Oxygen and strontium isotopic characteristics of calc-alkalic volcanic rocks from the central and western Japan arcs: Evaluation of contribution of crustal components to the magmas

Abstract

18O/ 16O and 87Sr/ 86Sr ratios were determined for Quaternary calc-alkalic volcanic rocks from six volcanic rock suites in the central and western Japan arcs. The δ 18O values relative to SMOW and 87Sr/ 86Sr ratios range from +6.3 to +9.9 0/ 00 and 0.70357 to 0.70684, respectively. Both the O- and Sr-isotopic compositions are higher than those for island-arc primitive magmas and their differentiates. The isotopic compositions of the calc-alkalic rocks cannot be derived by a simple fractional crystallization of the primitive magmas. On the other hand, the 18O- and 87Sr-enrichment is confined to the rock suites located in well-developed island arcs having thick continental-type crust with low or negative Bouguer anomalies. Involvement of 18O- and 87Sr-rich crustal material in the magma formation is suggested. The isotopic compositions vary remarkably within individual rock suites as well as from volcano to volcano. The data points in δ 18O vs. 87Sr/ 86Sr plot accord with a mixing model between primitive magmas and crustal material of dioritic composition on an average, assuming their comparative Sr contents. The primitive magmas involved could not be low-Sr tholeiites, but magmas more or less enriched in incompatible elements including Sr, which correspond to high-alkali tholeiites or alkali basalts and their evolved magmas. The nature of the primitive magmas seems to change from tholeiitic to more alkalic with progressing island-arc evolution. Mixing of crust-derived melts is more plausible than assimilation of solid-rocks for involving 20 to 30% crustal material in the magmas along simple mixing curves. Isotopic variations between the rock suites are ascribed to variable Sr concentration radio of the end-members, variable isotopic compositions of crustal material or variable mixing ratio of the end-members. Extremely high-δ 18O rocks with moderate increase in 87Sr/ 86Sr ratio suggest another mixing process in shallower magma chambers between andesite magmas and metasedimentary rocks having high δ 18O and 87Sr/ 86Sr values but low Sr content. Subsequent fractional crystallization of once-derived magmas would be the prominent process for the rock suites showing gradual increase in 18O up to 1 0/ 00 with uniform 87Sr/ 86Sr ratios.