Oxygen isotopic constraints on the geneses of the Cretaceous granitoids in the Kitakami and Abukuma terrains, Northeast Japan.

Abstract

Oxygen isotopic ratios (18 O/ 16 O) were measured on 48 whole rock samples for the Early Cretaceous granitoids of the Kitakami Mountains and the Abukuma Highland. Together with 10 published data, origins of these granitoids are considered. In the Kitakami Mountains, the granitoids generally belong to magnetite series and their initial Sr isotopic ratios are low (0.70363-0.70463). The Zone I granitoids are lowest in 18 O value, indicating they were originated in 18 O-depleted mafic rocks, such as underplated tholeiite. The 18 O values of the Zone II plutons where slab-melting has been proposed for the high-87 Sr granitoids and those of the Zone Va Tono-Kesengawa plutons are not particularly high but close to the average plots for the Kitakami granitoids. The 18 O values are relatively high in the K-alkalic Hinomiko and calc-alkaline Hitokabe plutons. Oxidized and K-rich metasomatized upper mantle may be needed to produce such a potassic magma at the Hinomiko body, but mafic igneous source rocks with intercalated terrestrial sediments may be enough to generate the calc-alkaline magma of the Hitokabe pluton. The Abukuma granitoids generally belong to ilmenite series and their initial Sr isotopic ratios are around 0.70518. Their 18 O values are higher than those of the Kitakami granitoids. The Abukuma granitoids of the western zone where metamorphic grade of the intruded rocks are higher than in the eastern zone, have slightly higher 18 O values than in the eastern zone. The two-mica granites of the western zone have the highest 18 O values and are least in magnetic susceptibility. These granites have the largest amount of sedimentary components of terrestrial origin in their protolith, while the other quartz diorite-granodiorite has had predominantly mafic igneous rocks in their source region.