Petrology of Holocene sub-alkaline basalts of Sedankinsky Dol, Kamchatka.

Shur M.Yu. *, Plechov P.Yu. *, Dirksen O.V. ** Bazanova L.I. **

* Geological department of Moscow State University,

** Institute of Volcanology and Seismology, Petropavlovsk-Kamchatsky, Russia

The province of Quaternary volcanism of the Middle Ridge of Kamchatka occupies its watershed and western slopes of the ridge. It covers more than 50% of the ridge area. The total length of the province is 450 km with width varying from 30 up to 100 km. Geological effect of the Quaternary volcanism of the Middle Ridge is quite comparable with those of the Eastern Volcanic Front. There are more than 120 polygenic volcanoes with the diameter of the base from 2-3 up to 45 km and about 1000 small monogenetic volcanoes and lava cones in this province (Ogorodov et al, 1972).

Sedankinsky Dole is situated on the northern watershed of the Middle Ridge of Kamchatka. It includes more than 300 eruptive centres - small shield volcanoes and monogenetic cones of Pleistocene and Holocene ages. The average density of eruptive centers here is about 0.4 center/km ², sometimes increasing to 1-2 center / km ².

The age division of the Holocene eruptive centers was conducted by tephra-chronological method with using of radiocarbon dating. Transit volcanic ash of different ages were discovered and dated in Sedankinsky Dol. The majority of them belongs to volcano Shiveluch, the closest active volcano with numerous explosive eruptions, which took place 950, 1900, 2550, 2800, 3800, 5700 and 8300 ¹⁴C years ago. Volcanic ash of several other volcanoes were also found there – ash from volcano Opala (with age 1500 ¹⁴C years), Ksudach (1800 ¹⁴C years) and Hangar (6900 ¹⁴C years). This plenty of datum marks gives us the opportunity to precisely estimate the age of Holocene volcanic eruptions in Sedankinsky Dol. It was determined that there were 21 eruptive events in this region in Holocene with a total volume of erupted material about 3 km³. This volcanic activity was not continuous, it manifested in the form of two pulses - in the Early Holocene (8000-9500 ¹⁴C years ago) and in the Late Holocene (2550-2800 ¹⁴C years ago) with a break more than 5000 years between them. There were 15 active eruptive centers in the territory of Sedankinsky Dol in the Early Holocene, and about 1.89 km³ of volcanic material was erupted on the surface during this period. In the Late Holocene 6 new volcanic buildings were formed and the total volume of erupted material was about 1 km³.

We chose representative samples of lava and tephra of both Holocene pulses of volcanism for a detailed study and comparison. They belong to 9 eruptive centers - 4 of the Early Holocene age and 5 of the Late Holocene age.

All the studied rocks are olivine basalts and are very similar to each other in their petrography. In the rocks of both age groups phenocrysts of the first generation are represented only by olivine and plagioclase. Clinopyroxene is completely absent, although it was detected in the form of solid inclusions in olivine crystals. Phenocrysts of olivine are strongly resorbed in the rocks of both age groups. In addition to rare inclusions of clinopyroxene, olivine cryatals also contain chrome-spinel, plagioclase and melt inclusions. The bulk of the studied rocks is composed of plagioclase, clinopyroxene, olivine, magnetite and a small amount of volcanic glass. It has a different degree of crystallization (reflecting the position of the sample in the beam). Porosity of the investigated rocks varies from 5 to 20%, pore size - from 0,1 up to 8 mm.

We studied the compositions of 330 olivine phenocrysts (130 from the Late Holocene basalts and 200 from the Early Holocene rocks). In all the studied samples olivine has no zonality. The range of variation of composition of olivine phenocrysts (Fo₇₅-Fo₈₇) is approximately the same for basalts of the both age groups. And besides, the extreme values of Mg (Fo_75-77 and Fo_86-87) are very rare in all the rocks. The contents of CaO (about 0,2 weight.%), MnO (about 0,3 weight.%), NiO (approx. 0.2 wt.%) and Cr₂O₃ (approx. 0.02%) are similar in olivine from the rocks of both age groups .

