Petrology of Mesozoic lamprophyres in Amami-Oshima, Kagoshima Prefecture, Japan

Large amounts of mafic igneous rocks are distributed in the Mesozoic formations of Amami-Oshima, Ryukyu Islands. Especially, the Yuwan Formation belonging to the upper Jurassic to lower Cretaceous contains large amounts of allochthones, and mafic rocks are intercalated in and/or intrude into allochthones and autochthonous part. Camptonite and spessartite occur in the Yuwan Formation as sheet and dyke forms. Camptonite is composed mainly of titan-augite, kaersutite, anorthoclase, plagioclase, and in parts aegirineaugite. Chief constituents of spessartite is Ti-rich pargasite, plagioclase, K-feldspar and chlorite after biotite. Chemically, camptonite belongs to typical alkali rock series of nenormative and is characteristically rich in Ti02, whereas spessartite is Q-normative. Titanaugite and kaersutite in camptonite have a wide compositional range. Titan-augite in camptonite shows the ranges of Mg/(Mg+Fe)=0.77-0.17, AlIV=0.22-0.51, AlVI=0.02-0, 13 and Ti=0.09-0.20, and shows hourglass structure of paler-coloured sector being rich in Mg and of deeper-coloured sector rich in Ti, Fe and AlVI. Ti/AlIV ratio of titan-augite are plotted between 1/2 and 1/3 lines. Compositional trend of titan-augite indicates substitutions of Ti+2Al for (Mg, Fe2+)+2Si and Fe2++Ti4++2Al for 2Mg+2Si. Aegirine-augite is the last of the mafic minerals to be crystallized, and shows wide ranges of Al=0.04-0.10, Fe3+=0.13-0.68, Fe2+=0.17-0.73, Ca=0.20-0.47, Na=0.39-0.82, whereas Mg is very low throughout wide compositional range of other components. The Mg-Fe2+-Na ratios of pyroxene in camptonite show a trend under extremely low fO2, condition similar to that of Ilimaussaq pyroxene. Kaersutite also has strong compositional zones from core to rim, showing distinct decrease of Mg and Ti, and increase of Fe, AlVI and Mn. It is suggested from the chemical characteristics of camptonite poor in SiO2 and extremely rich in TiO2 and alkalies that the situation of present area may have been oceanic islands or sea mounts during the late Jurassic to early Cretaceous.