Chemical compositions and evolution of alkaline melts of Gorely volcano (Southern Kamchatka): melt inclusions evidence.

Tolstykh M.L.*, Naumov V.B.*, Gavrilenko M.G.**, Ozerov A.Yu.**

* Vernadsky Institute of geochemistry and analytical chemistry, Moscow, Russia; ** Institute of Volcanology and Seismology (IVS), Petropavlovsk-Kamchatsky, Russia

Gorely volcano is known as a large long-living volcanic center at the Southern Kamchatka persisting its eruptive activity in recent time. This volcano is composed of two different volcanic structures, one of them being ancient and other is recent. The ancient structure (pra-Gorely) has a shield-like shape, its center is presented with a caldera of 12 x 13 km in diameter, the latter being formed along with enormous ignimbrite eruptions [2]. The recent structure (Young Gorely) occupies the central part of the caldera and is comprise of three conjuncted cones.

We studied samples of ignimbrites (caldera-forming stage), dacites (post-caldera stage), andesites and andesitic basalts of Young Gorely as well as olivine basalts presumably formed at the pra-Gorely stage. The bulk chemical and mineral composition, glass of melt inclusion composition in phenocrysts was studied. The melt inclusions in olivines and plagioclases were heated up to two phase (melt + gas) state and chilled. The homogeneous glasses were analysed by use of electrone microprobe (Cameca SX-100, Moscow) and ion microprobe (Cameca IMS-4f, Yaroslavl).

There is important difference in the compositions of the rocks and melts (tab.1). The melts demonstrate wider range in SiO₂ and alkalines.

NN	1	2	3	4	5	6	7	8	9	10
SiO₂	44,93	46,77	46,12	53,87	57,36	55,91	56,55	65,88	61,05	72,14
TiO2	0,6	0,31	1,29	2	1,98	0,59	1,62	0,75	0,46	0,65
Al₂O₃	17,02	20,76	17,8	15,66	15,85	20,89	14,99	17,4	16,91	12,48
FeO	9,54	8,9	7,9	9,28	7,22	1,67	8,8	2,77	2,36	2,69
MnO	0,17	0,17	0,17	0,19	0,17	0	0,19	0,09	0,04	0,1
MgO	6,44	8,95	9,24	2,36	2,19	3,22	2,3	0,67	1,47	0,53
CaO	10,18	7,96	11,09	7,13	6,41	5,23	6,1	3,26	2,91	1,39
Na2O	0,96	1,03	2,89	3,62	3,39	3,74	3,46	6,06	4,18	4,19
K₂O	5,55	3,28	0,8	2,5	2,8	6,14	2,6	2,74	7,79	3,49
P₂O₅	0,09	0,12	0,35	1,13	-	0,34	-	0,13	-	0,2
Cl	0,02	0	0,05	0,04	0,05	0	0,08	0,1	0	0,16
S	0,05	0	0,08	0,04	0,04	0	0,02	0,02	0	0,01
Total	95,55	98,25	97,78	97,82	97,46	97,73	96,71	99,87	97,17	98,03
Fo/An	Fo 83	Fo 84	Fo 82	54	57	60	59	42	40	37
Sample	Gor-161	Gor-161	Gor-161	Gor-46	Gor-188	Gor-60	Gor-60	Gor-19	Gor-19	Gor-11

Table 1. Representative chemical compositions of melt inclusions in olivine and plagioclase of the different rocks of Gorely.

All melt types of Gorely volcanic center are enriched of trace elements. LILE elements were found in highest concentrations excluding Sr as its behavior is related with the plagioclase fractionation; HFSE elements are also sufficiently high excluding Nb-minimum, being typomorphic for the island-arc rocks. The melts of Gorely volcano are specific in comparatively low Ti and Th concentration even in low differentiated melts as well as in high HREE elements.

***

In the TAS diagram (fig.1) the rocks are situated in the normal alkaline fields, but the part of the melts are of subalkaline types due to raised content of K₂O .

It is interesting, that the K- and Na-specialised varieties were found in the basic and acid melts. The maximum contents of K₂O in basalt melt is 5,5 wt.%, but in dacitic melt – 6,7%; it’s not too big excess. Hence, K-saturation is not connected with enrichment in the melts in the differentiation’s procecces. Though other incompatible elements (except Sr and the Ti directly participating in the fraction crystallisation) concentrate in the acid melts.

Sometimes the inclusions of high-K and low-K melts are found in one grain. Thus, it’s possible that such bimodality is a result of local heterogenity of the viscous alkalescent melt in the magma chamber or redistribution processes on a crystal-melt border.

The K₂O source is problematic. A presence of high-K varieties among high-Mg (MgO>8 wt.%, Cr> 400 ppm) from melt inclusions in Ol is assumes their deep origin. Probably, it’s result of reactions of a substratum with a subduction fluid, rich in LILE-elements.

But abnormal Li-enrichment of the andesitic melts (fig.2) might be explained by crust contamination [1]; thus high-K andesitic melts might contain a crust component too.

Fig. 1. TAS-diagram for rocks (markers) and melts (fields) of Gorely:

1 – basalt Gor-161; 2 – andesibasalts Gor-46, 188; 3 – andesite Gor-60; 4 – ignimbrite Gor-19; 5 – dacite Gor-11.

Fig. 2. Spider-diagram for melts of Gorely (by average composition):

1 – Na-basic melts; 2 – K-basic melts; 3 – andesitic melts; 4 – dacitic melts. P.m. by [3]

References:

Portnyagin M.V., Naumov V.B., Mironov N.L., Belousov I.A., Kononkova N.N. Melt’s composition and evolution of the eruption of 1996 y. in Karymsky lake (Eastern Kamchatka). Geochemistry, 2011, in press.

Selyangin, O.B., Ponomareva, V.V., 1999. Gorelovsky Volcanic Center, Southern Kamchatka: Structure and Evolution. Volcanol. Seismol. (in Russian) 2, 3–23.

Sun, S.-s., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins, Vol. 42. Geological Society, pp. 313–345.