Abstract This chapter describes the geological occurrences, mineralogical and geochemical diversity and petrogenetic importance of the ma?c and ultrama?c igneous (mainly plutonic) rocks in Japan. This chapter is divided intofive parts: (1) time-space distribution of ophiolitic rocks in Japan (AI); (2) ophiolitic rocks in SW Japan (AI); (3) ophiolitic rocks in Hokkaido (AI); (4) ophiolitic rocks in NE Honshu (KO); and (5) ultrama?c xenoliths from Japan (SA, SI, NA and MT).

Abstract LA–ICP-MS (laser ablation–inductively coupled plasma-mass spectrometry) has the potential to measure uranium concentration for fission-track (FT) chronometry as an alternative to thermal neutron-induced fission of 235 U. This study examines the effect that chemical etching, required to reveal spontaneous fission tracks of 238 U, has upon LA–ICP-MS analyses. Uranium concentrations were measured before and after etching for six large gem-quality apatite crystals and six zircon samples – three large crystals and three FT age standards. Comparison of the results shows no significant difference in 238 U concentrations measured on the etched and unetched mineral surfaces. The 238 U concentrations determined by the LA–ICP-MS provide reasonable FT ages for the zircon age standards, which, with the previously reported LA–ICP-MS apatite FT results, promotes the use of the LA–ICP-MS for FT chronometry.

Ultramafic and mafic xenoliths entrained in late Oligocene dikes of intraplate origin provide information about the composition of the lower crust and the processes operating beneath the eastern Rhodope metamorphic core complexes Biala Reka and Kesebir, in southeastern Bulgaria. The cumulates comprise a series of high- to medium-pressure rocks represented by olivine websterites, orthopyroxenites, clinopyroxenites, websterites, and gabbros. Thermobarometric studies and comparison with experimental works suggest that the cumulate sequence formed from hydrous (>3 wt%) mafic magma at pressures of 14–9 kilobars (45–30 km) and temperatures of 1200–850 °C. It is inferred that underplating of such hot, wet mafic intrusions modified the thermal and mechanical properties of the lower and middle crust, as is reflected in thermal metamorphism, associated extension, and hydrothermal activity producing low-sulfidation Au deposits. Findings of similar xenoliths in the alkaline basalts from other extended regions such as the eastern Mediterranean and the Basin and Range Province indicate that underplating of mafic magma plays an important role in core complex formation.