Mineral composition and features of geochemistry

 of the Koksharovsky massif carbonatites (Prymorye, Russia)

Moskalenko E.Yu.*, Vladykin N.V.**, Oktyabrsky R.A.*

*Far East Geological Institute FEB RAS, Vladivostok, Russia; ** Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia.

      The Jurassic alkaline-ultrabasic formation of Primorye comprises comagmatic volcanites of the meimechite-picrite composition and intrusions differentiated from dunites to gabbro. The Mesozoic rifting depression thins out from north to south, and in this direction the enrichment of the magmatic rocks of the formation in alkalis and the development of the leucocratic alkaline phases are observed. The Koksharovsky massif (44^o30'N, 134^o10'E) is dominated by the rocks of the syenite row, and only this intrusion in Primorye contains carbonatites.

     The massif is an elongated body about 17 km long with outcrops of 0.8 to 2 km in width extended north-eastward conformably to the strike of the country rock folding. The massif breaks the Triassic and Jurassic siliceous-terrigenous and volcanogene deposits with the formation of the contact hornfels. Maximal K-Ar age for the dike nepheline syenites and biotite from pyroxenite is 160-172 m.y. [1].

    The rocks of the massif are dominated by titanaugite pyroxenites. In the central part, the medium-coarse-grained kaersutite- and biotite-bearing (10-15%) pyroxenites with titanomagnetite (10-15%) and apatite (4-5 to 10-12%) often contain ore varieties with high titanomagnetite and sphene content. Marginal facies are finer-grained with high sphene content, and in them kaersutite prevails over augite.

    Findings of isoferroplatinum intergrown with low-aluminum (6-7mass. % Al₂O₃) ferrichromite (50-53 mass % Cr₂O₃) suggest the presence of dunites [2].

    The alkaline rocks of the Koksharovsky massif are represented by veins ( 0.5 to 0.7 m) and dikes (20 to 25 m). They are composed of fine-grained aegirine and medium-grained aegirine-augite (with hastingsite) ijolite-melteigites and ijolites, medium-coarse-grained faujasites, miaskites, lujaurites (with eudialyte), lujaurite porphyries, aegirine-augite-hastingsite nepheline and alkaline syenites, and nepheline syenite-pegmatites and syenite-aplites. As a whole the alkaline rocks occupy less than 10% of the massif area. Because of a thick (30-40 m) weathering crust it is impossible to improve the details of the massif structure and rock interaction.

     The only body of carbonatites of a subelipsoid form, 0.5-0.7 km across, has been recognized in the north-east part of the massif among the near-contact fine-grained kaersutite pyroxenites. Diagnostics of the initial composition and genetic features of carbonatites is very difficult due to the widespread hypergene transformations. We have established that the carbonatite structure of their body central part is medium-coarse-grained. In some samples, carbonate grains are lath-like and oriented in parallel to the crystal elongation. In mineral composition the Koksharovsky carbonatites are essentially calcite rocks. Of the admixtures determined by the microprobe analysis, calcite contains only 1.57-1.82 mass % SrO. In addition to calcite, carbonatites contain nepheline, albite, potassic feldsparapatite, sphene, aegirine, sphene, aegerine-augite, and more rarely hastingsite, biotite, titanomagnetite, and hematite. The content of these minerals is irregular. Chemical analysis of the carbonatite samples supported that they are calcite varieties with an insignificant MgO content (0.50-0.9 mass %). P₂O₅ amount in carbonatites varies within 0.9-2.4% that characterizes the calcitic carbonatites.

      The Koksharovsky carbonatites show a stable composition of rare elements. They contain maximal content of Sr (0.5-0.9 mass %) as well as (g/t) Y – 80-104 and Pb – 68-89 and have Ba/Rb ratio of 110-237, whereas ore pyroxenites, ijolites, and syenites have Y – 0.6-28, Rb – 0.4-5.6, and Ba/Rb = 17-120. Petrographically similar carbonatites of the Zhidoisky and Koksharovsky massifs contain similar concentrations of lead, copper, and iron group elements, and they are close in saturation with Sr (7700 and 7292 g/t), but the Koksharovsky carbonatites contain two times less Ba (582 g/t) and REE (1509 g/t) and much more concentrations of Zr and Nb, typical of carbonatites.

      In the carbonatites of the Koksharovsky massif, the REE summarized amount is 1394-1618 g/t that is 30-40 times as much as that in ore pyroxenites or nepheline syenites, but only 3-10 times as much as in sphene-hornblende pyroxenites, ijolites, and ijolite-melteigites. The carbonatites are also characterized by maximal enrichment in light REE relatively to heavy ones – (La/Yb)_N = 36.8-51.0, La amount being 1500-1800 times as much as its concentration in chondrites.

The line of the normalized distribution of REE in the Koksharovsky carbonatites is between the Ingili and Arbarastakh pyroxenite-carbonatite massifs of the Aldan province [3]. They turned out to be identical to the carbonatites of the Zhidoisky massif with regard to the content of light REE (La 360-447 and 419; Ce 609-758 and 804 g/t, correspondingly) and heavy REE (Yb 5.75-7.53 and 6.05; Lu 0.82-1.13 and 0.99).

The spectra of the rare-earth elements in the carbonatites showed that in the REE sum the carbonatites are similar to the perovskite and sphene-bearing hornblende-titanaugite pyroxenites. The plots of three main varieties of the Koksharovsky massif rocks show rather gentle and close inclination of the spectrum curves. Analysis of the REE contents has revealed the main common tendency of prevalence of light lanthanoids over heavy ones with their smooth interdistribution that indicates the comagmatic nature of the contrasting rocks of the massif. In all rocks, the effect of Eu fractionation is not observed. All these facts testify to the insignificant in time differentiation of the primary magma of the massif.

    Table 1. Chemical composition of calcite (1-3), sphene (4-5), and aegerine-augite (6-9) from carbonatites of the Koksharovsky massif

Data on isotopy of oxygen, carbon, neodymium, and strontium confirm the endogenous nature of the Koksharovsky carbonatites. By values of δ ¹³C  -5.2 - -4.9 and δ ¹⁸O 9.0-11.3 the Koksharovsky carbonatites correspond to the initial igneous carbonatites [4, 6]. The ratio of neodymium and strontium isotopes in them (εNd=5.6; ⁸⁷Sr/⁸⁶Sr=0.70363) [3] show that they were originated from the depleted source. Geological position of the massif in the ocean-continent transition zone (in the zone of subduction – into the mantle of the oceanic basalt material) does not contradict to this conclusion. Analysis of the REE distribution in the Koksharovsky massif rocks revealed the presence of all transitional varieties shown different saturation with lanthanoids of the leucocratic and melanocratic rocks that allows the conclusion that they resulted from magmatic differentiation.

This study was financially supported by FEB RAS grant 05-2-0-00-01.

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