Perovskite from the Tiksheozero carbonatite: age and genesis
*Lepekhina E. N., *Antonov A. V., **Savva E. V., ***Belyatsky B. V., *Sergeev S. A.
*Centre of Isotopic Research, VSEGEI, St.-Petersburg, Russia
The ultramafic–alkaline–carbonatite complex Tiksheozero is located near the NE continental margin of the Karelian Craton within Archaean tonalite–trondhjemite–granodiorite gneisses. The Tiksheozero carbonatite complex is a typical ring complex where carbonatites are associated with intrusions of ultramafic and alkaline rocks. Klyunin and Panichev (1987) postulated five separate intrusion phases: (1) olivinites; (2) pyroxenites; (3) theralites, jacupirangites and ijolites; (4) nepheline syenites; and (5) carbonatites. Carbonatites occur as large bodies which are more than several hundred meters wide and up to 3 km long. The main carbonatite associations are represented by calcite, apatite, phlogopite (tetroferrriflogopite), magnetite, amphibole – high-temperature, and ankerite, dolomite, amphibole and apatite – low-temperature carbonatite (Tichomirowa et al., 2006).
There were a lot of attempts to estimate of the real age of the massif by using various radiology methods. But the results were not good enough and all of this concerns the carbonatites but not ultramafic counterparts. The first age estimations of the Tiksheozero carbonatites were based on few K/Ar ages from biotites and amphiboles and corresponded to the range 2.0 – 1.8 Ga (Klyunin, Panichev, 1987). Lead-lead isochron for three apatite fractions was published by Shchiptsov et al. (1991) and the age of carbonatites was ascribed to 1980 ± 170 Ma. Further detail investigation of various isotope systems (Rb/Sr, Sm/Nd, U/Pb, C, O) for whole-rock samples and mineral separates were undertaken by Tikhomirova with coauthors (2006). Obtained Rb/Sr isochrons varied from 1680 to 1910 Ma, Sm-Nd – from 1700 to 1950 Ma, and Pb-Pb apatite–whole-rock isochron corresponded to 1850–1880 Ma. Thus, to summarize these data the emplacement of the complex can be related to Palaeoproterozoic activity of the Neoarchaean junction zone. But the exact time of massif intrusion has not been determined. Noteworthy there is not only large disagreement in age estimations obtained by different isotope systems but big age-errors, bad isotope data correlations and isochrons’ parameters are obvious as well. Such situation is quite typical for secondary metasomatic overprint on primary magmatic systems and such conclusions were done by Tichomirowa et al. (2006) but the source and time of this impact remained unknown.
We have made an attempt to estimate the time of carbonatite intrusion studying U-Pb system in zircons and baddeleyite from drill core samples of carbonatites (Antonov et al., 2008; Belyatsky et al., 2008; Rodionov et al., 2008). The obtained age estimation proved Proterozoic age of the carbonatites. But at the same time, the investigation of zircon internal structures demonstrated a recrystallization of metamict parts under hydrothermal (metasomatic) environments. This process probably took place much later during Caledonian orogenesis 450 – 410 Ma ago.
It is well known that perovskite is a rather ubiquitous mineral in alkaline-ultrabasic rocks and carbonatites. Also, it is mineral geochronometer which in contrast to zircon could not be xenogenous and due to its crystallization from the melt allows to estimate the time of intrusion emplacement properly. Therefore, we decided to support primary age estimation of carbonatite intrusion by study of perovskite U-Pb isotope system.
Perovskites were found in the mineral separates of carbonatite sample from core hole 146/177-187. It should be mentioned that perovskite was not observed in thin sections of the same sample under optical microscope revision and scanning electron microscopy study as well. Anhedral single perovskite crystals are typically 100 to 500 mkm in size (fig. 1). Chemical composition was analyzed by EDX technique (Link Pentafet Inka Energy with Si(Li) 10 mm2 detector). Studied perovskite has composition close to CaTiO3 and demonstrates only limited variations: Na – up to 1.2 wt.%, Fe – 0.6-1.7 wt.%, Sr – up to 0.9 wt.%, Ce – up to 3.7 wt.%, Nd – up to 1.6 wt.%. The most of perovskite grains contains inclusions of rock-forming and accessory minerals: calcite, dolomite, mica, diopside, apatite, ilmenite, pyrrhotite. A lot of holes after liquid inclusions were identified also.
SHRIMP analyses of perovskites were performed at Centre of Isotopic Research (VSEGEI, Sankt Petersburg, Russia). The procedure for SIMS U-Pb perovskite analysis is similar to that described by (Ireland, 1990). Grains were embedded in epoxy (probe mount) together with analytical standard, grinded and polished. As a standard we have used perovskite from skarn of the Tazheran complex, Baikal Lake area. Tazheran perovskite U-Pb isotope composition was previously analyzed by ID-TIMS method. Obtained U-Pb age corresponds to 458 ± 5 Ma that is in a good agreement with other labs' data. Our SHRIMP-II dating of this perovskite grains gave 465 ± 15.
