HR: 1340h
AN: V13B-1463    [Abstracts]
TI: Calc-alkalic vs. tholeiitic series revisited: new insight from isotopic micro-analyses of plagioclase phenocrysts
AU: * Tatsumi, Y
EM: tatsumi@jamstec.go.jp
AF: IFREE/JAMSTEC, Natsu-shima, Yokosuka, 237-0061 Japan
AU: Takahashi, T
EM: toshirot@jamstec.go.jp
AF: IFREE/JAMSTEC, Natsu-shima, Yokosuka, 237-0061 Japan
AU: Shimizu, N
EM: nshimizu@whoi.edu
AF: WHOI, Woods Hole, Woods Hole, MA 02543 United States
AB: Major, trace and Sr isotopic compositions of plagioclase phenocrysts in calc-alkalic and tholeiitic rocks from Zao volcano, NE Japan were mapped with micro-analyses techniques. Plagioclase in tholeiitic rocks possesses a constant Sr isotopic ratio of ~0.7042, regardless with its Ca/Na ratio or the degree of magmatic differentiation. On the other hand, Sr isotopic ratios of plagioclase in calc-alkalic rocks are lower (0.7034-0.7041) than those for tholeiites, and more importantly, increase with decreasing Ca/Na ratios. Generally accepted mechanisms for production of those two types of arc magmas are fractional crystallization of a mantle-derived basalt magma for tholeiitic rocks and mixing between tholeiitic, mafic magmas and crust-derived, felsic magmas for calc-alkalic rocks. However, the above observation does not support this. Alternatively, tholeiitic magmas could be produced by anatexis of isotopically enriched, lower crustal material; calc-alkalic andesites are mixing products between a mantle-derived, isotopically depleted basalt magma and crust-derived, tholeiitic felsic magmas. Trace element characteristics of such mantle-derived, primary O^calc-alkalicO basalt are deduced based on plagioclase compositions and inferred distribution coefficients.
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting