Isotope-geochemical paradox of acid alkaline rock compositions of Paleogene contrasting formation of Amguema - Kanchalan volcanic field, Chukotka

Polin V.F.*, Sakhno V.G.*, Maksimov S.O.*, Sandimirov I.V.**

*Far East geological Institute, Vladivostok, Russia; **Vinogradov Institute of geochemistry, Irkutsk, Russia

Within the Amguema - Kanchalan volcanic field (AKVF) of the Okhotsk - Chukotka volcanogenic belt, the Senonian-Paleogene contrasting series is distinguished including the Paleogene bimodal formation of the marginal-continental-rifting type (Polin, 1990; Polin, Moll-Stalcup, 1999).

The bimodal formation is related with the belt areas emplaced on the crust with a thick granite-metamorphic layer (Eskimos massif). It is dominated by trachybasalts, trachydacites, and trachydaciandesites. Trachyandesibasalts, trachyandesites, trachyrhyolites as well as pantellerites, comendites, and alkaline granites are found in fewer amounts.

Manifestations of the sodic alkaline acid volcano-plutonism in the AKVF were controlled by sublatitudinal and submeridional extension structures and were practically synchronous to the opening of the “pull-apart basins” type structures with a voluminous basaltic volcanism. The sequence of magmatic events has been supported by isotopic datings (Polin, Sakhno, et al., 2007).

Comendites and alkaline granites in the AKVF make up large volcanic and plutonic domes about the southern periphery of the Belouvalensky volcanic graben and on the eastern flange of the Varenaisky volcano-tectonic trough. Pantellerites, composing the subvolcanic domes and dikes, are found in the near-fault troughs and volcanic grabens within the relics of basaltic volcanoes.

Geochemical comparison of alkaline acid rocks of the Paleogene contrasting formation with the rocks of the type geodynamic regimes showed them to be almost analogous to acid members of the intraplate bimodal complexes (Polin, Moll-Stolcap, 1999). In some characteristics they are similar to anorogenic granites.

Acid alkaline rocks of this formation were earlier considered as a result of the in-depth differentiation of trachybasalts or as derivatives of the low degree partial melting of a mantle source common with trachybasalts. New isotope-geochemical characteristics obtained made it necessary to revise this point of view and to correct the ideas of the sources of alkaline-salic melts.

The model Sm-Nd age of the alkaline rock sources (Table) is consistent enough: T_DM = 481-500 m.y., T_DM2 = 516-580 m.y. The age for the one-stage model is close to the geological age of the Eskimos massif that is a complex of the AKVF structure basement. As to isotopes, the alkaline rocks belong to ε_Nd(+) type with the neodymium/neodymium ratio values similar to those of the moderately depleted mantle. Calculation of the initial strontium-strontium ratios (Table) showed unlikely low values of this parameter in them (in pantellerite and alkaline granite – close to BABI, and in comendite and alkaline granite-porphyry – half as in BABI). So it is impossible to determine the interdistribution of the neodymium and strontium isotopic ratios. When it is taken that the initial strontium ratios in the rocks of the alkaline association are similar to those in the ancient granitic rocks (~ 0.700-0.702), then their figurative points will be close to the field lower boundary of the (plume) HIMU component and will rather approximately correspond to the uppers of the hyperbola of mixing for the rocks of the continental crust and continental tholeiitic basalts (For, 1989). So it is difficult to explain the genesis of acid alkaline rocks of the AKVF as derivatives from the mixing of basaltic melts and anatectic crustal meltings.

Table. Isotopic characteristics of alkaline rocks of Paleogene contrasting formation of áëVF

Note. Samples: PN-29-263 – pantellerite, ï-1408-3 – alkaline granite, PN-201-1515 – alkaline granite-porphyry, PN-203-1530 – comendite. Calculations of epsilon neodymium were done for the depleted mantle using the method of the IGGD, RAS, Saint-Petersburg, in accordance with data on absolute age of acid alkaline rocks: 59-61 m.y. (Polin, Sakhno et al., 2007). The initial ⁸⁷Sr/⁸⁶Sr ratios were calculated for the obtained values of the absolute potassium-argon datings.

The pattern of distribution of the neodymium and strontium isotopes could take place if the Sm/Nd and Rb/Sr ratios in the source of these rocks were initially small and did not change markedly since the time when Nd and Sr separated from the mantle reservoir. At the same time, Rb/Sr ratio in the alkaline acid melts must be initially high to explain the determined high values of ⁸⁷Sr/⁸⁶Sr (Table). As the latter are defined only by the Sr anomalously low content, especially in comendite and alkaline granite-porphyry, it is important to understand the cause of the remarkable “depletion” of the pantellerite-comendite series in the alkaline-earth cations including strontium. Such depletion is a specific geochemical feature of pantellerites, comendites, and alkaline granites of the sodium profile.

Two mechanisms can be proposed to explain this phenomenon. Through the first mechanism the alkaline-earth cations and, to a lesser degree, aluminum could be evacuated from the alkaline melts at the closing stages of their evolution under the influence of a high-acid residual magmatic fluid (Polin, Moll-Stolcap, 1999). The second mechanism implies the fluid-magmatic enrichment of the acid melt in silicates and/or chlorides of alkalis that broke protocrystalline feldspar “motifs” (calcic, barium, and strontium feldspar clusters). This resulted in a mobile state and evacuation of the divalent strong cations and produced an albite minal less capacious regarding aluminum and calcium. The latter defines the increasing agpaite content of the system through its evolution that is characteristic of such series.

The acid alkaline melts could originate in the chambers of the subalkaline acid magma that had been preserved from the preceding (Campanian-Maastrichtian) period of development of the trachydacite-trachyrhyolite-alaskite formation that also belongs to the contrasting series (Polin, 1990; and others). Their enrichment in sodium is logically explained by the processes of ascending diffusion through the parataxis and syntexis of acid and basitic magmas (Dobretsov, Dobretsov, 1983; Borisov, 2008).

According to the lead, lead, and neodymium isotope ratio, the points of the acid alkaline rocks fall into the field of volcanites of Iceland. No marked variability of compositions by the lead isotopes is observed, and there is an essential departure of them from the field of the above-subduction rock compositions. As the lead isotopes are sensitive indicators of the crust contamination, then the picture observed can seemingly testify that the Paleogene pantellerites, comendites, and alkaline granites are neither the crust melts nor the derivatives of basalt contamination with a significant amount of the crust material. However, taking into account the great volumes of acid alkaline rocks and sharp bimodality of the formation in the structures of their occurrence (absence of the rocks of the intermediate between trachybasalts and pantellerites-comendites composition), it is difficult to explain their melting from the mantle substratum or origination through the basaltic melt differentiation. More real is the generation of the alkaline-salic melts from remelting of the crust substratum juvenile by isotopic characteristics.

The isotope data obtained allow the conclusions as follows:

1) the source of the Paleogene alkaline acid magmas in the AKVF is a juvenile continental crust supplying the subalkaline acid material transformed into the alkaline one through the interaction of basitic and acid magmas in the peripheral chambers under the conditions of a probable local compression. Positive values of ε_Nd(T) are explained by a relatively short time interval since the moment of the formation of the crust source of alkaline rocks;

2) the paper has raised an important problem of disruption of the isotope systems at the late magmatic stage of the existence of the acid alkaline melts of a sodic geochemical profile; this disruption is necessary to be considered in the interpretation of isotope ratios both measured and initial.

This study was financially supported by the Projects of FEB, RAS, ¹ 09-1-ð14-02 and ¹ 09-1-ð16-02

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