Petrogenesis of an Early Cretaceous lamprophyre dike from Kyoto Prefecture, Japan: Implications for the generation of high-Nb basalt magmas in subduction zones

Abstract

We studied a 107Ma vogesite (a kind of lamprophyre with alkali-feldspar>plagioclase, and hornblende±clinopyroxene±biotite) dike in the Kinki district of the Tamba Belt, Kyoto Prefecture, SW Japan, using petrography, mineralogy, K–Ar ages, and geochemistry to evaluate its petrogenesis and tectonic implications. The dike has the very specific geochemical characteristics of a primitive high-Mg basalt, with 48–50wt.% SiO2 (anhydrous basis), high values of Mg# (67.3–72.4), and high Cr (~431ppm), Ni (~371ppm), and Co (~52ppm) contents. The vogesite is alkaline and ne-normative with high concentrations of large ion lithophile elements (LILEs: Sr=1270–2200ppm, Ba=3910–26,900ppm), light rare earth elements (LREEs) [(La/Yb)n=58–62), and high field strength elements (HFSEs: TiO2=1.5–1.8wt.%, Nb=24–33ppm, Zr=171–251ppm), and the vogesite can be classified as a high-Nb basalt (HNB). The vogesite was formed by the lowest degree of melting of metasomatized mantle in the garnet stability field, and it may also have been formed at higher melting pressures than other Kyoto lamprophyres. The low degree of melting is the primary reason for the high-Nb content of the vogesite, not mantle metasomatism, and a higher degree of melting would have changed the primary magma composition from a HNB to a Nb-enriched basalt (NEB). The vogesite magma was contaminated at an early stage of its development by melts derived from sediments drawn down a subduction zone, as indicated by some geochemical indices and the initial Nd isotope ratios. The vogesite exhibits positive correlations between εSr(107 Ma) values (5.4–50.9) and its high Ba and Sr concentrations, and it has a limited range of εNd(107 Ma) values (+0.97 to +2.4). The fact that the vogesite contains centimeter-sized xenoliths of chert, which are composed of polycrystalline quartz, calcite, barite, pyrite, and magnetite, indicates that the barium contamination took place during the ascent of the lamprophyric magma through the upper crust. The episode of magmatism at c. 107Ma extended regionally from the Kinki district, through the Chugoku district and North Kyushu in SW Japan, to Korea as a result of slab roll-back at the eastern margin of Asia.