Sr and Nd isotopic compositions of mafic xenoliths and volcanic rocks from the Oga Peninsula, Northeast Japan Arc: Genetic relationship between lower crust and arc magmas

摘要

Whole-rock and constituent hornblende and plagioclase geochemical and isotopic compositions of 52 mafic xenoliths from the Ichinomegata maar in the Oga Peninsula, located on the backarc side of Northeastern Japan, were investigated to further understand the nature of lower crustal materials beneath the Oga Peninsula. The inter-rock variations in isotopic compositions (~(87)Sr/ssSr ratios of 0.703245-0.705246 and ~(143)Nd/~(144)Nd ratios of 0.512910-0.512608) correlate negatively with ~(87)Rb/s6Sr and ~(147)Sm/~(144)Nd ratios. A continuous and arcuate trend in a Sr-Nd isotope diagram suggests a two-component mixing curve is present; at lower ~(87)Sr/~(86)Sr and higher ~(143)Nd/~(144)Nd ratios, this trend extends towards and partly overlaps Quaternary volcanic rock compositions from the Toga, Ichinomegata (xenolith-hosting pumice), and Kampu volcanoes on the Oga Peninsula (herein, Oga volcanic rocks). This overlapping suggests a common control on the isotopic variations within both xenoliths and volcanic rocks. This common control is most likely to be the metasomatism of intact original lower crustal material by parental magmas of the Oga volcanic rocks, herein termed the Oga parental magma, in addition to the contamination of the Oga parental magma by the crustal material after contact between the two. This metasomatism also caused isotopic re-homogenization of these constituent minerals, meaning hornblende and plagioclase within individual xenoliths have the same Sr-Nd isotopic compositions; i.e., they show no intra-rock variations, suggesting thermal re-setting. However, inter-rock variations imply that full metasomatism and destruction of the original isotopic and geochemical characteristics of the lower crust did not occur. These inter-rock variations are consistently present as changes in the geochemistry of constituent minerals, with K2O, Rb, Sr, Sm, and Nd concentrations varying in hornblende, and anorthite contents (An %) varying in plagioclase. The original lower crustal material, as one end-member of a mixing curve, is assumed to have a ~(87)Sr/~(86)Sr ratio of 0.705250 and a ~(143)Nd/~(144)Nd ratio of 0.512570, whereas the Oga parental magma, forming the other end-member on the mixing curve, has a ~(87)Sr/~(86)Sr ratio of 0.702958 and a ~(143)Nd/~(144)Nd ratio of 0.512933, as represented by Oga volcanic rocks with the lowest ~(87)Sr/~(86)Sr and highest ~(143)Nd/~(144)Nd ratios (stage 3 lavas from Kampu volcano near Ichinomegata maar). The isotopic compositions of other Oga volcanic rocks vary from the most contaminated Toga rhyolitic pumices (~(87)Sr/~(86)Sr of 0.703723-0.703885 and ~(143)Nd/~(144)Nd of 0.512906-0.512847) through to the Ichinomegata host pumice (~(87)Sr/~(86)Sr of 0.703398 and ~(143)Nd/~(144)Nd of 0.512895). This research indicates that both the Ichinomegata mafic xenoliths and the Oga volcanic rocks have undergone isotopic changes, creating an overlap in compositions caused by metasomatism and contamination, respectively. This relationship between lower crustal and volcanic rocks can be also be applied to trench-side volcanic rocks. If the Ichinomegata lower crustal material extends to the trench side of the NE Japan Arc, the island-arc tholeiites typically found within the volcanic front probably also contain a similar, if not identical, lower crustal component.