Tholeiitic and calc-alkaline magma series at Adatara volcano, northeast Japan: 1. Geochemical constraints on their origin

Abstract

Low-alkali [“island arc” by Jakeš and Gill (1970) or “low-K” by Gill (1978)] tholeiitic rocks are closely associated with calc-alkaline rocks in the outer volcanic zone (called the Nasu volcanic zone) of northeast Japan. Major-element, trace-element (Ni, Cr, Co, Sc, V, Zr, Nb, Zn, Rb, Cs, Ba, Sr, Ce and Y) and Sr isotope compositions are presented for 9 low-alkali tholeiitic and 16 calc-alkaline samples of Adatara volcano belonging to the Nasu volcanic zone. In the trace elements vs. SiO 2 diagram the tholeiitic series and calc-alkaline series show different trends to each other for Co, Sc, V, Zr, Rb, Sc, Ba and Sr. As expected from K 2O contents, the calc-alkaline samples are enriched in incompatible elements such as Rb, Cs, Ba, Ce and Zr compared with the tholeiitic samples. On the other hand, contents of Nb in the calc-alkaline samples are somewhat lower than those in the tholeiitic samples. Plots of Ni and Cr contents vs. SiO 2 for the tholeiitic samples come near smooth trend lines whereas those for the calc-alkaline samples are scattered. Sr isotopic ratios in the tholeiitic series lie within the range between 0.7056 and 0.7058 and between 0.7048 and 0.7052 in the calc-alkaline series. Smooth major- and trace-element compositional trends in the tholeiitic series are successfully modelled by simple fractional crystallization of plagioclase, hypersthene, augite, titanomagnetite and minor olivine. Similarly, crystal fractionation models can closely reproduce major- and most of the trace-element variations in the calc-alkaline series, but failed to account for irregular variations of Ni and Cr. The scattering of Ni and Cr probably reflects simultaneous crystal fractionation and (subordinate) magma mixing in the subvolcanic chamber. The inferred fractionated phases for the calc-alkaline series are richer in augite and titanomagnetite and slightly poorer in plagioclase than those for the tholeiitic series. The tholeiitic magma and calc-alkaline magma are not derived from a common parental (tholeiitic) magma and the calc-alkaline parental magma is richer in SiO 2 and incompatible elements (in particular Rb and Zr), poorer in radiogenic Sr and possibly more oxidized than the tholeiitic parental magma.