ABSTRACT

Rare earth element (REE) concentrations were precisely determined by inductively coupled argon plasma/atomic emission spectrometry (ICP) for 9 tholeiitic and 13 associated calc-alkaline rocks from Adatara volcano, located at the volcanic front of Northeast Japan arc. The tholeiitic samples commonly display nearly flat REE patterns, whereas the calc-alkaline samples characteristically show concave light REE-enriched patterns. In both suites, the REEs, with the exception of Eu, are positively correlated to varying extents with K₂O and other incompatible elements. Eu concentrations in the calc-alkaline suite do not vary significantly, resulting in significant negative Eu anomalies. In the tholeiitic suite, chondrite-normalized (Ce/Yb)_CN ratios are low and show little variation (1.54–1.80) over a significant range in SiO₂ content (52–62 wt%). In contrast, (Ce/Yb)_CN in the calc-alkaline suite show much greater variation (2.17 to 2.94) over a smaller range in SiO₂ content (56 to 62 wt%) and increase systematically with SiO₂. REE data for the tholeiitic suite are consistent with a crystal fractionation model that successfully reproduces the observed major and trace-element variations. A similar fractionation model is compatible with major-element and some trace-element variations for the calc-alkaline suite, but results in excessively high heavy REE contents, and fails to reproduce the elevated (Ce/Yb)_CN ratios for the most acidic samples. The preferential enrichment of light REEs may reflect additional processes such as: (1) assimilation, (2) incorporation of a differentiated liquid (or evolved magma) that has undergone fractionation of heavy REE-rich minerals, and (3) mixing of primary magma derived by a small degree of partial melting from a common (amphibolitic or hornblende gabbroic) source. Although the tholeiitic and calc-alkaline suites possess distinct ⁸⁷Sr/⁸⁶Sr, Rb/Ba and Zr/Nb ratios, different magma sources are not required to produce the distinct REE patterns of the coexisting suites. An equilibrium batch partial melting model can reproduce both the flat (tholeiitic) and concave (calc-alkaline) REE patterns from a common peridotitic source by larger (10–15%) and smaller (5%) degrees of partial melting, respectively. Therefore, both tholeiitic and calc-alkaline suites could have been derived from a common lherzolitic magma source, provided that the calc-alkaline suite underwent incorporation (assimilation) of isotopically distinct (lower crust-derived) materials prior to evolution.