Geochemical diversity of Neogene plutonic rocks, North Fossa Magna, Japan

Abstract

Neogene plutonic rocks disperse in three areas, Makihata-Tanigawa (MT), Suzaka-Ueda (SU) and Utsukushigahara-Kirigamine (UK) areas in North Fossa Magna, central Japan. The rocks consist of tonalite and granodiorite with granite and become young to MT via SU from UK areas. The plutonic rocks in UK area show higher Sr, Nb, ΣLREE/ΣHREE and La_N/Yb_N than those of MT area and the rocks of SU have both characteristics. It is indicated that partial melting degree for UK area was smaller than that for MT area. Initial Sr isotopic ratios of the rocks display positive correlation with Tin contents, suggesting that source magma affected by crustal materials. In εSrI –εNdI diagram, the plutonic rocks from UK and SU areas plot between fields of the Izu arc volcanic rocks and gabbroic xenoliths, whereas those of MT area indicate the same trend of Neogene volcanic rocks in northeast Japan arc, which are affected by continental crust or originated from lower crustal materials. It is considered that the plutonic rocks of UK and SU areas were derived from source magma for the Izu arc volcanic rocks affected by gabbro or gabbroic magma, and those of MT area were formed by the source magma reacted with continental or lower crustal materials.