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Journal of Petrology | Volume 43 | Number 10 | Pages 1923-1946 | 2002
© Oxford University Press 2002

Chemical Diversity of the Ueno Basalts, Central Japan: Identification of Mantle and Crustal Contributions to Arc Basalts

JUN-ICHI KIMURA1,*, WILLIAM I. MANTON2, CHIH-HSIEN SUN2,{dagger}, SHIGERU IIZUMI1, TAKEYOSHI YOSHIDA3 and ROBERT J. STERN2

1DEPARTMENT OF GEOSCIENCE, SHIMANE UNIVERSITY, MATSUE 690-8504, JAPAN
2GEOSCIENCES DEPARTMENT, UNIVERSITY OF TEXAS AT DALLAS, DALLAS, TX 75083-0688, USA
3INSTITUTE OF MINERALOGY, PETROLOGY AND ECONOMIC GEOLOGY, TOHOKU UNIVERSITY, AOBAKU, SENDAI 980-7763, JAPAN

The Ueno Basalts of central Japan comprise a monogenetic volcanic cone complex that was active between 2·76 and 1·34 Ma. Basalts were erupted at more than 14 centers scattered over a region 40 km in diameter. Alkali basalt was erupted first, followed by sub-alkaline basalt. Quasi-concentric expansion of eruption centers coinciding with uplift and with decreasing alkalinity of the lavas suggests that Ueno magmatism originated from a mantle diapir as it mushroomed at the base of the lithosphere. Depleted asthenospheric mantle (alkali basalt), enriched lithospheric mantle (sub-alkaline basalt), and crustal components are identified as chemical end-members in the petrogenesis of the Ueno Basalts. Incompatible trace element abundances indicate that the Ueno alkali basalts are typical within-plate basalts, whereas the sub-alkaline basalts show strong affinities with normal arc lavas. Sr–Nd–Pb isotopic compositions indicate that the mantle source of the alkali basalts was more depleted than that of the sub-alkaline basalts. About 7% melting of asthenospheric mantle in the garnet-lherzolite stability field produced the primitive alkali basalts and 12% melting of spinel lherzolite within the subcontinental lithosphere produced the primitive sub-alkaline basalts. Isotopic compositions and fluid mobile/immobile element ratios broadly covary with SiO2 contents in the sub-alkaline suite, and increasing silica content is associated with stronger EMII (Enriched Mantle II) isotope affinities and fluid mobile element abundances. A progressive AFC (assimilation–fractional crystallization) model assuming assimilation of a low-K silicic melt reproduces the chemical variations observed in the sub-alkaline suite. Melting of a flattening mantle diapir at the base of the lithosphere is the dominant cause of Ueno magmatism, accompanied by the assimilation of older arc crust.

KEY WORDS: arc basalt; crustal assimilation; mantle heterogeneity; Ueno Basalts


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