Volcanoclastic carbonatites from Chrebtovy volcano, Kamchatka

Landa E.A., Markovsky B.A.

VSEGEI, S.Peterburg,Russia

The problem of extrusive carbonatites is actual now [Wolley, Church, 2005]. Specific products of such extrusive process are revealed in East Kamchatka among widely developed upper cretaceous ultramafic volcanites [íÁrkÏvsky, Rotman, 1981]. In this area it was reconstructed Chrebtovy paleovolcano generated pillow lavas and volcanoclastic rocks with picrite composition (average composition: SiO₂ - 39,7, TiO₂ - 0,25, Al₂O₃ - 4,3, Fe₂O₃-2,95, FeO - 7,9, MgO - 30,6, CaO - 5,4, Na₂O - 0,2, K₂0 – 0,13, Sr – 134 ppm). Ultramafic volcanites are present in volcanoclastic layers asš fragments ofš various sizes. Many fragments are in full or in part «bleached», the size of the «bleached» fragments usually from 1 to 10-15 cm, andš first mm in the fine fragmentary tuffes. The total amount of such fragments in separate layers rans up to 20-30 %. Bleaching is result of replacement of initial picrite mineralogy by aggregate with Ándradite, diopside, carbonates, and also apatite, strontianite, barite, sulfides. Thus someš initial features of initial rocks, in particular amygdaloidal structure are kept.

Pure carbonate component of such secondary product (calcite, sometimes magnesite) is presented only in very thin veins, spots and in amygdaloidal infilling. Carbonate-silicate rocksš saturated by carbonates with óÁï content at a level ofš 42 % and SiO₂ š-_š 11-12 % -are very rare. For the first time they were described [Rass, Frih-Har, 1987] as carbonatites šcompared with such effusive rocks of some regions (Mongolia etc.). However in overwhelming majority of samplesš _šSiO₂ content in carbonate-bearingš rocks is at a level of 25-40 % (averageš composition: of 16 samples, %: SiO₂ - 34,6, TiO₂ - 0,53, Al₂O₃ - 2,73, Fe₂ O₃ - 7,6, FeO - 5,8, MgO-12,1, CaO - 21,1, Na₂O - 0,14, ë₂ï - 0,03, ò₂ï₅ - 0,57).

But the main point isš that in many volcanoclastic fragments one can see transitions without sharp contacts from such carbonate-bearingš rock to picrites šand picrobasaltes kept both structural features and initial composition .So carbonate-bearingš rock šis a product of the metasomatic process produced by carbonate rich fluid during formation of volcanic units (in parallel with autometasomatic changes – serpentinization and chloritization). Earlier this process has been named «rodingitization», and these rocks – «rodingites» [íÁrkÏvsky, Rotman, 1981; Seliverstov, ïsipenko, 1999]. However composition of garnets (Ándradite, not grossular) and in a first time some mineralogical and geochemical features, such as: presencešš Sr-apatite, Sr-carbonates (table 1) and very highš Sr content (up to 60-80 thousands ppm) and ÷Á content (up to 4,5 thousand ppm) testify, that these rocks are not rodingites, butš specific metasomatic carbonatites. Probably, after E.M.Epstein such rocks must be called «carbonatitÏides», in our case ÁpÏpicrobasaltic and apopicritic. Among typical extrusive carbonatites enrichment of silicates is known š(Fort-Portal etc)

According to our isotopic data (ε_Nd = +8, ⁸⁷Sr/⁸⁶Sr=0,703743), carbonate-bearing fluid was formed as well as ultramafic volcanitesš in depleted mantle from a source functionedš simultaneously with a source of volcanites. The specificity of this source which probably reflects to regional features of mantle processes under Kamchatka has defined very lowš Nb content and rather moderate REE Óontent in described carbonatitÏides. Our data indicate that carbonatites can be metasomatic in extrusive systems and can be generated without connection with foidites.

Table 1

Composition of some minerals from carbonatitÏides, %.

1 Sr-apatite, 2- barite, 3,4 – Sr-carbonates (calciostroncianite?).

References:

íÁrkÏvsky B.A., Rotman V.K., Geology and petrology of ultramafic volcanism. Leningrad: Nedra, 1981.247p (in Russia).

Rass I.T., Frih-Har D.N., About find of carbonatitesš in upper cretaceous ultramafic volcanites, Kamchatka || Dokl. AN SSSR,1987,Vol. 1987. 294, P. 182-186. (in Russia).

Seliverstov V.A., ïsipenko A.V/, Petrology of Kamchatka rodingites.Vladivostok: Dalnauka, 1999. 146 p (in Russia).

Wooley A., Church A. Extrusive carbonatites: A brief review.|| Lithos,2005,Vol.85. P.1-14