Endogenous and sedimentary carbonate rocks from the Belomorian province: new geochemical, isotopic and geochronological data

 

K.I. Lokhov*, .S. Sibelev**, .I. Slabunov**, E.S. Bogomolov*, E.B. Prilepsky*

* Isotopic Research Centre,VSEGEI, St.Petersburg, Russia; ** Institute of Geology, KarRC, RAS, Petrozavodsk, Russia

 

In the Belomorian province of the Baltic Shield carbonate rocks (CR) are fairly scarce. One of the best-known CR occurrences is located on Mramorny Island in the White Sea and was considered earlier [1, 4] an example of highly metamorphosed sedimentary rocks. Together with this occurrence, we have studied veined, genetically obscure CRs near the Town of Gridino and marbles discovered recently [5] in the River Pongoma - Lake Korzhino area and interpreted as metasediments.

On Mramorny Island, CRs constitute a ca. 5-40 m thick and ca. 170 m long lens-shaped body thrown into isoclinal folds. They are associated, interbedded and deformed jointly with amphibolites and are cut by granite-aplites [4]. Kyanite-garnet-biotite gneisses, exposed in the central part of the islet, seem to be a fragment of host rocks. In the Gridino area, CRs fill several closely-spaced, steeply dipping, branched veins that vary in thickness from 0.1 to 0.7 m and are visible over a distance of ca. 10 m. Host rocks are represented by pyroxene-amphibole gneisses and amphibolites. Skarnification processes are observed at vein contacts. The mineral composition and structural characteristics of CRs from the above localities are similar. The rocks are medium-grained, coarse-grained and xenomorphic-granular (heterogranoblastic, occasionally lepidogranoblastic). Their major minerals are dolomite (ca. 70-90%), calcite (5-10%), biotite (up to 10%) and hornblende (up to 7%). Their minor minerals are clinopyroxene, chlorte, apatite, plagioclase (up to 100% n) and quartz. Epidote, sphene, muscovite, rutile, zircon, ilmenite, pyrite and thorite are present as accessories.

In the River Pongoma and River Sigma areas, CRs occur as abundant lenticular bodies, up to 5 2 m in size. Together with alumina gneisses and amphibolites, they are a part of the clastic constituent of a tectonic melange supported by migmatized garnet-amphibole gneisses (blastomylonites). CR bodies are spatiously restricted to the axial part of the zone. Their mineral composition is dominated by dolomite (over 90%), calcite, quartz and chlorite being less abundant. Amphibole and muscovite are encountered in the exocontact zones of lenses.

Isotopic data show that Gridino and Marble Island rocks are endogenous, possibly igneous carbonatites with a considerable contribution of a mantle constituent (Fig. 1), and rocks from the mélange zone (Rivers Pongoma and Sigma) are typical metasedimentary carbonate rocks commonly understood as marble.

 

Fig.1. Isotopic parameters of carbonate rocks from the White Sea region. Composition fields: metacarbonate rocks [6], mantle carbonatites [7].

 

It is obvious from the isotopic composition of strontium and the εNd(T) value that Gridino carbonatites are less heavily contaminated with crustal matter. However, the isotopic composition of their carbon is similar to that of mantle carbonatites and their oxygen is slightly heavier presumably due to interaction with crustal matter, probably upon metamorphic alteration in Svecofennian time, and Mramorny Island carbonatites contain carbonates (dolomite) with significantly isotopically fractionated carbon and oxygen. Such an isotopic fractionation could have occurred either in a calcite-carbon dioxide system or in a calcite-dolomite system [2, 3 et al.]. For igneous carbonatites the latter option seems more preferable and implies the presence of complementary, largely calcitic rocks relatively light with respect to carbon and oxygen isotopes. The strontium isotope ratio of metasedimentary carbonate rocks agrees with that of oceanic water in Neoarchean time [6], and we estimate the age of sedimentation at ca. 2800 Ma.

We extracted zircons from Mramorny Island dolomitic carbonatites. They consist of two matter generations: 1) cores that show a thin oscillatory zoning and Th/U >0.1 and 2) dark high-uranium homogeneous crusts with Th/U<0.1. Matter from both zircon generations was analysed on SIMS SHRIMP-II. Both generations show concordant U-Pb age values, the former marking a 242020 Ma magmatic stage and the latter being indicative of 184515 Ma metamorphic alterations.

Thus, for the first time we have revealed endogenous carbonatites in the Belomorian complex the age of which (ca. 2420-2450 Ma) agrees with the rifting epoch.

 

References:

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