High-Mg# andesitic lavas of the Shisheisky Complex,Northern Kamchatka: implications for primitive calc-alkaline magmatism |
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| Authors: | J A Bryant G M Yogodzinski T G Churikova |
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| Institution: | (1) Department of Earth and Ocean Sciences, University of South Carolina, 701 Sumter Street, Columbia, SC 29208, USA;(2) Electron Microscopy Center, University of South Carolina, 715 Sumter Street, Columbia, SC 29208, USA;(3) Institute of Volcanology and Seismology Far East Division, Russian Academy of Sciences, 9 Piip Avenue, Petropavlovsk-Kamchatsky, 683006, Russia; |
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| Abstract: | Primitive arc magmatism and mantle wedge processes are investigated through a petrologic and geochemical study of high-Mg#
(Mg/Mg + Fe > 0.65) basalts, basaltic andesites and andesites from the Kurile-Kamchatka subduction system. Primitive andesitic
samples are from the Shisheisky Complex, a field of Quaternary-age, monogenetic cones located in the Aleutian–Kamchatka junction,
north of Shiveluch Volcano, the northernmost active composite volcano in Kamchatka. The Shisheisky lavas have Mg# of 0.66–0.73
at intermediate SiO2 (54–58 wt%) with low CaO (<8.8%), CaO/Al2O3 (<0.54), and relatively high Na2O (>3.0 wt%) and K2O (>1.0 wt%). Olivine phenocryst core compositions of Fo90 appear to be in equilibrium with whole-rock ‘melts’, consistent
with the sparsely phyric nature of the lavas. Compared to the Shisheisky andesites, primitive basalts from the region (Kuriles,
Tolbachik, Kharchinsky) have higher CaO (>9.9 wt%) and CaO/Al2O3 (>0.60), and lower whole-rock Na2O (<2.7 wt%) and K2O (<1.1 wt%) at similar Mg# (0.66–0.70). Olivine phenocrysts in basalts have in general, higher CaO and Mn/Fe and lower Ni
and Ni/Mg at Fo88 compared to the andesites. The absence of plagioclase phenocrysts from the primitive andesitic lavas contrasts
the plagioclase-phyric basalts, indicating relatively high pre-eruptive water contents for the primitive andesitic magmas
compared to basalts. Estimated temperature and water contents for primitive basaltic andesites and andesites are 984–1,143°C
and 4–7 wt% H2O. For primitive basalts they are 1,149–1,227°C and 2 wt% H2O. Petrographic and mineral compositions suggest that the primitive andesitic lavas were liquids in equilibrium with mantle
peridotite and were not produced by mixing between basalts and felsic crustal melts, contamination by xenocrystic olivine,
or crystal fractionation of basalt. Key geochemical features of the Shisheisky primitive lavas (high Ni/MgO, Na2O, Ni/Yb and Mg# at intermediate SiO2) combined with the location of the volcanic field above the edge of the subducting Pacific Plate support a genetic model
that involves melting of eclogite or pyroxenite at or near the surface of the subducting plate, followed by interaction of
that melt with hotter peridotite in the over-lying mantle wedge. The strongly calc-alkaline igneous series at Shiveluch Volcano
is interpreted to result from the emplacement and evolution of primitive andesitic magmas similar to those that are present
in nearby monogenetic cones of the Shisheisky Complex. |
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