Variations in isotopic and trace-element composition of lavas from volcanoes of the Northern group, Kamchatka, in relation to specific features of subduction
On. Volynets et al., Variations in isotopic and trace-element composition of lavas from volcanoes of the Northern group, Kamchatka, in relation to specific features of subduction, GEOCHEM INT, 38(10), 2000, pp. 974-989
Two associations were revealed among the Northern group volcanoes in Kamcha
tka by study of chemical and isotopic composition of their lavas. The first
association includes volcanoes located north of the Kamchatka River, e.g.,
Shiveluch, Zarechnyi, Kharchinskii volcanoes, and Kharchinskii regional zo
ne of cinder cones. The second association comprises volcanoes located sout
h of this river, e.g., Klyuchevskoi, Ploskie Sopki, Tolbachik, Nikolka volc
anoes, as well as Tolbachik and Ploskie Sopki regional zones of cinder cone
s. The volcanic rocks of the first association differ from those of the sec
ond one in higher magnesium contents; elevated Sr; lower Ca, Sc, Y, and Yb;
higher Sr/Y, K/Ti, La/Yb, Zr/Y, Th/Yb, Ni/Sc, Cr/Sc and lower Ca/Sr and U/
Th ratios. Variations of Sr and Nd isotopic ratios in volcanics of these as
sociations overlap. The covariations between Sr and Nd isotope characterist
ics and isotopic ratios with some major- and trace-element contents differ
in these two associations. We concluded that the parental melts for the roc
ks of these associations were formed during the metasomatic transformation
of mantle wedge under the influence of two different agents: partial melts
generated within the subducted slab, for the northern volcanoes; and fluids
derived from the subducted slab, for the southern ones. This discrepancy i
s probably related to different subduction conditions: oblique, gently dipp
ing, slow subduction (like in the western Aleutians) beneath the northern v
olcanoes; and orthogonal subduction of ancient oceanic crust beneath the so
uthern ones.