,4; TiO2 ‑ 0,2-0,3; Al2O3 ‑ <0,1; FeO ‑ 3,2-6,1; MnO ‑ 0,05-0,13; MgO ‑ 20,3-23,0; CaO ‑ 5,5-7,5; Na2O ‑ 4,8-6,1; K2O ‑ 1,4-1,6; F ‑ 0,6-0,9; BaO ‑ <0,05. Base on this data amphiboles should be realted to low-potassic and a low-titanium variety of high-magnesium richterite. The value of Na / (Na + K) ratio is 0,83-0,88.
Apatite is rather widely distributed. It forms idiomorphic crystals of prismatic habit which are confined mainly to segregations of calcite. Apatite is earlier formation in relation to calcite. On composition it related to fluoapatite (F=3,0-4,8 %). Its characteristic admixture is SrO. Two varieties of apatite, low-strontium and high-strontium, are distinguished. SrO makes less than 1 % in the former variety, and 2,5-8,0 % in the latter one. Such admixtures as Na2O (0,45-0,85 %), FeO (0,1-0,4 %), SiO2 (0,2-0,6 %), MnO (<0,1 %) and TR2O3 (<0,1 %) are also established in apatite.
Calcite is one of the main minerals found in matrix of kimberlites. According to the results of petrographic investigations (Tsymbal et. al., 1999), its amount locally reaches 20 %. The calcite is homogeneous in composition and also shows almost whole absence of in such admixtures as FeO, MnO, MgO, BaO, SrO and TR2O3. It is final product of crystallisation of residual kimberlite melt.
Thus, studying of main part of kimberlites of the central part of the Kirovograd megablock has allowed us to obtain original data on morphology, composition and sequence of formation of minerals composing it and on this basis to define pattern of evolution of kimberlite melt in low-deep conditions. The established typochemical features of actually kimberlite minerals are greatly important for understanding of geochemical specialisation of kimberlites in general.
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