Phenocrysts of plagioclase in all rocks have strong oscillating zoning, which makes the estimation of the differences in the compositions of plagioclase from two age groups of basalts very difficult. We have studied the features of zoning of 32 plagioclase phenocrysts from the Early Holocene basalts and 16 phenocrysts from basalts of the Late Holocene age. The range of variation of plagioclase composition in them is roughly the same. Often plagioclases from rocks of the same age group differ significantly greater than plagioclases from rocks of different age. The following zoning is typical to the plagioclase phenocrysts of both age groups: The most basic plagioclase occurs in the central parts of crystals, its composition varies from An₇₂ to An₇₉ (and one relict of plagioclase An₈₉ was found). This zone is followed by the rim of more acid plagioclase An₆₅ - An₆₉ (in rare cases up to An₅₀), which in turn is enclosed in the rim of enhance of the content of the anorthite component up to An₇₂ - An₇₄. Sometimes crystals may have some more rims of increasing and decreasing of the anorthite content (it varies within the same range of An₆₅ - An₇₄). The composition of microlites of plagioclase and the marginal parts of its phenocrysts varies from An₄₀ up to An₅₈.

As it was mentioned above, phenocrysts of olivine often contain grains of chrome-spinel. The composition of 67 of such chrome-spinel crystals was studied (44 from the Early Holocene basalts and 23 from the Late Holocene rocks). It was determined that the composition of chrome-spinels vary within wide limits, regardless of age of rocks. The content of chromium (Cr/R3) in all chrome-spinels is relatively low (0,1-0,3), with a weak inverse correlation with the content of magnesium. The dominant trivalent cation is aluminum (Al/R3 is 0,3-0,8). The content of ferric iron (Fe³⁺ / R3) varies in the same range as the chrome (0,1-0,3) and is directly correlated with it. The content of TiO₂ in the studied chromites differs from 0,5 up to 5,6 wt.% and has a clear inverse correlation with magnesium. The content of magnesium (Mg/Mg+Fe²⁺) in chrome-spinels varies from 0,4 to 0,7 and directly correlates with the content of forsterite in olivine host.

The oxygen fugacity and the temperature of crystallization were calculated, using the compositions of coexisting spinel and olivine (Ballhaus et al., 1991). Oxygen fugacity was estimated as Ni-NiO - Ni-NiO +1 for basalts of the both age groups .

Olivine phenocrysts in all studied rocks contain melt inclusions. We examined 150 melt inclusions (99 from the Early Holocene and 51 from the Late Holocene basalts). Recalculation of compositions of melt inclusions to the compositions of initial melts was carried out using the program Petrolog-3 with the model «Fe-loss» (Plechov, Danyushevsky, 2006). The compositions of derived melts form a rather large field - the content of SiO₂ ranges from 46 to 55 weight%, TiO₂ – from 1 to 2,5 weight%, Al₂O₃ – from 13 to 19 weight%, MgO – from 5 to 8, 5 weight%, CaO – from 6 to 11 weight%, Na₂O - from 2,5 to 5,5 weight% and K₂O from 0,6 to 2,5 weight%. And besides the extreme values of ranges belong to melts from the Late Holocene eruptive centers, while compositions of melts from the Early Holocene basalts are located between them. Thus, there is no distinct dependence of the composition of melts and the time of eruption. But some correlation between the composition of melts and the altitude of the eruptive center was found. These relationships are illustrated here by the graph of distribution of the content of K₂O in melts, averaged for each of the studied rocks. Similarly, with the increasing of altitude of the eruptive center the content of SiO₂ in melts increases. The content of MgO, CaO and Al₂O₃ have a distinct reverse trend.

References:

1. Ogorodov N.V., Kozhemyaka N.N., Vazheyevskaya A.A., Ogorodova A.S. (1972) Volcanoes and Quaternary Volcanism of the Middle Ridge of Kamchatka. Moscow: Nauka, 1972. 192 pp. (in Russian)

2. Ballhaus C., Berry RF, et al. (1991) "High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle." Contributions to Mineralogy and Petrology 107: 27-40, 1991.

3. Plechov P.Y., Danyushevsky L.V. "PETROLOG III. Simulation of equilibrium and fractional crystallization" / / Proceedings ESMPG-2006, Electronic Scientific Information Journal <Herald of the Department of Earth Sciences RAS "1 (24), 2006 (in Russian)