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,
Within the Amguema - Kanchalan volcanic
field (AKVF) of the
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: TDM = 481-
Table. Isotopic
characteristics of alkaline rocks of Paleogene
contrasting formation of áëVF
Sample |
143Nd/144Nd |
ε60Nd |
Sr, ppm |
87Sr/86Sr |
(87Sr/86Sr)i |
206Pb/204Pb |
207Pb/204Pb |
208Pb/204Pb |
PN-29-263 |
0,512832 |
4.44 |
17 |
0.717699 |
0.698936 |
18.146000 |
15.394000 |
37.928001 |
ï-1408-3 |
0,512806 |
4.0 |
15 |
0.717857 |
0.689869 |
18.358000 |
15.478000 |
38.209999 |
PN-201-1515 |
0,512803 |
3.91 |
2 |
0.735698 |
0.475475 |
18.319000 |
15.418000 |
38.028000 |
PN-203-1530 |
0,512789 |
3.64 |
5 |
0.738847 |
0.649953 |
18.325001 |
15.367000 |
37.865002 |
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-
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
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 εNd(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
References:
Borisov A.A. Experimental
study of K and Na distribution between mixed liquids// Petrologiya
(Petrology). 2008. V. 16. N 6. P.
593-605 (in
Dobretsov G.L., Dobretsov N.L. Problem of genesis of alkaline-salic rocks// Geologiya i Geophyzika (Geology and
Geophysics). 1983. N 1. (277). P. 69-73 (in
Polin
V.F. Petrology of contrasting series of the Amguemo-Kanchalansky
volcanic field, Chukotka.
Polin V.F., Moll-Stalcup E. J. Petrological-geochemical
criteria of tectonic conditions of formation of Chukotka
member of the
Polin V.F., Sakhno V.G., Ekimova N.I., Sandimirova G.P. Pantellerite – comendite - alkaline-granite association of Paleogene bimodal formation of the Okhotsk - Chukotka volcano-plutonic belt // Doklady
Akademii Nauk (Reports of
the Russian Acad. Sci.). 2006. V. 407. N 3. P. 388-393 (in
Epelbaum M.B. Silicate
melts with volatile components.