Isotope Characteristics of Withinplate Geochemical Type Basalts From Kamchatka Island-Arc System:
Evidence of Composite Magma Source

O. N. Volynets Institute of Volcanic Geology & Geochemistry RAN Piip blvd. 9

Petropavlovsk-Kamchatsky 683006, Russia

S. F. Karpenko Institute of Geochemistry RAN Kosygina ave. 19 Moscow 117975 Russia

V. I. Vinogradov Geologic Institute RAN Pyzhevsky per. 17 Moscow 109017 Russia

B. G. Pokrovsky Geologic Institute RAN Pyzhevsky per. 17 Moscow 109017 Russia

R. W. Kay Department of Geological Sciences, Snee Hall, Cornell University, Ithaca, NY 14853, USA

M. Gorring Department of Geological Sciences, Snee Hall, Cornell University, Ithaca, NY 14853, USA

Withinplate (WP) geochemical type lavas were found among the Late Cenozoic volcanics of Kamchatka, although island-arc (IA) geochemical type volcanics dominate in this region. WP volcanic rocks are divided into K-Na alkali basalt, K-Na alkali olivine basalt and K-Na basalt-comendite geochemical series. Detailed study of mineralogy, major and rare elements chemistry of both WP and IA volcanic rocks permitted to speculate on the various magma sources of their primary melts and study of geological position of IA and WP lavas in Kamchatka tectonic structure let us to suppose that these magma types are related to different geodynamic processes. Now Sr, O, Nd and Pb isotopes composition of WP basalts was studied to check this speculation. For comparison IA basalts isotope composition was also studied. The summary results are shown below.

-The basalts of IA rock series: 87Sr/86Sr = 0.70304 - 070382 (n = 45); d18O = 5.2 ½ 6.5” (n = 20); eNd = +7.3 - +9.6 (n=6).

-The basalts of alkali olivine basalt and basalt-comendite rock series: 87Sr/86Sr = 0.70313 - 0.70386 (n = 20); d18O = 5.5 - 6.6” (n = 5); eNd = +4.4 - +8.7 (n = 9).

- The basalts of alkali basalt series: 87Sr/86Sr = 0.70362 - 070512; d18O = 6.0 - 10.4”(n = 10); eNd = +2.0 - +7.1 (n = 14); 206Pb/204Pb = 17.880 - 18.005 (n = 5); 207Pb/204Pb = 15.470 - 15.537 (n = 5); 208Pb/204Pb = 37.950 - 38.239 (n=5).

These results indicate that Sr and O isotope composition in the basalts of alkali olivine basalt and basalt-comendite series is similar to this of IA basalts although eNd values in the rocks of alkali olivine basalt series are intermediate between the values typical for IA and alkali basalt rock series. The basalts of K-Na alkali basalt series differ from IA basalts in isotope composition of all studied elements. The correlation of Sr , Nd, O and Pb isotope ratioes with some geochemical parameters of lavas (Mg#, Na2O+K2O, LaN/SmN, Sm/Nd) is also found for K-Na alkali basalts. Less magnesian lavas with higher alkali content, higher LaN/SmN and Sm/Nd ratioes are characterized by higher values of 87Sr/86Sr, d18O, 208Pb/204Pb and 207Pb/204Pb but less eNd and 206Pb/204Pb values compared to more magnesian ones. Probably these features indicate the participation of mixing processes in primary alkali basalt magma genesis and composite magma source. We think that this source includes the matter of mantle plume which uplifted from the deep undepleted mantle level and the matter of depleted mantle wedge. The primary alkali basalt magmas were produced
by the melting of uplifted mantle plume as a result of decompression and then they interacted with adjacent plume depleted mantle.


This research was supported by Russian Foundation of Fundamental Investigations, Grant 93-05-8521.