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 [rkvsky, 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 as fragments of various sizes. Many fragments are in full or in part bleached, the size of the bleached fragments usually from 1 to 10-15 cm, and 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 ndradite, diopside, carbonates, and also apatite, strontianite, barite, sulfides. Thus some 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 rocks saturated by carbonates with content at a level of 42 % and SiO2 - 11-12 % -are very rare. For the first time they were described [Rass, Frih-Har, 1987] as carbonatites compared with such effusive rocks of some regions (Mongolia etc.). However in overwhelming majority of samples SiO2 content in carbonate-bearing rocks is at a level of 25-40 % (average 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, 2 - 0,03, 25 - 0,57).

But the main point is that in many volcanoclastic fragments one can see transitions without sharp contacts from such carbonate-bearing rock to picrites and picrobasaltes kept both structural features and initial composition .So carbonate-bearing rock is 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 [rkvsky, Rotman, 1981; Seliverstov, sipenko, 1999]. However composition of garnets (ndradite, not grossular) and in a first time some mineralogical and geochemical features, such as: presence Sr-apatite, Sr-carbonates (table 1) and very high Sr content (up to 60-80 thousands ppm) and content (up to 4,5 thousand ppm) testify, that these rocks are not rodingites, but specific metasomatic carbonatites. Probably, after E.M.Epstein such rocks must be called carbonatitides, in our case ppicrobasaltic and apopicritic. Among typical extrusive carbonatites enrichment of silicates is known (Fort-Portal etc)

According to our isotopic data (εNd = +8, 87Sr/86Sr=0,703743), carbonate-bearing fluid was formed as well as ultramafic volcanites in depleted mantle from a source functioned 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 low Nb content and rather moderate REE ontent in described carbonatitides. 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 carbonatitides, %.

 

BaO

CaO

SrO

SO3

P2O5

La2O5

CeO2

1.

-

38,24

7,77

-

53,55

0,12

0,27

2.

63,45

-

1,62

33,95

-

-

-

3.

0,78

7,80

59,71

-

-

-

-

4.

0,98

9,66

57,20

 

 

 

 

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

 

References:

rkvsky 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 carbonatites in upper cretaceous ultramafic volcanites, Kamchatka || Dokl. AN SSSR,1987,Vol. 1987. 294, P. 182-186. (in Russia).

Seliverstov V.A., sipenko 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


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