Episodic magmatism at 105 Ma in the Kinki district, SW Japan: Petrogenesis of Nb-rich lamprophyres and adakites, and geodynamic implications

Abstract

Cretaceous episodic magmatism produced Nb-rich lamprophyres and adakitic granitoids in the Kinki district of SW Japan. K–Ar dating of minerals from the lamprophyres, adakites, and hornblende peridotite xenoliths yielded ages of 109–99Ma, indicating a short-lived episodic magmatism. The lamprophyres generally display primitive high-Mg basaltic to basaltic andesite compositions with high Mg# and high Cr and Ni contents that preclude substantial differentiation. Some high-Nb basalt (HNB) and Nb-enriched basalt (NEB) compositions also occur. The lamprophyres have high large-ion lithophile element (LILE) and high field-strength element (HFSE) contents and variable (La/Yb)n ratios, and can be divided into high-(La/Yb)n (12.5–22.1) and low-(La/Yb)n (3.6–6.1) groups. The former contains nepheline-normative rocks with positive initial εNd(T) values, whereas the latter contains hypersthene-normative subalkaline rocks with negative initial εNd(T) values. The adakitic granitoids have relatively high TiO2, Nb, and Ta contents compared to more typical high-silica adakites elsewhere, indicating that they were produced by high temperatures (ca. 920 to 970°C) during slab melting.Early Cretaceous slab rollback and the accompanying asthenospheric upwelling at 105Ma could form a transitory thermal anomaly, and hence induce melting of the subducted slab to form adakitic granitoids, and produce metasomatized wedge mantle to form the lamprophyres. The high-(La/Yb)n lamprophyres originated from small degrees of partial melting of an enriched metasomatized mantle wedge within the garnet stability field at depths of ≥70km, whereas the low-(La/Yb)n lamprophyres originated from a different mantle source by a comparatively larger degree of partial melting in a relatively shallow part of the mantle wedge. The magmatic diversity of the Kyoto lamprophyres thus derives primarily from a heterogeneous mantle source that has been variably affected by the results of subduction.