Sixteen perovskite grains from Tiksheozero carbonatites were analyzed. The measured U-Pb isotope data, uncorrected for common Pb, were plotted on a Tera-Wasserburg diagram. The obtained spread in isotope composition was really enough for construction best-fit regression line through all analysis data and a common Pb composition (207Pb/206Pb=0.836). The age of studied perovskite corresponds to 381 ± 10 Ma (MSWD= 0.63) (fig.2).
The obtained perovskite age is in contradiction with generally accepted Proterozoic age of the Tiksheozero carbonatite and coincides with the age of the zircon recrystallization caused by overprinted hydrothermal (metasomatic) process. So, it is possible to suppose that the studied perovskite is not primary magmatic mineral and its formation is directly connected with the processes of recrystallization and metasomatic transformation of the carbonatite rocks. Similar perovskite origin is described by Kukharenko et al. (1965) for Devonian alkaline-ultrabasic massifs with carbonatites of the Kola Peninsula and northern Karelia. Moreover there are a lot of examples for metasomatic origin of perovskite in skarns formed over carbonaceous rocks at the contact with alkaline and basic intrusions – typical localities are Shishim Mountains, Ural, Tazheran complex, Siberia, etc (Mitchell, 2002). So, it is cannot be excluded that metasomatic event which caused perovskite formation in Tiksheozero carbonatites is connected with the well known alkaline magmatism, which was widespread in the Kola region 360-380 Ma ago (Kramm et al., 1993) and is the plausible result of the upper mantle Paleozoic plume activity (Arzamastzev et al., 2002).
Fig. 1. Back-scattered electron image of perovskite from the Tiksheozero carbonatite.
Cc – calcite, Ilm – ilmenite, Apt - apatite
Fig. 2. U-Pb SHRIMP perovskite data plotted on a Tera-Wasserburg concordia diagram.
Analytical results are plotted as a total ratios, uncorrected for common Pb. Solid line indicates mixing trend between radiogenic lead and common lead of present-day composition .
Antonov A.V. et al., 2008. Mineralogical and geochemical peculiarity of accessory minerals from Tiksheozero carbonatites complex. Geochemistry of magmatic rocks–2008. School «Geochemistry of Alkaline rocks». Absract Volume, 4–5. (in Russian).
Arzamastsev, A.A et al., 2002. Palaeozoic processes of plume–lithosphere interaction in the Northeast Baltic Shield: prolongation, volumes, conditions of magma generation. In: F.P. Mitrofanov, Editor, Geology and Raw Materials of the Kola Peninsula, Apatity, KSC RAS, vol. 2, 104–145 (in Russian).
Belyatsky et al., 2008. Hydrothermal zircons from Proterozoic carbonatite Tiksheozero massif. 33 IGC. Abstract CD-ROM. Oslo. MPI-07, 1257121.
Ireland, T.R., Compston, W., Williams, I.S. & Wendt, I., 1990. U-Th-Pb systematics of individual perovskite grains from the Allende and Murchison carbonaceous chondrites. Earth and Planetary Sci. Lett., 101, 379–387.
Klyunin, S.F., Panichev, V.V., 1987. Geological building and mineral resources from the Panaryarvin zone and its framework. North-West Geological Survey. Reprint, Monchegorsk. 56 p. (in Russian).
Kramm, U., Kogarko, L.N., Kononova, V.A. and Vartiainen, H., 1993. The Kola Alkaline Province of the CIS and Finland: Precise Rb-Sr ages define 380-360 Ma age range for all magmatism. Lithos, 30, 33–44.
Kukharenko, A.A. et al., 1965. The Caledonian Complex of Ultrabasic, Alkaline Rocks and Carbonatites of the Kola Peninsula and northern Karelia. Nedra, Moscow. 550 p. (in Russian).
Mitchell, R. H., 2002. Perovskites. Modern and ancient. Almaz Press. Canada. 320 p.
Rodionov, N. et al., 2008. Baddeleyite SHRIMP age: application for carbonatite massifs dating. 4th International SHRIMP Workshop. Abstract Volume. 103–105.
Shchiptsov, V.V. et al., 1991. The distribution of U–Th–Pb and rare earth elements in apatites of Karelia. Mineral. Zh. 13 (4), 92–98 (in Russian).
Tichomirowa, M. et al., 2006. The mineral isotope composition of two Precambrian carbonatite complexes from the Kola Alkaline Province – Alteration versus primary magmatic signatures. Lithos, 91, 229–249.