ABSTRACT: Magmatic Zonation of the Kurile Island Arc
During the past decade Soviet r/v Volcanolog made six cruises in the Kurile Island arc conducting multipurpose geological-geophysical investigations practically on each of its 86 submarine volcanoes. The obtained results enabled us to decipher spatial-structural distribution of the Kurile Quaternary volcanoes and compositional patterns of their lavas. Correlating available information on submarine and onland volcanics one can notice that concentrations of LIL and HFS elements just as Mg, Ni, Cr quite distinctively increase from volcanic front toward the arc's rear while concentrations of Fe and V, just as 87Sr/86Sr and 143Nd/144Nd ratios distinctively decrease. Substantial difference is also reported for modal compositions of la as and their phenocryst-microlith assemblages. Typical of the intermediate-acid rocks from the frontal zone are two-pyroxene assemblages while those from the back-arc zone are amphybole-biotite, the latter two minerals typical even to some basalts and basaltic andesites.
Chemical differences between magmas generated in the two structural zones of the Kuriles are stressed by composition of inclusions. Volcanic front lavas are practically free of ultramafic nodules while in the back-arc zone they are fairly common (Chirinkotan, Brouton Islands, etc). Gabbroic and metamorphic inclusions in the frontal zone like lavas do not contain any H2O-bearing phases while those of the back-arc zone are usually amphibole-mica bearing.
The available data show that compositional dissimilarity of Quaternary lavas across the Kurile volcanic chain do not depend on the thickness of the crust but rather are related to chemical differences of magmatic sources and melting conditions at depths. Most likely Kurile magmas were generated within the mantle wedge with the substantial contribution from the fluids which composition in the frontal zone was determined by dehydration of the down-going oceanic slab (very close to seawater) and in the back-arc zone by phase transitions at higher PT conditions.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990